Novel bilayer aberration-inducing gel phantom for high-intensity focused ultrasound applications

Aberrations induced by soft tissue inhomogeneities often complicate HIFU therapies. In this work, a bilayer phantom made from polyvinyl alcohol hydrogel (PVA) and ballistic gelatin (BG) was built to mimic alternating layers of water-based and lipid tissues characteristic for a body wall and reproducibly distort HIFU fields. The density (1.04 g/ml PVA, 0.86 g/ml BG), sound speed (1520 m/s PVA, 1450 m/s BG), attenuation coefficient (0.016 dB/cm MHz PVA, 0.052 dB/cm MHz BG) of gel materials, and nonlinearity coefficient close to that of water were measured using homogeneous gel layers of 4 cm thickness. A 3-D-printed mold was fabricated to generate the random interface between the gel layers. The interface pattern was designed using a 2-D Fourier spectrum approach replicating different spatial scales of tissue inhomogeneities. Distortion of the field of a 256-element 1.5 MHz HIFU array by the phantom was characterized through hydrophone measurements for both linear and nonlinear beam focusing. Both spatial shift (up to 2.3 mm axially and 0.35 mm transversely) and widening of the focus were observed, as well as dramatic reduction in focal pressures caused by aberrations (up to fourfold decrease in the peak positive pressure for a focal waveform with developed shock). [Work supported by NIH R01EB007643, R01GM122859, and R01EB025187.]