Use of depth dependent attenuation characteristics of thermal energy deposition to increase spatial uniformity of HIFU ablation in large uterine fibroids

Abstract

Purpose : In high intensity focused ultrasound (HIFU) treatment of large fibroid, depth dependent attenuation of thermal deposition leads to insufficient temperature rise for necrosis in deeper tissue. The purpose is to determine the relationship between temperature rise (T) and sonication parameters focal depth (F), applied energy (E) and spot size for inducing more uniform heat distribution throughout the fibroid. Materials and methods : Measurements were performed on 10 patients with large fibroids and a phantom using magnetic resonance guided focused ultrasound (MRgFUS) system ExAblate 2000 ® . Peak temperatures were measured from various size sonication spots placed at different depths and empirically modeled using linear regression T(F,E) = a*F+ b*E+c , where (a,b) are constant coefficients and c is zero crossing. Results : Phantom study yielded T(F,E) = -0.3458 *F +0.03653 *E +57.2. For the fibroids, a varied from -0.396 to -0.162 and b varied from 0.009 to 0.038 while c ranged between 27.81 and 103.22, depending on the selected spot size. The values for the coefficient of determination were close to 1 in all cases. This indicated that the model represented the measurements reasonably well and could reliably be used for predicting necessary sonication energy to produce temperature rise required at any desired depth. Based on this, a simple algorithm was outlined to estimate the constants (a,b,c) from only three sets of measurements (T,F,E) for a given sonication condition. Conclusion : In clinics, predicting temperature profile prior to treatment enables compensating attenuation via increasing applied sonication energy, planning optimal strategies with a primary goal of ensuring more uniform distribution of thermal ablation in large fibroids, and thereby improving therapeutic efficiency.