Analysis and reduction of thermal dose errors in MRgFUS treatment.

ExAblate 2000 MRgFUS system (InSightec) installed in Ospedale Maggiore Niguarda Ca' Granda (Milano, Italy) is currently used to treat uterine fibroids. Through the magnetic resonance thermometry (PRF method), it is possible to monitor the temperature in the target in real-time and compute the treated region calculating the thermal dose. The purpose of this work is to investigate the errors in the temperature measurements and their effect on thermal dose. A low pass filtering of temperature maps is proposed to reduce the errors and therefore to improve the reliability of the treated regions calculated. The PRF method was studied through a calibration experiment on ex vivo pig muscle. The outcome resulted to be a very good linearity (p value 0.03) between phase and temperature in the range of interest, and an α value of -0.0109 ± 0.0002 ppm/°C. Temperature statistical uncertainty was evaluated by analyzing the temperature readout variability in specific gel provided by InSightec for daily quality assurance control. It resulted to be 1.89 ± 0.32 °C. A Monte Carlo simulation of the MRI temperature measurement and thermal dose calculations in our specific conditions of geometry and statistical uncertainty revealed that a low-pass filtering process on each temperature map can strongly reduce systematic errors in thermal dose evaluations (1.11 overestimation factor instead of 2.62 without filter); consequently the systematic errors on the size of the predicted ablated area are reduced as well.