Predicting lesion size by accumulated thermal dose in MR-guided focused ultrasound for essential tremor.

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To correlate the accumulated thermal dose (ATD) with lesion size in magnetic resonance (MR)-guided focused ultrasound (MRgFUS) thalamotomy to help guide future clinical treatments. Thirty-six patients with medication-refractory essential tremor were treated using a commercial MRgFUS brain system (ExAblate 4000, InSightec) in a 3T MR scanner (MR750, GE Healthcare). Intraoperative MR-thermometry was performed to measure the induced temperature and thermal dose distributions (thermal coefficient=-0.00909ppm/°C). The ATD was calculated over multiple sonications with appropriate corrections for spatial-shifting artifacts. The ATD profile sizes obtained for dose values of 17, 40, 100, 200, and 240 cumulative equivalent minutes at 43°C (CEM) were correlated with the corresponding lesion sizes measured via axial T1- and T2-weighted MR images acquired 1day post-treatment. Of a total of 232 included sonications, 83 required corrections for off-resonance-induced spatial-shifting artifacts (correction range=[1.1,2.2]mm). The mean lesion sizes measured on T2-weighted MR images (6.2±1.3mm, mean±SD) were 15% larger than those measured on corresponding T1-weighted MR images (5.3±1.2mm, mean±SD). The ATD values that provided the best correlations with the measured lesion sizes on T2- and T1-weighted MR images were 100 and 200 CEM, respectively. The ATD was correlated with lesion size measured 1day following MRgFUS thalamotomy for essential tremor. These data provide useful information for predicting brain lesion size and determining treatment endpoints in future clinical MRgFUS procedures.