High-intensity focused ultrasound (HIFU) based longitudinal shear wave elastography for tissue ablation planning

Abstract

Accurately determining the margins of prostate cancer (PCa) during planning of endorectal high-intensity focused ultrasound (HIFU) ablation under ultrasound (US) imaging guidance is challenging. Shear wave elastography imaging (SWEI) is a promising method to outline stiff PCa tumors but has depth limitations and lacks built-in arrangements with HIFU systems. The objective here was to develop and validate a HIFU push method of SWEI that would overcome the limitations in the context of PCa ablation planning. A 1.5 MHz HIFU transducer integrated coaxially with an US imaging probe produced a 500-μm push at its focus through acoustic radiation force in homogenous tissue-mimicking phantoms with different stiffness representative of benign and cancerous prostatic tissue. The US probe was used to track the resulting speckle displacement along the HIFU axis over a 50 ms time window. The Young's moduli of the phantoms were calibrated by a clinical SWEI system (SSI Aixplorer) and were 7.5, 30, and 59 kPa. Finite differences numerical modeling of SW generation and propagation in the phantoms revealed both “classic” side-propagating SW and those propagating along the HIFU axis, toward and away from the transducer—“longitudinal” SW (LSW). These LSW formed due to constructive interference of the SW generated throughout the conical HIFU beam, and their propagation speed was higher than SW velocity by a constant factor dependent on the HIFU field shape. Measurement of LSW propagation speed allowed us to map Young's modulus distribution along the HIFU axis, with average values in close agreement with the calibration for the three phantoms.