Quality assurance and system stability of a clinical MRI-guided focused ultrasound system: four-year experience

To retrospectively evaluate the four-year experience of a quality assurance method for a MRI-guided focused ultrasound system that uses temperature maps acquired during heating in an ultrasound/MRI phantom. This quality assurance method was performed before 148 clinical uterine fibroid thermal ablation treatments. The stability of the peak temperature rise, the targeting accuracy, the shape of the heated zone, and the noise level in the imaging was evaluated. The peak temperature rise was mostly stable for the first three years. An increase in heating was observed when the system was replaced after year three. Detection of this increase was taken into account in the subsequent clinical treatments. A small secondary hotspot was detected by the temperature maps and was seen to be resolved after system calibration. The average standard deviation in unheated regions of the phantom in the temperature maps was 0.5 +/- 0.2 degrees C; it was less than 1 degrees C in all but one procedure. The average initial targeting error was 2.8 +/- 1.8 and 2.8 +/- 2.1 mm in two radial directions and 7.7 +/- 2.9 mm along the ultrasound beam direction. The width of the heating profile was consistent over the four years. This simple method to evaluate the performance appeared to be sensitive to small changes in system performance, which was adequately stable over a four-year time period.