WO1997043606A1

WO1997043606A1 - Ultrasound phantoms

Info

Publication number: WO1997043606A1
Application number: PCT/EP1997/002364
Authority: WO
Inventors: Lars Vincents JØRGENSEN; Christian Christiansen BÜRGER
Original assignee: Osteometer Meditech A/S
Priority date: 1996-05-09
Filing date: 1997-05-08
Publication date: 1997-11-20
Also published as: AU2894697A; GB9609678D0

Abstract

A phantom for use in calibrating an ultrasound scanner has a portion (34) made from a first material such as an acrylic plastics having a speed of ultrasound transmission similar to bone encasing a portion (42) made from a second material such as tungsten powder bound in an epoxy resin having a broad band attenuation of ultrasound resembling that of bone.

Description

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ULTRASOϋND PHANTOMS

The present invention relates to phantoms for use in the calibration of ultrasound scanners and in particular for use in connection with ultrasound scanners for use in the evaluation of the condition of bone. Such measurements are of value in diagnosing bone diseases and other abnormal bone conditions including principally osteoporosis. As disclosed in numerous Patent specifications including US-A-4774959 it is known to evaluate the condition of bone by the measurement of the speed of ultrasound transmission through a bony body member and additionally or alternatively to measure the broad band ultrasound attenuation (BUA) of such a bony body member. The broad band ultrasound attenuation is the rate of change with frequency of the attenuation of ultrasound in the member.

In order to ensure that the performance of an ultrasound scanner is reproducible it is necessary to conduct a scan of a so-called phantom, i.e. an artificial member simulating the bony body members on which the ultrasound scanner machine is used which has fixed ultrasound characteristics. Where the ultrasound scanner will be making measurements both of the speed of sound SOS and of BUA, it is necessary that scans of phantoms be carried out in respect of both of these parameters. Naturally, it is preferred that the phantom exhibits a speed of ultrasound transmission which resembles that of bone when checking the SOS measurement performance of the scanner. Similarly, it is preferred that the phantom exhibit a BUA measurement similar to that of bone when one is checking the BUA measurement performance of the scanner. The values measured on the phantom are saved in a file on the computer controlling the scanner and are used for calculating the stability of the scanner over time. This is important because a defect in the scanner must be detected before it adversely affects patient measurements, possibly giving rise to a wrong diagnosis.

As it has not been possible to devise a material which resembles bone both in SOS and BUA, the previous practice has been to use two separate phantoms for the two calibration measurements and generally it has been assumed that the measurement of one value of SOS and one value of BUA is sufficient for verifying the stability of the scanner.

We have now appreciated that it is highly advantageous to use a single phantom containing both a material suitable for SOS measurement and a material suitable for BUA measurement and that it may be further advantageous to measure different thicknesses of the phantom to establish more than one SOS measurement and/or more than one BUA measurement . Accordingly, the present invention provides a phantom for use in the calibration of an ultrasound scanner comprising a first material having a speed of ultrasound transmission (SOS) resembling that of bone and a second material having a broad band ultrasound attenuation (BUA) resembling that of bone. A material may be considered to resemble bone in SOS if the SOS measurement conducted on such a material at an ultrasound frequency of 300 kHz is within one percent more than the maximum or less than the minimum of measurements conducted at the same frequency on bone of equivalent thickness. Generally, having regard to the normal range of SOS measurements in bone, this will imply a value in the range of 1500 to 1650 ms^"1.

The BUA of the material of the phantom may be considered to resemble that of bone if it is within 1 percent greater than the maximum or less than the minimum of similar BUA measurements conducted on bone. Once again, having regard to the normal range of such measurements in bone, this will normally imply a BUA in the range of 10 to 120 dB/MHz units.

In order that both the BUA and SOS can be measured at the same location in the phantom, the phantom preferably has at least one layer of said first material and at least one layer - 3 -

of the second material. Most conveniently, either the first or the second material is encased in or sandwiched between layers of the other material.

There may be at least one region in which the first material has a first, greater thickness and at least one region in which said first material has a second, smaller thickness, in the direction of ultrasound propagation in use. The same may also or instead be true of the second material and preferably the regions of greater thickness of the first and second materials at least partially coincide when both are present. Thus, one may conduct an SOS and a BUA measurement at a first location through a greater thickness of each material and at a second location through a lesser thickness of each material to provide ones calibrating measurements. It is desirable that the measurements are always made in exactly the same place in the phantom. To avoid having to provide mechanical means for positioning the phantom so that its position is always reproducible and for ensuring that the ultrasound scanner always scans the same fixed location, according to a preferred aspect of the present invention the phantom has at least one zone detectable by ultrasound scanning in which one of the first and second materials is absent or is significantly thinner or thicker than in the surrounding areas, whereby the position and optionally the attitude of the phantom may be determined by said scanning. There may be at least two, more preferably three said detectable areas spaced from one another so as to provide a wholly unambiguous spacial location determination for the phantom. Preferably, the scanner is adapted to make calibration measurements at locations defined with reference to said position determining location dr locations.

Such a location may be provided by the second material being shaped as a lens, especially as a plano-concave or biconcave lens, the centre of which can then be accurately detected. Materials suitable for the first material in the phantom have been found to include acrylics, for instance polymethyl methacrylate (PMMA) . Silicone rubber may also be used as may vulcanised ('hard') rubber. Materials suitable for use as the second material in the phantom have been found to include tungsten powder, a mixture of tungsten powder and aluminium powder, or hydroxyapatite in a suitable binder, e.g. an epoxy resin or silicone rubber or vulcanized rubber. The invention will be further described and illustrated with reference to the accompanying drawings in which:

Figure 1 shows a phantom according to the invention in plan view;

Figure 2 shows the phantom of Figure 1 in transverse cross-section.

Figure 3 shows in plan view a second embodiment according to the invention; and Figure 4 shows a section on the line iv- iv of Figure 3.

As shown in the figures, a suitable phantom according to the invention comprises a first material 1 encasing a body of a second material 2. The body of first material 1 is in the form of a circular moulding having a first half cylinder portion of a greater thickness and a second half cylinder portion of a lesser thickness. The two portions which are integral with one another meet along a diameter of the phantom. A body of second material 2 of similar shape but of reduced size is encased within the material 1 with the borderline between the thicker and thinner portions of the material 2 being coincident with that of the body of material 1.

As shown in Figure 1, there are three spaced circular bores through the second material 2 each of which is filled by the material 1 to define three circular zones 3. With reference to these, the location of two circular measurement zones 4 and 5 is specified in a computer program controlling the ultrasound scanner for which the phantom is adapted. In use, the location of the zones 3 is detected by an attenuation scan and the location of the areas 4 and 5 is calculated. SOS measurements and BUA measurements are then conducted in the areas 4 and 5, one of which lies in the thicker region of the phantom and the other of which lies in the thinner region of the phantom and the required calibration BUA and SOS measurements are recorded.

As shown in Figure 3 , a second phantom according to the invention comprises a main body made from a first material, suitably an acrylic as described above. The main body is shaped to comprise a flat plate portion 30 comprising a handle portion 32 at one end, a central disc like portion 34 and at the other end a rectangular tongue 36. At the end of tongue 36 is a circular plate 38 set at right angles to the plane of th plate portion 30. A key 40 extends from the side of plate 38 opposite to the plate portion 30.

Contained within the disc like portion 34 of the main body is a lens shaped mass 42 of a second material, suitably a tungsten powder bound in expoxy as described above. The diameter of the lens shaped mass 42 is suitably about 50mm and the thickness suitably varies from about 4mm at the edge to 2.5mm at the centre.

This produces an effect on the transmission of ultra sound allowing the position of the centre of the lens shaped mass to be identified.

A mounting may be provided in the water bath of an ultrasound scanner taking the form of an upstanding cylindrical boss, having a slot in its plane upper surface.

The key 40 is designed to fit into the slot in the boss so that the plate 38 can come to abut against the surface of the cylindrical boss.

This provides a secure mounting for the phantom ensuring that it is not free to move and can be repeatably positioned.

Many variations and modifications of the invention as described with reference to the specific embodiment are possible within the scope of the invention.

Claims

1. A phantom for use in calibration of an ultrasound scanner comprising a first material having a speed of ultrasound transmission resembling that of bone and a second material having a broadband ultrasound attenuation resembling that of bone .

2. A phantom as claimed in Claim 1, wherein there is at least one layer of said first material and at least one layer of said second material.

3. A phantom as claimed in Claim 1 or Claim 2, wherein said second material is encased or sandwiched in said first material, or wherein said first material is encased or sandwiched in said second material .

4. A phantom as claimed in any one of Claims 1 to 3 , comprising at least one region in which said first material has a first, greater thickness and at least one region in which said first material has a second, smaller thickness, in the direction of ultrasound propagation in use.

5. A phantom as claimed in any one of Claims 1 to 4, comprising at least one region in which said second material has a first, greater thickness and at least one region in which said second material has a second, smaller thickness, in the direction of ultrasound propagation in use.

6. A phantom as claimed in Claim 5 when dependant on Claim 4, wherein said regions of greater thickness of the first and second materials at least partially coincide.

7. A phantom as claimed in any preceding claim, comprising at least one relatively small zone detectable by ultrasound scanning in which one of said first and second materials is - 7 -

absent or is thinner or thicker than in surrounding areas, whereby the position and/or attitude of the phantom may be determined by said scanning.

8. A phantom as claimed in Claim 7, wherein there are at least two said areas spaced from one another.

9. A phantom as claimed in Claim 1, wherein said second material is formed in the shape of a lens and is encased in said first material.

10. A phantom as claimed in Claim 1, wherein said first material is formed in the shape of a lens and is encased in said second material.