Nonlinear imaging methods for characterization of HIFU-induced lesions

Nonlinear ultrasonic imaging methods (like pulse inversion¹ and quadratic imaging based second-order Volterra filter⁸) are used in visualization of lesion formation in freshly excised tissue. Both of these methods are more sensitive to nonlinear echoes (e.g., due to micro-bubbles) than standard B-mode imaging. While all three methods typically show increased echogenicity at the lesion location, the nonlinear methods exhibit more localized echo enhancement than B-mode imaging. Therefore, nonlinear methods are potentially better suited to lesion mapping for purposes of image guidance. Quadratic images have the added advantage of a significant increase in image dynamic range and noise reduction (a major limitation of pulse inversion imaging). The results shown in this report continue to support the hypothesis that micro-bubbles play an important role of lesion formation. In this paper, we present imaging results before and after volumetric lesion formation in ex vivo tissue. The results illustrate the advantage of nonlinear imaging methods compared to conventional B-scan imaging in terms of accurate mapping of lesion size and location.