Purpose: Elastosonography (ES) is an ultrasound (US) technique which allows to evaluate elastic properties of tissues under external compression  and has been first proposed in the 1990' s for the early diagnosis of breast cancer. ES phantoms are useful to test and optimize the performance of ES devices and for training purposes but also to validate software ES models . Some breast phantoms for ES training purposes are already available on the market but they cannot be used to investigate the quality of ES devices or software models because no information regarding the mechanical properties of their constitutive materials is provided. On the other hand phantoms for ES machine quality certification are quite expensive. The aim of this paper is to identify and to mechanically characterize low cost and simply available materials to build ES breast phantoms. Methods: Polyvinyl alcohol (PVA) hydrogel and four different types of room temperature vulcanizing (RTV) silicones (Sylgard 184 by Dow Coming, DragonSkin 10 Medium, DragonSkin FX-Pro and Ecoflex by Smoothon) were selected as potential candidates for the development of ES breast phantoms, since they are commonly employed for the manufacturing of US phantoms . The former has low acoustic attenuation and sound impedance and speed similar to those of biological tissue, since they mainly consist of water. Silicone phantom are particularly interesting for their deformability and durability, and the possibility of reproducing specific tissue properties and shapes. The echogenicity of the selected materials was qualitatively assessed by means of US examination, while their mechanical properties were evaluated by means of compression tests. Samples 12.5 ± 0.5 mm in thickness and 29.0 ± 0.5 mm in diameter were prepared according to the ASTM D395-03. Silicone samples were obtained mixing the base and curing agent in the standard ratio and performing vacuum degassing cycles to remove trapped air from the mixtures. DragonSkin and Ecoflex components were cooled to slow down the curing speed and reduce mixture viscosity in order to facilitate air removal. A 30% w/w aqueous solution was prepared with PVA (87% degree of hydrolysis) by heating and mixing with a magnetic stirrer. The PVA solution was poured into a mold, frozen to a temperature of -80°C and thawed back to room temperature obtaining the formation of crystallites . The number of freezing and thawing cycles (FTC) (that influence the degree of hydro gels crystallinity) was varied (Series A, B, C respectively: 3, 5 and 8 cycles) to determine if the tensile elastic modulus was affected. The echogenicity of each material was qualitatively assessed through US examination, embedding samples inside a commercial echogenic phantom with hyperechogenic inclusions and vessels. Mechanical compression tests, in displacement control, were performed leading samples to a deformation of up to 25% of their original thickness for four consecutive cycles of loading and unloading, except for Sylgard which was compressed to a deformation of up to 13% due to its greater stiffness. The samples were compressed at three different speeds: 12, 50 and 100 mm/min. Collected data were analyzed and a stress-strain curve was plotted for each sample, finally the elastic modulus was calculated as the slope of the initial linear part of the curve. Results: US examination showed that Sylgard and Dragon Skin FX-Pro have a marked hypoechogenecity and exhibit acoustic shadowing (Fig. l a, b), so they are good candidates to simulate breast malignant lesions which attenuate the US signal producing a black shadow under the nodule. Moreover, Dragon Skin FX-Pro samples showed an echogenic halo which is typical of infiltrating carcinomas.
Keywords: Elastosonographic phanthoms, Elastosonography, material characterization