Skull bone marrow injury caused by MR-guided focused ultrasound for cerebral functional procedures

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OBJECTIVE

One patient for whom an MR-guided focused ultrasound (MRgFUS) pallidotomy was attempted was discovered to have multiple new skull lesions with the appearance of infarcts on the MRI scan 3 months after his attempted treatment. The authors conducted a retrospective review of the first 30 patients treated with MRgFUS to determine the incidence of skull lesions in patients undergoing these procedures and to consider possible causes.

METHODS

A retrospective review of the MRI scans of the first 30 patients, 1 attempted pallidotomy and 29 ventral intermediate nucleus thalamotomies, was conducted. The correlation of the mean skull density ratio (SDR) and the maximum energy applied in the production or attempted production of a brain lesion was examined.

RESULTS

Of 30 patients treated with MRgFUS for movement disorders, 7 were found to have new skull lesions that were not present prior to treatment and not visible on the posttreatment day 1 MRI scan. Discomfort was reported at the time of treatment by some patients with and without skull lesions. All patients with skull lesions were completely asymptomatic. There was no correlation between the mean SDR and the presence or absence of skull lesions, but the maximum energy applied with the Exablate system was significantly greater in patients with skull lesions than in those without.

CONCLUSIONS

It is known that local skull density, thickness, and SDR vary from location to location. Sufficient energy transfer resulting in local heating sufficient to produce a bone lesion may occur in regions of low SDR. A correlation of lesion location and local skull properties should be made in future studies.

ABBREVIATIONS FUS = focused ultrasound; MRgFUS = MR-guided FUS; SDR = skull density ratio.

Article Information

Correspondence Michael L. Schwartz: Sunnybrook Health Sciences Centre, Toronto, ON, Canada. m.schwartz@utoronto.ca.

INCLUDE WHEN CITING Published online May 4, 2018; DOI: 10.3171/2017.11.JNS17968.

Disclosures Dr. Lozano: ownership in Functional Neuromodulation; and consultant for Medtronic, St. Jude, and Boston Scientific. Dr. Hynynen: patent holder with InSightec.

The care of these patients was supported by InSightec Ltd., Tirat Carmel, Israel, and the Focused Ultrasound Foundation, Charlottesville, Virginia.

© AANS, except where prohibited by US copyright law.

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Figures

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    Index case. This patient underwent sonication increasing to a maximum power of 1100 W for a duration of 31 seconds. During the procedure, the scalp and skull were constantly cooled with flowing degassed water at 13°C. Despite this sonication, the target locus in the internal globus pallidus reached only 48°C. No brain or skull lesions are visible on this day 1 posttreatment axial MRI scan.

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    Axial MRI scan obtained at 3 months posttreatment, showing ovoid lesions throughout the calvaria indicated by the arrow, which were new since the pretreatment MRI scan. Their appearance resembles that of bone infarcts.

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    Axial MRI scan obtained 8 months posttreatment, showing no change in the appearance of the skull lesions.

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    The mean maximum energy for patients with skull lesions was 25.8 kJ. The mean maximum energy for patients without lesions was 15.5 kJ.

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    Coronal temperature map through the target point in the left ventral intermediate nucleus. At the cursor over the left parietal bone, the temperature reaches a maximum of 65°C when a baseline temperature of 37°C is used. The baseline temperature, however, is reduced by the skin cooling of 13°C, and thus the peak temperature is likely to be below the threshold of thermal damage in the tissue on the bone surface.

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