CA2044713A1 - Animal body parts phantom - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An animal body parts phantom comprised of a substrate sheet which is shaped in the form of an animal body part e.g. a thyroid, including a radioactive material substantially uniformly deposited on the substrate, and a tissue simulating material encapsulating and supporting the substrate. A container may be included to provide a mold for the tissue simulating material and to protect the phantom. Anadditional fat simulating overlay may also be provided to extend the capability of the device.

Description

2 ~ 3 Back~round of the Invention Field of the Inventiorl S The present invention relates to animal body parts calibration phantoms, and in particular to a thyroid calibration phantom ~or calibration of radiation detection equipment used to measure the uptake of radioac~ive tracer by the thyroid gland.Descript-on of the Prior Art Conventional thyroid phantoms employ liquid solu~ions or dispersions of radio-iodine as the tracer material. Radio-iodine is an extremely toxic material.
Accordingly, spills or leaks of such material are hazardous to the health of laboratory personnel.
One such thyroid phantom is manufactured and sold by Radiology Support Devices Inc. of Long Beach, California. This device comprises a hollow-shell synthetic resin material in the three-dimensional characteristic butterI1y-shape of the thyroid.
Posterior ports are provided for ~llling the device with a liquid solution/dispersion of radio-iodine or the like. A further drawback of this device is that it cannot becompletely filled with the liquid. As a result an air bubble is formed which affects the accuracy of the d~vice. Moreover, the device is made to fit snugly into a companion neck ph~ultom which has no capacity to accept other sizes of thyroid phantoms or fat overlay. This presents a problem in calibrating for patients with other than average sized thyroids or heavy patients who have more than the average amount of fat tissue covering their thyroid gland.
Sllmmary of the Invention It is thus an object of the invention to provide an animal body parts phantom,and in particular a thyroid phantom, which avoids the handling of radioactive liquids by laboratory personnel.

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According to the invention, an animal body part phantom is provided compAsing, a substrate sheet shaped in the form of an animal body part; a predeterminecl known amount of a radioactive tracer material substantially uniformly deposited on said substrate; and a tissue simulating material encapsulating and supporting said substrate.
s Brief I)escription of the Accompanvin~ Drawin~
Figure 1 is a plan view of the substrate in the form of a thyroid.
Figure 2 is a plan view of the substrate illustrating the spacer pins and their location on the substrate.
Figure 3 is a schematic representation, partly in section, of the container, illustrating the location of the subskate.
Figure 4 is a plan view of the container partly in section, taken along the line 4-4 in figure 3, illustrating the location of the paper substrate and the supporting material.
Figure 5 is a graph illustrating the radiation attenuation of the supporting material.
Figure 6 is a schematic representation, partly in section of a neck phantom, illustrating the location of the thyroid phantom in use.
Figure 7 is a bottom view of a neck phantom illustrating the retainer.
Figure 8 is a bottom view of a neckphantom illustrating the fat simulating overlay.

Det~iled Descr;ption of the Preferred Embod~ments As seen in Figure 1, the outline of the substrate 10 is first traced on a sheet of suitable material using a template 12. Specifilcally, the substrate sheet is cut to the shape of the thyroid, including two lobes 11 ~oined by an isthmus 13. i.e. thc characteristic butterfly shape and size of the average thyroid is involved. Different sized thyroid shapes are also contemplated. Suitable materials include those which will absorb or adsorb the radioactive tracer. Liquid absorbent cellulosic-based materials such as paper and cardboard have been folmd to be most suitable. Suitable adsorbent matenals - : , , ~ , . .;

-2~7~ 3 include synthetic resins such as polyethylene. A preferred paper material is the highly liquid absorbent Whatman 0 No. 541 fast filter paper. Excess handling of the paper should be avoided to prevent oily deposits that could impede absorption of radioactive solutions.
As seen in Figure 2, holes 14 are punched in the paper substrate 10.
Spacer pins 16 are inserted half way through the holes and glued in place, to position the paper as will be apparent hereinafter. Wooden toothpicks cut to the appropriate lengths have been found acceptable for this purpose.
The paper substrate is then placed in a Petri dish and spiked with a predetermined known amount of radioactive tracer material i.e. 125 I or 131 I in standard solutions in water. As will be apparent hereinafter, a known amount of the radioactive material is required to provide a specific activity of tracer material in the phantom. Both of these radio-isotopes are used to permit calibration at different energy levels i.e. 125 I
emits photons around 30 KeV whereas 53~ I emits at about 364.5 KeY. The former is typically used in research applications and the latter for medical purposes. Since the half-lifes of these isotopes are relatively short, simulants may be used. In the case of the former, 129 I is often employed. ~or the latter, a mixture of ~33 Ba and 137 Cs iS used.
The shelf-life of these simulants is considerably longer. The use of simulants thus extends the useful life of the device. The paper substrate absorbs the radioactive solution. As a result, the radio-iodine is substantially uniformly distributed in the paper substrate. Thorough drying is then effected under a heat lamp to completely eliminate any moisture. Otherwise, any remaining water could chemically react with the encapsulating material. Adsorbent substrates must be kept in a horizontal position during the drying step to optimize the distribution of Mdio-iodine.
Turning to figure 3, the outer container 13 for enclosing the thyroid phantom is illustrated. It is seen that the container is generally in the shape of a section of a cylinder, defined by a convex front wall 20, a flat rear wall 22, and a semi-circular bottom 24, and top 26 portions. A convex wall section 28 is provided centrally (but not concentrically with the front wall) and adjxent to the rear wall 22. The rear wall ~2 is typically 68 mm wide and 57 mm in height. The depth at the centre of the top 26 and , 7 ~ 3 bottom portions 24 is 30 mm. The convex front wall and the convex centre wall are cut from 69.85 X 3.2 mm and 25.4 X 3.2 mm tubing material, respectively, and the other portions from 3 mm sheet material and adhesively attached with 3M CA-5 glue. Thematerial used is typically an acrylic resin material. Such materials are sold under the 5 Trademarks Plexiglas and Lucite.
Respecting the inner construction of the container, the central convex wall 28 includes an empty cavity 29 which simulates the location of the trachea and esophagus. A frontal cavity 30 is provided for location and positioning of the paper substrate. The frontal cavity 30 is defined by the convex wall 28 and front convex wall 10 20. It will be apparent that the frontal cavity is wider adjacent to the rear wall 22 This is to approximate the shape "in plan" of an actual thyroid. The paper substrate 10 is thus seen to be vertically positioned, with the isthmus 13 closer to the bottom, in the frontal cavity 30 substantially equidistant from the front wall 20 and the central wall `~ 28. Location in this manner is achie~ted by means of the aforementioned spacer pins 16.
The idea is that by positioning the radioactive substrate 10 inside the container 18 equi-distance from the walls, one would most closely mimic the homogeneously distributed radioactivity in the actual thyroid gland.
Referring to figure 4, it is seen that the paper substrate 10 presents a convex curved configuration which generally ~ollows the contour of the front convex wall 20. In operation it is this view which faces the radiation detecting equipment to effectively simulate the real homogeneous distribution of radioactive material in a thyroid. The supporting tissue simulating material 32 is pour cast to encapsulate and completely fill the ~ront cavity 30 which acts as a mold. In this manner, support and protection (e.g. ~rom mechanical shock) of the paper substrate are effected.
In order to effectively simulate human tissue, the al~erage density (1.07 g/cm3) and radiological properties (mass attenuation coef~lcient) of the tissues are substantially matched. The typical amount of adipose tissue coverin~g the thyroid in the body is also considered i.e. about 1 cm.
One useful tissue simulating material is Scotchcast ~9 Blectrical Resin 226, a rubbery castor-based polyurethane liquid resin available from 3M of St. Paul, .:
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2~47~3 Minnesota. For example, its density (1.06 g/cm3), exotherm, low cure shrinkage, low viscosity, low volatility at room temperature and room temperature curing properties are suited for this application.
Other synthetic resin materials with similar properties could also be used, for example, the prior art RSD phantom uses adiprene which is a trademark for a four-part polyether based urethane pre-polymer available from Uniroyal Chemical.
The Scotchcast resin is a two-part system i.e. Parts A and B. Par~ A
contains a polyisocyanate and diphenyl methane diisocyanate. The mixing proportion are 2 parts of component A to 5 parts of component B. The mix~ure of Parts A and B
is poured into the frontal cavity 30 to fill the cavity and cured at room temperature overnight. The top portion 26 is then glued in place.
It will be appreciated that the container ;s essential to the invention. It is contemplated that once molded in the container, the container could be discarded and the phantom used on its own. However, it is preferabls to use the container to protect the phantom and facilitate mounting in a neck phantom as described below.
Turning to figure 6, a solid neck phantom 40 constructed from a mold of a Rando body phantom is illustrated. The neck phantom is conveni~ently made of the same Scotchcast 226 polyurethane resin. A central cavity 42 is provided for receiving the thyroid phantom. The cavity 42 is accessed through an opening 44 provided in the base 46 of the neck phantom. The cavity 42 is of the same general shape and size of the container 18 and includes a front convex portion 48 of the same contour as the front surface section 20 of the container.
As best seen in Figure 7, a synthetic resin material retainer block 50, preferably made from Scotchcast 226, placed behind the container is used to ensure a snug fit of the container 18 in the cavity with the front wall 2û contacting the front wall 48 of the cavity. The container can accommodate substrates of di~ferent sizes tosimulate different sized thyroids.
Moreover, in some cases involving heavy patients, a thicker than l cm layer of adipose tissue is likely to be present. To best simulate such conditions, an additional overlay of acrylic plastics material may be included. l~or example, using the ~ ,.

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~4~3 same 3.2 mm (wall thickness) acrylic tubing, a frontal overlay may be constructed. As seen in figure 8, a convex frontal overlay 52 i5 located in front of and in contact with the front section 20 of the container. In this case a retainer is not required. It has been determined that a 6.4 mm thick overlay (e.g. composed of two convex pieces of tubing S one 76.2 mm of OD X 3.2 mm W and the other 82.6 mm OD X 3.2 mm W) glued together i.e. 6.4 mm tubing effectively simulates 8 mm of adipose tissue. (This is the ~ase because of the almost identical values of mass attenuation coefficient for adipose and acrylic.) The photons emitted by l25I and l29I have the average en~rgy around 30 KeV whereas the main photon peak for the decaying '3lI occurs at 364.5 KeV. The corresponding mass attenuation coef~lcients for adipose and acrylic are 0.03û6 and 0.0303 m2!kg for 30 KeV photons, and 0.0106 and 0.0103 m2/kg for 400 KeY photons, respectively. In this manner a total of 18 mm of tissue is simulated. For both figure 7 and figure 8 embodiments, a semi-circular synthetic resin material lid, preferably made from Scothcast 226, is used to close the cavity to hold the container in place. It is important to note that any number of convex pieces of acrylic tubing (not just two) that fit inside each other can be used in combination with retainers of different thicknesses to create fat overlays of different thicknesses. The cavity size would have to be enlarged if thick overlays are desired. This enlarged cavity would be universal regardless of the patient. This would allow calibration for people with different amount of fat overlaying the thyroid.
It will be thus appreciated that the thyroid phantom according to the invention is easily adaptable to simulate different sizes of thyroids and different amounts of overlaying fat tissue without having to alter the neck phantom.
F.xperimentnl 25 It has been determined mathematically that for Scotchcast 226, a 7.2mm thickness of front wall of the neck phantom plus the 3.2mm thickness of acrylic container wall is effective. Specifically, for '25I (average peak photon energy ~ 30 KeV) xad 1 172 (1) .
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and for '31I (main peak photon energy 364. KeV ~ 400 KeV) ad 1.1 8 3 ( 2 ) The convex acrylic front wall of the container plus the neck phantom 5 covering thickness of Scothcast 226, will simulate l cm of adipose tissue. For the purposes of this calculation, the thickness of supporting material covering the paper substrate maybe ignored in view of the positioning of the substrate as described above.
It was determined that ?.2mm of the resin plus 3.2mm of the acrylic do just that.
At 30 KeV: calculations show that 3.2mm of acrylic simulates 3.9mm of adipose; 7.2mm of Scotchcast 225 simulates (according to (1):

7 2 = 6 . lmm of adipose Total simulated adipose thickness is thus

3.9mm ' 6.1~ 10.0mm= lcm At 364.5 KeV ~ 400 KeV: calculations show tha$ 3.2mm of acrylic simulates 3.8mm lS of adipose; 7.2mm of Scotchcast 226 simulates (according to (2):

7 2 = 6 lmm of adipose The total simulated adipose thickness is thus:
3.8mm ~ 6.1n~m = 9.9mm ~ lcm It has also been determined mathematically (see figure 5) that the mass radiation attenuation coef~lcient of Scotchcast 226 is 2.34 X 10~2 ma/kg at 30 KeV photon energy, charrcteristic of ~25 I.

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Moreover, from the curve defined by circles (see figure S) we see that the transmission of 30 KeV photons by a lcm thick slab of 3M Scotchcast 226 is 78%, i.e.
0.78. Transmission is defined by I = exp (-~PX) 5 where:
Io - intensity of a bare radioactive source measured by a detector.
I - intensity of the radioactive source attenuated by a lcm thick Scotchcast 226 slab (measured by the same detector) ~ - mass attenuation coef~lcient of Scotchcast 226.
p - density of Scotchcast 226 (1.06 g/cm3) X - thickness of the slab (lcm in this case) Putting all the known quantities into equation # ~3) we get:
0.78 = exp(-,u x 1.06 g/cm3 X lcm) Now the above equation can be selected for ~. This gives us the value for mass attenuation coefficient, ~, of Scotchcast 226.
~u = 0.234 cm2/g = 2.34 x lo-2 m2/kg The adiprene based material from which the RSD phantom is made has also been tested for transmission properties. This data is provided in figure 5 as well.
20 It is denoted by diamonds ~.L.N.L. stands for Lawrence Livermore Na~ional Lab, - the place where the phantom was originally fabricated).
Also shown in figure S are transmission values for adipose (square~s) based on the data from the Report 44 of the International (: ommission on Radiation Units and Measurements (I.C.R.U.) , " ~

2~713 Similarly, the mass attenuation coef~lcient of Scotchcast 226 at phantom energy of 364.5 KeV has been determined to be:
,u = o . 88 x lo-2 mZ/kg (This calculation does not come frorn Fig.5. It was calculated in another - but very 5 similar experiment.) A further expe~iment was conducted to compare the performance of the thyroid phantom according to the invention to two other commercially available thyroid phantoms. The results are tabulated in Table 1 below.
In the experiment, the other phantoms tested are identified as RSD and 10 ANSI. The RSD phantom is the one described above in the Description of the Prior Art. The A~SI (American National Standards Institute) phantom is a rather crude device comprising a large solid cylindrical acrylic body having a cylindrical cavity which accommodates a sample bottle containing the radio-iodine held in an acrylic carrier.

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'l 2~7~3 Procedure:
1. The three phantoms are prepared with the precisely known amounts of radio-iodine (i.e. 125I and l3lI or simulants therefore using commercially available standard solutions in water. For 125 I, a simulant 129 I, from Amersham Laboratories -Product Code lSZ.44. For 131 I, a simulant mixture of 133 Ba and 137 Cs in approximate ratio of 9.3:1 by activity, also from Amersham - Product Code lZy.55 2. Each phantom is then placed at the same precise distance from a radiation detector. (In this case, a Phoswich Detector 15.2 cm diameter, containing NaI(Th) crystal 0.3 cm thick and CsI Crystal 5.1 cm thick, manufactures by Horsaw Co. Crystal and Electronics Products of Solon, Ohio.
3. ~ The output signal (expressed as number of a radiation counts per second cps) is then measured for each phantom at various distances of phantom ~rom detector.
The ef~lciency of the phantoms is then calculated according to the mathematical expression ~=~
activlty The uptake of radio-iodine by an actual thyroid will ~ary with the degree of exposure. It could be lower or higher than the activity deposited onto the paper 20 substrate. What is important is that the activity deposited onto the substrate be known very precisely in order to be able to peIform accurate calibration of radiation detecting equipment. In this case high and low activity thyroid standards were made (approxi-mately 9200 Bq and 1500 Bq ~or 129 I, and 4800 Bq and 1100 Bq for simulated 131 I).
The amount of activlty deposited ls measured by weighing the "baby"
25 bottle (pipette-like device containing the stand:ard soluhon~ be~ore and after deposit. The deposited weight is then multiplied by the knQwn activity per unit weight o~ the standard soluhon to obtrdn the total d~osited ætivity as tabulated in table 1.

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2 ~ 1 3 Distance betweenEfficiency Efficiency Efficiency Phantom & deteetorRSD ~ BRMD -eps AMSI-nCi nCi nCi 5.730 6.100 6.791 5.603 5.757 7.088 5.883 5.860 6.855 ON CONIACT 5.956 5.g60 6.968 5.7~ S.919 6.~70 1 5 - -----_ AVERAGE 5.780+0.062 S.899+0.057 6.914~0.052 0.837 0.764 0.790 0.837 0.768 0.792 18 CM 0.801 0.787 0.786 0.823 0.781 0.784 0.824 0.78~ 0.787 ------~
AVERAGE 0.824+0.007 0.776+0.004 0.788+0.0(~1 0.341 0.331 0.322 0.331 0.332 0.329 30 CM 0.334 0.325 0.319 0.332 0.326 0.330 0.337 0.333 0.322 - - _ AVERAGE 0.335 +0.0020.329~ 0.002 0.324+0.002 nCi is the abbreviation for nano-Curie = 10 -9Ci. lnCi = 37 Bq (Beequerel).
It will be seen ~rom table 1 that the effieiency results for all three phantoms are fairly elose at eaeh distanee from the deteetor, (mostly within 2-3 standard deviations) and essentially the same at 30 em from the deteetor. Accordingly, the 45 phantom aceording to the inventi~n works equally as well as the eonventional phantoms but avoids the various drawbaeks of these deviees as diseussed above.

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It will be appreciated by those skilled in the art of that while the invention has been described in relation to a thyroid phantom, it is also applicable to other animal body parts phantoms. Specifically, this technique of simulating the incorporation of radioactivity into tissue could easily be applied to other organs or body parts. For S example, radionuclides of Pu, Co, Th, etc. tend to accumulate in the bones and the liver. A hollow shell could be fabricated in the shape of the liver. A substrate spiked with a radioactive tracer could then be inserted inside the hollow shell (liver in this case). The substrate could be, for example, attached with glue to the walls of the shell.
The hollow shçll can then be filled wi~h tissue simulating material ~such as Scotchcast 10 226 for example3. Similarly, contaminated bone could be simulated with a bonephantom. To simulate radio-activity deposited into ribs (for example), a hollow rib phantom could be fabricated and a radioactive substrate inserted therein. The hollow rib phantom would then be filled with tissue simulating material.
Another way to simulate contaminated bones would be to wrap radioactive 15 substrate around parts of the real skeleton and attach with glue. Those parts could then be inserted inside a mold. The mold will then be filled with tissue simulating material.

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Claims (20)

WE CLAIM:

1. An animal body parts phantom, comprising:
- a substrate sheet, shaped in the form of an animal body part;
- a predetermined known amount of a radio-active tracer material substantially uniformly deposited on said substrate; and - a tissue simulating material encapsulating and supporting said substrate.

2. A phantom according to claim 1, wherein a radio-active tracer materialis selected from 125I, 131I, 129I, a mixture of 133Ba and 137Cs, and radio nuclides of Pu, Co and Th.

3. A phantom according to claim 2, wherein the tissue stimulating material is a polyurethane material.

4. A phantom according to claim 3, wherein the animal body parts are selected from the thyroid, the liver and bones.

5. A thyroid phantom comprising:
- a substrate sheet, shaped in the form of a thyroid; a predetermined known amount of radio-active tracer material substantially uniformly deposited on said substrate ;and - a tissue simulating material encapsulating and supporting said substrate.

6. A thyroid phantom according to claim 5, including a snug fitting outercontainer enclosing said encapsulated substrate.

7. A thyroid phantom according to claim 6, wherein the outer container includes a cavity, having convex frontal and rear walls, the substrate being vertically positioned in said cavity by spacer pins to locate said substrate substantially equidistant from said frontal and rear walls.

8. A thyroid phantom according to claim 7, wherein said tissue simulatingmaterial is pour cast into said cavity, which acts as a mold for said material, to encapsulate said substrate.

9. A thyroid phantom according to claim 8, wherein the tissue simulating material has a density and mass attenuation coefficient substantially matching those of human tissue.

10. A thyroid phantom, according to claim 9, wherein the tissue simulatingmaterial is a rubbery polyurethane material.

11. A thyroid phantom according to claim 10, wherein the substrate is a cellulosic material.

12. A thyroid phantom according to claim 11, wherein the cellulosic material is a highly liquid absorbent paper material.

13. A thyroid phantom according to claim 12, wherein the radioactive tracer is radioactive iodine.

14. A thyroid phantom according to claim 13, wherein the radioactive iodine selected from 125I and 131I, in a water solution.

15. A thyroid phantom according to claim 14, wherein the 125I and I131 aresimulated by 129I and a mixture of 133Ba and 137Cs, respectively.

16. A thyroid phantom according to claim 15, including a convex overlay contacting the frontal wall of the container to simulate the presence of additional tissue covering an active thyroid.

17. A thyroid phantom according to claim 16, wherein the container and overlay are made of an acrylic resin material.

18. A thyroid phantom according to claim 8, in combination with a neck phantom, said neck phantom having a base and a central cavity of substantially the same shape as said container, said cavity being accessible through an opening in the base, and including a retainer block to ensure a snug fit of the container in the cavity.

19. A thyroid phantom according to claim 8, in combination with a neck phantom, said neck phantom having a base and a central cavity of substantially the same shape as said container, said cavity being accessible through an opening in the base, and including a frontal overlay to simulate additional tissue covering an actual thyroid.

20. A thyroid phantom according to claim 18, wherein the thickness neck phantom covering the container, together with the thickness of the frontal wall simulates the about 1cm tissue thickness covering an actual thyroid.