Anti-focus driving signals to mitigate bone heating during focused ultrasound ablation of uterine fibroids with phased arrays.

Objective. During magnetic resonance-guided focused ultrasound (MRgFUS) treatment of uterine fibroids, ablating tissue near the spine is challenging as unwanted far-field bone heating can cause pain and potentially injure nearby nerves. This study investigates an anti-focus (AF) scheme for minimizing bone heating when targeting fibroid tissue in proximity to the spine.Approach.Acoustic and thermal simulations were conducted using 3D MRI-derived anatomical models from ten patients who underwent MRgFUS for uterine fibroids using a low-frequency (0.5 MHz) 6144-element flat fully-populated modular phased array during a clinical trial (NCT03323905). AF points were distributed over patient spine surfaces with varying densities, transducer array element-wise phase and amplitude controls were obtained via Tikhonov regularization, and the resulting temperature fields were compared to those obtained via both conventional focusing (CF) and a transducer module partial blocking (PB) approach developed to mitigate far-field bone heating (Goudarziet al(2024Med. Phys.518670-87)). Optimal regularization parameter values were chosen to maximize the focal-to-bone heating ratio (ΔTratio) while maintaining focal integrity.Results. For high point densities (0.25 AF mm-2), AF driving signals provided significant improvements in ΔTratiocompared to CF (fold improvement: 13 ± 16,p= 0.0009) and the optimal PB approach (fold improvement: 11 ± 11,p= 0.0009), at the cost of more diffuse focusing (focal heating volumes increased 77% ± 23% vs CF; 50% ± 33% vs PB;p< 0.0001) and thus requiring higher acoustic power levels to maintain focal heating (increased 73% ± 19% vs CF; 15 ± 20% vs PB;p< 0.0001). Soft-tissue aberration corrections improved focal targeting accuracy and reduced acoustic power demands, complementing the AF strategy.Significance. AF driving signals show promise for enabling MRgFUS tissue ablation close to the spine without overheating the bone or nearby nerve bundles.