Transducer module apodization to reduce bone heating during focused ultrasound uterine fibroid ablation with phased arrays: A numerical study.

Background: During magnetic resonance‐guided focused ultrasound (MRgFUS) surgery for uterine fibroids, ablation of fibrous tissues in proximity to the hips and spine is challenging due to heating within the bone that can cause patients to experience pain and potentially damage nerves. This far‐field bone heating limits the volume of fibroid tissue that is treatable via MRgFUS. Purpose: To investigate transducer module apodization for improving the ratio of focal‐to‐bone heating (ΔTratio$\Delta T_{\mathrm{ratio}}$) when targeting fibroid tissue close to the hips and spine, to enable MRgFUS treatments closer to the bone. Methods: Acoustic and thermal simulations were performed using 3D magnetic resonance imaging (MRI)‐derived anatomies of ten patients who underwent MRgFUS ablation for uterine fibroids using a low‐frequency (0.5MHz$0.5 \ \text{MHz}$) 6144‐element flat fully‐populated modular phased array system (Arrayus Technologies Inc., Burlington, Canada) at our institution as part of a larger clinical trial (NCT03323905). Transducer modules (64elements$64 \ \text{elements}$ per module) whose beams intersected with no‐pass zones delineated within the field were identified, their output power levels were reduced by varying blocking percentage levels, and the resulting temperature field distributions were evaluated across multiple sonications near the hip and spine bones in each patient. Acoustic and thermal simulations took approximately 20min$20 \ \text{min}$ (7min$7 \ \text{min}$) and 1min$1 \ \text{min}$ (30s$30 \ \text{s}$) to run for a single near‐spine (near‐hip) target, respectively. Results: For all simulated sonications, transducer module blocking improved ΔTratio$\Delta T_{\mathrm{ratio}}$ compared to the no blocking case. In just over half of sonications, full module blocking maximized ΔTratio$\Delta T_{\mathrm{ratio}}$ (increase of 82% ±$\pm$ 38% in 50% of hip targets and 49% ±$\pm$ 30% in 62% of spine targets vs. no blocking; mean ± SD), at the cost of more diffuse focusing (focal heating volumes increased by 13% ± 13% for hip targets and 39% ± 27% for spine targets) and thus requiring elevated total (hip: 6% ± 17%, spine: 37% ± 17%) and peak module‐wise (hip: 65% ± 36%, spine: 101% ± 56%) acoustic power levels to achieve equivalent focal heating as the no blocking control case. In the remaining sonications, partial module blocking provided further improvements in both ΔTratio$\Delta T_{\mathrm{ratio}}$ (increased by 29% ± 25% in the hip and 15% ± 12% in the spine) and focal heating volume (decrease of 20% ± 10% in the hip and 34% ± 17% in the spine) relative to the full blocking case. The optimal blocking percentage value was dependent on the specific patient geometry and target location of interest. Although not all individual target locations saw the benefit, element‐wise phase aberration corrections improved the average ΔTratio$\Delta T_{\mathrm{ratio}}$ compared to the no correction case (increase of 52% ± 47% in the hip, 35% ± 24% in the spine) and impacted the optimal blocking percentage value. Transducer module blocking enabled ablative treatments to be carried out closer to both hip and spine without overheating or damaging the bone (no blocking: 42±1mm$42\pm 1 \ \text{mm}$/17±2mm$17 \pm 2 \ \text{mm}$, full blocking: 38±1mm$38\pm 1 \ \text{mm}$/8±1mm$8\pm 1 \ \text{mm}$, optimal partial blocking: 36±1mm$36\pm 1 \ \text{mm}$/7±1mm$7\pm 1 \ \text{mm}$ for hip/spine). Conclusion: The proposed transducer apodization scheme shows promise for improving MRgFUS treatments of uterine fibroids, and may ultimately increase the effective treatment envelope of MRgFUS surgery in the body by enabling tissue ablation closer to bony structures. [ABSTRACT FROM AUTHOR]