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 [Goudarzi et al., Med. Phys. 51(12):8670-87 (2024)]. 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/mm²), AF driving signals provided significant improvements in ΔTratio compared 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. .