Alexandre Boutet, Dave Gwun, Robert Gramer, Manish Ranjan, Gavin J. B. Elias, David Tilden, Yuexi Huang, Stanley Xiangyu Li, Benjamin Davidson, Hua Lu, Pascal Tyrrell, Ryan M. Jones, Alfonso Fasano, Kullervo Hynynen, Walter Kucharczyk, Michael L. Schwartz and Andres M. Lozano
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.
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.
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.
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.
Alexandre Boutet, Gavin J. B. Elias, Robert Gramer, Clemens Neudorfer, Jürgen Germann, Asma Naheed, Nicole Bennett, Bryan Li, Dave Gwun, Clement T. Chow, Ricardo Maciel, Alejandro Valencia, Alfonso Fasano, Renato P. Munhoz, Warren Foltz, David Mikulis, Ileana Hancu, Suneil K. Kalia, Mojgan Hodaie, Walter Kucharczyk and Andres M. Lozano
Many centers are hesitant to perform clinically indicated MRI in patients who have undergone deep brain stimulation (DBS). Highly restrictive guidelines prohibit the use of most routine clinical MRI protocols in these patients. The authors’ goals were to assess the safety of spine MRI in patients with implanted DBS devices, first through phantom model testing and subsequently through validation in a DBS patient cohort.
A phantom was used to assess DBS device heating during 1.5-T spine MRI. To establish a safe spine protocol, routinely used clinical sequences deemed unsafe (a rise in temperature > 2°C) were modified to decrease the rise in temperature. This safe phantom-based protocol was then used to prospectively run 67 spine MRI sequences in 9 DBS participants requiring clinical imaging. The primary outcome was acute adverse effects; secondary outcomes included long-term adverse clinical effects, acute findings on brain MRI, and device impedance stability.
The increases in temperature were highest when scanning the cervical spine and lowest when scanning the lumbar spine. A temperature rise < 2°C was achieved when 3D sequences were modified to 2D and when the number of slices was decreased by the minimum amount compared to routine spine MRI protocols (but there were still more slices than allowed by vendor guidelines). Following spine MRI, no acute or long-term adverse effects or acute findings on brain MR images were detected. Device impedances remained stable.
Patients with DBS devices may safely undergo spine MRI with a fewer number of slices compared to those used in routine clinical protocols. Safety data acquisition may allow protocols outside vendor guidelines with a maximized number of slices, reducing the need for radiologist supervision.
Clinical trial registration no.: NCT03753945 (ClinicalTrials.gov).