MR-Guided Focused Ultrasound Thalamotomy in the Setting of Aneurysm Clip

Case

The patient is a 75-year-old, right-handed woman with a history of hypertension, seizure disorder, and aneurysmal subarachnoid hemorrhage treated by MCA clip (Yasargil FE784K, conditional B₀ ≤ 3T) ligation. The patient presented for a medically refractory right-hand tremor that was diagnosed 10 years prior. The patient had a preprocedural diagnosis of poststroke parkinsonism. The tremor was characterized by a prominent resting and postural tremor (right upper extremity: Clinical Rating Scale for Tremor [CRST]-A 7 [rest 3, postural 3, kinetic 1], CRST-B 11, CRST-C 16). The symptoms, including imbalance, affected her activities of daily living. After discussing the results from the main randomized controlled trial of MRgFUS for essential tremor, including the expected 50% reduction in tremor⁶ as well as the risks, benefits, and alternatives, the patient agreed to proceed with MRgFUS of the left VIM.

Before referral to our institution, the patient had a head CT scan done, according to the specifications of the MRgFUS vendor (Insightec). The day before the procedure, diagnostic MR imaging was performed for the structural evaluation and assessment of the degree of susceptibility artifact from the aneurysm clip in the right Sylvian fissure (CT shown in Online Supplemental Data). All MR imaging scans were performed at 3T (Skyra, Siemens). The MR imaging revealed extensive encephalomalacia and gliosis in the right MCA territory from aneurysmal subarachnoid hemorrhage and associated ischemic injury.

On the day of the treatment, the standard MRT protocols were tested with the patient in a focused ultrasonography transducer (Exablate Neuro, Insightec) without the coupling water bath. Before shaving her head or placing the stereotactic frame, the patient’s head was stabilized with pads, and imaging was performed with the embedded 2-channel head coils (Insightec). To investigate the homogeneity of the magnetic field and its impact on the MRT in the region of the left VIM target, 30-second duration MRT scans were acquired (8 dynamics). Scans were acquired in 3 orthogonal orientations, and for each orientation, both phase-encoding directions were acquired. Scans were performed with vendor-supplied single-echo (TMAP) and multi-echo (MEMP) protocols, for a total of 12 scans (Figure 1). TMAP has a frequency encoding bandwidth of 45 Hz/pixel, whereas MEMP has a bandwidth of 250 Hz/pixel, and hence, less susceptibility to off-resonance effects. For the multi-echo protocol, the echoes were optimally combined by weighting the signal by (TE_j*M_j), where TE is the echo time, M is the magnitude image, and j = 1:N for N echoes.⁷ To investigate the precision of the measurement as a function of the spatial location, the standard deviation through time was calculated over each 30-second scan, as shown in Figure 1.

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FIG 1.

CT scan and precision of MRT scans. MRT precision is measured as the standard deviation through time. For all 3 orientations, both phase-encoding directions for both the single-echo and multi-echo MRT sequences are shown. The scans were performed in the Exablate transducer (without the coupling water bath) with a 2-channel radiofrequency receiver coil. The patient’s head was stabilized with pads, but no stereotactic frame was used. The scans were performed before the patient’s head was shaved.

Following the verification of adequate MRT precision in the target area, the patient’s head was shaved, secured in a stereotactic head frame, and placed in a cold bath of degassed water. The patient was placed and secured on the MR imaging scanner table within the Exablate Neuro System (Insightec). After turning off the transducer elements that were interacting with the aneurysm clip, calcifications, etc, 861 of 1024 elements were active (Online Supplemental Data), with a skull attenuation ratio of 0.60. Next, test doses of ultrasonography were delivered while monitoring side effects and tremor reduction (Online Supplemental Data). No side effects were observed during the alignment sonications. During the verification sonications, substantial tremor reduction was not observed in the initially targeted location. Therefore, the target was moved 1 mm posterior and, later, 1.5 mm lateral, given that the patient did not have any sensory changes. The target was moved a total of 4 times, until a tremor reduction was seen with sonications at a temperature of ∼49–52°C. A total of 11 sonications with power ranging from 200–952 W, lasting between 11.5–31.4 seconds, and delivering between 2002–22202 J, were performed. The last 3 sonications were stopped prematurely by the patient because of discomfort, but it was not believed to be related to the aneurysm clip. MRT was successfully acquired during all sonications (Figure 2 and Online Supplemental Data), with a maximum temperature rise of 53.0°C (Online Supplemental Data).

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FIG 2.

MRT of the hottest time point for one heating in each orientation (sonications 2, 7, and 11, respectively, are shown). The inlay in each panel zooms in on a 33 × 33-mm region around the focal spot.

Posttreatment T2-weighted images (axial sampling perfection with application-optimized contrasts by using different flip angle evolution [SPACE sequence; Siemens]) that were acquired immediately posttreatment in 20-channel head coil (Siemens) showed a hyperintense signal at the treatment site (Online Supplemental Data).