CA2202323C - Method and equipment for the characterisation of suspensions - Google Patents
Method and equipment for the characterisation of suspensions Download PDFInfo
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- CA2202323C CA2202323C CA002202323A CA2202323A CA2202323C CA 2202323 C CA2202323 C CA 2202323C CA 002202323 A CA002202323 A CA 002202323A CA 2202323 A CA2202323 A CA 2202323A CA 2202323 C CA2202323 C CA 2202323C
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/44—Processing the detected response signal, e.g. electronic circuits specially adapted therefor
- G01N29/52—Processing the detected response signal, e.g. electronic circuits specially adapted therefor using inversion methods other that spectral analysis, e.g. conjugated gradient inversion
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume, or surface-area of porous materials
- G01N15/02—Investigating particle size or size distribution
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/02—Analysing fluids
- G01N29/032—Analysing fluids by measuring attenuation of acoustic waves
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/04—Analysing solids
- G01N29/11—Analysing solids by measuring attenuation of acoustic waves
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/22—Details, e.g. general constructional or apparatus details
- G01N29/24—Probes
- G01N29/2456—Focusing probes
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/36—Detecting the response signal, e.g. electronic circuits specially adapted therefor
- G01N29/40—Detecting the response signal, e.g. electronic circuits specially adapted therefor by amplitude filtering, e.g. by applying a threshold or by gain control
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/44—Processing the detected response signal, e.g. electronic circuits specially adapted therefor
- G01N29/4409—Processing the detected response signal, e.g. electronic circuits specially adapted therefor by comparison
- G01N29/4427—Processing the detected response signal, e.g. electronic circuits specially adapted therefor by comparison with stored values, e.g. threshold values
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/44—Processing the detected response signal, e.g. electronic circuits specially adapted therefor
- G01N29/4472—Mathematical theories or simulation
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/01—Indexing codes associated with the measuring variable
- G01N2291/015—Attenuation, scattering
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/02—Indexing codes associated with the analysed material
- G01N2291/024—Mixtures
- G01N2291/02408—Solids in gases, e.g. particle suspensions
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/02—Indexing codes associated with the analysed material
- G01N2291/024—Mixtures
- G01N2291/02416—Solids in liquids
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/02—Indexing codes associated with the analysed material
- G01N2291/028—Material parameters
- G01N2291/02818—Density, viscosity
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/04—Wave modes and trajectories
- G01N2291/044—Internal reflections (echoes), e.g. on walls or defects
Abstract
Method and equipment for the detection and identification of particles in a suspension with the aid of acoustic signals, directed at at least one measurement volume within the suspension, via reception of acoustic reflection signals, conversion of the acoustic reflection signals into electrical reflection signals, counting the number of electrical reflection signals which have an amplitude in excess of a predetermined value and converting the count into numbers of particles which are larger than a certain size, at least one curve being composed on the basis of a cumulative count of the number of reflection signals which have an amplitude in excess of a specific value as a function of the amplitude and the at least one curve being compared with predetermined standard cumulative count curves and material properties, particle concentration, particle size distribution and/or particle characteristics, such as particle shape, particle size and standard deviation thereof, are deduced from the comparison.
Description
METHOD AND EQUIPI~NT FOR THE C~iARACTEFtISATION OF
SUSPENSIONS
The present invention relates to a method for the detection and identification of particles in a' suspen-sion, comprising the following steps;
a. generation of acoustic signals having the form of a beam using an acoustic source;
b. directing the acoustic signals at at least one measurement volume Within the suspension, the bound aries of the measurement volume in the axial direction pith respect to the acoustic source being defined With the aid of time windows;
c. reception of acoustic zeflectiorr signals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. conversion of the acoustic reflection signals into electrical reflection signals;
e. counting numbers of electrical reflection signals Which have ae amplitude in excess of a predetermined value and conversion thereof into numbers of particles which are larger than a certain size.
A method of this type is disclosed in Hritiah pat-ent 1,012,010, which describes a method arid equipment for counting and measuring such particles, wherein acoustic samples are taken in various measurement vol-umes along the acoustic axis of the acoustic transducer in the suspension. 8y using suitable time windows when receiving reflected acoustic signals, the particles in, 5. for example, four predetermined measurement volumes, which are each located a predetermined distance away from the transducer, are counted. By making use of a threshold voltage which the electrical signals produced from the acoustic signals must exceed is order to be t0 counted, which threshold voltage is different for each zone, a minimum size for the particles to be counted is selected for each zone. Assuming that the particle dis-tribution is the same is each zone, a rough estimate of the number of particles, subdivided according to par-15 title size, can be obtained using this known method and using aimp~e mathematical methods.
US Patent 3 774 717 describes a method and equip-ment for the~detection and identification of small par-20 titles, for example biological cells, Which, for example, are located in a medium which flows transversely to the direction of propagation of an acoustic signal. Each of the particles gives a specific scatter of the acoustic signal, depending on the size, 25 the shape and the acoustic impedance of the particles.
The acoustic signal has a wavelength and an effective cross-section of the order of magnitude of the par-ticles to be detected and identified. Blood cells, for example, are detected with the aid of an acoustic sig-30 nal of 860 ~lz. Therefore, particles can be identified with the aid of techniques which are known from radar technology. The technique disclosed in this patent is unsuitable for in vivo detection and identification because the wavelengths used allow only very restricted 35 depths of penetration in biological tissue.
SUSPENSIONS
The present invention relates to a method for the detection and identification of particles in a' suspen-sion, comprising the following steps;
a. generation of acoustic signals having the form of a beam using an acoustic source;
b. directing the acoustic signals at at least one measurement volume Within the suspension, the bound aries of the measurement volume in the axial direction pith respect to the acoustic source being defined With the aid of time windows;
c. reception of acoustic zeflectiorr signals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. conversion of the acoustic reflection signals into electrical reflection signals;
e. counting numbers of electrical reflection signals Which have ae amplitude in excess of a predetermined value and conversion thereof into numbers of particles which are larger than a certain size.
A method of this type is disclosed in Hritiah pat-ent 1,012,010, which describes a method arid equipment for counting and measuring such particles, wherein acoustic samples are taken in various measurement vol-umes along the acoustic axis of the acoustic transducer in the suspension. 8y using suitable time windows when receiving reflected acoustic signals, the particles in, 5. for example, four predetermined measurement volumes, which are each located a predetermined distance away from the transducer, are counted. By making use of a threshold voltage which the electrical signals produced from the acoustic signals must exceed is order to be t0 counted, which threshold voltage is different for each zone, a minimum size for the particles to be counted is selected for each zone. Assuming that the particle dis-tribution is the same is each zone, a rough estimate of the number of particles, subdivided according to par-15 title size, can be obtained using this known method and using aimp~e mathematical methods.
US Patent 3 774 717 describes a method and equip-ment for the~detection and identification of small par-20 titles, for example biological cells, Which, for example, are located in a medium which flows transversely to the direction of propagation of an acoustic signal. Each of the particles gives a specific scatter of the acoustic signal, depending on the size, 25 the shape and the acoustic impedance of the particles.
The acoustic signal has a wavelength and an effective cross-section of the order of magnitude of the par-ticles to be detected and identified. Blood cells, for example, are detected with the aid of an acoustic sig-30 nal of 860 ~lz. Therefore, particles can be identified with the aid of techniques which are known from radar technology. The technique disclosed in this patent is unsuitable for in vivo detection and identification because the wavelengths used allow only very restricted 35 depths of penetration in biological tissue.
The methods described above are based on the ultrasonic pulse-echo technique. With this technique use can be made of a so-called ultrasonic transducez, which converts an applied electrical pulse into an acoustic (ultrasonic) signal and which is also capable of converting an acoustic (ultrasonic) signal which is incident on the reception surface back into an electri-cal signal. Therefore, the transducer serves as trans-mitter and as receiver foz ultrasonic signals. It is also possible to use independent transmitters and receivers. A high-frequency pulse-echo'reeording of the flowing suspension is made. A focusing transducer can be used for this. In Figure 1 the "illuminating" sound signal is shown diagrammatically in an arrangement known per se.
As is shown in Figure 1, the principal axis z of the sound beam 20 generated by a transducer 23 is per-pendicular to the direction of flaw P of the suspension 21 flowing in a channel 24 acrd, consequently, to the direction of movement of the particles 22 present in the suspension: When a particle 22 passes through the sound beam 20, the incident sound field till be reflected by the particle and the reflected signal will be captured by the transducer 23. The received signal is converted by the transducer 23 into an electrical signal, which is transmitted to the transmission and reception electronics 25. The transmission and recep-tion electronics 25 transmit the signal to a computer 26, which is connected to a memory 27 for storing measurement data. The computer 26 is provided with suitable software for evaluation of the measurement data. The. electrical signal froth a single measurement is indicated diagrammatically by s and is a time sig-nal, the time axis t indicating the propagation time of the sound. The response of the particle 22 in the meas-urement volume can be detected in the recording at that moment in time which corresponds to the' propagation time of the ultrasonic pulse between transducer I3 and reflecting particle 22 and back again.
Only reflections within an applied time ~riadow [t~, t2] (see Figure 1) are processed fn the analysis. The measurement volume is thus lien~.ted in the axial direc-tion by zi = ti.c/2 and z2 - tz.c/Z (c i6 the spae8 of ptopagation of the sound). In the lateral direction the measurement volume is limited by the shape of the acoustic beam.
A methed for counting the number of particles using a measurement set-up of this type is described in the above-mentioned patents and in Groetsch, J.G.:
"Theory and application of acoustic particle monitoring systems", Jr. Advances in Instrumentation and Control 45 ('1990), Part 1. Generally speaking, with this method a specific threshold value is chosen for the amplitude of the reflected signal. If the recorded signal within the time window [ t~ , tz ] is in excess of this threshold value, this is interpreted as the presence of a par-ticle in the measurement volume. On condition that the particle concentration is so low that the risk of the simultaneous presence of more than one particle within the measurement volume is negligible, the particle con-centration can be estimated by counting the number of recordings in excess of the set threshold value.
This method takes no account of variations in par-tiele size and no distinction is made between different types of particles which are possibly present fn the suspension.
S
The aim of the present invention is to provide a reliable and more precise method for the characterisation of a suspension and the particles pre sent in the suspension. In this context characterisation is understood to be estimation of the particle concentration, the particle size distribution, the shape of the particles and the reflectivity of the particles.
A further aim of the invention is to provide a method for the characterisation of suspensions and the particles contained therein, with which method account does not necessazily have to be taken of the condition that no more than one particle may be present in the measurement volume at any one time.
The first-mentioned objective is achieved with a method of the above-mentioned typ., which method is characterized by the following steps:
f. composing at least one curve on the basis of a cumulative count of the number of reflection signals which have an amplitude in excess of a specific value as a function of the amplitude;
g. comparison of the at least one curve with prede-termined standard cumulative count curves and deduction of at least one feature from a set of features compris-ing: material properties, particle concentration, par-ticle shapes, particle size and standard deviation thereof and particle size distribution.
3S Haterial properties thus deduced may, e.g. be den-sity and compressibility.
A method of this type can be used successfully where the suspension flows relative to the acoustic source, where the acoustic source is moved relative to the suspension (for example along the suspension), where, for rthatever reason, the properties of the sus-pension change as a function of time and the acoustic source is fixed, and ~rhere an array of acoustic sources is used instead of a single acoustic source, the acous-tit sources always being activated in succession.
For the purposes of the further objective, one embodiment of the method according to the invention is characterized in that:
15 - in step a, the acoustic beam generated has such a lazge apezture, and the time which elapses between two successive measurements is so short, that each par-ticle is exposed several times wha.Ie passing through the beam and that, depending on the lateral position of ZO a particle in the measurement volume, a varying angle-dependent reflection of the acoustic signal is pro-duced;
- prior to step f, one or more different types of ZS particles present in the suspension are identified on the basis of a series of angle-dependent reflection signals zeceived successively over time;
- when composing the curve in step f, the maximum 30 value of the amplitudes of a series of successive angle-dependent reflection signals, received over time, from a detected particle from the one or more groups is taken as the amplitude of the electrical signals, which are produced after conversion of the acoustic reflec-35 tion signals from that particle.
with a method of this type, the measurement volume chosen is so large that, within the measurement volume, the angle of~incidence varies as a function of the lat-eral position. If a particle in the flovir~g ~ttepension is "exposed" various times in succession by as acoustic signal in pulse form, the successive reflection signals differ as a Consequence of angle-dependent reflection_ The angle-dependent behaviour is highly dependent on the shape of the particle, aS can be seen from, inter alias M.G.M. de ICroon: "Acoustic backscatter in arteries - Measurements and modelling of arterial wall and blood", thesis 1993, ISBN 90-9006182, Section II.
Therefore, a particle can be characterized by this means.
In an alternative embodiertent the present invention relates to a method for. the detection and identifica-tion of particles in a suspension, comprising the fol-lowing steps:
a. generation of acoustic signals using an acous-tic source;
b. directing the acoustic signals at at least one measurement volume trithin the suspension, the bound-aries of the measurement volume in the axial direction with respect to the acoustic source being defined With the aid of time ~indoWS:
c_ reception of acoustic reflection signals pro-duced by reflection of the acoustic signals by the par-ticles in the at least one measurement volume;
d. conversion of the acoustic reflection signals into electrical reflection signals;
e. counting numbers of electrical zeflection sig-nals which have an amplitude in excess of a predetez-mined value and conversion thereof into numbers of par-ticles which are larger than a certain size;
wherein the method also comprises the step of applying ass inversion algorithm on the amplitudes of the electrical reflection signals to deduce at least one feature from a set of features comprising: material properties, particle concentration, particle shapes, particle size and standard deviation thereof a~ad par-tiele size distribution.
The present invention also relates to equipment foz the detection and identification of particles in a suspension, comprising:
a. an acoustic source for the generation of acoustic signals in pulse form:
b. means for directing the acoustic signals at at least one measurement volume within the suspension, the boundaries of the measurement volume in the axial direction With respect to the acoustic source being defined with the aid of time windows;
c. means for receiving acoustic reflection signals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. means for converting the acoustic reflection sig-nals into electrical reflection signals;
e. means for counting numbers of electrical reflec-lion signals, which have an amplitude in excess of a predetermined value and for converting the count into numbers of particles which are larger than a certain size;
wherein the equipment also comprises:
f. means foz composing at least one curve on the basis of a cumulative count of the number of zeflection signals which have an amplitude in excess of a specific value as a function of the amplitude;
g. means foz comparing the at least one curve with predetermined standard cumulative count curves and for deducing at least one feature from a set of features comprising: material properties, particle eoncen-tration, particle shapes, particle size and standard deviation thereof and particle size distribution.
In one embodiment, such equipment is characterized in that - during operation, the acoustic source generates an acoustic beam which has such a large aperture, and makes the time Which elapses between two successive measurements so short, that each particle is exposed several times while passing through the beam and that, depending on the lateral position of the particles in the measurement volume, a different angle-dependent reflection of the acoustic signal is produced;
- identification means are also present for identi-fication of one or more different groups of particles present in the suspension on the basis of a series of successive angle-dependent reflection signals received over time;
- the means for composing the cumulative curve eom-~a pose the curve, the maximum value of the amplitudes of a series of successive angle-dependent reflection sig-nals, received over time, from a detected particle being taken as the amplitude of the electrical signals, which are produced after conversion of the acoustic reflection signals from that particle.
In the last-mentioned equipment, use is advantage-ously made of the angle-dependent reflection behaviour of acoustic signals reflected by particles and it is no longer necessary to meet the condition that only one pazticle may be present in the measurement volume at any one time, as has been explained above.
In an alternative embodiment the invention relates to equipment for the detection and identification of particles in a suspension, comprising:
a. an acoustic source for the generation of acoustic signals;
b. means for directing the acoustic signals at at least one measurement volume within the flowing suspen-sion, the boundaries of the measurement volume in the axial direction with respect to the acoustic source being defined with the aid of time windows;
c. means for receiving acoustic reflection sig-nals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. means for converting the acoustic reflection signals into electrical reflection signals;
e. means for counting numbers of electrical reflectiod signals which have an amplitude in excess of a predetermined value and for converting the count into numbers of particles which are larger theft a certain size;
characterized in that the equipment also comprises:
f. means for applying an inversion algorithm on the amplitudes of the electrical reflection signals to deduce at least one feature from a set of features com-prising: material properties, particle concentration, particle shapes, particle size and standard deviation thereof and particle size distribution.
Brief Description of the Drawings The invention gill be explained in more detail belo~r with reference to a few drawings and illustrative embodiments, which are intended solely for the purposes of illustration and not as a limitation of the inven=
ties concept.
In the drawings:
Figure 1. shows a diagrammatic measurement set-up for the determination of the particle concentration and particle size distribution in a flowing suspension con-taining equivalent particles; .
Figure 2 shows a measured cumulative count curve for oil droplets in water (133 ppm), for an oil droplet average diameter of 40 yun;
Figure 3 shoes a diagrammatic measurement set-up for the detez~mination of the particle concentration and particle size distribution in a flowing suspension, where several particles may be present in the measure-meet volume;
Figure 4 shows simulated cumulative Count curves for oil droplets in water, at a concentration of 9.109 S m3, far a standard deviation of the oil droplet 8iam-eters of 1 ~.im and an average of oil droplet 8iameter varying from 15 yuN to 20 dun;
Figure 5 shows simulated cumulative count curves for oil droplets in water, for a standard deviation of the oil droplet diameters of 1 y,na and for an average of oil droplet diameter of 15 7sm at a concentration vary-ing from 5. 109 m'3 t0 11 . 1 09 ai 3.
Figure 6 shows simulated cumulative count curves for oil droplets in water, at a concentration of 9.109 m'3, for an average of oil droplet diameter of 15 ym and for a standard deviation of the oil droplet diameters varying from 0.5 to 2.5 Vim;
Figure 7 shows an inversion result of a inverted simulated count curve. The particle diameter is distributed according to a Gaussian distribution with an average particle diameter ~ of 10 yun, a standard deviation ao of 1 um and the particle concentration equals 7 .1 Og nt 3.
Figure 8 shows an inversion result of a suspension with two particle components: one has a average diam-eter of 6 ~rrn, the other of 10 um.
Figure 9 shays successive recordings for a single spherical particle in a flowing suspension using the measurement set-up according to Figure 3;
Figure 10 shows successive recordings of a single eiengated particle with a length of 1 mm in a flowing suspension using the measurement set-up according to Figure 3;
Figure 't 7 shoals a sequence of recordings for tsr~o particles 0.5 mm apart.
First of all a method for the determa.nation of the particle size distribution and particle concentration in a suspension containing equivalent particles Will be explained. Equivalent particles are understood to be particles of the same material and of the same shape, Whilst the size of the particles may vary.
Not only is the number of recordings above the threshold value determined, but a complete histogram of recorded amplitudes is made. The histogram is then con-verted to give a cumulative count NQ s~rith the highest recorded amplitude as the start value. The cumulative count N~(A) is defined as the number of recordings with an amplitude greater than or equal to 7~. The cumulative count curve is simple to calculate from the histogram.
The particle concentration and the particle sire dis-tribution in a suspension containing equivalent par-ticles can be estimated from the histogram or from the count curve.
Cumulative count curves for suspensions containing equivalent particles can be simulated on the basis of the model given below.
First of all it has to be checked whether the assumption that the likelihood of finding two or more particles is negligibly small is realistic.
i r wrr ~rli",ir4r"v~. il ri The Pc~~.~on distxibutican can be used for the p~csk~--ab~lit~ density function ~~m~ for finding m parta.Cles ~.r~ the measurement ~rt~la~~
,~cl ~ ~~ ft>
~a~ this equation ~ zs i~he expectation value for the num3aer c~f particles an the measurement uc~lume. For sus pensionwith lr~w particle cc~nCentr~tions anc~ fir a sufficiently small aneasurement volume, tha likelihood r~~ t~wc~ p~rt~cles being present in the measurement vc~l~
ume at the same time inegligibly small. in this case it can be r3eduCed from equation ~'f ) that the f'c~llowanc~
apprc~~ir~a~icsn applies f~~ ~hlike2ihc~r~d t~f the pres-enCe of no gar~iCles ( f ( (l ) ~ and of cane gar~,ie.~.e ( f ( 1 ) ~ , respectively:
f t i } = i -~ ~ '~
'the likelihocad c~f finding a paz~icl~ Can also be calCUlated as follows:
C~
~rr~~~ ~ # 3 ) ~ah~~e "V"~~~ zs the easuz°~ent volume and ~ the number of particles per unit ~,~c~lume.
2Fquatirtg f 4 9 ) fra~n aguatior~s ~ 2 ) and ( 3 ) 9xves arz estimate for ~:S=~
i i ~Nii i i I~ a ~.i il.n u.. ~ W i i , i~
~uu~ =~ - 3.n ( ~ ~ ~''m~as ~~ ( ~ 3 Using this equ~tic~r~ it is then possible, with the a.~c~ of ec~uatie~n ( ~ ) , tc~ make an esta.ate cai~ the 3.ake~.~.-h~ao~ of fir~dinc~ two or more p>arta.eles, ~~ t,~t it possible to ~rwestac~ate whether the assumption th>~t this ~iel~hcaod is negligibly small is realistic.
The shape of tt~e cumulative count curve depend not only pan the properties c~f the susper~s~.on but also r~ra the acoustic pressure distribution in the e~ure-~~7 went volume. The probability density function for the me~xsa~~°ed ap~.~.tue3e for a particle of riven diameter I~
depenc~;s on the properties of tt~e acoustic ieic~ ~nr~
will be represented by g(~(I~). If the suspension con-s~.sts of a cc~ilection ref part~.c2es of different dimen-sioras, the resultant. Cumulative court curve will be the we~.ghted sum of the ar~divS.~Iua~. cumu.~~tive count curves c~h~ch are associated with the garti.Cles of a constant r~~~eter. The weight~.nc~ ~~~sit~ ~n the apps ~Catic~n of the particle size dzstrib~:tit~n for the parti.Cies in the 2~ suspension ct~ncerned, The fo2lowing equsti.on then >~ppli~:s for the prc~babi~.sty density f~znctic~n ptA? fear the amplitude:
pf.~~ m ~ ~tr9j~i ~:~~,' c'3s'? ~51 In this e~uatic~n hdT~) is the probability densaty Function for the particle diameter I~.
If the measurement volume is chosen to be around the foCUS c~f the acout~c beam 2~ (figure t), the ~t3 amplitude of tt~e incident aCOUStic pressure can be apprr~~cimated by a Gaussi an iunctic~r~:
....,.,r. ~"~,.""b".,~~.,~
P(r)~ ~p e~'~~2 where x~
3Z f,~
In this squatit~n r is the lateral distance t'ro~n the principal axis (see ~'xguxe 1), k is the wave number and f~ is the f-number of the tr~cnsducer, which is defined as the quotient ~~ the distance from the focus, ~.~, and the t~ransduce~ diameter df~ = z~~2a~, where ar is 1CJ the radius of the transducery.
~'he likeiicod 0~ finding no particles, given in equation ( ~ ) , indicates which fraction of the total number c~f measurements ~*ields an amplitude which ~.s 1 ~ Less than or equal tc~ ~a~; (amplitude of the naise) . Tf a partiClc~ is positioned that far form the central axis of the acr~usti~: beam that the reflection amplitude is below the noise ~.cvel An, the particle is the tc~ he outside the measurement volume Vas. As such the ampi i~-~U tulle of the noise determines the 2ateral dimensions of the measurement uoiume; after all, with the aid of equaticsn ~~? it fc~~.3~r~ws that:
t8) ~bWcBS -~' StB35 " 'JC ~~df8 '~ c. " y' 1 ~ 't C"''~~ 2 Try this equation R,~a~ ~.s the radius of the measurement 5 uolume in the lateral direction and Ais the maximum i i ~wii ~,~~i.,~.~,1.M,.".~ il~ ii t7 amplitude detected far the suspension (largest particle located an the principle axis of the beam), ~1~ is the rt~ea~uxement valurne which is delimited in the direction e~f the principal axis z by the chaser time ~ri.nda~w It"
t~ ~ .
"car Chase ~nessu~°ements where there is ~: particle found in the bean, the measured amplitude will depend r~~a the precise location of the particle concerned in ~Q the measurement volume and on the dimen~ians of the particle concerned.
A~~ur~inc~ a Gaussian acoustic beam profile in the lateral direction ~Equatian (6)) in the measurement 15 volume grad negligible amplitude variation in the axial direction within the selected tine window, it oa,n be deduced that the conditional probability density func---tian g(A~D) for the measuxed amplitude A t~riginati.ng from are arbitraz°~r particle with diameter I~ is given by:
~ C1 <A <A,~t~) ~~'~t:~) =~'In~ A'' ~ tt0l ~(A~r~) = 3 i 0 if A > ApiD) l1 .Ac:9~, The follow~.nc~ relationship can be derived far the conditional cumulative count G(A > A):
r ,I?AO~I1) ~ iriA
~ A > A f ~ ; ~ ~ n A0 . ~ .~ f ,A~ < A E AO ~ D ) ( ~ l ) -- ~ 1 .z f ,J3 < 1~., if A 5 A~~D) 25 ~r~ these equations A~ is the arn~litude ~rhich would be detected from the particle concerned if the particle were lo~atss~ an the principal axis of the beam. ~*h~ A.~
depends on the size of the particle i'or sphera.ca~ par tides, the diameter of which is much smaller than the ~ravelength, ~,~ is froportionai to the tharr~ power of the diaaneter i~ (Rayleigh seattex~.ng) ~ # ~,~,s ~ 12 ) Tn this equation y is a proportionality constant, which defends b~rtk~ on the transducer ~arr~perties and 1 (3 material characteristics of the partic3.e, li3~e density and cc~m~a~-essa.ta~.ity.
The probability density ~unct~~an c~~ equation ~ 5 is the so-called forward model caf the experiment, from '15 which the measured amplitude histogram, c~~.ven the prop-erties of the suspension (Particle d~a~neter distribu-tion h(~) ) and the pr~perta.es of the accaustic field, cars be ~alcu~.ated. "T"his forward rnod~:~. needs to be inverted to use able to deduce the prc~ferties of the 2~ susgensic~n ~h(~) ) from the measured amialitude. ~'or a Gaussiapressure field, a meth~ad for inversion is described below.
The ir~tegrati~n in equatiara ES) is rep3aCed lay 25 summation and equations ~~G) and t12) are applied:
;,~~~ -',~ ' -~ a _hfaD
's ~.i lr. a ~ ) ~rhere the summation is over al.~. parti.ce diameters D~, which might give a reflection acnglitude equal to A:.
~. L I . ~,i eYll.~ i I II yyi:ld~ll.:rt~:iLI~l i 'he 1.a.l~~li~scac~ fc~r a pa.rti~l~ to have a diameter in the interval Icy-6~l ~ . . y+~L~ J is given by h ( I~~ ) ~tb, and will lie denoted as q ~ L~~ ~ . So equation ( 1 ~ ) b~crames ~?i~'s9~ ~' ~ q~.L?
~ n ~ ~~~~ ;~
~Ft This equatic~~ Can be written as a atrax product uforward matrix W wit?: prr~~a3~ility vector q.
P(~~ ~ ~zt~i.L~> a,~~ ~'~5~
1 (~ Nc~ a~~~t~ptic~ns caere made about the parti~Ie aiameter distribution, it was r~~rr~.y assumed that there exists a ~xi~nur~ diameter, which is n~,t a li~nitinc~ restriction.
~quat:.s~n ~ 1 ~ way; de~l~zce.ci fc~r a Gaussian pressure field, however, in general it is possible to ~~ciu~e a matrix presentation fc~r the rel.atic~n between the par-tiC~e size distribution q(I~~ and the amplitude distra.butz.c~n p~A) .
2~l There are several ways tc~ find a tstable) pseuc3c~-inverse ref the forward matrix W which fits the parame-.
tars ~ i » a . the elements of vector ? tc~ the data in a least squares sense, A standard appraach for this prob--.
le~r~ is singular value decomposition (S~I~) .
2~
Anather, mare superior method, uses ~ trachasti.
approach, which is r3esCribed by Franklin (Franklin, ;
N. , 19TH, "Well-pr~sec~ sterchastiC extension c~f ill~posed ~.inear prablem~", ,journal of r~athematzcal analysis and . _ ~... i , ~ ~ ~w.~m ~ i ~i ~i m"ik~n,: e. i i ~~ i i applications, val . ~1 . pp ~82m71 ~ ( "I ~'~0 ) ~ . ~ stc~ChaStl.c approach mans that it is assumed that the parameters ~q) and noise are samples drawn Pram random processes.
The assumption yields an a s~aoathness constraint for the desired solution. The parameters that control the s~nc~athzaes of the scalutian depend an the salutic~r~
itse3.~, therefore an fterative method can be used to find the apti.r~al set of parameters.
'fhe calcui~ted particle size di~tr.ibuta.c~n, us~.r~g one of above-mentioned techniques is not the true size distribution of particle in the suspension (q~~~,), but the apparent size distribution (q8~a? . 'this distribution deviates from the true size distribution since smaller 1S particles canz~at be detected throughout the ca~nplete c~easure~cer~t volume as defined by ec~u~tir~n ~ 9 ) . Applying a correction factor results in the true particle size distribution of particles in the suspensi.an. Assuming a ~aussaaz~ pressure field, the cc~rrectior~ factor is given ~0 by:
In ( ~'; ) ne _ l Y{ 16 ) ~tr;;e ~ 3''~
? n ( ~~', ) ~( :.,~
Tn this equa~.ic~n ~~~" is the diameter of the largest p~~rticle in suspension. 'the factcar ~ is applied to nor-~alize the area below q~~.U~ equal to unity.
The number of measurements with an amplitude larger than ~~, is indicative for the number of particles C per unit volume in the suspension. Assuming a Gaus-~0 sian pressure field, it can be deduced that the number of particles per unit volume is riven by:
i I.~Pw",I II.~,..iMno i iI ii N{A>An) ~tr~~~~) d~
~N:~~ ~,a Zn this equation ~1(...) gives the nuc~er of ~e~uremen~s for which the conc~itaon given in brackets ~pp~.ies. N~~~ is the total number c~f measurements; '~,~~
is the measurement volume as defined in eq~~tic~n tg~.
The factor within straie~ht brackets ~~ is a crrrectic~n factor xahiCh has tc~ be applied because smaller par-ticles cannot be detected within the whole measurement ~rr~lume , F~c~~are 2 gives an example of a measured Cumulative ccsunt Curve fc~r oil drc~~lets in wa~:er f 1 ~~ ppm~ . fr~r an oil. droplet average diame~.er c~f 4(~ ~Zm.
The count C~zrve~ given in Figures 9 to 6 are the results of ilatior~based on the cr~mbina~aoz~ Qf equations (S~, (3~) arrc~ (~~).
The c~~ l ~iro~lets are hoac~enc~usly dis~ributec~
2Q throughout the s~xsper~~ion. For the simulations the dia~
meter of the droplets is normally distributed with an average of ~~, and standard deviation c~~,. The number caf drcalets per unif vClume is ~. The cumulative count Curve for such a suspension depends on these three ~~rariables u~;, r~~ and C. 'his is illustrated in Figures 4, ~ and 6, which show sima~lateti count curves can ~.he basis of fhe model described above.
~°ic~ure 4 gives sfmula~ted count Curves for water--~0 oii suspension where ~Y, ~rarses between °iS end U ~un~
where cry, is kept cc~ns~ant at 1 ~m and C is constant 9.'iC~~ m-~. Figure ~ c~~.ves simulated count curves fear water-oil suspension where aD is varied from 0.5 to Z.5 fun, the concentration is 9.1 O9 m 3, and the average drop diameter ( yb) is 15 tun .
Comparing Figure 4 and 5 sho~r clearly that the effect of a variation in uo diffeza from the effect of a variation of ap. The effect of a variation in C on the cumulative count curve is illustrated in Figute 6. In this Figure, the cumulative count curves for a, concen-1 0 tzation of 5 . 1 O9 ni 3, 7 . 109 m 3, 9. 1 O9 ~ 3 and 1 1 . 1 O9 ni 3, ~rhere y~ has been kept constant at 15 um and ao has been kept constant at 1 ym, are shown. The effect of a higher C is to raise the height of the cumulative count curve for all 1~r ~ A~. Comparing Figure 6 arid Figure 4 shows that the effect of a variation in C can be read-ily differentiated from that of a variation in whilst comparing Figure 6 and Figure 5 shoWS that the effect of C can also be readily differentiated from the effect of cp.
It follows from this example that the C, up and op can be derived from the measured cumulative count curve for a suspension in which an unknown quantity of oil droplets of unknown ~ and ap are distributed. For this purpose measured cumulative count curves can be com-pared with predetermined standard cumulative count curves for known particles. Th! standard cumulative count curves can, for example, be determined via simu-lation, as has been explained above with reference to Figures 4, 5 and 6. In this cane, for example, a Bayesian inversion can be used to derive the particle size distribution from the cumulative count curves. As an alternative, standard cumulative count curves of this type can be determined experimentally, Standard count curves of this type can be stored in the memory 27 of computer 26, the computer 26 being equipped with suitable softirare for comparing measured count curves with the standard count curves. standard methods, such as the least squares fit, can be used for thin purpose.
The method can, og course, also be used for suspensions containing pazticles ~rhich have a particle size distribution other than normal particle size distribu-tion ~rith characteristic values other than ~ and ap.
Another method to derive C and the particle size distribution (e. g. defined by y.~ and Qo) from a measured cumulative count curve is to apply an inversion algorithm, such like the one as given in equation (15).
The capabilities of such an invezsion algorithm are illustrated in Figures 7 and 8.
Figure 7 gives the inversion result using the described inversion technique given by equations (14), (15), (16)_ The line (curve 16) shots the exact size distribution, the cizcular markers (curve 17) are the ZO result obtain using singular value decomposition and the cross markers (curve 18) are the inversion result using the stochastic approach. The Figure shows that more accurate results are obtained using the stochastic approach. To illustrate that any kind of particle size distribution can be detezmined using this technique, an inversion result of a non-gaussian size distribution is shown in Figure 8 . The line ( curve 19 ) gives the exact distribution and the cross markers (curve ZO) give the inversion result using the stochastic approach.
A method and set-up for the characterisation of various types of particles in a suspension will nov be described.
In the above it has been assumed that there fa only one type of particle in the $uspeasion, only the size of the particles being allowed to vary. 7~ further assumption was that the likelihood of more than one particle being present in the measurement volume at the same time is negligibly small. On these grounds, a 3 small measurement volume in the focus region of the acoustic beam 20 was preferably chosen.
However, there are sometimes advantages in, in contrast, selecting a large measurement volume beyond the focus. This is illustrated in Figure 3, fn which the same reference numerals as in Figure 1 refer tv the same elements. The measurement volume is delimited in the axial direction of the principal axis z by z3 and zb associated with time window [t3, t4~. In this case the angle of incidence within the measurement volume varies as a function of the lateral position r with respect to the principal axis z. Zf several reflection measure-ments are carried out in succession on one particle 22 while the particle is passing through the measurement 2Q volume (see Figure 3), the angle-dependent reflection behaviour of the particle can be determined from the change fn the reflection signal as a function of the lateral position. The angle-dependent behaviour is highly dependent on the shape of the particle and therefore the particle can be charactezized on the basis of this behaviour.
Figure 9 shoes a succession of simulated record-ings for a spherical particle which passes through the beam at a fixed distance beyond the focus. Simulated recordings for an elongated particle aze given in Fig-ure 10. These Figures clearly illustrate the effect of the shape of the particle on the reflection behaviour as a function of the angle.
For this method, the larger the aperture of the beam from the transducer 23, the more clearly visible is the difference in angle-dependent behaviour between different types of particles. As a result, the measure-ment volume therefore also increases. However, the 5 requirement that no more than one particle at a time enay be present irt the lateral direction in the measure-went volume is dispensed with with this method because different particles located laterally alongside one another can be individually recognised in the suc-10 cession of recordings. This is illustrated in Figure 11, where two spherical particles pass through the beam just behind one another. Although the lateral distance (75 mm) between these particles is much smaller than the beam diameter, the reflections originating from the 15 first particle can be separated from the reflections from the second particle in the succession of record-ings. The most important characteristic of: the last-mentioned embodiment is that the beam has a large aper-ture and that the time which elapses between two suc-ZO cessive measurements is so short that each particle is exposed several times during its presence fn the beam.
As has been explained above, this can be achieved, for example, with the aid of a focused transducer. Theor-etically, the use of a point source ar a relatively Z5 small source (of the order of magnitude of the wave-length used) is also possible. It is only at high fre-quencies that this cannot be achieved in practice. Op-tionally, use can be made of a Radon transformation or a 2D Fourier transformation to the 1~-k domain in order to quantify the angle-dependent reflection behaviour of each recorded particle.
Following the characterisation of a particle on the basis of its angle-dependent behaviour, the cumu-lative count curve can be determined for each type of particle. For this purpose the maximum amplitude for zs the series of recordings originating from the particle concerned ithis is the atmplitude when the particle is located on the principal axis s of the beam 20) is taken as the amplitude per detected particle. This ' S maximum amplitude can still vary for one type of par-ticle because of variations in particle size acrd vari-ations in the minimum lateral distance from the princi-pal axis a at which the particle passes through the beam. A cumulative count curve similar to that illus-trated above With reference to Figures 4, 5 and 6 will be found for each type of particle.
A possible application of the method described above is the measurement of thrombus particles in a blood stream. Patients who have a heart valve prothesis have an increased risk of thrombosis because the arti-ficial valve promotes the production of thrombus par-ticles. Therefore, these patients have an increased risk of acute vascular occlusion as a result of too large a thrombus particle. This can lead, for example, to cerebral infarction.
Anticoagulants are administered to the patient to counteract thrombus formation. The risk of too high a dosage of these anticoagulants is the occurrence of haemorrhaging (for example cerebral haemorrhaging).
currently the dosage is determined on the basis of the coagulation measured on a blood sample. Ho~rever, this is an unreliable method because the coagulation is dependent on many more factors than solely the concen-tration of thrombi. Moreover, a measurement of this type provides only a snapshot.
It is therefore desirable to have available a method with which the concentration of thrombi in the blood can be measured reliably. The measurement method described here is suitable for this. purpose.
$y using a transmission frequency of the sound craves which ie relatively high for medical epplfc-ations, for example in the range of 10-40 l~Iz, prefer-ably ZO-30 lZHz, it is possible to detect small par-ticles present in the b~.ood, The depth of penetration is still found to be sufficiently great at these fre-quencies. By making use of the high efficiency of, for example, a composite transducer and of sophisticated transmission and reception electronics, it is possible to achieve an adequate depth of penetration, so that an echo-acoustic recording of the blood can be made using a non-invasive technique. However, blood also contains other particles which will be detected by the ultra-sonic reflection method, for example red blood cells.
It is knoxm that red blood cells form elongated aggregates during a certain period of the heart cycle (end of diastolic), which aggregates align With the flow. See, for example, M.G.M. de Kroon: "Acoustic backscatter in arteries -- Measurements and modelling of arterial wall and blood", thesis 1993, ISBN 90-9006182, Section III., It is possible to distinguish these long structures from thrombi, Which are of a jagged shape (see: S. Chien: "Clinical Naemorheology", l~Jartinus Hijhoff), on the basis of the angle-dependent behaviour.
After making this distinction, the concentration and size distribution of thrombi, tan be estimated on the basis of the cumulative count cuzves, as has been explained above.
As is shown in Figure 1, the principal axis z of the sound beam 20 generated by a transducer 23 is per-pendicular to the direction of flaw P of the suspension 21 flowing in a channel 24 acrd, consequently, to the direction of movement of the particles 22 present in the suspension: When a particle 22 passes through the sound beam 20, the incident sound field till be reflected by the particle and the reflected signal will be captured by the transducer 23. The received signal is converted by the transducer 23 into an electrical signal, which is transmitted to the transmission and reception electronics 25. The transmission and recep-tion electronics 25 transmit the signal to a computer 26, which is connected to a memory 27 for storing measurement data. The computer 26 is provided with suitable software for evaluation of the measurement data. The. electrical signal froth a single measurement is indicated diagrammatically by s and is a time sig-nal, the time axis t indicating the propagation time of the sound. The response of the particle 22 in the meas-urement volume can be detected in the recording at that moment in time which corresponds to the' propagation time of the ultrasonic pulse between transducer I3 and reflecting particle 22 and back again.
Only reflections within an applied time ~riadow [t~, t2] (see Figure 1) are processed fn the analysis. The measurement volume is thus lien~.ted in the axial direc-tion by zi = ti.c/2 and z2 - tz.c/Z (c i6 the spae8 of ptopagation of the sound). In the lateral direction the measurement volume is limited by the shape of the acoustic beam.
A methed for counting the number of particles using a measurement set-up of this type is described in the above-mentioned patents and in Groetsch, J.G.:
"Theory and application of acoustic particle monitoring systems", Jr. Advances in Instrumentation and Control 45 ('1990), Part 1. Generally speaking, with this method a specific threshold value is chosen for the amplitude of the reflected signal. If the recorded signal within the time window [ t~ , tz ] is in excess of this threshold value, this is interpreted as the presence of a par-ticle in the measurement volume. On condition that the particle concentration is so low that the risk of the simultaneous presence of more than one particle within the measurement volume is negligible, the particle con-centration can be estimated by counting the number of recordings in excess of the set threshold value.
This method takes no account of variations in par-tiele size and no distinction is made between different types of particles which are possibly present fn the suspension.
S
The aim of the present invention is to provide a reliable and more precise method for the characterisation of a suspension and the particles pre sent in the suspension. In this context characterisation is understood to be estimation of the particle concentration, the particle size distribution, the shape of the particles and the reflectivity of the particles.
A further aim of the invention is to provide a method for the characterisation of suspensions and the particles contained therein, with which method account does not necessazily have to be taken of the condition that no more than one particle may be present in the measurement volume at any one time.
The first-mentioned objective is achieved with a method of the above-mentioned typ., which method is characterized by the following steps:
f. composing at least one curve on the basis of a cumulative count of the number of reflection signals which have an amplitude in excess of a specific value as a function of the amplitude;
g. comparison of the at least one curve with prede-termined standard cumulative count curves and deduction of at least one feature from a set of features compris-ing: material properties, particle concentration, par-ticle shapes, particle size and standard deviation thereof and particle size distribution.
3S Haterial properties thus deduced may, e.g. be den-sity and compressibility.
A method of this type can be used successfully where the suspension flows relative to the acoustic source, where the acoustic source is moved relative to the suspension (for example along the suspension), where, for rthatever reason, the properties of the sus-pension change as a function of time and the acoustic source is fixed, and ~rhere an array of acoustic sources is used instead of a single acoustic source, the acous-tit sources always being activated in succession.
For the purposes of the further objective, one embodiment of the method according to the invention is characterized in that:
15 - in step a, the acoustic beam generated has such a lazge apezture, and the time which elapses between two successive measurements is so short, that each par-ticle is exposed several times wha.Ie passing through the beam and that, depending on the lateral position of ZO a particle in the measurement volume, a varying angle-dependent reflection of the acoustic signal is pro-duced;
- prior to step f, one or more different types of ZS particles present in the suspension are identified on the basis of a series of angle-dependent reflection signals zeceived successively over time;
- when composing the curve in step f, the maximum 30 value of the amplitudes of a series of successive angle-dependent reflection signals, received over time, from a detected particle from the one or more groups is taken as the amplitude of the electrical signals, which are produced after conversion of the acoustic reflec-35 tion signals from that particle.
with a method of this type, the measurement volume chosen is so large that, within the measurement volume, the angle of~incidence varies as a function of the lat-eral position. If a particle in the flovir~g ~ttepension is "exposed" various times in succession by as acoustic signal in pulse form, the successive reflection signals differ as a Consequence of angle-dependent reflection_ The angle-dependent behaviour is highly dependent on the shape of the particle, aS can be seen from, inter alias M.G.M. de ICroon: "Acoustic backscatter in arteries - Measurements and modelling of arterial wall and blood", thesis 1993, ISBN 90-9006182, Section II.
Therefore, a particle can be characterized by this means.
In an alternative embodiertent the present invention relates to a method for. the detection and identifica-tion of particles in a suspension, comprising the fol-lowing steps:
a. generation of acoustic signals using an acous-tic source;
b. directing the acoustic signals at at least one measurement volume trithin the suspension, the bound-aries of the measurement volume in the axial direction with respect to the acoustic source being defined With the aid of time ~indoWS:
c_ reception of acoustic reflection signals pro-duced by reflection of the acoustic signals by the par-ticles in the at least one measurement volume;
d. conversion of the acoustic reflection signals into electrical reflection signals;
e. counting numbers of electrical zeflection sig-nals which have an amplitude in excess of a predetez-mined value and conversion thereof into numbers of par-ticles which are larger than a certain size;
wherein the method also comprises the step of applying ass inversion algorithm on the amplitudes of the electrical reflection signals to deduce at least one feature from a set of features comprising: material properties, particle concentration, particle shapes, particle size and standard deviation thereof a~ad par-tiele size distribution.
The present invention also relates to equipment foz the detection and identification of particles in a suspension, comprising:
a. an acoustic source for the generation of acoustic signals in pulse form:
b. means for directing the acoustic signals at at least one measurement volume within the suspension, the boundaries of the measurement volume in the axial direction With respect to the acoustic source being defined with the aid of time windows;
c. means for receiving acoustic reflection signals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. means for converting the acoustic reflection sig-nals into electrical reflection signals;
e. means for counting numbers of electrical reflec-lion signals, which have an amplitude in excess of a predetermined value and for converting the count into numbers of particles which are larger than a certain size;
wherein the equipment also comprises:
f. means foz composing at least one curve on the basis of a cumulative count of the number of zeflection signals which have an amplitude in excess of a specific value as a function of the amplitude;
g. means foz comparing the at least one curve with predetermined standard cumulative count curves and for deducing at least one feature from a set of features comprising: material properties, particle eoncen-tration, particle shapes, particle size and standard deviation thereof and particle size distribution.
In one embodiment, such equipment is characterized in that - during operation, the acoustic source generates an acoustic beam which has such a large aperture, and makes the time Which elapses between two successive measurements so short, that each particle is exposed several times while passing through the beam and that, depending on the lateral position of the particles in the measurement volume, a different angle-dependent reflection of the acoustic signal is produced;
- identification means are also present for identi-fication of one or more different groups of particles present in the suspension on the basis of a series of successive angle-dependent reflection signals received over time;
- the means for composing the cumulative curve eom-~a pose the curve, the maximum value of the amplitudes of a series of successive angle-dependent reflection sig-nals, received over time, from a detected particle being taken as the amplitude of the electrical signals, which are produced after conversion of the acoustic reflection signals from that particle.
In the last-mentioned equipment, use is advantage-ously made of the angle-dependent reflection behaviour of acoustic signals reflected by particles and it is no longer necessary to meet the condition that only one pazticle may be present in the measurement volume at any one time, as has been explained above.
In an alternative embodiment the invention relates to equipment for the detection and identification of particles in a suspension, comprising:
a. an acoustic source for the generation of acoustic signals;
b. means for directing the acoustic signals at at least one measurement volume within the flowing suspen-sion, the boundaries of the measurement volume in the axial direction with respect to the acoustic source being defined with the aid of time windows;
c. means for receiving acoustic reflection sig-nals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. means for converting the acoustic reflection signals into electrical reflection signals;
e. means for counting numbers of electrical reflectiod signals which have an amplitude in excess of a predetermined value and for converting the count into numbers of particles which are larger theft a certain size;
characterized in that the equipment also comprises:
f. means for applying an inversion algorithm on the amplitudes of the electrical reflection signals to deduce at least one feature from a set of features com-prising: material properties, particle concentration, particle shapes, particle size and standard deviation thereof and particle size distribution.
Brief Description of the Drawings The invention gill be explained in more detail belo~r with reference to a few drawings and illustrative embodiments, which are intended solely for the purposes of illustration and not as a limitation of the inven=
ties concept.
In the drawings:
Figure 1. shows a diagrammatic measurement set-up for the determination of the particle concentration and particle size distribution in a flowing suspension con-taining equivalent particles; .
Figure 2 shows a measured cumulative count curve for oil droplets in water (133 ppm), for an oil droplet average diameter of 40 yun;
Figure 3 shoes a diagrammatic measurement set-up for the detez~mination of the particle concentration and particle size distribution in a flowing suspension, where several particles may be present in the measure-meet volume;
Figure 4 shows simulated cumulative Count curves for oil droplets in water, at a concentration of 9.109 S m3, far a standard deviation of the oil droplet 8iam-eters of 1 ~.im and an average of oil droplet 8iameter varying from 15 yuN to 20 dun;
Figure 5 shows simulated cumulative count curves for oil droplets in water, for a standard deviation of the oil droplet diameters of 1 y,na and for an average of oil droplet diameter of 15 7sm at a concentration vary-ing from 5. 109 m'3 t0 11 . 1 09 ai 3.
Figure 6 shows simulated cumulative count curves for oil droplets in water, at a concentration of 9.109 m'3, for an average of oil droplet diameter of 15 ym and for a standard deviation of the oil droplet diameters varying from 0.5 to 2.5 Vim;
Figure 7 shows an inversion result of a inverted simulated count curve. The particle diameter is distributed according to a Gaussian distribution with an average particle diameter ~ of 10 yun, a standard deviation ao of 1 um and the particle concentration equals 7 .1 Og nt 3.
Figure 8 shows an inversion result of a suspension with two particle components: one has a average diam-eter of 6 ~rrn, the other of 10 um.
Figure 9 shays successive recordings for a single spherical particle in a flowing suspension using the measurement set-up according to Figure 3;
Figure 10 shows successive recordings of a single eiengated particle with a length of 1 mm in a flowing suspension using the measurement set-up according to Figure 3;
Figure 't 7 shoals a sequence of recordings for tsr~o particles 0.5 mm apart.
First of all a method for the determa.nation of the particle size distribution and particle concentration in a suspension containing equivalent particles Will be explained. Equivalent particles are understood to be particles of the same material and of the same shape, Whilst the size of the particles may vary.
Not only is the number of recordings above the threshold value determined, but a complete histogram of recorded amplitudes is made. The histogram is then con-verted to give a cumulative count NQ s~rith the highest recorded amplitude as the start value. The cumulative count N~(A) is defined as the number of recordings with an amplitude greater than or equal to 7~. The cumulative count curve is simple to calculate from the histogram.
The particle concentration and the particle sire dis-tribution in a suspension containing equivalent par-ticles can be estimated from the histogram or from the count curve.
Cumulative count curves for suspensions containing equivalent particles can be simulated on the basis of the model given below.
First of all it has to be checked whether the assumption that the likelihood of finding two or more particles is negligibly small is realistic.
i r wrr ~rli",ir4r"v~. il ri The Pc~~.~on distxibutican can be used for the p~csk~--ab~lit~ density function ~~m~ for finding m parta.Cles ~.r~ the measurement ~rt~la~~
,~cl ~ ~~ ft>
~a~ this equation ~ zs i~he expectation value for the num3aer c~f particles an the measurement uc~lume. For sus pensionwith lr~w particle cc~nCentr~tions anc~ fir a sufficiently small aneasurement volume, tha likelihood r~~ t~wc~ p~rt~cles being present in the measurement vc~l~
ume at the same time inegligibly small. in this case it can be r3eduCed from equation ~'f ) that the f'c~llowanc~
apprc~~ir~a~icsn applies f~~ ~hlike2ihc~r~d t~f the pres-enCe of no gar~iCles ( f ( (l ) ~ and of cane gar~,ie.~.e ( f ( 1 ) ~ , respectively:
f t i } = i -~ ~ '~
'the likelihocad c~f finding a paz~icl~ Can also be calCUlated as follows:
C~
~rr~~~ ~ # 3 ) ~ah~~e "V"~~~ zs the easuz°~ent volume and ~ the number of particles per unit ~,~c~lume.
2Fquatirtg f 4 9 ) fra~n aguatior~s ~ 2 ) and ( 3 ) 9xves arz estimate for ~:S=~
i i ~Nii i i I~ a ~.i il.n u.. ~ W i i , i~
~uu~ =~ - 3.n ( ~ ~ ~''m~as ~~ ( ~ 3 Using this equ~tic~r~ it is then possible, with the a.~c~ of ec~uatie~n ( ~ ) , tc~ make an esta.ate cai~ the 3.ake~.~.-h~ao~ of fir~dinc~ two or more p>arta.eles, ~~ t,~t it possible to ~rwestac~ate whether the assumption th>~t this ~iel~hcaod is negligibly small is realistic.
The shape of tt~e cumulative count curve depend not only pan the properties c~f the susper~s~.on but also r~ra the acoustic pressure distribution in the e~ure-~~7 went volume. The probability density function for the me~xsa~~°ed ap~.~.tue3e for a particle of riven diameter I~
depenc~;s on the properties of tt~e acoustic ieic~ ~nr~
will be represented by g(~(I~). If the suspension con-s~.sts of a cc~ilection ref part~.c2es of different dimen-sioras, the resultant. Cumulative court curve will be the we~.ghted sum of the ar~divS.~Iua~. cumu.~~tive count curves c~h~ch are associated with the garti.Cles of a constant r~~~eter. The weight~.nc~ ~~~sit~ ~n the apps ~Catic~n of the particle size dzstrib~:tit~n for the parti.Cies in the 2~ suspension ct~ncerned, The fo2lowing equsti.on then >~ppli~:s for the prc~babi~.sty density f~znctic~n ptA? fear the amplitude:
pf.~~ m ~ ~tr9j~i ~:~~,' c'3s'? ~51 In this e~uatic~n hdT~) is the probability densaty Function for the particle diameter I~.
If the measurement volume is chosen to be around the foCUS c~f the acout~c beam 2~ (figure t), the ~t3 amplitude of tt~e incident aCOUStic pressure can be apprr~~cimated by a Gaussi an iunctic~r~:
....,.,r. ~"~,.""b".,~~.,~
P(r)~ ~p e~'~~2 where x~
3Z f,~
In this squatit~n r is the lateral distance t'ro~n the principal axis (see ~'xguxe 1), k is the wave number and f~ is the f-number of the tr~cnsducer, which is defined as the quotient ~~ the distance from the focus, ~.~, and the t~ransduce~ diameter df~ = z~~2a~, where ar is 1CJ the radius of the transducery.
~'he likeiicod 0~ finding no particles, given in equation ( ~ ) , indicates which fraction of the total number c~f measurements ~*ields an amplitude which ~.s 1 ~ Less than or equal tc~ ~a~; (amplitude of the naise) . Tf a partiClc~ is positioned that far form the central axis of the acr~usti~: beam that the reflection amplitude is below the noise ~.cvel An, the particle is the tc~ he outside the measurement volume Vas. As such the ampi i~-~U tulle of the noise determines the 2ateral dimensions of the measurement uoiume; after all, with the aid of equaticsn ~~? it fc~~.3~r~ws that:
t8) ~bWcBS -~' StB35 " 'JC ~~df8 '~ c. " y' 1 ~ 't C"''~~ 2 Try this equation R,~a~ ~.s the radius of the measurement 5 uolume in the lateral direction and Ais the maximum i i ~wii ~,~~i.,~.~,1.M,.".~ il~ ii t7 amplitude detected far the suspension (largest particle located an the principle axis of the beam), ~1~ is the rt~ea~uxement valurne which is delimited in the direction e~f the principal axis z by the chaser time ~ri.nda~w It"
t~ ~ .
"car Chase ~nessu~°ements where there is ~: particle found in the bean, the measured amplitude will depend r~~a the precise location of the particle concerned in ~Q the measurement volume and on the dimen~ians of the particle concerned.
A~~ur~inc~ a Gaussian acoustic beam profile in the lateral direction ~Equatian (6)) in the measurement 15 volume grad negligible amplitude variation in the axial direction within the selected tine window, it oa,n be deduced that the conditional probability density func---tian g(A~D) for the measuxed amplitude A t~riginati.ng from are arbitraz°~r particle with diameter I~ is given by:
~ C1 <A <A,~t~) ~~'~t:~) =~'In~ A'' ~ tt0l ~(A~r~) = 3 i 0 if A > ApiD) l1 .Ac:9~, The follow~.nc~ relationship can be derived far the conditional cumulative count G(A > A):
r ,I?AO~I1) ~ iriA
~ A > A f ~ ; ~ ~ n A0 . ~ .~ f ,A~ < A E AO ~ D ) ( ~ l ) -- ~ 1 .z f ,J3 < 1~., if A 5 A~~D) 25 ~r~ these equations A~ is the arn~litude ~rhich would be detected from the particle concerned if the particle were lo~atss~ an the principal axis of the beam. ~*h~ A.~
depends on the size of the particle i'or sphera.ca~ par tides, the diameter of which is much smaller than the ~ravelength, ~,~ is froportionai to the tharr~ power of the diaaneter i~ (Rayleigh seattex~.ng) ~ # ~,~,s ~ 12 ) Tn this equation y is a proportionality constant, which defends b~rtk~ on the transducer ~arr~perties and 1 (3 material characteristics of the partic3.e, li3~e density and cc~m~a~-essa.ta~.ity.
The probability density ~unct~~an c~~ equation ~ 5 is the so-called forward model caf the experiment, from '15 which the measured amplitude histogram, c~~.ven the prop-erties of the suspension (Particle d~a~neter distribu-tion h(~) ) and the pr~perta.es of the accaustic field, cars be ~alcu~.ated. "T"his forward rnod~:~. needs to be inverted to use able to deduce the prc~ferties of the 2~ susgensic~n ~h(~) ) from the measured amialitude. ~'or a Gaussiapressure field, a meth~ad for inversion is described below.
The ir~tegrati~n in equatiara ES) is rep3aCed lay 25 summation and equations ~~G) and t12) are applied:
;,~~~ -',~ ' -~ a _hfaD
's ~.i lr. a ~ ) ~rhere the summation is over al.~. parti.ce diameters D~, which might give a reflection acnglitude equal to A:.
~. L I . ~,i eYll.~ i I II yyi:ld~ll.:rt~:iLI~l i 'he 1.a.l~~li~scac~ fc~r a pa.rti~l~ to have a diameter in the interval Icy-6~l ~ . . y+~L~ J is given by h ( I~~ ) ~tb, and will lie denoted as q ~ L~~ ~ . So equation ( 1 ~ ) b~crames ~?i~'s9~ ~' ~ q~.L?
~ n ~ ~~~~ ;~
~Ft This equatic~~ Can be written as a atrax product uforward matrix W wit?: prr~~a3~ility vector q.
P(~~ ~ ~zt~i.L~> a,~~ ~'~5~
1 (~ Nc~ a~~~t~ptic~ns caere made about the parti~Ie aiameter distribution, it was r~~rr~.y assumed that there exists a ~xi~nur~ diameter, which is n~,t a li~nitinc~ restriction.
~quat:.s~n ~ 1 ~ way; de~l~zce.ci fc~r a Gaussian pressure field, however, in general it is possible to ~~ciu~e a matrix presentation fc~r the rel.atic~n between the par-tiC~e size distribution q(I~~ and the amplitude distra.butz.c~n p~A) .
2~l There are several ways tc~ find a tstable) pseuc3c~-inverse ref the forward matrix W which fits the parame-.
tars ~ i » a . the elements of vector ? tc~ the data in a least squares sense, A standard appraach for this prob--.
le~r~ is singular value decomposition (S~I~) .
2~
Anather, mare superior method, uses ~ trachasti.
approach, which is r3esCribed by Franklin (Franklin, ;
N. , 19TH, "Well-pr~sec~ sterchastiC extension c~f ill~posed ~.inear prablem~", ,journal of r~athematzcal analysis and . _ ~... i , ~ ~ ~w.~m ~ i ~i ~i m"ik~n,: e. i i ~~ i i applications, val . ~1 . pp ~82m71 ~ ( "I ~'~0 ) ~ . ~ stc~ChaStl.c approach mans that it is assumed that the parameters ~q) and noise are samples drawn Pram random processes.
The assumption yields an a s~aoathness constraint for the desired solution. The parameters that control the s~nc~athzaes of the scalutian depend an the salutic~r~
itse3.~, therefore an fterative method can be used to find the apti.r~al set of parameters.
'fhe calcui~ted particle size di~tr.ibuta.c~n, us~.r~g one of above-mentioned techniques is not the true size distribution of particle in the suspension (q~~~,), but the apparent size distribution (q8~a? . 'this distribution deviates from the true size distribution since smaller 1S particles canz~at be detected throughout the ca~nplete c~easure~cer~t volume as defined by ec~u~tir~n ~ 9 ) . Applying a correction factor results in the true particle size distribution of particles in the suspensi.an. Assuming a ~aussaaz~ pressure field, the cc~rrectior~ factor is given ~0 by:
In ( ~'; ) ne _ l Y{ 16 ) ~tr;;e ~ 3''~
? n ( ~~', ) ~( :.,~
Tn this equa~.ic~n ~~~" is the diameter of the largest p~~rticle in suspension. 'the factcar ~ is applied to nor-~alize the area below q~~.U~ equal to unity.
The number of measurements with an amplitude larger than ~~, is indicative for the number of particles C per unit volume in the suspension. Assuming a Gaus-~0 sian pressure field, it can be deduced that the number of particles per unit volume is riven by:
i I.~Pw",I II.~,..iMno i iI ii N{A>An) ~tr~~~~) d~
~N:~~ ~,a Zn this equation ~1(...) gives the nuc~er of ~e~uremen~s for which the conc~itaon given in brackets ~pp~.ies. N~~~ is the total number c~f measurements; '~,~~
is the measurement volume as defined in eq~~tic~n tg~.
The factor within straie~ht brackets ~~ is a crrrectic~n factor xahiCh has tc~ be applied because smaller par-ticles cannot be detected within the whole measurement ~rr~lume , F~c~~are 2 gives an example of a measured Cumulative ccsunt Curve fc~r oil drc~~lets in wa~:er f 1 ~~ ppm~ . fr~r an oil. droplet average diame~.er c~f 4(~ ~Zm.
The count C~zrve~ given in Figures 9 to 6 are the results of ilatior~based on the cr~mbina~aoz~ Qf equations (S~, (3~) arrc~ (~~).
The c~~ l ~iro~lets are hoac~enc~usly dis~ributec~
2Q throughout the s~xsper~~ion. For the simulations the dia~
meter of the droplets is normally distributed with an average of ~~, and standard deviation c~~,. The number caf drcalets per unif vClume is ~. The cumulative count Curve for such a suspension depends on these three ~~rariables u~;, r~~ and C. 'his is illustrated in Figures 4, ~ and 6, which show sima~lateti count curves can ~.he basis of fhe model described above.
~°ic~ure 4 gives sfmula~ted count Curves for water--~0 oii suspension where ~Y, ~rarses between °iS end U ~un~
where cry, is kept cc~ns~ant at 1 ~m and C is constant 9.'iC~~ m-~. Figure ~ c~~.ves simulated count curves fear water-oil suspension where aD is varied from 0.5 to Z.5 fun, the concentration is 9.1 O9 m 3, and the average drop diameter ( yb) is 15 tun .
Comparing Figure 4 and 5 sho~r clearly that the effect of a variation in uo diffeza from the effect of a variation of ap. The effect of a variation in C on the cumulative count curve is illustrated in Figute 6. In this Figure, the cumulative count curves for a, concen-1 0 tzation of 5 . 1 O9 ni 3, 7 . 109 m 3, 9. 1 O9 ~ 3 and 1 1 . 1 O9 ni 3, ~rhere y~ has been kept constant at 15 um and ao has been kept constant at 1 ym, are shown. The effect of a higher C is to raise the height of the cumulative count curve for all 1~r ~ A~. Comparing Figure 6 arid Figure 4 shows that the effect of a variation in C can be read-ily differentiated from that of a variation in whilst comparing Figure 6 and Figure 5 shoWS that the effect of C can also be readily differentiated from the effect of cp.
It follows from this example that the C, up and op can be derived from the measured cumulative count curve for a suspension in which an unknown quantity of oil droplets of unknown ~ and ap are distributed. For this purpose measured cumulative count curves can be com-pared with predetermined standard cumulative count curves for known particles. Th! standard cumulative count curves can, for example, be determined via simu-lation, as has been explained above with reference to Figures 4, 5 and 6. In this cane, for example, a Bayesian inversion can be used to derive the particle size distribution from the cumulative count curves. As an alternative, standard cumulative count curves of this type can be determined experimentally, Standard count curves of this type can be stored in the memory 27 of computer 26, the computer 26 being equipped with suitable softirare for comparing measured count curves with the standard count curves. standard methods, such as the least squares fit, can be used for thin purpose.
The method can, og course, also be used for suspensions containing pazticles ~rhich have a particle size distribution other than normal particle size distribu-tion ~rith characteristic values other than ~ and ap.
Another method to derive C and the particle size distribution (e. g. defined by y.~ and Qo) from a measured cumulative count curve is to apply an inversion algorithm, such like the one as given in equation (15).
The capabilities of such an invezsion algorithm are illustrated in Figures 7 and 8.
Figure 7 gives the inversion result using the described inversion technique given by equations (14), (15), (16)_ The line (curve 16) shots the exact size distribution, the cizcular markers (curve 17) are the ZO result obtain using singular value decomposition and the cross markers (curve 18) are the inversion result using the stochastic approach. The Figure shows that more accurate results are obtained using the stochastic approach. To illustrate that any kind of particle size distribution can be detezmined using this technique, an inversion result of a non-gaussian size distribution is shown in Figure 8 . The line ( curve 19 ) gives the exact distribution and the cross markers (curve ZO) give the inversion result using the stochastic approach.
A method and set-up for the characterisation of various types of particles in a suspension will nov be described.
In the above it has been assumed that there fa only one type of particle in the $uspeasion, only the size of the particles being allowed to vary. 7~ further assumption was that the likelihood of more than one particle being present in the measurement volume at the same time is negligibly small. On these grounds, a 3 small measurement volume in the focus region of the acoustic beam 20 was preferably chosen.
However, there are sometimes advantages in, in contrast, selecting a large measurement volume beyond the focus. This is illustrated in Figure 3, fn which the same reference numerals as in Figure 1 refer tv the same elements. The measurement volume is delimited in the axial direction of the principal axis z by z3 and zb associated with time window [t3, t4~. In this case the angle of incidence within the measurement volume varies as a function of the lateral position r with respect to the principal axis z. Zf several reflection measure-ments are carried out in succession on one particle 22 while the particle is passing through the measurement 2Q volume (see Figure 3), the angle-dependent reflection behaviour of the particle can be determined from the change fn the reflection signal as a function of the lateral position. The angle-dependent behaviour is highly dependent on the shape of the particle and therefore the particle can be charactezized on the basis of this behaviour.
Figure 9 shoes a succession of simulated record-ings for a spherical particle which passes through the beam at a fixed distance beyond the focus. Simulated recordings for an elongated particle aze given in Fig-ure 10. These Figures clearly illustrate the effect of the shape of the particle on the reflection behaviour as a function of the angle.
For this method, the larger the aperture of the beam from the transducer 23, the more clearly visible is the difference in angle-dependent behaviour between different types of particles. As a result, the measure-ment volume therefore also increases. However, the 5 requirement that no more than one particle at a time enay be present irt the lateral direction in the measure-went volume is dispensed with with this method because different particles located laterally alongside one another can be individually recognised in the suc-10 cession of recordings. This is illustrated in Figure 11, where two spherical particles pass through the beam just behind one another. Although the lateral distance (75 mm) between these particles is much smaller than the beam diameter, the reflections originating from the 15 first particle can be separated from the reflections from the second particle in the succession of record-ings. The most important characteristic of: the last-mentioned embodiment is that the beam has a large aper-ture and that the time which elapses between two suc-ZO cessive measurements is so short that each particle is exposed several times during its presence fn the beam.
As has been explained above, this can be achieved, for example, with the aid of a focused transducer. Theor-etically, the use of a point source ar a relatively Z5 small source (of the order of magnitude of the wave-length used) is also possible. It is only at high fre-quencies that this cannot be achieved in practice. Op-tionally, use can be made of a Radon transformation or a 2D Fourier transformation to the 1~-k domain in order to quantify the angle-dependent reflection behaviour of each recorded particle.
Following the characterisation of a particle on the basis of its angle-dependent behaviour, the cumu-lative count curve can be determined for each type of particle. For this purpose the maximum amplitude for zs the series of recordings originating from the particle concerned ithis is the atmplitude when the particle is located on the principal axis s of the beam 20) is taken as the amplitude per detected particle. This ' S maximum amplitude can still vary for one type of par-ticle because of variations in particle size acrd vari-ations in the minimum lateral distance from the princi-pal axis a at which the particle passes through the beam. A cumulative count curve similar to that illus-trated above With reference to Figures 4, 5 and 6 will be found for each type of particle.
A possible application of the method described above is the measurement of thrombus particles in a blood stream. Patients who have a heart valve prothesis have an increased risk of thrombosis because the arti-ficial valve promotes the production of thrombus par-ticles. Therefore, these patients have an increased risk of acute vascular occlusion as a result of too large a thrombus particle. This can lead, for example, to cerebral infarction.
Anticoagulants are administered to the patient to counteract thrombus formation. The risk of too high a dosage of these anticoagulants is the occurrence of haemorrhaging (for example cerebral haemorrhaging).
currently the dosage is determined on the basis of the coagulation measured on a blood sample. Ho~rever, this is an unreliable method because the coagulation is dependent on many more factors than solely the concen-tration of thrombi. Moreover, a measurement of this type provides only a snapshot.
It is therefore desirable to have available a method with which the concentration of thrombi in the blood can be measured reliably. The measurement method described here is suitable for this. purpose.
$y using a transmission frequency of the sound craves which ie relatively high for medical epplfc-ations, for example in the range of 10-40 l~Iz, prefer-ably ZO-30 lZHz, it is possible to detect small par-ticles present in the b~.ood, The depth of penetration is still found to be sufficiently great at these fre-quencies. By making use of the high efficiency of, for example, a composite transducer and of sophisticated transmission and reception electronics, it is possible to achieve an adequate depth of penetration, so that an echo-acoustic recording of the blood can be made using a non-invasive technique. However, blood also contains other particles which will be detected by the ultra-sonic reflection method, for example red blood cells.
It is knoxm that red blood cells form elongated aggregates during a certain period of the heart cycle (end of diastolic), which aggregates align With the flow. See, for example, M.G.M. de Kroon: "Acoustic backscatter in arteries -- Measurements and modelling of arterial wall and blood", thesis 1993, ISBN 90-9006182, Section III., It is possible to distinguish these long structures from thrombi, Which are of a jagged shape (see: S. Chien: "Clinical Naemorheology", l~Jartinus Hijhoff), on the basis of the angle-dependent behaviour.
After making this distinction, the concentration and size distribution of thrombi, tan be estimated on the basis of the cumulative count cuzves, as has been explained above.
Claims (26)
1. A method for the detection and identification of particles in a suspension, comprising the following steps:
a. generation of acoustic signals having the form of a beam using an acoustic source;
b. directing the acoustic signals at at least one measurement volume within the suspension, boundaries of the measurement volume in an axial direction with respect to the acoustic source being defined with the aid of time windows;
c. reception of acoustic reflection signals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. conversion of the acoustic reflection signals into electrical reflection signals;
e. counting numbers of electrical reflection signals which have an amplitude in excess of a predetermined value and conversion thereof into numbers of particles which are larger than a certain size;
wherein the method also comprises the following steps:
f. composing at least one curve on the basis of a cumulative count of the number of reflection signals which have an amplitude in excess of a specific value as a function of the amplitude;
g. comparison of the at least one curve with pre-determined standard cumulative count curves and deduction of at least one feature from a set of features comprising: material properties, particle concentration, particle shapes, particle size and standard deviation thereof and particle size distribution.
a. generation of acoustic signals having the form of a beam using an acoustic source;
b. directing the acoustic signals at at least one measurement volume within the suspension, boundaries of the measurement volume in an axial direction with respect to the acoustic source being defined with the aid of time windows;
c. reception of acoustic reflection signals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. conversion of the acoustic reflection signals into electrical reflection signals;
e. counting numbers of electrical reflection signals which have an amplitude in excess of a predetermined value and conversion thereof into numbers of particles which are larger than a certain size;
wherein the method also comprises the following steps:
f. composing at least one curve on the basis of a cumulative count of the number of reflection signals which have an amplitude in excess of a specific value as a function of the amplitude;
g. comparison of the at least one curve with pre-determined standard cumulative count curves and deduction of at least one feature from a set of features comprising: material properties, particle concentration, particle shapes, particle size and standard deviation thereof and particle size distribution.
2. A method according to Claim 1, wherein the standard cumulative count curves are derived as follows:
- definition of lateral boundaries of the measurement volume as being the boundaries beyond which no amplitudes of the reflection signals smaller than a predetermined minimum value can be recorded;
- derivation of a probability density function for the amplitude of the electrical reflection signals within the measurement volume as a function of a given particle shape, and particle size distribution thereof, and - derivation of the standard count curves from the probability density function for the amplitude.
- definition of lateral boundaries of the measurement volume as being the boundaries beyond which no amplitudes of the reflection signals smaller than a predetermined minimum value can be recorded;
- derivation of a probability density function for the amplitude of the electrical reflection signals within the measurement volume as a function of a given particle shape, and particle size distribution thereof, and - derivation of the standard count curves from the probability density function for the amplitude.
3. A method according to Claim 2, wherein the following equation is used for the probability density function for the amplitude:
P(A) = .intg. g(A¦(D) dD
where: g(A¦D) = probability density function for a measured amplitude A originating from an arbitrary particle with diameter D;
h (D) = probability density function for the particle diameter, the equation being integrated between two predetermined limits for the particle diameter.
P(A) = .intg. g(A¦(D) dD
where: g(A¦D) = probability density function for a measured amplitude A originating from an arbitrary particle with diameter D;
h (D) = probability density function for the particle diameter, the equation being integrated between two predetermined limits for the particle diameter.
4. A method according to Claim 3, wherein a Gaussian amplitude profile of the acoustic signals in the lateral direction of the measurement volume and a substantially negligible amplitude variation in the acoustic signals in the axial direction of the measurement volume are assumed for determination of g(A¦D), resulting in the relation:
where A o is the amplitude which is detected from the particle concerned with said particle located on a principal axis of the beam.
where A o is the amplitude which is detected from the particle concerned with said particle located on a principal axis of the beam.
5. A method according to Claim 1, wherein standard cumulative count curves are determined experimentally with the aid of suspensions containing particles of a pre-known shape, size and standard deviation of the size.
6. A method for the detection and identification of particles in a suspension, comprising the following steps:
a. generation of acoustic signals using an acoustic source;
b. directing the acoustic signals at at least one measurement volume within the suspension, boundaries of the measurement volume in an axial direction with respect to the acoustic source being defined with the aid of time windows;
c. reception of acoustic reflection signals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. conversion of the acoustic reflection signals into electrical reflection signals;
e. counting numbers of electrical reflection signals which have an amplitude in excess of a predetermined value and conversion thereof into numbers of particles which are larger than a certain size;
wherein the method also comprises the step of applying an inversion algorithm on the amplitudes of the electrical reflection signals to deduce at least one feature from a set of features comprising: material properties, particle concentration, particle shapes, particle size and standard deviation thereof and particle size distribution.
a. generation of acoustic signals using an acoustic source;
b. directing the acoustic signals at at least one measurement volume within the suspension, boundaries of the measurement volume in an axial direction with respect to the acoustic source being defined with the aid of time windows;
c. reception of acoustic reflection signals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. conversion of the acoustic reflection signals into electrical reflection signals;
e. counting numbers of electrical reflection signals which have an amplitude in excess of a predetermined value and conversion thereof into numbers of particles which are larger than a certain size;
wherein the method also comprises the step of applying an inversion algorithm on the amplitudes of the electrical reflection signals to deduce at least one feature from a set of features comprising: material properties, particle concentration, particle shapes, particle size and standard deviation thereof and particle size distribution.
7. A method according to Claim 6, wherein the following equation is used for the inversion algorithm:
~(A) = W(A,D) ~(D) where W is a forward matrix of size j x I, I and j being predetermined integer numbers, which relates likelihood q(D j) for a particle to have a diameter in a predetermined interval D j-.DELTA.D/2.. D j+.DELTA.D/2 to probability p(A1) of measuring an amplitude A1.
~(A) = W(A,D) ~(D) where W is a forward matrix of size j x I, I and j being predetermined integer numbers, which relates likelihood q(D j) for a particle to have a diameter in a predetermined interval D j-.DELTA.D/2.. D j+.DELTA.D/2 to probability p(A1) of measuring an amplitude A1.
8. A method according to Claim 7, where the forward matrix W is inverted applying either a Singular Value Decomposition or a stochastic approach.
9. A method according to Claim 6, wherein a Gaussian amplitude profile of the acoustic signals in a lateral direction of the measurement volume and a substantially amplitude variation in the acoustic signals in the axial direction of the measurement volume are assumed for determination of a forward matrix W, resulting in the following expression for the probability density for the amplitude P(A1):
wherein q(D j) is a likelihood for a particle to have a diameter in the interval D j - .DELTA.D/2..D j + .DELTA.D/2 y is a proportionality constant D is a particle diameter; and A n is a noise amplitude
wherein q(D j) is a likelihood for a particle to have a diameter in the interval D j - .DELTA.D/2..D j + .DELTA.D/2 y is a proportionality constant D is a particle diameter; and A n is a noise amplitude
10. A method according to Claim 6, where a calculated particle size distribution q app resulting from the inversion algorithm is corrected for the fact that smaller particles cannot be detected throughout the whole measurement volume, resulting in a true particle size distribution q true.
11. A method according to Claim 10, wherein a Gaussian amplitude profile of the acoustic signals in a lateral direction of the measurement volume and a substantially negligible amplitude variation in the acoustic signals in the axial direction of the measurement volume are assumed for determination of a correction to be applied, resulting in the following relation:
where D max is the diameter of the largest particle in suspension and E is a factor applied to normalize the area below q true equal to unity and A n is a noise amplitude.
where D max is the diameter of the largest particle in suspension and E is a factor applied to normalize the area below q true equal to unity and A n is a noise amplitude.
12. A method according to Claim 1 where the particle concentration is calculated from the number of measurements with amplitude above a particular noise level.
13. A method according to Claim 12, where a calculated particle concentration is corrected for the fact that smaller particles cannot be detected through the whole measurement volume, resulting in a true particle concentration.
14. A method according to Claim 13, wherein a Gaussian amplitude profile of the acoustic signals in a lateral direction of the measurement volume and a substantially negligible amplitude variation in the acoustic signals in the axial direction of the measurement volume are assumed for determination of the said correction to be applied, resulting in the following relation for the true particle concentration C:
where (N(...) gives the number of measurements for which a condition given in brackets applies, N tot is the total number of measurements, V meas is the measurement volume .xi.
is a normalizing factor and q true is a true particle size distribution and D max is the diameter of the largest particle in the suspension.
where (N(...) gives the number of measurements for which a condition given in brackets applies, N tot is the total number of measurements, V meas is the measurement volume .xi.
is a normalizing factor and q true is a true particle size distribution and D max is the diameter of the largest particle in the suspension.
15. A method according to Claim 1, wherein the measurement volume is chosen to be around a focus of the acoustic beam and to be so small that a likelihood of the presence of more than one particle in the measurement volume during a measurement is negligibly small.
16. A method according to Claim 6, wherein the measurement volume is chosen to be around a focus of the acoustic beam and to be so small that the likelihood of the presence of more than one particle in the measurement volume during a measurement is negligibly small.
17. A method according to Claim 1, wherein:
- in step a, the acoustic beam generated has such a large aperture, and the time which elapses between two successive measurements is so short, that each particle is exposed several times while passing through the beam and that, depending on a lateral position of a particle in the measurement volume, a varying angle-dependent reflection of the acoustic signal is produced;
- prior to step f, one or more different types of particles present in the suspension are identified on the basis of a series of angle-dependent reflection signals received successively over time;
- when composing the curve in step f, the maximum value of the amplitudes of a series of successive angle-dependent reflection signals, received over time, from a detected particle from the one or more groups is taken as the amplitude of the electrical signals, which are produced after conversion of the acoustic reflection signals from that particle.
- in step a, the acoustic beam generated has such a large aperture, and the time which elapses between two successive measurements is so short, that each particle is exposed several times while passing through the beam and that, depending on a lateral position of a particle in the measurement volume, a varying angle-dependent reflection of the acoustic signal is produced;
- prior to step f, one or more different types of particles present in the suspension are identified on the basis of a series of angle-dependent reflection signals received successively over time;
- when composing the curve in step f, the maximum value of the amplitudes of a series of successive angle-dependent reflection signals, received over time, from a detected particle from the one or more groups is taken as the amplitude of the electrical signals, which are produced after conversion of the acoustic reflection signals from that particle.
18. A method according to Claim 17, wherein in step a. a focused acoustic beam with a predetermined focus is generated and in step b the focus remains outside the measurement volume.
19. A method according to Claim 17, wherein an acoustic source having dimensions smaller than a wavelength of the generated acoustic signals is used.
20. A method according to Claim 1, wherein the acoustic signal comprises sound signals having a frequency of 10-40 MHz.
21. A method according to Claim 20, wherein the sound signals have a frequency of 20-30 MHz.
22. A method according to Claim 6, wherein the acoustic signal comprises sound signals having a frequency of 10-40 MHz.
23. A method according to Claim 22, wherein the sound signals have a frequency of 20-30 MHz.
24. Equipment for the detection and identification of particles in a suspension, comprising;
a. an acoustic source for the generation of acoustic signals;
b. means for directing the acoustic signals at at least one measurement volume within a flowing suspension, boundaries of the measurement volume in an axial direction with respect to the acoustic source being defined with the aid of time windows;
c. means for receiving acoustic reflection signals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. means for converting the acoustic reflection signals into electrical reflection signals;
e. means for counting numbers of electrical reflection signals which have an amplitude in excess of a predetermined value and for converting said count into numbers of particles which are larger than a certain size;
wherein the equipment also comprises:
f. means for composing at least one curve on the basis of a cumulative count of the number of reflection signals which have an amplitude in excess of a specific value as a function of the amplitude;
g. means for comparing the at least one curve with predetermined standard cumulative count curves and for deducing at least one feature from a set of features comprising: material properties, particle concentration, particle shapes, particle size and standard deviation thereof and particle size distribution.
a. an acoustic source for the generation of acoustic signals;
b. means for directing the acoustic signals at at least one measurement volume within a flowing suspension, boundaries of the measurement volume in an axial direction with respect to the acoustic source being defined with the aid of time windows;
c. means for receiving acoustic reflection signals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. means for converting the acoustic reflection signals into electrical reflection signals;
e. means for counting numbers of electrical reflection signals which have an amplitude in excess of a predetermined value and for converting said count into numbers of particles which are larger than a certain size;
wherein the equipment also comprises:
f. means for composing at least one curve on the basis of a cumulative count of the number of reflection signals which have an amplitude in excess of a specific value as a function of the amplitude;
g. means for comparing the at least one curve with predetermined standard cumulative count curves and for deducing at least one feature from a set of features comprising: material properties, particle concentration, particle shapes, particle size and standard deviation thereof and particle size distribution.
25. Equipment for the detection and identification of particles in a suspension, comprising:
a. an acoustic source for the generation of acoustic signals;
b. means for directing the acoustic signals at at least one measurement volume within a flowing suspension, boundaries of the measurement volume in an axial direction with respect to the acoustic source being defined with the aid of time windows;
c. means for receiving acoustic reflection signals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. means for converting the acoustic reflection signals into electrical reflection signals;
e. means for counting numbers of electrical reflection signals which have an amplitude in excess of a predetermined value and for converting said count into numbers of particles which are larger than a certain size;
wherein the equipment also comprises:
f. means for applying an inversion algorithm on the amplitudes of the electrical reflection signals to deduce at least one feature from a set of features comprising: material properties, particle concentration, particle shapes, particle size and standard deviation thereof and particle size distribution.
a. an acoustic source for the generation of acoustic signals;
b. means for directing the acoustic signals at at least one measurement volume within a flowing suspension, boundaries of the measurement volume in an axial direction with respect to the acoustic source being defined with the aid of time windows;
c. means for receiving acoustic reflection signals produced by reflection of the acoustic signals by the particles in the at least one measurement volume;
d. means for converting the acoustic reflection signals into electrical reflection signals;
e. means for counting numbers of electrical reflection signals which have an amplitude in excess of a predetermined value and for converting said count into numbers of particles which are larger than a certain size;
wherein the equipment also comprises:
f. means for applying an inversion algorithm on the amplitudes of the electrical reflection signals to deduce at least one feature from a set of features comprising: material properties, particle concentration, particle shapes, particle size and standard deviation thereof and particle size distribution.
26. Equipment according to Claim 24, wherein - during operation, the acoustic source generates an acoustic beam which has such a large aperture, and makes the time which elapses between two successive measurements so short, that each particle is exposed several times while passing through the beam and that, depending on a lateral position of the particles in the measurement volume, a different angle-dependent reflection of the acoustic signal is produced;
- identification means are also present for identification of one or more different groups of particles present in the suspension on the basis of a series of successive angle-dependent reflection signals received over time;
- the means for composing a cumulative curve compose the curve, the maximum value of the amplitudes of a series of successive angle-dependent reflection signals, received over time, from a detected particle being taken as the amplitude of the electrical signals, which are produced after conversion of the acoustic reflection signals from that particle.
- identification means are also present for identification of one or more different groups of particles present in the suspension on the basis of a series of successive angle-dependent reflection signals received over time;
- the means for composing a cumulative curve compose the curve, the maximum value of the amplitudes of a series of successive angle-dependent reflection signals, received over time, from a detected particle being taken as the amplitude of the electrical signals, which are produced after conversion of the acoustic reflection signals from that particle.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| NL1002836 | 1996-04-10 | ||
| NL1002836 | 1996-04-10 | ||
| NL1003595A NL1003595C2 (en) | 1996-04-10 | 1996-07-15 | Method and device for characterizing suspensions. |
| NL1003595 | 1996-07-15 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2202323A1 CA2202323A1 (en) | 1997-10-10 |
| CA2202323C true CA2202323C (en) | 2006-06-06 |
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ID=26642374
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002202323A Expired - Lifetime CA2202323C (en) | 1996-04-10 | 1997-04-10 | Method and equipment for the characterisation of suspensions |
Country Status (6)
| Country | Link |
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| US (2) | US6029507A (en) |
| EP (2) | EP0801305B1 (en) |
| CA (1) | CA2202323C (en) |
| DE (1) | DE69739434D1 (en) |
| NL (1) | NL1003595C2 (en) |
| NO (1) | NO322667B1 (en) |
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| SE516861C3 (en) * | 2000-07-14 | 2002-04-17 | Abb Ab | Raffinörstryrning |
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1996
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| US6205848B1 (en) | 2001-03-27 |
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| NO971649D0 (en) | 1997-04-10 |
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