Variation in Skull Density Ratio When Planning Bilateral Magnetic Resonance-guided Focused Ultrasound

Introduction: Staged bilateral MR-guided focused ultrasound (MRgFUS) thalamotomy for tremor is increasing. Generally, it is presumed that the skull density ratio (SDR) is conserved bilaterally and after MRgFUS treatment. However, occasionally second side treatments are more technically challenging than expected. We sought to understand variation in SDR as a possible explanation.

Methods: We analyzed a retrospective cohort of 23 patients who underwent staged bilateral MRgFUS for tremor from 2020 to 2024. Planning CTs were obtained before each stage (i.e. CT1 = pre-first side; and CT2 = post-first side, pre-second side). All CTs were postprocessed using recommended bone kernals and SDRs were calculated using Insightec Exablate software. SDRs were compared both from left-to-right and between CT1 and CT2 for each patient.

Results: The mean SDR across all scans and sides was 0.54 (SD = 0.10, range 0.38 to 0.79). The mean left-to-right SDR difference was 0.005 (SD 0.006, range 0 to 0.02). Averaging bilaterally, the mean SDR difference from CT1 to CT2 was 0.04 (SD 0.04, range 0 to 0.12). Notably, 6 of 23 patients (26%) had a large (≥ 0.05) difference in SDRs between CT1 and CT2. In 6 of 6 of these cases (100%), different bone kernals were used for CT1 and CT2, compared with only 6 of 17 patients (35%) with different bone kernals where there were not large changes in SDR (p = 0.014). In 5 of 6 cases with large changes, the SDR decreased, with the bone kernal changed from either GE BONEPLUS to Philips C (4 cases) or Siemens Hr60f to Siemens Hr60s (1 case). In the 6th case, the SDR increased, with the bone kernal changed from Siemens Hr60s to Siemens Hr60f.

Conclusion : The bone kernal used to postprocess MRgFUS planning CTs may substantially affect the calculated SDR, which may impact who is considered eligible for MRgFUS treatment.