The relevance of skull density ratio in selecting candidates for transcranial MR-guided focused ultrasound

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  • 1 University Health Network, Toronto;
  • 2 Krembil Research Institute, Toronto;
  • 4 Physical Sciences Platform, Sunnybrook Research Institute, Sunnybrook Health Sciences Centre, University of Toronto;
  • 5 Department of Statistical Sciences, University of Toronto;
  • 6 Joint Department of Medical Imaging, University of Toronto;
  • 7 Edmond J. Safra Program in Parkinson’s Disease, Morton and Gloria Shulman Movement Disorders Clinic, Toronto Western Hospital, UHN, Division of Neurology, University of Toronto;
  • 8 Department of Medical Biophysics, University of Toronto;
  • 9 Institute of Biomaterials and Biomedical Engineering, University of Toronto;
  • 10 Division of Neurosurgery, Sunnybrook Health Sciences Center, University of Toronto, Ontario, Canada; and
  • 3 Insightec, Ltd., Dallas, Texas
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OBJECTIVE

Transcranial MR-guided focused ultrasound (MRgFUS) is a minimally invasive treatment for movement disorders. Considerable interpatient variability in skull transmission efficiency exists with the current clinical devices, which is thought to be dependent on each patient’s specific skull morphology. Lower skull density ratio (SDR) values are thought to impede acoustic energy transmission across the skull, attenuating or preventing the therapeutic benefits of MRgFUS. Patients with SDR values below 0.4 have traditionally been deemed poor candidates for MRgFUS. Although considerable anecdotal evidence has suggested that SDR is a reliable determinant of procedural and clinical success, relationships between SDR and clinical outcomes have yet to be formally investigated. Moreover, as transcranial MRgFUS is becoming an increasingly widespread procedure, knowledge of SDR distribution in the general population may enable improved preoperative counseling and preparedness.

METHODS

A total of 98 patients who underwent MRgFUS thalamotomy at the authors’ institutions between 2012 and 2018 were analyzed (cohort 1). The authors retrospectively assessed the relationships between SDR and various clinical outcomes, including tremor improvement and adverse effects, as well as procedural factors such as sonication parameters. An SDR was also prospectively obtained in 163 random emergency department patients who required a head CT scan for various clinical indications (cohort 2). Patients’ age and sex were used to explore relationships with SDR.

RESULTS

In the MRgFUS treatment group, 17 patients with a thalamotomy lesion had an SDR below 0.4. Patients with lower SDRs required more sonication energy; however, their low SDR did not influence their clinical outcomes. In the emergency department patient group, about one-third of the patients had a low SDR (< 0.4). SDR did not correlate with age or sex.

CONCLUSIONS

Although lower SDR values correlated with higher energy requirements during MRgFUS thalamotomy, within the range of this study population, the SDR did not appreciably impact or provide the ability to predict the resulting clinical outcomes. Sampling of the general population suggests that age and sex have no relationship with SDR. Other variables, such as local variances in bone density, should also be carefully reviewed to build a comprehensive appraisal of a patient’s suitability for MRgFUS treatment.

ABBREVIATIONS ET = essential tremor; MRgFUS = MR-guided focused ultrasound; SDR = skull density ratio; VIM = ventral intermediate nucleus.

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Contributor Notes

Correspondence Andres M. Lozano: Toronto Western Hospital, Toronto, ON, Canada. lozano@uhnresearch.ca.

INCLUDE WHEN CITING Published online May 3, 2019; DOI: 10.3171/2019.2.JNS182571.

Disclosures The study was partially supported by Insightec. Insightec assisted by calculating the SDR, but otherwise, had no role in data acquisition, analysis, or interpretation. The corresponding author confirms that they had full access to all the data in the study and had final responsibility for the decision to submit for publication.

David Tilden: clinical or research support for this study from and employee of Insightec. Dr. Hynynen: inventor/patent holder of intellectual property owned by Brigham and Women’s Hospital (BWH), which BWH has licensed to Insightec; receives royalties from BWH. Dr. Lozano: consultant to Medtronic, St. Jude, Boston Scientific, and Insightec.

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