Focal beam distortion and treatment planning in abdominal focused ultrasound surgery.

Recent clinical trials show promising results in using MRI and MRI-based thermometry to guide focused ultrasound surgery to treat uterine fibroids. During treatment, large variation in the focal temperature distribution has been observed. It is possible that some of this variation is due to abdominal tissue inhomogeneity, which might be causing focal beam distortion, and might largely decrease the focusing ability in deep-seated tissues. The purpose of this study was to numerically demonstrate this effect and also show the feasibility of restoring the focal beam patterns by employing the phase correction procedure for phased arrays. Abdominal MR data from four uterine fibroid patients were obtained to reconstruct the three-dimensional meshes of interfaces used in simulations, and one patient was selected to perform the analysis of key parameters in focused ultrasound surgery. Results show that, without phase correction, the focused beam can be severely distorted while using a frequency above 1 MHz or delivering ring-shape focal patterns. Different focal positions at the same depth may require a different power to induce the same ultrasonic intensity level (up to 179% among the different focal patterns). After adding a phase correction procedure, the distorted focal beams can be restored, and the peak intensity can be largely recovered (up to 85% among the different focal patterns). This study may offer important implications and information for treatment planning toward optimizing focused ultrasound surgery in uterine fibroid or other abdominal tumor treatments.