Multi-imaging modality tissue mimicking materials for imaging phantoms

Tissue-mimicking material suitable for phantoms for use with at least ultrasound and MRI have sections of material in contact with each other which mimic ultrasound and magnetic resonance imaging properties of human tissues, and preferably also computed tomography properties, so that the phantom can be used for the testing of imaging by various types of medical imagers. A suitable tissue-mimicking material for use in phantoms of this type includes an aqueous mixture of large organic water soluble molecules, a copper salt, a chelating agent for binding the copper ions in the salt, and a gel-forming material. Small glass beads may be intermixed therewith to provide a selected ultrasound attenuation coefficient without substantially affecting the MRI properties of the material. Larger glass beads may be used in a section to control primarily the ultrasound backscatter coefficient without significant effect on the ultrasound attenuation coefficient. Tissue-mimicking material that simulates muscle may have smaller glass beads and a higher concentration of gel-forming material than an adjacent section. Such similar materials in contact with one another show relative stability over extended periods of time.

FIELD OF THE INVENTION
 This invention pertains generally to the field of phantoms for use with
 medical imaging such as ultrasound scanning, magnetic resonance imaging
 and computed tomography.
 BACKGROUND OF THE INVENTION
 There has been a tremendous surge in the number of ultrasound guided
 transperineal prostate implants performed in recent years. Effective
 implants require involved treatment planning based on three-dimensional
 multi-modality (magnetic resonance imaging, ultrasound, computed
 tomography) images used in combination with one another. A multi-modality
 prostate imaging phantom could have applications in quality assurance,
 image registration and treatment planning. Three human soft tissues
 relevant to a prostate phantom that should be mimicked for magnetic
 resonance imaging (MRI), ultrasound, and computed tomography (CT) are
 prostate parenchyma, skeletal muscle and adipose (fat) tissue.
 Tissue-mimicking (TM) materials must exhibit the same properties relevant
 to a particular imaging modality as actual human soft tissues.
 Tissue-mimicking materials for use in magnetic resonance imaging phantoms
 should have values of characteristic relaxation times, T1 and T2, which
 correspond to those of the tissue represented at the Larmor frequency of
 concern. Soft tissues exhibit T1 values ranging from about 200-1200 ms and
 T2 values from about 40-200 ms. For given soft tissue, T1 in particular
 can exhibit a significant dependence on frequency as well as on
 temperature. However, for multi-modality imaging phantoms, in general,
 measurements may be performed near the available clinical Larmor frequency
 of an MRI system (typically 64 MHz or 85 MHz) and at room temperature.
 Phantoms must be assumed to be useful at room temperature even though
 their properties must mimic those of soft tissues at the normal body
 temperature of 37.degree. C.
 The ideal tissue-mimicking material for use in ultrasound should have the
 same ranges of speeds of sound, attenuation coefficients, and backscatter
 coefficients as soft tissue. These parameters should be controllable in
 the manufacturing process of the phantom material, and their variation
 within the range of room temperatures should be small. Speeds of sound in
 human soft tissues vary over a fairly small range with an average value of
 about 1540 m/s. The speed of sound in fat is thought to be about 1470 m/s.
 The amplitude attenuation coefficients appear to vary over the range from
 0.4 dB/cm to about 2 dB/cm at a frequency of 1 MHz in these tissues. The
 frequency dependencies of the attenuation coefficient of some soft tissues
 have been studied and, typically, it has been reported that the
 attenuation coefficient is approximately proportional to the ultrasonic
 frequency in the diagnostic frequency range of 1 to 10 MHz. An exception
 is breast fat, in which the attenuation coefficient is proportional to the
 frequency to the 1.7 power.
 F. T. D'Astous and F. S. Foster, "Frequency Dependence of Attenuation and
 Backscatter in Breast Tissue," Ultrasound in Med. & Biol., Vol. 12, pp.
 795-808 (1986).
 For use in computed tomography (CT), the tissue-mimicking materials must
 exhibit the same CT number as that of the tissue being mimicked. The CT
 numbers for most soft tissues lie in the range of about 20-70 at the
 typical effective x-ray energy of a clinical CT scanner except for fat
 where the CT number is about -100.
 In addition to the individual imaging modality parameters listed above,
 tissue-mimicking materials must also exhibit long term stability and ease
 of storage without which they are rendered useless in an imaging phantom.
 An ultrasound phantom containing tissue-mimicking material is disclosed in
 U.S. Pat. No. 4,277,367, to Madsen, et al., entitled Phantom Material and
 Methods, in which both the speed of sound and the ultrasonic attenuation
 properties could be simultaneously controlled in a mimicking material
 based on water based gels, such as those derived from animal hides. In one
 embodiment, ultrasound phantoms embodying the desired features for
 mimicking soft tissue were prepared from a mixture of gelatin, water,
 n-propanol and graphite powder, with a preservative. In another
 embodiment, an oil and gelatin mixture formed the basis of the
 tissue-mimicking material.
 Tissue-mimicking material is typically used to form the body of an
 ultrasound scanner phantom. This is accomplished by enclosing the material
 in a container which is closed by an ultrasound transmitting window cover.
 The tissue-mimicking material is admitted to the container in such a way
 as to exclude air bubbles from forming in the container. Tissue-mimicking
 materials may contain scattering particles, spaced sufficiently close to
 each other that an ultrasound scanner is incapable of resolving individual
 scattering particles. Testing spheres of tissue-mimicking material, or
 other targets, may be located within the phantom container, suspended in
 the tissue-mimicking material body. The objective is for the ultrasound
 scanner to resolve the testing spheres or other targets from the
 background material and scattering particles. This type of ultrasound
 phantom is described in U.S. Pat. No. 4,843,866, to Madsen, et al.,
 entitled Ultrasound Phantom.
 U.S. Pat. No. 5,625,137 to Madsen, et al. discloses a tissue-mimicking
 material for ultrasound phantoms with very low acoustic backscatter
 coefficient that may be in liquid or solid form. A component in both the
 liquid and solid forms is a filtered aqueous mixture of large organic
 water soluble molecules and an emulsion of fatty acid esters, which may be
 based on a combination of milk and water. Hydroxy compounds, such as
 n-propanol, can be used to control the ultrasonic speed of propagation
 through the material and a preservative from bacterial invasion can also
 be included. The use of scattering particles allows a very broad range of
 relative backscatter levels to be achieved.
 Hydrogen magnetic resonance imaging (MRI) (also known as nuclear magnetic
 resonance, or NMR, imaging) is generally a more complicated imaging
 procedure than X-ray or ultrasound since it does not measure just one
 dominant property, such as electron density in the case of X-ray computed
 tomography, but is affected by the hydrogen atom density, flow, and two
 relaxation phenomena. The contrast, or differences in image brightness, in
 an MRI image is primarily due to differences in the relaxation times of
 tissues. It has been found that there are relaxation time differences
 between normal tissue and certain tumors, which makes MRI imaging
 potentially very valuable in early detection of such tumors.
 A satisfactory MRI phantom must satisfy several requirements. First, the
 material of which the phantom is made should mimic the hydrogen density
 and relaxation times of several types of tissues. Second, the relaxation
 times of the material should not change over time, such as over several
 months or years, so that the phantom can be used in tests of imager
 reproducibility. Third, if the phantom includes inclusions of materials
 within the surrounding matrix which have different NMR characteristics
 than the surrounding matrix, these inclusions must be stable over time in
 both shape and in NMR relaxation times, T1 and T2.
 Soft tissues exhibit T2's from about 40 ms to 200 ms. Typical values for
 the ratio T1/T2 lie between 4 and 10 for soft tissues. For a given soft
 tissue parenchyma, T1 in particular can exhibit a significant dependence
 on frequency as well as temperature.
 Materials which have been proposed for use in phantoms to mimic soft
 tissues with respect to one or more NMR properties include aqueous
 solutions of paramagnetic salts and water based gels of various forms.
 Such gels may also contain additives such as a paramagnetic salt for
 control of T1. Aqueous solutions of paramagnetic salts can be used in
 phantoms to produce a desired value of either T1 or T2. The ratio of T1/T2
 in the salt solutions is almost always less than 2, however, rendering
 such solutions inadequate for the close mimicking of soft tissue, with the
 possible exception of body fluids.
 Phantom materials composed of water based agar gels doped with MnCl.sub.2
 to control T1 have been reported. R. Mathur-DeVre, et al., "The Use of
 Agar as a Basic Reference for Calibrating Relaxation Times and Imaging
 Parameters," Magn. Reson. Med., Vol. 2, 1985, p. 176. Agar gels doped with
 CuSO.sub.4 have also been reported. M. D. Mitchell, et al., "Agarose as a
 Tissue-Equivalent Phantom Material for NMR Imaging," Magn. Reson. Imag.,
 Vol. 4, 1986, p. 263.
 A phantom material consisting of mixtures of agar gel and animal hide gel
 in which CuSO.sub.4 was used to lower T1 has also been reported.
 Unfortunately, a long-term instability manifested itself in that a steady,
 very slow rise in T1 was observed over a period of months. This
 instability precludes the use of this material in MRI phantoms. The rise
 in T.sub.1 was perhaps due to the slow formation of metal-organic
 complexes, removing the Cu.sup.++ paramagnetic ions. J. C. Blechinger, et
 al., "NMR Properties for Tissue-Like Gel Mixtures for Use as Reference
 Standards or in Phantoms," Med. Phys., Vol. 12, 1985, p. 516 (Abstract).
 More recently, the problem of gradual increase in T1 in the agar, animal
 hide gel, Cu.sup.++ SO.sub.4.sup.- gel has been eliminated by addition of
 the chelating agent EDTA (ethylenediaminetetraacetic acid). This stable
 material is excellent for use in MRI phantoms. See J. R. Rice, et al.,
 "Anthropomorphic .sup.1 H MRS Head Phantom," Medical Physics, Vol. 25,
 1998, pp. 1145-1156.
 U.S. Pat. No. 5,312,755 to Madsen et al. discloses a tissue-mimicking NMR
 phantom that utilizes a base tissue-mimicking material which is a gel
 solidified from a mixture of animal hide gelatin, agar, water and
 glycerol. The amount of glycerol could be used to control the T1. The
 preferred base material included a mixture of agar, animal hide gelatin,
 distilled water (preferably deionized), glycerol, n-propyl alcohol,
 formaldehyde, and p-methylbenzoic acid. The contrast resolution phantom
 could include inclusions which have NMR properties which differ from the
 base tissue-mimicking material. Differences in contrast between the
 surrounding base material and the spherical inclusions could also be
 obtained by the use of a solid such as powdered nylon added to the base
 material and the inclusions that has little NMR response but displaces
 some of the gelatin solution, decreasing the apparent .sup.1 H density to
 the NMR instrument.
 As noted above, phantoms for use in MRI systems made from water-based
 agarose gels along with a copper salt have been made previously. M. D.
 Mitchell, et al., supra. The T1 and T2 relaxation rates are strongly
 dependent on the concentrations of agarose and copper ions in the
 tissue-mimicking sample with the T1 depending more on the copper and the
 T2 depending more strongly on the concentration of dry weight agarose in
 the sample. Burlew et al. "A New Ultrasound Tissue-Equivalent Material,"
 Radiology, Vol. 134, 1980, pp. 517-520, have described a polysaccharide
 gel (agar) for ultrasound phantoms that can be made to exhibit speeds of
 sound over the range of 1498 m/s to 1600 m/s at 22.degree. C.
 A prostate phantom based on CT slices and made from solid water
 (Gammex/RMI, Madison, Wis.) for imaging, volume rendering, treatment
 planning, and dosimetry applications has also been constructed. B. B.
 Paliwal, et al., "A Solid Water Pelvic and Prostate Phantom for Imaging,
 Volume Rendering, Treatment Planning, and Dosimetry for an RTOG
 Multi-Institutional, 3-D Dose Escalation Study," International Journal of
 Radiation Oncology, Biology, Physics, Vol. 42, 1998, pp. 205-211.
 An earlier investigation had reported on whether tissue-mimicking (TM)
 materials for ultrasound might be appropriate for use in magnetic
 resonance imaging (MRI) phantoms as well. See, E. L. Madsen, et al.,
 "Prospective Tissue-mimicking Material For Use In NMR Imaging Phantoms,"
 Magn. Reson. Imaging, Vol. 1, 1982, pp. 135-141. These materials consisted
 of powdered graphite and preservatives in water-based proteinaceaous gels.
 Though the materials looked promising initially, later measurements
 revealed that, although T1 was mimicked adequately, it was the T2* which
 was being controlled through concentration of graphite, not T2 itself. For
 tissue-mimicking materials, it is the T2 which must be controlled because
 T2 is intrinsic to the material whereas T2* is influenced by the involved
 imager instrumentation.
 SUMMARY OF THE INVENTION
 In accordance with the invention, a tissue-mimicking material is provided
 for imaging phantoms that can be used with two or more types of imaging
 modalities, such as ultrasound scanning, magnetic resonance imaging, and
 computed tomography. The tissue-mimicking material may be adjusted to
 appropriately mimic human tissue in the several modes of imaging for
 particular tissues such as organs, skeletal muscle, and fat. The materials
 mimicking the various tissues may be incorporated in direct contact with
 one another in an imaging phantom and remain stable in their multi-modal
 imaging properties over time, allowing such phantoms to be used for
 long-term calibration and evaluation of the imaging instruments. Phantoms
 in accordance with the invention have particular application in simulating
 prostate tissue which is surrounded by and adjacent to muscle and fat
 tissue.
 Each component material in a tissue-mimicking material influences
 ultrasound, CT and MRI properties to a greater or lesser extent. There is
 at least one combination that yields good representation of the essential
 properties for all three modalities for that tissue-mimicking material
 (e.g., prostate parenchyma). In addition, different tissue-mimicking
 materials should be capable of remaining in direct contact without changes
 in their ultrasound, CT and MRI properties for long periods of
 time--months or years--to allow construction of anthropomorphic phantoms
 without the need for unrealistic image-degrading diffusion barrier between
 tissue-mimicking materials.
 A preferred multi-imaging modality tissue-mimicking material for use in
 phantoms with at least ultrasound and MRI comprises an aqueous mixture of
 large organic water soluble molecules, a copper salt, a chelating agent
 for binding the copper ions in the salt, a gel-forming material, and glass
 or plastic beads intermixed therewith to provide a selected ultrasound
 attenuation coefficient, the glass or plastic beads selected and treated
 to have a low effect on the MRI T1 and T2 properties of the
 tissue-mimicking material. Such a material is particularly suitable for
 mimicking skeletal muscle tissue in both MRI and ultrasound imaging. A
 preferred gel-forming material is agarose, a preferred copper salt is
 CuCl.sub.2, and the large organic water soluble molecules are preferable
 derived from condensed milk. EDTA may be utilized as the chelating agent.
 The glass beads are utilized to adjust the ultrasound attenuation
 coefficient of the material to the desired level but have no substantial
 effect on MRI properties. The glass beads may be treated, such as by
 soaking in nitric acid to clean the surfaces thereof, to reduce the effect
 of any surface contamination on the glass beads on MRI properties.
 An imaging phantom for use with at least ultrasound and MRI in accordance
 with the invention includes a phantom container and a tissue-mimicking
 material within the container, the tissue-mimicking material comprising at
 least two distinct sections in contact with each other, the
 tissue-mimicking material in the at least two sections in contact with
 each other including an aqueous mixture of large organic water soluble
 molecules, a copper salt, a chelating agent for binding the copper ions in
 the salt, and a gel-forming material, and wherein one of the sections
 includes glass beads intermixed therewith to provide a selected ultrasound
 attenuation coefficient to mimic muscle tissue, the glass beads treated to
 have a low effect on the MRI properties of the tissue-mimicking material.
 A section in contact with that section has glass beads or a larger size
 organ tissues such as prostate. The tissue-mimicking material may comprise
 at least two distinct sections in contact with each other, the two
 sections having first, small diameter glass or plastic beads (less than 20
 .mu.m diameter) intermixed therewith to provide a selected ultrasound
 attenuation coefficient, and wherein one of the sections includes larger
 beads (greater than 30 .mu.m mean diameter) intermixed therewith to
 provide a selected backscatter coefficient therein. A preferred
 gel-forming agent is agar, and the dry weight concentration of agar in the
 section having glass beads therein is preferably higher than the dry
 weight concentration in the adjacent section to mimic the MRI properties
 of muscle tissue. A further section mimicking fat may also be in contact
 with one or both of the sections mimicking muscle tissue and organ tissue.
 Fat tissue may be mimicked by various materials including liquid vegetable
 oils such as safflower oil. In a region for mimicking fat, an open-cell
 reticulated mesh material that holds oil, such as the polyurethane
 material used in air filters, can be employed, with the liquid vegetable
 oil filling the interstices within the polyurethane material. Such a
 structure provides realistic ultrasound backscatter, simulating that due
 to the connective tissue matrix in real adipose tissue.
 Further objects, features and advantages of the invention will be apparent
 from the following detailed description when taken in conjunction with the
 accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION
 A multi-imaging modality phantom incorporating the tissue-mimicking
 material of the present invention is shown generally at 10 in FIG. 1 for
 purposes of illustrating the invention. The phantom 10 includes a
 container 12 having a bottom 14 and walls 15. Preferably the walls 15
 include opposed faces 16, and opposed ends 18. Taken together, the bottom,
 faces, and ends 14, 16 and 18 form a hollow, box-like container structure.
 Margins of the walls 15 remote from the bottom 14 define a window 20. The
 window 20 is closed with an ultrasound-transmitting window cover 22. The
 window cover 22 may be made of any cohesive ultrasound transmitting
 material of suitable physical durability. A thin sheet of polyurethane or
 saran is preferred.
 The phantom 10 further includes a body 24 of the tissue-mimicking material
 of the present invention. This material substantially fills the container
 12 up to the level of the window 20, except as discussed below. The
 phantom body 24 includes several distinct sections, shown for illustration
 as three sections 25, 26 and 27, of the tissue-mimicking material of the
 present invention. As discussed further below, the three sections 25, 26
 and 27 may comprise materials which mimic at least the MRI and ultrasound
 properties of three different body tissues; for example, the section 25
 may have properties mimicking prostate parenchyma, the section 26 may
 mimic muscle, and the section 27 may mimic fat tissue.
 The container 12 may be filled with the sections 25, 26 and 27 of
 tissue-mimicking material as desired, for example, in the manner described
 in U.S. Pat. No. 5,625,137. Although the sections 25, 26 and 27 have been
 shown for simplicity of illustration in FIG. 1 as rectangular blocks in
 contact with each other, as discussed further below, they may be and
 generally will be formed of other shapes, including shapes simulating
 human body structures such as a rounded inclusion of the material of the
 sections 25 surrounded by material of the sections 26 and 27.
 The tissue-mimicking material of the present invention contains water and
 is subject to drying by escape of the water to the atmosphere. This can
 result in changes in the acoustic or NMR properties that make the material
 a less effective tissue mimicker. Consequently, the container 12 must be
 fluid tight and preferably also water vapor tight. The window cover 22
 must include means for reducing water transfer therethrough. To this end,
 the window cover 22 may be made of a flexible plastic material that does
 not readily transmit water vapor. An alternative means for reducing water
 transfer through the window cover 22 includes a layer 28 of an oil-based
 gel that completely closes the window 20, adhering to the uppermost
 portions of the faces 16 and ends 18 in water and water vapor tight
 relation. The layer 28 of oil-based gel preferably is also covered with a
 thin and flexible plastic sheet 30 that forms part of the window cover 22
 and that serves to form and protect the surface of the layer 28 of
 oil-based gel.
 In practice, the bottom 14, faces 16, and ends 18 may be molded as a unit
 or formed of flat pieces of plastic or other material and be glued or
 otherwise joined so as to constitute the container 12. If the window cover
 22 is to include the layer of oil-based gel, the plastic sheet 30 may
 first be glued or otherwise attached to the container 12 so as to close
 the window 20 in fluid-tight relation. At least one of the bottom 14,
 faces 16, or ends 18 includes a filling hole, shown at 32 and located in
 an end 18 of the ultrasound phantom 10 shown in FIG. 1. The layer 28 of
 oil-based gel may then be created by inserting through the filling hole 32
 a sufficient quantity of the oil-based gel to make the layer 28, the
 oil-based material being in molten form. With the container 12 oriented so
 that the window 20 is downward most, the molten oil-based gel may then be
 allowed to cool and solidify. The exact thickness of the layer 28 is not
 critical. After the layer 28 of oil-based gel has been formed, the
 remainder of the container 12 may then be filled with the tissue-mimicking
 material by inserting the material in liquid form through the filling hole
 32 and, for the solid form, allowing it subsequently to solidify as
 described above.
 Exemplary prostate and muscle tissue-mimicking materials in accordance with
 the invention that may be used for the sections 25 and 26 contain agarose
 (Type A-4679, Sigma Chemical Co., St. Louis, Mo.), condensed milk (Diehl
 Company, Defiance, Ohio), distilled water deioinized to 18 M-.OMEGA.,
 n-propyl alcohol, CuCl.sub.2 salt, EDTA (ethylenediaminetetraacetic acid,
 a chelating agent), and thimersal (preservative). CuCl.sub.2 salt is used
 to lower T1. The use of a copper salt and chelating agent to lower T1 has
 been shown previously. J. R. Rice, et al., "Anthropomorphic .sup.1 H MRS
 Head Phantom," Medical Physics, Vol. 25, 1998, pp. 1145-1156. An
 appropriate chelating agent is EDTA. EDTA binds to the Cu.sup.++ and
 prevents imobilization of the Cu.sup.++ through the formation of
 metal-organic complexes with the rigid agarose or other gel that may be
 used. Thus, the use of CuCl.sub.2 and EDTA together, forming mobile
 paramagnetic particles, results in a stable T1. Thimerosal prevents any
 bacterial invasion in the materials. Microscopic glass beads are added to
 the material mimicking muscle to augment the ultrasound attenuation. The
 differences in the materials mimicking prostate and muscle are the dry
 weight concentrations of agarose and the inclusion of glass beads in the
 material mimicking muscle. Fat tissue for the section 27 may be mimicked
 with vegetable oils such as pure safflower oil (The Hain Food Group,
 Gardena, Calif.) which may be suffused into an open cell mesh material
 that will hold oil, e.g., such as the materials used for air filters in
 air conditioners. A preferred example of such a material is polyurethane
 mesh, e.g., polyether polyurethane mesh, product code PDQZ-14A, "zether"
 natural mesh, pore size of 14 per linear inch, manufactured by Foamex,
 Eddystone, Pa. The oil is absorbed into this material and held in the
 interstices of the mesh. The values for the parameters specifying the
 composition of the three soft tissue-mimicking materials are given in
 Table 1.
 TABLE 1
 Material Composition of TM Material
 Tissue-Mimicking Prostate 50% (volume percent) condensed milk, 50%
 (Section 25) (volume percent) agarose solution (2% dry
 weight percent) and 7.9 cm.sup.3 n-propyl
 alcohol per 100 cm.sup.3 agarose solution,
 0.103 g EDTA per 100 cm.sup.3 of total volume,
 0.06 g CUCl.sub.2 per 100 cm.sup.3 of total volume
 Tissue-Mimicking Muscle 50% (volume percent) condensed milk, 50%
 (Section 26) (volume percent) agarose solution (6% dry
 weight percent) and 7.9 cm.sup.3 n-propyl
 alcohol per 100 cm.sup.3 agarose solution,
 0.0618 g EDTA per 100 cm.sup.3 of total
 volume, 0.036 g CuCl.sub.2 per 100 cm.sup.3 of total
 volume, 5 g microscopic glass beads per
 100 cm.sup.3 total volume
 Tissue-Mimicking Fat Pure Safflower oil and polyurethane mesh
 (Section 27) (alternatively, pure safflower oil)
 The process of making the prostate and muscle tissue-mimicking materials is
 described as follows. Thimerosal is added to the condensed milk after
 which it is filtered. N-propyl alcohol (n-propanol) is added to deionized
 water (7.9% by volume). A known dry weight percent of dry agarose added is
 2% for the material mimicking prostate and 6% for the material mimicking
 muscle. The resulting mixture is heated in a water bath until the agarose
 solution clarifies. The molten agarose solution is then cooled to
 55.degree. C. while the condensed milk is simultaneously heated to the
 same temperature. The agarose solution is then added to the condensed milk
 to make a 50-50 volume mixture. The mixture is stirred and air bubbles are
 discarded. 0.103 g per 100 cm.sup.3 of EDTA and 0.06 g of CuCl.sub.2 per
 100 cm.sup.3 of the mixture are then added and the mixture is stirred to
 ensure homogeneous distribution of all the constituent materials. In
 addition to all of the materials described above, microscopic glass beads
 are added to the material mimicking muscle. The glass beads are previously
 treated with nitric acid for a period of 24 hours to remove any
 paramagnetic impurities that might affect T1. They are then washed with
 water, dried, and homogenized by stirring. The purpose of homogenizing the
 glass beads is to ensure that they are uniformly distributed when added to
 the agar and milk mixture. Each material is then poured into a first
 cylinder which is placed on a mechanical rotator for a period of 24 hours
 to prevent the buildup of any gradients of components within the
 tissue-mimicking materials. Each first cylinder has an inner diameter of
 7.6 cm with a 6 mm thick curved acrylic wall. 25 .mu.m thick saran windows
 cover each end of the cylinder. The alternative tissue mimicking fat
 material (pure safflower oil) is made by pouring the oil into the
 cylinder. The preferred fat tissue-mimicking material is made by placing a
 7.6 cm diameter, 2.5 cm thick disc of polyurethane mesh in the cylinder
 followed by pouring safflower oil into the mesh and gluing the second
 saran window in place.
 For MRI, the parameters of interest are hydrogen T1 and T2 relaxation
 times. Measurements were performed on small samples in 5 mm diameter NMR
 tubes of tissue-mimicking prostate, muscle, and safflower oil (alternative
 fat tissue-mimicking material) using a 40 MHz Minispec spectrometer
 (Bruker, Canada) along with supporting equipment consisting of an IBM
 computer, a storage oscilloscope, and a constant temperature water bath
 maintained at a temperature slightly below 22.degree. C. The 40 MHz
 spectrometer probe is maintained at 40.degree. C. In order to make
 measurements at 22.degree. C., the sample placed in the water bath
 initially is then inserted in the spectrometer probe. Data is acquired
 within 1.5 minutes to avoid significant temperature rise of the sample. It
 has been shown that the temperature rise within the first minute is less
 than 2.degree. C. The spectrometer was interfaced with the computer which
 uses software from IBM Instruments (Danbury, Conn.) for pulse programming
 and data acquisition. The optimum pulse durations were found by maximizing
 the initial signal for a 90.degree. pulse and minimizing the absolute
 value of the entire free induction decay (FID) for the 180.degree. pulse.
 An inversion recovery (IR) sequence was used to obtain the data for the
 longitudinal relaxation time. A relaxation time (TR) of at least five
 times the expected T1 was used. The T1 experiment was repeated ten times.
 Data reduction was done by curve fitting to an expression of the form:
EQU M(t)=M.sub.0 (1-2 exp(-t/T1)) (1)
 where M(t) is the instantaneous magnetization, M.sub.0 is the initial
 longitudinal magnetization (thermal equilibrium), and t is the time at
 which each data point is acquired in the experiment. The uncertainty in
 the measurement of M(t) is calculated and this uncertainty is propagated
 to calculate the estimated uncertainty in T1.
 The CPMG spin-echo pulse sequence was used to measure the transverse
 relaxation time. The relaxation delay (repetition time) was set to 7 s and
 data was acquired for .tau. (.tau.=one-half the echo time, TE) values of
 25 .mu.s, 125 .mu.s, 250 .mu.s, and 500 .mu.s. 255 echo peaks were
 recorded in each CPMG sequence. The data obtained was fitted to a single
 exponential of the form:
EQU M(t)=M.sub.0 exp(-t/T2) (2)
 where M(t) is the instantaneous magnetization at time t, M.sub.0 is the
 initial magnetization and T2 is the transverse relaxation time.
 For ultrasound phantom purposes, the material should exhibit the same speed
 of sound and ultrasonic attenuation as prostate tissue, skeletal muscle
 and fatty tissue. The backscatter parameter was adjusted by the addition
 of 45-53 .mu.m glass beads, for example, as described in E. L. Madsen, et
 al., "Liquid or Solid Ultrasonically Tissue-mimicking Materials With Very
 Low Matter," Ultrasound in Med. and Bio., Vol. 24, 1998, pp.535-542.
 The ultrasound parameters of the tissue-mimicking materials were measured
 as follows on the cylinders of tissue-mimicking material described above
 (one prostate, one muscle, one fat and one alternative fat).
 Tissue-mimicking material cylindrical samples are placed in a constant
 temperature water bath (maintained at 22.degree. C.) between the
 transmitting transducer and receiving transducer. The parallel faces of
 the samples are maintained perpendicular to the ultrasound beam direction.
 The speed of sound was measured by measuring the difference in the pulse
 arrival time for the cases in which the sample is present and absent
 between the transmitting transducer and the receiving hydrophone. The
 speed of sound in the tissue-mimicking material sample was then calculated
 relative to the speed of sound in distilled water. The ultrasonic
 attenuation coefficient at four discrete frequencies was measured with the
 same experimental setup. This was done by noting the pulse amplitudes when
 the sample is present and absent from the path of the ultrasound beam.
 Corrections for the nonzero thickness of thin plastic layers over the
 parallel sample faces are significant for frequencies above about 2 MHz
 and are included in the data reduction.
 For evaluation of the materials for use as a CT phantom, the x-ray
 attenuation coefficient was measured with three different beam qualities
 at the Accredited Dosimetry Calibration Laboratory (ADCL), University of
 Wisconsin-Madison. The four sample cylinders discussed above were
 employed. The x-ray beams are calibrated and traceable to the National
 Institute of Standards and Technology (NIST). The beams used were M-150
 and M-200 classified according to the filtration used with mean energies
 of 67 keV and 100 keV respectively. The M-150 x-ray beam is representative
 of a typical clinical CT beam. The x-ray system (Advanced X-ray, Atlanta,
 Ga.) uses a 14 kHz constant potential generator and has a tungsten anode
 with 3 mm inherent beryllium filtration.
 The charge was collected with a spherical graphite walled ion chamber (Far
 West Technology, Calif.) and measured using an electrometer (Keithley
 Measurements, Inc., Cleveland, Ohio). The tissue-mimicking material sample
 was placed in the x-ray beam with its parallel face perpendicular to the
 direction of the x-ray beam. The attenuation coefficient was calculated by
 measuring the charge collected with and without the sample in the path of
 the x-ray beam and applying exponential attenuation to the beam due to the
 presence of the tissue sample.
EQU I=I.sub.0 exp(-.mu..sub.eff t) (3)
 where I is the charge collected with the sample in the beam, Io is the
 charge collected with the open beam, .mu..sub.eff is the effective
 attenuation coefficient, and t is the thickness of the sample.
 Clinically, the CT number of a tissue-mimicking sample may be of more
 relevance than the measured effective attenuation coefficient. The CT
 number was measured using a Siemens CT scanner at 133 kVp and mean photon
 energy of 78 KeV. The tissue mimicking materials were scanned and the CT
 number measured from a selected region of interest from the CT images of
 the samples.
 The foregoing materials are suitable for use in an anthropomorphic prostate
 phantom. In a phantom, where different components containing different
 concentrations of materials are in direct contact, it is a paramount
 importance that materials in one component do not cross the interface into
 an adjacent component. Such diffusion across the interfaces of various
 components of the phantom would lead to its degradation.
 To test precisely this phenomenon, three test phantoms as shown in FIGS. 22
 and 23 were constructed. These test phantoms had a container formed of a
 cylindrical wall 35 of acrylic plastic and top and bottom windows 36 and
 37 of 25 micron thick saran. Each test phantom had a container of the type
 shown in FIGS. 22 and 23, but was first half filled with one type of
 tissue-mimicking material 37 and then with a second type of
 tissue-mimicking material 38 after the first tissue-mimicking material was
 allowed to stand and congeal for 24 hours, with the materials 37 and 38 in
 contact with each other at an interface 40. Each test phantom contained
 50% (by volume) of one type of tissue-mimicking material and 50% of a
 different type of tissue-mimicking material. The first phantom contained
 material mimicking prostate tissue and material mimicking fat tissue; the
 second phantom contained material mimicking muscle tissue and material
 mimicking fat tissue; and the third phantom contained material mimicking
 prostate tissue and material mimicking muscle tissue. The composition of
 each of the long term stability test phantoms is listed in Table 2. In
 addition, three phantoms each filled with a single tissue-mimicking
 material were constructed to serve as controls and also to check the
 inherent stability of the tissue-mimicking materials.
 TABLE 2
 Phantom Volume Composition of Phantom
 1 50% TM Prostate, 50% alternative TM Fat
 2 50% TM Muscle, 50% alternative TM Fat
 3 50% TM Prostate, 50% TM Muscle
 To investigate if diffusion does occur across the interface between two
 different tissue-mimicking materials, the characteristic parameters were
 monitored using the long term stability phantoms described above over a
 period of months on each imaging modality. Measurements were made at
 monthly intervals of T1 and T2 relaxation times for MRI, speed of sound
 and ultrasonic attenuation for ultrasound, and CT numbers for computed
 tomography. This was done for each of the two different tissue-mimicking
 materials in contact in each of the three test phantoms, as well as in the
 control phantoms containing each individual tissue-mimicking material.
 T1 and T2* were measured using a 1.5T GE Signa MRI scanner. T2* is a
 relaxation time that is unavoidably influenced by the measuring
 instrumentation, with T2*.ltoreq.T2. To measure T1, six T1-weighted images
 were obtained with repetition times (TR) of 116 ms, 250 ms, 500 ms, 1000
 ms, 2000 ms, and 4000 ms respectively with an echo time (TE) of 15 ms.
 Regions of interest were selected for each tissue-mimicking material in
 all three phantoms, and the mean pixel values along with the standard
 deviations were recorded. The data was curve fitted to an exponential of
 the form of Equation 1. T2* was measured by using a CPMG multi-echo pulse
 sequence with TR=2000 ms and echos were acquired at 20 ms, 40 ms, 60 ms
 and 80 ms. This data was curve fitted to an equation of the form of
 Equation 2.
 The speed of sound and ultrasonic attenuation for the long term stability
 phantoms were measured in the same manner as was previously described. The
 phantom was placed in between the transmitting transducer and the
 receiving hydrophone in such a way that the ultrasound beam passed through
 only one of the two tissue-mimicking materials in the phantom. This was
 then repeated for the second tissue-mimicking material in the phantom. The
 CT number was monitored for each tissue-mimicking material in the three
 long term stability phantoms through sequential CT scans. Measurements of
 the same parameters were also made with the control phantoms on all three
 imaging modalities. These measurements were repeated at regular intervals
 over a five month period on each imaging modality to assess long term
 stability.
 T1 and T2 relaxation times for actual human tissue is shown in Table 3. The
 T1 and T2 values measured using a relaxometer for the tissue-mimicking
 (denoted TM in the tables) materials are shown in Table 4 and Table 5
 respectively (in Table 5, T2 values are indicated with .+-.
 uncertainties), and T1 and T2* times measured using a 1.5 T GE Sigma MRI
 scanner are given in Table 11 and Table 12, respectively. Relaxometer T2
 times were found to vary with the echo time (2.tau.). T1 was calculated
 using Equation 1. The uncertainty in the T1 measurement was determined by
 propagating the standard deviation associated with M.sub.(t) measurements
 obtained from repeating the T1 experiment (using IR pulse sequence) ten
 times. It must be noted that although the samples are maintained in a
 water bath at 22.degree. C. there may be a slight temperature rise (say
 2.degree. C.) in them because the 40 MHz spectrometer is maintained at
 40.degree. C. Data obtained from the T2 experiment was curve fitted in a
 least squares manner to Equation 2 to obtain T2 and the uncertainty
 associated with it.
 Good agreement is found between the measured values for the
 tissue-mimicking materials and the literature values for actual human
 tissue shown in Table 3. It is important to note that T1 depends somewhat
 on the Larmor frequency, typically with a square-root-of-frequency
 dependence. Measurements were done with a 40 MHz spectrometer and most
 clinical MRI units operate at a Larmor frequency of 60 MHz.
 Reproducibility of the relaxation times is an indication of the overall
 precision of the measurement. The T1 measured by relaxometer for
 tissue-mimicking prostate was 937 ms with a standard deviation of 13 ms.
 T1 times for tissue-mimicking muscle and tissue-mimicking fat are shown in
 Table 4. T2 for each tissue-mimicking material was measured for different
 .tau. values. The average T2 for tissue-mimicking prostate was 88.0 ms
 with a standard deviation of 1.2 ms. Similarly the average T2 values for
 tissue-mimicking muscle and tissue-mimicking fat were measured as 36.7 ms
 and 154.4 ms with a standard deviations of 0.7 ms and 3.4 ms respectively.
 TABLE 3
 Human Frequency
 tissue-type Temp. (.degree. C.) In vivo? (MHz) T1 (ms) T2 (ms)
 prostate 40 no 20 808 98
 muscle 37 yes 6 -- 47 .+-. 3
 " 37 yes 15 514 .+-. 138 --
 " 37 no 43.5 650 - 800 --
 fat 37 yes 12 209 .+-. 17 135 .+-. 16
 " 37 yes 15 266 .+-. 45 57 .+-. 3
 TABLE 3
 Human Frequency
 tissue-type Temp. (.degree. C.) In vivo? (MHz) T1 (ms) T2 (ms)
 prostate 40 no 20 808 98
 muscle 37 yes 6 -- 47 .+-. 3
 " 37 yes 15 514 .+-. 138 --
 " 37 no 43.5 650 - 800 --
 fat 37 yes 12 209 .+-. 17 135 .+-. 16
 " 37 yes 15 266 .+-. 45 57 .+-. 3
 TABLE 5
 TM prostate TM muscle TM fat
 .tau. (.mu.s) T2 T2 T2
 25 84.2 .+-. 0.2 37.4 .+-. 0.1 154.8 .+-. 0.4
 125 91.1 .+-. 0.3 39.1 .+-. 0.3 154.6 .+-. 0.7
 250 91.5 .+-. 0.5 35.3 .+-. 0.3 149.6 .+-. 2.9
 500 85.3 .+-. 1.0 35.1 .+-. 0.5 158.6 .+-. 1.6
 Ultrasound attenuation was measured at four different transducer
 frequencies 2.5, 4.5, 6.2, and 8.0 MHz. FIG. 2 shows the ultrasonic
 attenuation dependence on frequency for each of the TM materials.
 A compilation of ultrasound propagation speeds and attenuation values for
 human soft tissues relevant to a prostate phantom is shown in Tables 6 and
 7, respectively. The corresponding measured values at 22.degree. C. for
 the tissue-mimicking materials developed are shown in Table 8. More
 detailed attenuation values for human and animal fat are given in Table 9.
 Data for prostate tissue was not found in the literature so only values
 for muscle and fat are given in Tables 6, 7 and 9. Sound speeds in muscle
 and fat correspond reasonably well with those in the tissue-mimicking
 versions. Attenuation coefficient/frequency values in muscle are a little
 higher than in the TM muscle. Attenuation coefficient/frequency values for
 fat (Table 9--see values in parentheses) are comparable to those in TM fat
 (safflower oil plus polyurethane mesh).
 Regarding ultrasound backscatter coefficients, comparison of relative
 values for prostate muscle and fat in ultrasound patient scans with
 relative values in the TM materials (using TM fat with oil and
 polyurethane) show good agreement; this is important for use in
 anthropomorphic phantoms.
 TABLE 6
 Human Tissue Speed of sound
 Type Temperature (.degree. C.) In vivo? (m/s)
 muscle 37 yes 1580
 fat 35 no 1476
 TABLE 6
 Human Tissue Speed of sound
 Type Temperature (.degree. C.) In vivo? (m/s)
 muscle 37 yes 1580
 fat 35 no 1476
 TABLE 6
 Human Tissue Speed of sound
 Type Temperature (.degree. C.) In vivo? (m/s)
 muscle 37 yes 1580
 fat 35 no 1476
 TABLE 9
 Atten
 Tissue Temperature Frequency Atten. Coeff. Coeff./freq.
 type (.degree. C.) (MHz) (dB/cm) (dB/cm/MHz)
 Human 37 5 2.3 0.46
 Human 18.2 4 4.2 1.05
 Human 18.2 5.6 6.1 1.09
 Bovine 37 5 6.0 1.20
 Bovine 37 6 7.0 1.17
 Bovine 37 7 8.0 1.14
 Porcine 37 4 5.5 1.38
 Porcine 37 6 8.5 1.42
 Porcine 37 8 12.5 1.56
 Porcine 37 4 3 0.75
 Porcine 37 6 4.9 0.82
 Porcine 37 7 7 1.0
 CT numbers obtained with a CT scanner at 133 kVp and mean photon energy of
 78 keV, and x-ray attenuation coefficients measured with the ADCL system
 for two different effective energies, are shown in Table 10 for the four
 tissue-mimicking materials. The x-ray beams are classified according to
 the filtration present in the beam. For comparison with CT numbers in
 corresponding human tissues, see Table 11 (i.e., compare the right-most
 columns of Tables 10 and 11).
 TABLE 10
 CT number at
 78 keV (133
 kVp) on UW
 Attenuation coefficient (cm.sup.-1) radiotherapy
 Material M-150, E = 67 keV M-200, E = 100 keV Ct scanner
 TM 0.204 .+-. 0.008 0.173 .+-. 0.008 47 .+-. 5
 prostate
 TM 0.214 .+-. 0.009 0.179 .+-. 0.008 88 .+-. 5
 muscle
 TM fat -- -- -115 .+-. 3
 (oil and
 poly-
 urethane)
 Alter- 0.179 .+-. 0.008 0.160 .+-. 0.007 -120 .+-. 4
 native
 TM fat
 (pure oil)
 TABLE 10
 CT number at
 78 keV (133
 kVp) on UW
 Attenuation coefficient (cm.sup.-1) radiotherapy
 Material M-150, E = 67 keV M-200, E = 100 keV Ct scanner
 TM 0.204 .+-. 0.008 0.173 .+-. 0.008 47 .+-. 5
 prostate
 TM 0.214 .+-. 0.009 0.179 .+-. 0.008 88 .+-. 5
 muscle
 TM fat -- -- -115 .+-. 3
 (oil and
 poly-
 urethane)
 Alter- 0.179 .+-. 0.008 0.160 .+-. 0.007 -120 .+-. 4
 native
 TM fat
 (pure oil)
 The degree of correspondence of CT numbers between the tissue-mimicking
 materials and in vivo human tissue values can be assessed using Tables 9
 and 10. The level of agreement is reasonably good, comparing CT numbers
 attained under identical conditions (same scanner, kVp and mean photon
 energy). For TM prostate, CT#=47.+-.5, while for human tissue,
 CT#=36.+-.10. For TM muscle, CT#=61.+-.7. For TM fat (oil only),
 CT#=-120.+-.4 and TM fat (oil and polyurethane), CT#=-115.+-.3, while
 human fat, CT#=-97.+-.9. Thus, the contrast between TM materials mimics
 that for the actual in vivo human tissues rather well.
 TABLE 12
 Sample T1(ms)
 TM prostate 1032 .+-. 8
 TM muscle 750 .+-. 2
 TM alternative fat (pure oil + 306 .+-. 15
 polyurethane mesh)
 TM fat (pure safflower oil) 302 .+-. 16
 TABLE 12
 Sample T1(ms)
 TM prostate 1032 .+-. 8
 TM muscle 750 .+-. 2
 TM alternative fat (pure oil + 306 .+-. 15
 polyurethane mesh)
 TM fat (pure safflower oil) 302 .+-. 16
 FIGS. 3-21 show the results of the long term stability measurements on each
 phantom sample for MRI, ultrasound, and CT. The initial concentrations (at
 the time of production) of Cu.sup.++ and EDTA were the same in the
 prostate and muscle mimicking material. There is no change in the
 ultrasound and CT parameters as well as the T2* of the tissue-mimicking
 materials in the long term stability phantoms. The graphs for T1 however
 (see FIGS. 3 and 4) show changes in the T1 relaxation times for the
 phantom containing TM prostate and TM muscle. The T1 instability was
 attributed to the possibility that, for tissue-mimicking muscle and
 tissue-mimicking prostate in direct contact, the equilibrium
 concentrations of Cu.sup.++ and EDTA in the two materials are not equal.
 Three new long term stability phantoms were constructed in which the ratio
 of the concentration of Cu.sup.++ /EDTA in TM muscle to that in TM
 prostate was lowered to 0.6, 0.7 and 0.8. FIG. 5 shows the time
 dependencies of T1 of tissue-mimicking prostate and tissue-mimicking
 muscle in the new long term stability phantoms.
 MRI, ultrasound and CT parameters were monitored for tissue-mimicking
 materials in direct contact with each other in the long term stability
 phantoms with equal concentrations of Cu.sup.++ /EDTA in tissue-mimicking
 prostate and tissue-mimicking muscle. The ultrasound attenuation, speed of
 sound and CT number did not show any variation over a course of five
 months relative to the controls (containing isolated tissue-mimicking
 materials) which were monitored in the same manner. T2* values obtained
 with the MR scanner did not show any change over the same period of time.
 It must be noted that the values for T2* obtained are significantly lower
 than the true T2 values of the tissue-mimicking materials measured with a
 relaxometer. T2* was measured using a multi-echo sequence where the
 successive 180.degree. refocusing pulses are not exact. Hence, the spins
 towards the edge of the slice may or may not see the 180.degree. pulse. As
 a result, the slice thickness effectively decreases with each successive
 180.degree. pulse and there is a loss of signal with each successive echo.
 Since T2* does not change with time, it can be assumed that T2 does not
 change either.
 The scanner determined that T1 changed significantly over the same time
 frame for tissue-mimicking prostate and tissue-mimicking muscle when they
 were placed in direct contact with each other and initially had the same
 concentration of Cu.sup.++ and EDTA (see FIGS. 3 and 4). This may be
 ascribed to the lack of equilibrium in the Cu.sup.++ /EDTA concentrations
 between the two tissue-mimicking materials. T1 for tissue-mimicking
 prostate steadily declined and increased for tissue-mimicking muscle
 relative to the controls. An increase in Cu.sup.++ /EDTA tends to decrease
 the T1. The changes in T1 seen in the tissue-mimicking prostate and
 tissue-mimicking muscle were explained by the diffusion of Cu.sup.++ /EDTA
 from the tissue-mimicking muscle side to the tissue-mimicking prostate
 side causing a lowering of the T1 in tissue-mimicking prostate and
 consequently a increase in T1 for tissue-mimicking muscle. To lower the
 concentration of Cu.sup.++ /EDTA in tissue-mimicking muscle, long term
 stability phantoms were made with 60%, 70%, and 80% of the Cu.sup.++ /EDTA
 concentration as compared with the original tissue-mimicking muscle
 sample. From FIGS. 6-9 it can be seen that the tissue-mimicking muscle
 sample containing 60% Cu.sup.++ /EDTA compared to tissue-mimicking
 prostate is the preferred material for mimicking skeletal muscle in an
 anthropomorphic phantom where tissue-mimicking muscle are in direct
 contact. FIG. 7 shows the time dependence of Tls for muscle mimicking
 material in two environments: (1) isolated from other tissue-mimicking
 material with Cu.sup.++ and EDTA concentrations at 60% of those in
 reference prostate-mimicking material; and (2) in direct contact with
 reference prostate-mimicking material with the muscle-mimicking material
 initially having Cu.sup.++ and EDTA concentrations at 60% of those in
 reference prostate-mimicking material.
 Phantoms in accordance with the present invention can be formed as
 anthropomorphic phantoms which simulate complex body structures in which
 multiple types of tissues are in contact with one another. A particular
 example of an anthropomorphic phantom simulating the prostate and
 surrounding tissue is shown generally at 50 in the view of FIGS. 24-26.
 The phantom 50 has a generally rectangular container 51 formed of rigid
 walls of, e.g., 6.3 mm thick acrylic plastic. One wall 52 of the container
 has a round opening 53 therein to which is secured a closed cylinder 55
 of, e.g., 0.7 mm thick polymethyl pentene. A sphere 57 of tissue-mimicking
 material is embedded within surrounding tissue-mimicking material 58 which
 simulates muscle/fascia and in contact with a slab of tissue-mimicking
 material 60 which simulates fat. The tissue-mimicking material 58
 simulating muscle also surrounds the open cylinder 55. A thin cylinder of
 tissue-mimicking material 61 extends through the muscle simulating
 material 58 and through the center of the prostate simulating sphere 57.
 In this arrangement of structures, the sphere 57 simulates the prostate
 gland in contact with muscle 58 and fat 60, adjacent to the closed
 cylinder 55 simulating the rectum, with the material simulating the
 prostate 57, of the muscle 58, and the fat 60 formed, e.g., as described
 above. The tissue-mimicking material 61 simulating the urethra may be
 formed of the same material comprising the prostate simulating section 57
 with a higher concentration of (e.g., 45-53 .mu.m diameter) glass beads,
 four times the concentration in the material 57.
 The phantom 50 may be used to compare images obtained with various imaging
 equipment, e.g., ultrasound scanners, MRI imagers and CT scanners,
 allowing a standardized comparison of the images obtained with each
 modality. Ultrasound scans may be taken through the walls of the closed
 cylinder 55 to simulate ultrasound images of the prostate from the rectum.
 When the phantom is not in use, an appropriate solution is preferably
 maintained in the cylinder 55 to inhibit desiccation of the gel material
 in contact with the walls of the cylinder.
 The glass beads that are added to the tissue-mimicking muscle material are
 extremely small, with a mean diameter of about 18 .mu.m. These beads raise
 the ultrasound attenuation coefficient of the tissue-mimicking muscle and
 the backscatter coefficient. Larger beads (45-53 .mu.m diameter range) may
 be added in a much smaller concentration to the tissue-mimicking prostate
 material with little effect on the tissue-mimicking prostate attenuation
 coefficient while raising the backscatter coefficient to a range such that
 the contrast between the tissue-mimicking prostate material and
 tissue-mimicking muscle material simulates that in a human prostate region
 on ultrasound images. In accordance with the invention, the addition of
 beads with various different diameter distributions allows adjustment of
 attenuation coefficients and backscatter coefficients to clinically
 representative values.
 It is understood that the invention is not confined to the particular
 embodiments set forth herein, but embraces all such forms thereof as come
 within the scope of the following claims.