Elastic modulus contrast enhancement in shear wave imaging using mechanical nonlinearity: In vitro tissue mimicking phantom study

Shear wave elasticity imaging (SWEI) has been widely used to assess the elasticity of tissues. However, the shear modulus estimated in SWEI is often less sensitive in some cases, especially to a subtle change of the stiffness that produces only small mechanical contrast to the background tissues. This small mechanical contrast can be enhanced if the tissues are compressed, exhibiting mechanical nonlinearity. In this study, we propose a new approach of nonlinear SWEI and evaluate its feasibility through experiments using a tissue mimicking phantom. SWEI was performed while a tissue mimicking phantom was continually deformed over a relatively large dynamic range of strains. A 1.5% agar mixed with 5% gelatin inclusion of a long cylinder (D: 8 mm, hard) embedded in 0.5% agar mixed with 5% gelatin phantom block (soft) was fabricated. The average shear modulus of the inclusion exhibited noticeable nonlinearity after >10% overall applied strain and sharply increased to 40 kPa at 30% overall applied strain. On the other hand, the average shear modulus of the surrounding phantom block increased almost linearly from 4 to 16 kPa over the same applied strain range. The elastic modulus contrast of the inclusion to the surrounding phantom block was increased from 0.40 at 0% overall applied strain to 1.52 at 30% applied strain, which displays detecting the inclusion better.