WO2001043639A1

WO2001043639A1 - Determination of at least one material in an object

Info

Publication number: WO2001043639A1
Application number: PCT/NL2000/000919
Authority: WO
Inventors: Johan Willem Haarman
Original assignee: Haarman Innovation Management Bvba
Priority date: 1999-12-16
Filing date: 2000-12-14
Publication date: 2001-06-21
Also published as: AU5439501A; NL1013865C2

Abstract

Method for the determination of at least one material in an object, which consists of at least two materials, by means of energy subtraction, whereby the object under investigation is at least partially irradiated in at least essentially one direction by a number of in practice different energy spectra, which is lower than necessary for the usual application of energy subtraction and whereby the for the usual application of energy subtraction in the first instance missing data are derived from information obtained from the measurement of the transmission of the radiation through at least one other part of the object. For this method an apparatus can be used, which irradiates with the appropriate alternation of in practice different spectra and with which bone density can be determined, an X-ray image can be made, the fat-bone meat ratio of animals in a slaughter process can be determined or inspection of luggage can be performed.

Description

determination of at least one material in an object

The invention relates to a method for the determination of at least one material in an object, more in particular to the analysis of organic and inorganic materials with ionising radiation such as X-rays and γ-rays, comprising these materials in humans and animals. In this way it is possible to determine the material quantitatively or qualitatively. The invention also relates to apparatus suitable for carrying out the method.

Apparatus and methods have been developed already to determine more in particular the amount of bone in the human body. Thus for example it is possible to determine the presence of osteoporosis which may lead to a bone fracture, for example of the wrist, hip or vertebrae.

Thus in US-A-Re.34.511 an apparatus is described in which a patient is scanned with a so-called pencil beam and in US-A-5.432.834 a so called fan beam of ionising radiation, usually X-rays, is used. In fact the analysed part is scanned twice, first with radiation with a specific energy spectrum and the second time with radiation with another energy spectrum. If the object is scanned with a pencil beam, one detector-element is positioned behind the part to be analysed and in the case of a fan beam an array of detector elements is used. Usually the detector(s) is (are) mechanically coupled with the radiation source in order to track the scanning beam. By measuring the absorption at two different energy spectra and inserting these data into the equations

2 ) i₂/I₂ = e^~γH1 x e^'SK2

wherein Ml represents the distance the radiation must travel through material 1 ,

M2 represents the distance the radiation must travel through material 2 , ii represents the amount of radiation in the beam with energy spectrum 1 which falls on the relevant detector, exiting the object, i₂ represents the amount of radiation in the beam with energy spectrum 2 which falls on the relevant detector, exiting the object,

Ij_. represents the amount of radiation in the beam as defined under i_{l f} falling on the object, I₂ represents the amount of radiation in the beam as defined under i₂, falling on the object, α, β, γ and δ represent the coefficients of absorption belonging to the relevant materials and the energy spectra 1 and 2 of the used radiation respectively, the amount of bony material can be determined for instance in the part of the body concerned. For the measurement of the bone density most apparatus contain a bed on which the patient can lie in an appropriate position and a scanning mechanism, that can move relative to the patient and the bed. Such an apparatus is described in U.S. patent 5.778.045. If a pencil beam is used, the radiation beam scans, according to U.S. patent 4.811.373, first in the direction from one shoulder to the other then the radiation beam is shifted in the longitudinal direction of the patient and then the beam is scanned in opposite direction from shoulder to shoulder. This method is repeated until the intended area has been scanned completely. For the measurement of bone density in every position the absorption must be measured two times at two different radiation spectra . Because the scanning with a pencil beam and only one detector takes a lot of time relatively, other types of equipment have been developed which scan the patient with a fan beam, of which the dimension in the scanning direction is relatively small, normally the size of one detector of the array on which the radiation is received, and extends in the direction perpendicular to the scanning direction so that at least a part of the dimensions of the patient perpendicular to the scan direction can be scanned. The full dimension of the patient perpendicular to the scan direction can be covered if the detector array is large enough or this dimension can be covered by a smaller detector array by a repeated combination of scanning and swinging the detector array around an axis parallel to the direction of scanning. In order to get the results of measurements for two different energy spectra, the patient can first be scanned with one energy spectrum and than with another energy spectrum, with the risk that the patient moves between the two measurements so that the reliability of the measurements can be compromised, or at each position two measurements are executed one after the other with different energy spectra. EP 0713677 describes as prior art a CT scanner imaging technique wherein two reconstructed images are generated through the same slice, but with different energy spectra, e.g. by pulsing the X-ray tube alternately at high and low energy levels. It is also possible to collect the two images sequentially. In both techniques two rotations per slice are required in order to collect all data required. Accordingly these techniques are not suited to linear weighted helical scanning. A solution for the latter technique is provided, wherein rotations are required for each complete cycle from high kV to low kV and back to high kV.

It is an object of the present invention to provide an improved method and apparatus using the principle of dual energy subtraction, whithout having to irradia- te each part of the object in one direction with as many energy spectra as the amount of different materials to be analysed, and thus reducing the amount of radiation and time necessary for the measurement, by using at least two distinct radiation energy spectra alternately and reconstructing the missing data necessary for performing the calculation associated with dual energy subtraction. Usually an X-ray source such as an X-ray tube is used for the creation of a fan beam or a pencil beam. In order to get two different energy spectra two different high tensions may be used to feed the X-ray source or one high tension may be used and the spectrum can be modulated by positioning different absorption filters in the beam. A combination of the two methods can be used as well.

With the methods used until now for the scanning of a certain area it is necessary that each point is irradiated with two beams with different energy spectra in order to get a good result.

It would be desirable to use for each part of the object only one energy spectrum. Then the measurement can be speeded up and the amount of radiation used can be diminished. To that end adjacent areas to be measured, called strips, are essentially irradiated with radiation of different energy spectra. Using only two different energy spectra for adjacent areas, these energy spectra can be used in an alternating fashion. Consequently it is possible to obtain the desired analysis results, such as the measurement of bone density in humans, by using only one energy spectrum per position. In this manner two different radiation images can be obtained, which are shifted with respect to each other by at least one position. For each radiation energy the radiation image can be completed by filling in the missing strips by interpolating between two relevant strips of the other radiation image.

Strips as used herein are longitudinal strips which correspond with the dimensions of a detector system, that consists of one or more rows of detector elements, wherein one strip corresponds with all rows of detector elements. Consequently the invention refers to the determination of at least one material in an object, which consists of at least two materials, by means of energy subtraction, whereby the object investigated is at least partially irradiated in essentially at least one direction by a number of in practice discernable distinct energy spectra that is smaller than required for the usual application of energy subtraction and whereby the data missing in first instance are derived from the measurement of the transmission of at least one other part of the object.

Preferably the spatial modulation of the radiation pattern is realized by a relative movement of a radiation beam and the object with respect to each other and modifying the energy spectrum of the beam. With this method separate strips of ratio numbers (numbers which indicate the ratio of quantity of incident and exiting radiation) are created, which belong to one energy spectrum. The information for the intermediate strips is reconstructed by interpolation. This interpolation is not necessarily linear. Other interpolation schemes can be applied as well, if desired. It will be obvious that strips belonging to different energy spectra may overlap but may also be adjacent or be separated by areas which are not irradiated. In practice a partial overlap is preferred as in that way easier reciprocal reference points can be obtained.

Additionally it is possible, if desired, to use more than two different energy spectra so that as many differently absorbing materials can be distinguished as the number of different penetrating types of radiation which are used. The limiting factor is that the succeeding positions referring to the same radiation energy should not be so far apart, that interpolation of the intermediate positions leads to unacceptable errors in measurements.

The invention is illustrated by means of the accompanying drawing.

In the drawing Fig. 1 shows an outline of an apparatus for scanning according to the invention.

Fig. 2 shows the radiation patterns belonging to alternating spectra and the corresponding pulse durations. Fig. 1 shows an apparatus in which a fan beam is used. An assembly of X-ray source (1) with a focus (2) of the X-ray source (1) which is also the axis of rotation of the fan beam-detector system, which axis is perpendicular to the plane of the drawing, a collimator system (3) and a fan shaper (4) of the collimator system which moves together with the fan beam-detector system and a detector system (8) consisting of several detector elements (9), between which the object (6) is placed on a bed (7). The length of the detector system is large enough to capture the total transmitted radiation generated by the fan beam. In the current case the width (45 cm by 1 cm) is constituted by a number of rows of detector elements, each detector element having a size of 100 micron by 100 micron. The detector system consists of 100 rows of detector elements. The dimensions of this detector system correspond with a strip. Perpendicular to the direction of the rows of detector elements, thus in the direction of the movement relative to the object, the detector elements are connected to each other in such a way that the signal of the first detector is shifted after a short period to the second detector element in opposite direction of the scanning movement and the result of the second detector element is shifted to the third detector element and so on, the detector elements beginning to start a new measurement. This is a so called TDI detector. The detector is tightly coupled with the X- ray source mechanically or electrically in such a way that it captures all of the transmitted fan beam.

The distance between the X-ray source and the detector may be varied, if desired. In this way the absorption of the radiation in the patient is measured in succeeding strips. These strips are adjacent in the direction of the relative movement of the detector system. Consequently a complete X-ray image is being formed.

Fig. 2 shows in the lower diagram in vertical direction the X-ray dose and along the horizontal axis the dimension in cm. The diagram shows that alternately spectra A and B are being emitted. In the example provided the distance between the beginning of the maxima of successive spectra is 1 cm.

The corresponding upper diagram in Fig. 2 shows in vertical direction the X-ray energy spectrum and along the horizontal axis the time in s. The duration of the pulses, which means the width of each tooth, is 2.5 milliseconds.

If the scanning of 40 cm in the longitudinal direction of the patient requires about one second and if the X-ray source creates a relatively short pulse each 25 ms, the X-ray image created consists of a series of strips of images with a width of 1 cm which are approximately adjacent in longitudinal direction which are generated in alternating fashion with a different spectrum. Technically it is difficult to make the X-ray pulses so short in time that the movement of the detector system is small in comparison with the size of one detector element. Therefore the detector is provided with a TDI-mode of which the velocity of shifting the charge to adjacent detector elements, is synchronised with the velocity of the detector and the patient relative to each other. Consequently, the transmitted radiation of one element of the object (i.e. a part of the object that corresponds with the size of one detector element) is captured by all successively passing detector elements . However the charge , which corresponds with the radiation transmitted, is each time shifted to the next detector element so that the charge, which is measured at the end, corresponds with the transmitted radiation of one element of the object during the time the detector passes the element of the object. During the X-ray pulse of 2.5 ms ten (10) detector elements will shift their electrical charge in opposite direction to the relative movement of the detector system and the object into the device, that measures the charge. This implies, that the first charge, which is measured immediately after the beginning of the X-ray pulse, corresponds with about one tenth of the dose compared with the dose which corresponds with the charge measured at the last shift before the next X-ray pulse is created. The second charge, which is being measured, corresponds with about two tenths of such a dose. With each following charge the dose increases until the eleventh time a charge is being measured after the X-ray pulse has been created. From that measurement on the dose remains approxi- mately constant until the last charge is being measured before the next X-ray pulse is created.

After the X-ray pulse has ended, the charges are shifted and measured in much shorter time in order to make it possible to read out the total detector and to reset the system before the next X-ray pulse. In this way an X-ray image of the object can be made consisting of strips with a width of 1 cm if the intensity of the first ten rows of detector elements corresponding with every X-ray pulse are corrected for the lower initial dose with which the image is made. Then two complete images of the object can be reconstructed by interpolating the space between two rows of detector images belonging to the same X-ray energy spectrum. In this way two complete X-ray images are obtained corresponding with two different energy spectra with which for instance the amount of bone can be determined by using formulas 1 and 2.

Nearly all elements in above mentioned elucidation can be varied. For instance the detector elements may be smaller or larger. The number of detector elements in the direction of the relative movement may be smaller or larger. The speeds of movement can be varied in such a way that the strips overlap or are spaced apart. Also the speed of movement can be varied during one measurement. The duration of the pulses can be made shorter or longer relatively or absolutely. The output of the X-ray tube can be varied during the measuring process in order to make the signal-to-noise ratio optimal. The total measuring process can be made shorter or longer dependant on the output of the X-ray tube.

Moreover an embodiment can be made in which the X- ray tube need not to be pulsed but can produce continuous radiation, though varying in energy spectrum or in energy spectrum and output.

It is possible to build a detector system consisting of only one row of detector elements which scan the object alternately with strips with high and with lower energy spectra with which data for the amount of bone can be determined, for instance after interpolation.

It is not necessarily restricted to two energy spectra. In principle more energy levels may be used, allowing that more than two materials can be analysed. In this way a detector consisting of one row of detector elements with a relative short read out time can be moved with a constant speed relative to the object while the X- ray energy spectrum varies periodically. The specific distance belonging to this periodicity has to be large relative to the size of the detector element but also small enough to enable interpolation with the required accuracy.

Consequently the invention refers more particularly to a method in which the radiation is presented to the object in the form of pulses, but a method in which the radiation is applied to the object continuously with time but simultaneously varying the energy spectrum is also possible.

This principle can be extended to the use of a TDI detector consisting of several rows of detector elements, which moves uniformly relative to the object to be scanned while the X-ray energy spectrum varies with time, preferably in a periodical manner. The specific dimension belon- ging to the periodical X-ray pattern must preferably be more then two times larger than the dimension of the detector element in the direction of the relevant movement but not so large, that interpolation, necessary for the analysis of the different materials, can not be done with sufficient accuracy.

The read out of the charge, which corresponds with the radiation transmitted by one object element, is a superposition of charges, which during the relative move- ments are created in the succeeding detector elements by the transmitted radiation. This transmitted radiation is the superposition of the radiation changing in energy spectrum which is sent through said object element by the radiation source during the passage of the detector system. This superposition of different spectra is to be considered as a spectrum in itself. The charges which in this way are read out successively belong to in effect different spectra, because the specific dimension of the periodic radiation pattern in the direction of the relevant movement is preferably more than twice the dimension of the detector system in the same direction. In this way the transmission is measured of many rows of object elements (perpendicular to the relative movement) , which belong to an effective spectrum changing from row to row. In order to improve the signal-to-noise ratio the output of the radiation source can be changed simultaneously with the energy spectrum.

It is also possible to use X-ray energy spectra which change non-periodically. The above methods can also be used in part in equipment which uses only one detector element and a so- called pencil beam by using in different positions, where the detector element measures radiation with different energy spectra. The scan movement is achieved preferably by swinging the system consisting of radiation source and detector system about an axis, which runs through the focal point of the radiation source. However it is also possible to move the object of investigation past the system consisting of radiation source and detector array. This movement should preferably be a swinging movement around the focal point of the radiation source, such that the object under investigation is irradiated with adjacent and not overlapping conically shaped radiation beams. However in practice in many cases a linear movement past the radiation source-detector system is sufficient. This makes the mentioned methods suitable for scanning of for instance luggage in airports or for the scanning of animals in slaughter systems in order to determine the fat-bone-meat ratio.

Another application using the method of interpola- tion as set out above is one in which the spatial modulation of the radiation pattern is achieved by radiating larger parts of the object of investigation by means of a radiation source provided with a radiation filter being modulated in a direction approximately perpendicular to the direction of the radiation beam in such a way that different parts of the object of investigation are irradiated with radiation with a different energy spectrum.

A CT apparatus is particularly suitable for the application of the described methods in which a radiation source and the detector system belonging to it are rotated around the object of investigation and in which successive planes are scanned with X-ray radiation of different energy spectra.

In an alternative to this practice the energy spectrum of the X-ray radiation is varied in a CT apparatus which uses the method of spiral scanning.

More particularly a CT apparatus can be used in which the succeeding measured profiles are generated with different energy spectra. Then the radiation source is controlled in such a way that during the turning of the apparatus the created modulation preferably contains at least one periodical component. Similarly the method can be applied in an apparatus which is suited for the creation of rotational angio exposures .

For the creation of exposures conform the general method described in this document in particular an apparatus is suited in which the object of investigation and the radiation source with the detector system belonging to it move with respect to each other and in which the detector system consists of more than one row of detector elements wherein the detector elements are connected to each other in a TDI mode in the direction of the relative movement of the object.

Claims

Claims :

1. Method for the determination of at least one material in an object consisting of at least two materials by means of energy subtraction, characterised in that the relevant object is at least partially irradiated in at least essentially one direction by a number of distinct energy spectra which is lower than normally necessary if the usual method of energy subtraction is applied and the missing data for the usual application of energy subtracti- on are being derived from information obtained from the measurement of the transmission through at least one other part of the object.

2. Method according to claim 1, characterised in that the spatial modulation of the radiation pattern is being created by moving a radiation beam, which irradiates a part of the object, and the object with respect to each other and modifying the energy spectrum of the radiation beam.

3. Method according to claim 2, characterised in that the radiation is presented to the object in the form of pulses which are created with intervals between them.

4. Method according to claim 3 , characterised in that the energy spectrum is different in different radiation pulses.

5. Method according to claim 4 , characterised in that the radiation is continuous but changing in energy spectrum with time.

6. Method according to claim 5, characterised in that the modulation of the radiation pattern contains spatially at least one periodical component.

7. Method according to claim 1, characterised in that the detector system does not move with respect to the object and is large enough to capture and measure the transmitted radiation pattern of the total part of the object which is subject to analysis.

8. Method according to claim 1 , characterised in that the radiation source and the detector system belonging thereto turn around the object of investigation and that the energy spectrum of the radiation source changes during this movement.

9. CT-apparatus for use in the method of claim 8, characterised in that during the scanning process the energy spectrum of the X-ray radiation changes.

10. Apparatus for use in the method according to claim 8, characterised in that it is suitable for performing rotational angio procedures.

11. Apparatus for use in a method according to one of the claims 1-6, characterised in that the object of investigation and the radiation source with the detector system belonging to it move with respect to each other and that the detector system consists of more than one row of detector elements whereby the detector elements are connected in TDI-mode in the direction of the relative movement.

12. Apparatus for use in a method according to one of the claims 1-7, characterised in that it is adapted for the measurement of bone density.

13. Apparatus for use in a method as claimed in one of the Claims 1-7, characterised in that it is suited for simultaneously measuring bone density and making an X-ray image of at least part of the body.

14. Apparatus used with a method as claimed in one of claims 1-7, characterised in that it is adapted for measuring the ratio of fat-meat-bone in slaughter procedures.

15. Apparatus used with a method according to one of claims 1-7, characterised in that it is suitable for the determination of the presence of at least one material in luggage inspection.