The Potential Role of the Regional Skull Conditions in Predicting the Efficacy of Transcranial Magnetic Resonance-guided Focused Ultrasound in Patient with Low Skull Density Ratio
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Objective
The therapeutic effect of magnetic resonance-guided focused ultrasound is limited to patients with a low skull density ratio (SDR). We explored the skull conditions associated with successful treatment among low-SDR patients, and, to compensate for the small sample size, performed analyses using all cases irrespective of SDR. This is the first report to examine the significance of regional skull conditions.
Methods
We retrospectively analyzed 171 consecutive cases. Descriptive statistics for the entire skull, and averages for 10 regions, were obtained for variables including the SDR, skull thickness, and ultrasound incident angle (IA; smaller = more vertical). The 1,024 ultrasonic transducer elements were divided into 10 regions predefined by ExAblate4000. Symptoms were evaluated by Clinical Rating Scale for Tremor for essential tremor and Unified Parkinson’s Disease Rating Scale tremor score for Parkinson’s tremor. Successful treatment was defined as <half preoperative symptom score at 6 months postoperatively. First, univariate analysis of cases with 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
Of the cases, 26 had SDR < 0.40, and 15 were successful. Among the cases with SDR < 0.40, IA of the parietal region on the sonication side and SDR of the bilateral temporal region tended to be smaller in the success group (not statistically significant). The maximum temperature was more accurately predicted when IA of the parietal region on the sonication side was included in the model (Akaike information criterion, 777 [from 757]). Furthermore, replacing SDR with SDR excluding the bilateral temporal region enhanced predictions (Akaike information criterion, 777 [from 767]).
Conclusions
Even if SDR is low, treatment success may be more attainable if the IA in the parietal region on the sonication side is smaller, or if the SDR excluding the bilateral temporal regions is large.
