Avoiding Poor Skull Areas Improves Heating Efficiency in MR-guided Focused Ultrasound Therapy

Transcranial magnetic resonance-guided focused ultrasound surgery (MRgFUS) is an effective and safe treatment for drug-resistant symptoms of Parkinson’s disease and essential tremor. In some patients, MRgFUS is difficult to perform because the temperature in the target area does not easily increase. In this study, we evaluated the effectiveness of avoiding the sonication of skull regions with poor conditions and redistributing energy to relatively good-quality regions.

We retrospectively analyzed MRgFUS data at our facility. In some patients, when it was considered difficult to achieve the required temperature in the target area and thermal coagulation after starting treatment, the ultrasound transducer elements that passed through areas with poor conditions, considered as such on the basis of the skull density ratio and skull thickness, were turned off. A linear regression model was created for all ultrasound sonications in all patients, with heating efficiency as the objective variable and various treatment and patient factors, including whether cranial areas with poor conditions were excluded from the sonication field, as explanatory variables.

In total, 213 patients underwent MRgFUS treatment, with 1,891 sonications. Eleven patients had 30 sonications excluding skull areas with poor conditions. The median number of excluded elements was 37.5 (maximum 74, minimum 6). Linear regression analysis showed that excluding cranial regions with poor conditions from the sonication field significantly improved the heating efficiency ( P = 0.0125, R ² = 0.769). The regression coefficient was 0.286°C/kJ, which means that for every additional 10,000 J of sonication energy, the temperature increases by 2.86°C. The difficulty of controlling coagulation foci and the number of adverse events were not obviously increased using this method.

When sonication was conducted while avoiding areas with a low skull density ratio and a thick skull, the heating efficiency of MRgFUS significantly improved. Using this approach, even difficult-to-treat cases may be successfully managed.