Volumetric MR-HIFU ablation of uterine fibroids: Role of treatment cell size in the improvement of energy efficiency

Purpose To evaluate the energy efficiency of differently sized volumetric ablations in MR-guided high-intensity focused ultrasound (MR-HIFU) treatment of uterine fibroids. Materials and methods This study was approved by the institutional review board and informed consent was obtained from all participants. Ten symptomatic uterine fibroids (mean diameter 8.9 cm) in 10 women (mean age 42.2) were treated by volumetric MR-HIFU ablation under binary feedback control. The energy efficiency (mm3/J) of each sonication was calculated as the volume of lethal thermal dose (240 equivalent minutes at 43 °C) per unit acoustic energy applied. Operator-controllable parameters and signal intensity ratio of uterine fibroid to skeletal muscle on T2-weighted MR images were tested with univariate and multivariate analyses to discern which parameters significantly correlated with the ablation energy efficiency. Results We analyzed a total of 236 sonications. The energy efficiency of the ablations was 0.42 ± 0.25 mm3/J (range 0.004–1.18) with energy efficiency improving with the treatment cell size (4 mm, 0.06 ± 0.06 mm3/J; 8 mm, 0.29 ± 0.12 mm3/J; 12 mm, 0.58 ± 0.18 mm3/J; 16 mm, 0.91 ± 0.17 mm3/J). Treatment cell size (r = 0.814, p < 0.001), distance of ultrasound propagation (r = −0.151, p = 0.020), sonication frequency (1.2 or 1.45 MHz; p < 0.001), and signal intensity ratio (r = −0.205, p = 0.002) proved to be significant by univariate analysis, while multivariate analysis revealed treatment cell size (B = 0.075, p < 0.001), US propagation distance (B = −6.928, p < 0.001), and signal intensity ratio (B = −0.024, p = 0.001) to be independently significant. Conclusion Energy efficiency in volumetric MR-HIFU ablation of uterine fibroids improves with increased treatment cell size, independent of other significant contributors such as distance of ultrasound propagation or signal intensity of the tumor on T2-weighted MR imaging.