The Potential Role of the Regional Skull Conditions in Predicting the Efficacy of Transcranial Magnetic Resonance–Guided Focused Ultrasound in Patients With a Low Skull Density Ratio

BACKGROUND AND OBJECTIVES: The therapeutic effect of magnetic resonance–guided focused ultrasound is generally not available to patients with a low skull density ratio (SDR). In this study, we investigated factors associated with treatment success in low-SDR cases with essential tremor and tremor-dominant Parkinson disease, and predictors of maximum temperature rise, with particular regard to regional skull conditions. METHODS: We retrospectively analyzed 171 consecutive cases. Descriptive statistics for the entire skull and averages across 10 regions were obtained for SDR, skull thickness, and ultrasound incident angle (IA; smaller = more vertical). We divided 1024 ultrasonic transducer elements into 10 regions predefined by ExAblate 4000. Successful treatment was defined as < half the preoperative symptom score at 6 months postoperatively. First, univariate analysis of cases with an SDR < 0.40 was conducted to explore candidates for skull conditions associated with successful treatment. Subsequently, for all cases, regardless of SDR, several multiple regression models were built to predict the maximum temperature rise, and their performance was compared. RESULTS: Twenty-six cases had an SDR < 0.40, and treatment was successful in 15. Of those with an SDR < 0.40, the IA of the parietal region on the sonication side and the SDR of the bilateral temporal region tended to be smaller in the successful treatment group (not statistically significant). The maximum temperature was more accurately predicted when the model included the IA of the parietal region on the sonication side (Akaike Information Criterion, 777 [from 757]). Furthermore, replacing SDR with SDR excluding the bilateral temporal region enhanced predictions (Akaike Information Criterion, 777 [from 767]). CONCLUSION: The parietal IA on the sonication side and bilateral temporal SDR were associated with treatment success in low-SDR patients, which enabled more accurate prediction of maximum temperature rise. This finding will help identify suitable candidates among those with low-SDR.