A pre-treatment planning strategy for high-intensity focused ultrasound (HIFU) treatments

Most operational protocols for HIFU procedures do not incorporate a pre-treatment planning stage comparable to the rigorous pre-treatment planning that is mandated for other modalities of radiation therapy. Ongoing studies investigating pre-treatment strategies that would improve the efficiency and effectiveness of HIFU treatments are complex in nature. Most are based on the incorporation of predictive phase-aberration corrections of propagation-path-specific phase changes. We have shown that the substantially simpler method of optimizing HIFU source placement in relation to layered tissue structures can have a significant effect on focal integrity, and that MRI scans and a wave-vector time-domain linear propagation model can potentially be used to plan for optimized source orientation. Five ex vivo bovine tissue specimens with heterogeneous tissue structures were each mounted in rigid frames with acoustic windows for HIFU transmission. A spherically-focused HIFU source (F₀ = 1.502 MHz, D = 100 mm, R_c = 100 mm) was positioned to transmit though each specimen at pre-selected orientations, and the transmitted ultrasound pressure fields were scanned for a series of orientations, followed by a series of MRI scans. Ultrasound transmission simulations were performed and compared with experimental results. Analyses performed on the acoustic field scans to quantify the level of focal distortion [distortion index (DI) = 1- (the ratio between the acoustic energy within a focal zone and the total acoustic energy within the measured area] demonstrated that over the 5 specimens, at least an average of 7.3% (range 5.6% to 12.3%, SD = 2.8%) improvement in DI could be expected by source placement optimization. The accurate simulation of ultrasound propagation through heterogeneous tissue layers using MRI data was also achieved.