Medical Physics Supplemental Material - Phantom Materials N. Hungr, J. A. Long, V. Beix, J. Troccaz
SUPPLEMENTAL MATERIAL 1 – PHANTOM MATERIALS
Paper: “A realistic deformable prostate phantom for multi-modal imaging and needle-insertion procedures.”
Authors: Nikolai Hungr, Jean-Alexandre Long, Vincent Beix, JocelyneTroccaz.
Note:Numbered refs. refer to references in the paper. Lettered refs.refer to reference list at the end of this document.
The following is a detailed description of the various materials considered during the design of our deformable prostate phantom.
Agarose is a plant-based jellifying agent typically used in microbiology as a bacterial growth medium, in molecular biology for protein separation by electrophoresis and in the food industry. It is sold in the form of a white powder that, once mixed with water, heated above 85˚C and then cooled to below around 35˚C, becomes a lightly opaque gelatinous substance. Its strength can be adjusted based on the dry-weight concentration of agarose used, its Young’s modulus ranging from 7.6 to 195 kPa according to Hall et al.22 De Brabandereet al9 used an agarose phantom to evaluate the accuracy of a brachytherapy seed detection algorithm in CT and MR images. They chose agarose because its density and T1 and T2 MRI relaxation times are comparable to prostatic tissue. Baxet al.,13 on the other hand, used agarose for its tissue-equivalent speed of sound, to study the accuracy of an ultrasound guided biopsy robot. Although agarose has mechanical and imaging characteristics similar to those of soft tissues, its main limitation is its low toughness, making it fragile during handling.27
Gelatin is another type of jellifying agent, derived from animal collagen and used primarily in the food and pharmaceutical industries. It is prepared identically to Agarose gel, but at lower temperatures, as it dissolves at around 40˚C. Its strength depends not only on the dry-weight concentration used in the mixture, but also on the Bloom value of the gelatin used. Hall et al.22 report a similar range of Young’s modulus to that of agarose gels (4.8-159 kPa). They did, however, find that gelatin has a linear stress-strain curve, compared to the non-linear behavior of agarose, suggesting that gelatin is more suitable for elastography phantoms. A gelatin phantom was used by Lefrançoiset al.6 to test the response of their needle insertion drive, while McGahanet al.17 constructed a gelatin-based phantom to explore the effect of external ultrasound probe pressure on prostate localization. Doyleyet al.a describe the testing of a prototype elastographic imaging system, using gelatin phantoms prepared with polyethylene granules as acoustic scattering agents. Like agarose gels, gelatin gels must be stored in either air-tight containers to prevent water evaporation, or in liquid baths at low temperatures.21,27
A different class of tissue-equivalent materials used in phantoms is based on polymers. One such material is commercialized under the name of Zerdine® (Computerized Imaging Reference Systems, Inc.) and was developed specifically for ultrasound calibration phantoms. It is a clear polyacrylamide substance, whose formulation can be adjusted to match a variety of soft-tissue acoustic properties. Its primary advantages over agarose and gelatin are its increased stability over time in room-temperature conditions as well as its improved strength and temperature resistance. As it is water based and its density can be controlled, it can also be used for CT and MR imaging, as demonstrated by Cunha et al.,16 for example, who used a Zerdine prostate phantom to validate their MRI robot’s range of motion in both modalities. The same phantom was used by Wen et al.15 to validate an ultrasound brachytherapy seed detection algorithm. No published information was found on the range of elasticity that can be obtained with this material.
Another polymer-based hydrogel is polyvinyl alcohol cryogel (PVA-C), a solution that combines PVA compound with water. By subjecting the solution to repeated freeze-thaw cycles, a semi-transparent gel of various strengths can be made. The more cycles the material experiences, the harder it becomes. As summarized in Table I of the paper, a number of studies have shown its very similar mechanical and acoustic characteristics to those of human soft tissues. For this reason, it has often been recommended for use with ultrasound imaging.21,23-25 Its extensive use, however, in multi-component phantoms and needle insertion phantoms has been hindered by its complex preparation. The temperature cycles during preparation must be carefully controlled, taking on the order of 20 hours per cycle, and requiring specialized equipment, such as controlled rate freezer. To prevent dehydration over time, it must also be stored in a water bath at low temperatures.
A third polymer gel often used in phantoms is silicone gel. Primarily composed of silicon, oxygen and other organic side-groups, silicone gel comes in many varieties, depending, among others, on the curing characteristics, strength and environmental compatibility required. It is used in a large variety of applications, from industrial, to home, to medical. A typical variety used for phantom construction, is a room temperature vulcanization (RTV) silicone consisting of two compounds: a catalyst and a crosslinker. The compounds can be mixed in variable proportions to produce a clear gel of the required strength. Bubbles must be removed using a vacuum chamber. In their phantom material study, Zell et al.21 conclude that silicone materials are appropriate for long-term, stable phantoms, but their acoustic properties are not ideal for ultrasound applications. Kerdoket al.10 used silicone for soft-tissue modeling experiments, using Teflon beads scattered throughout the phantom as reference markers for CT-imaging. Ottensmeyeret al.8 used silicone for initial validation on a new soft-tissue mechanical property measurement instrument.
The final type of gel that we will present here is soft polyvinyl chloride (PVC), or plastisol. PVC is a combination of liquid polyvinyl chloride resin with a plasticizer, such as ethyl hexyl adipate. Different ratios of resin-to-plasticizer can be mixed and heated to around 150 - 200˚C, which, upon cooling back to room temperature, turn into clear rubber-like plastics with a range of different elasticities. Soft PVC in its uncured form is sold for home fabrication of soft fishing lures. Both Spirouet al.26 and Madsen et al.27 found its speed of sound for acoustic applications to be on the lower side of standard tissue values, hovering around 1400 m/s. Detailed information on its Young’s modulus was not found in the literature, although DiMaioet al.11 mention a range between 10 and 100kPa is attainable. However, with its inexpensive and simple preparation, its resistance to rough handling, as well as its stability over time, it has often been used as a phantom material. Indeed, DiMaioet al.11 used PVC to validate a needle insertion force model that they developed. The same team a few years later,18 again used a more complex PVC phantom to introduce a new experimental method of modeling needle-tissue interactions, using ultrasound. Another team used PVC phantoms for testing the needle insertion accuracy of a brachytherapy robot they designed,7 as well as for studying the effect of stabilizing the prostate with hooked and angled needles for brachytherapy.19
The materials detailed above are generally described in their pure form; however, many additives and mixtures have been reported in the literature. Additives are typically used to change the acoustic scatter properties for ultrasound applications,13,24,a-c to modify the mechanical characteristics,22 or to improve the stability of the material over time.d Other materials have also been presented for more specific uses, such as thermal ablatione-g and others.14,h,i
References:
aM. M. Doyley, J. C. Bamber, F. Fuechsel and N. L. Bush, “A freehand elastographic imaging approach for clinical breast imaging: system development and performance evaluation,” Ultrasound Med. Biol. 27(10), 1347-1357 (2001).
bW. D. D'Souza, E. L. Madsen, O. Unal, K. K. Vigen, G. R. Frank and B. R. Thomadsen, “Tissue mimicking materials for a multi-imaging modality prostate phantom,” Med. Phys. 28(4), 688-700 (2001).
cE. L. Madsen, M. A. Hobson, H. Shi, T. Varghese, and G. R. Frank, “Tissue-mimicking agar/gelatin materials for use in heterogeneous elastography phantoms,” Phys. Med. Biol. 50(23), 5597-55618 (2005).
dB. W, Pogue and M. S. Patterson, “Review of tissue simulating phantoms for optical spectroscopy, imaging and dosimetry,” J. Biomed. Opt. 11(4), 1-16 (2006).
eZ. Bu-Lin, J. Bing, K. Sheng-Li, Y. Huang, W. Rong and L. Jia, “A polyacrylamide gel phantom for radiofrequency ablation,” Int. J. Hyperthermia, 24(7), 568-576 (2008).
fU. Lindner, N. Lawrentschuk, R. A. Weersink, O. Raz, E. Hlasny, M. S. Sussman, S. R. Davidson, M. R. Gertner and J. Trachtenberg, “Construction and evaluation of an anatomically correct multi-image modality compatible phantom for prostate cancer focal ablation,” J. Urol. 184(1), 352-357 (2010).
gM. McDonald, S. Lochhead, R. Chopra and M. J. Bronskill, “Multi-modality tissue-mimicking phantom for thermal therapy,” Phys. Med. Biol. 49(13), 2767-2778 (2004).
hH. Kato, M. Kuroda, K. Yoshimura, A. Yoshida, K. Hanamoto, S. Kawasaki, K. Shibuya and S. Kanazawa, “Composition of MRI phantom equivalent to human tissues,” Med. Phys. 32(10), 3199-3208 (2005).
iT. Kondo, M. Kitatuji and H. Kanda, “New Tissue Mimicking Materials for Ultrasound Phantoms,” IEEE Ultrasonics Symposium, (Rotterdam, The Netherlands, 2005) pp. 1664 – 1667.
Page 1 of 4