Objective: To investigate whether fractional anisotropy (FA) value can predict the outcome of magnetic resonance-guided focused ultrasound (MRgFUS) for essential tremor.

Background: MRgFUS achieves its therapeutic effect by ablating the ventralis intermedius(VIM) nucleus of the thalamus . Over 55℃ is the achieved temperature to obtain effective outcomes. Thus, MR thermometry was used to measure temperature during treatment. However, MR thermometry cannot accurately measure temperature when magnetic susceptibility changes due to microhemorrhage occur during the treatment process. Past studies have shown that FA on the day after MRgFUS is associated with treatment outcome of essential tremor. However, FA values on more than 1 day may have been affected by edematous changes. On the other hand, edema is often absent or scarce immediately after surgery. Therefore, we investigated whether the FA value immediately after surgery can predict the outcome  than MR thermometry.

Method: This study included twenty-one consecutive cases with essential tremor who underwent MRgFUS from September 2020 to April 2021, and in the follow up investigation in the our patient clinic.The clinical rating scale for tremor (CRST) of the treatment side was used to determine clinical efficacy. The improvement rate was calculated using the preoperative score (X) and the 3-month postoperative score (Y) using the following formula (X-Y/X). The FA map was created from the diffusion tensor imaging immediately after finishing MRgFUS. The region of interest (ROI) was set as a circle of 5 mm in diameter centered on the presumed VIM nucleus on the treatment side in the axial image (FAtarget). The ratio of FA values on the treated side to the contralateral side (relativeFA: FArelative) were also calculated.

Results: A linear regression analyses were performed with the CRST score improvement rate as the dependent variable and the FAtarget and FArelative values as explanatory variables, and significant differences were observed (p<0.05 and p<0.01 ). In the ROC analysis of whether the treatment effect was greater than 70%, the AUCs of FAtarget and FArelative (0.82, 0.83) were better than those of maximum attained temperature and maximum mean temperature (0.71, 0.74).

Conclusion: FAtarget and FArelative immediately after finishing MRgFUS are useful in predicting the outcome of MRgFUS.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/outcome-prediction-using-fractional-anisotropy-map-immediately-just-after-magnetic-resonance-guided-focused-ultrasound-surgery-for-essential-tremor/