Investigation of PVA cryogel Young’s modulus stability with time, controlled by a simple reliable technique François Duboeuf, a Adrian Basarab, Hervé Liebgott, Elisabeth Brusseau, Philippe Delachartre, and Didier Vray The authors are researchers with CREATIS-LRMN, INSA-Lyon, UCB Lyon 1, Université de Lyon, CNRS UMR 5220, INSERM U 630, 69621 Villeurbanne, France Received 21 April 2008; revised 15 December 2008; accepted for publication 15 December 2008; published 27 January 2009 We describe a quasistatic method for mechanical characterization of tissue-mimicking material used in elastography. We demonstrate that it is possible to assess the elasticity modulus with a reasonable reproducibility using simple and easy tools and methods. Possessing a simple relevant technique with evaluated relative error to assess Young’s modulus of these phantoms could deeply improve the quality of the research in the field of elastography. The method was tested and validated with four samples of polyvinyl alcohol PVA cryogel with different elasticity values corresponding to those of stiffer soft biological tissues. Young’s moduli, varying from 70 to 180 kPa depending on the number of freeze-thaw cycles two to five, were measured within strict measurement conditions and found to have a reproducibility varying from 4% to 8%. Relative error, estimated as the ratio between observed and reference values, varied from 16% to 32%. Besides, measurement stability over 4 months was evaluated. The method demonstrated good feasibility and acceptable reproduci- bility for mechanically characterizing and controlled over time phantoms used for validating new potential ultrasound imaging techniques in the field of elastography. Nevertheless, in this study, investigation was performed on gel possessing young’s modulus values ranging from 80 to 215 kPa. Some tissue values of Young’modulus were reported to be lower, ranging from 0.6 to 28 kPa as liver or glandular values. Consequently, further validation of this static method for mechanical characterization of phantom gels should be performed using softer PVA cryogel. © 2009 American Association of Physicists in Medicine. DOI: 10.1118/1.3065031 I. INTRODUCTION Medical imaging is currently undergoing major improve- ments in the quantification of diagnostic parameters. Elasto- graphy, one example, is a medical imaging technique used in ultrasound and magnetic resonance imaging MRI, 1,2 which estimates the elasticity properties of soft biological tissues. To validate these methods, medical imaging phantoms with well-characterized elasticity are needed. 3–5 A number of different materials mimicking biological tis- sues have been proposed in the literature. Well-characterized commercialized phantoms could be useful to calibrate clini- cal systems, but in most cases, those available do not offer adequate geometry and elasticity for research studies. More- over, they are often expensive. Therefore, many research laboratories working in elastography construct their own phantoms using Agar-Agar, gelatin, 6 or PVA cryogel. 7,8 Because the phantom’s elastic properties depend on vari- ous parameters such as the molecular weight of the gel, the concentration of the aqueous solution, the temperature of the preparation, and for PVA cryogel, the number of freeze-thaw cycles, and the duration of each cycle, 5 it is interesting to have a simple and validated Young’s modulus measurement method. Moreover, because of the natural origin of these products, their mechanical properties change with time and these variations depend on the gel composition. Researchers need to control this evolution. Agar-Agar is a plant powder extracted from red sea algae polysaccharide, gelatin is an animal protein and PVA is a commercialized synthetic poly- mer. Consequently, phantoms with reproducible elasticity ap- pear difficult to obtain because a number of parameters come into play. Therefore, each constructed tissue-mimicking phantom should be characterized when it is used. These dif- ferent materials are often characterized in external specia- lized laboratories, a time-consuming and costly process. The goal of this study is to validate a simple and cheap static method for mechanical characterization of phantom gels, possessing acceptable reproducibility and relative error according to the large variation encountered in the biological tissue values. This method could be used in laboratories that are not specialized in mechanics. We investigated the repro- ducibility, the relative error, and the limits of the method, using PVA cryogel, which is a widely used biological tissue- mimicking material in the field of ultrasound and MRI ima- ging. II. METHOD In the approximation of a uniform isotropic medium com- pressed axially, and a free slip of the sample at top and bottom of the compression plate, Young’s modulus E can be calculated as the ratio between the stress  and the strain , as follows: E =   = F/A 0 L/L 0 , 1 where E is the Young’s modulus modulus of elasticity, in pascals, F is the force applied to the object in newtons, A 0 656 656 Med. Phys. 36 „2…, February 2009 0094-2405/2009/36„2…/656/6/$25.00 © 2009 Am. Assoc. Phys. Med.