Introduction: MRI-guided focused ultrasound (FUS) is a new technique that allows for the creation of lesions in the brain non-invasively. FUS thalamotomy targeting the ventral intermediate nucleus of the thalamus (Vim) has been shown to be an effective treatment of medication-resistant essential tremor. This project aimed to measure the location, volume and evolution of the FUS thalamotomy lesion and correlate it with clinical outcome.
Methods: Immediate and 24-hour post-treatment MRI studies were analyzed for 7 patients treated at Brigham and Women’s Hospital. Imaging studies were analyzed using open-source software Slicer 3D. Each patient’s studies were registered to each other using ACPC registration. Distances were measured on thin-slice T2-weighted images and volumetric segmentation was measured on T2 and FLAIR studies. Bivariate Pearson Correlation statistical analysis was done on SPSS. Clinical outcome was assessed using the CRST.
Results: Lesion locations in the LR and AP plane were within 1mm of each other on the immediate and 24-hour post-op MR studies. Volume of necrosis, cytotoxic edema and perilesional edema increased by an average of 0.018cc, 0.135cc and 0.852cc, respectively over 24 hours; constituting a percent change of 239%, 393% and 355% respectively. Total signal (lesion+edema) was on average 0.4cc larger on FLAIR imaging compared to T2-weighted imaging, constituting a 27% difference in volume. Improvement in the motor component of the CRST score in the treated upper extremity (contralateral to thalamotomy) at one month was correlated with immediate post-op volumes of necrotic core (r=-0.860, p=0.028) and cytotoxic edema (r=-0.841, p=0.036), but not with their volumes at 24 hours. No such correlation was seen for the tremor component of the CRST score of the treated upper limb or either component of the untreated side.
Conclusions: Post-treatment lesion location was stable on all images but volumes of the necrotic core and surrounding edema increased substantially in the 24hour post-treatment period. Thin-slice T2-weighted imaging underestimates the extent of edema 24-hour post-op relative to FLAIR studies. There was a statistically significant correlation between improvement in the CRST score for the treated upper extremity with the volume of necrotic core and cytotoxic edema immediately post-op.
Patient Care: The results from this study will contribute to the current understanding of focused-ultrasound thalamotomy lesion features and their evolution, and provide information on how to optimize clinical benefit and minimize adverse effects. Lessons learned from this cohort can also be applied to further applications of FUS under study, such as treatment of Parkinsons Disease.
Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe the optimal FUS thalamotomy location, 2) Identify the different anatomic zones in a FUS thalamotomy, 3) Discuss the early MR evolution of FUS thalamotomy