High-intensity focused ultrasound (HIFU) ablation by frequency chirp excitation: reduction of the grating lobe in axial focus shifting

Abstract

High-intensity focused ultrasound (HIFU) ablation has gained popularity in tumor treatment. However, the pre-focal heating due to the presence of the grating lobe may generate unintended heating in clinical applications and limit the steering abilities of the phased array design. In order to solve this problem, frequency chirp excitation was proposed and evaluated both numerically and experimentally. First, propagation of an HIFU burst through multi-layer media was simulated in a nonlinear acoustic model using the angular spectrum approach. Influences of axial focus shifting on the acoustic intensity at the both grating and main lobes at different excitation frequencies were investigated. It was found that different excitation frequencies result in variations in the position and magnitude of the grating lobe in the pre-focal region, but have little influence on the main lobe. Thus, the frequency chirp excitation can spread the acoustic energy in the pre-focal region, and subsequently reduce the temperature elevation. The performances of downward and upward frequency chirps in different sweeping times were compared to those of conventional excitation at the constant frequency. Then, HIFU plane ablation was evaluated. Even without the cooling time between treatment spots when using the frequency chirps, the pre-focal heating was still less than that at the constant frequency. Finally, the thermocouple measurement in the gel phantom showed an ~40% reduction in pre-focal temperature elevations produced by a single-element concave HIFU transducer using the frequency chirps. Altogether, the frequency chirp excitation could effectively reduce the pre-focal grating lobe at a large axial focus shifting distance, which may enhance both the efficacy and safety of clinical HIFU ablation.