Nonlinear distortion associated with high‐intensity focused ultrasound creates apparent displacements in acoustic radiation force imaging.

Abstract

An acoustic radiation force imaging (ARFI) system has been developed which employs a 1.1‐MHz high‐intensity focused ultrasound (HIFU) source to create the pushing beam (0.5–7‐MPa peak pressure) and a focused 7.5‐MHz transducer (mounted concentrically in a hole of the HIFU transducer) to create the imaging pulse. The motion of tissue is determined by recording multiple A‐lines from the imaging transducer and using speckle tracking to estimate displacement. The HIFU was amplitude modulated to result in a time‐varying radiation force and imaging pulses transmitted simultaneously in order to monitor tissue motion during the push phase. Measurements of the pressure waveforms in water indicated that nonlinear distortion of the HIFU could advance the arrival time of the imaging pulse if it was on a pressure peak and retard it if it was on a pressure trough, resulting in apparent displacements on the order of 5 μm. The apparent displacement varied with the phase at which the ARFI was transmitted and increased with the HIFU pressure amplitude. Experiments with ARFI in chicken breast indicated that the apparent displacement varied with depth which appeared to be related to differences in the diffraction patterns of the two sources. [Support NSF CenSSIS EEC‐9986821.]