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Ryan Alkins, Yuexi Huang, Dan Pajek and Kullervo Hynynen

Object

Transcranial focused ultrasound is increasingly being investigated as a minimally invasive treatment for a range of intracranial pathologies. At higher peak rarefaction pressures than those used for thermal ablation, focused ultrasound can initiate inertial cavitation and create holes in the brain by fractionation of the tissue elements. The authors investigated the technical feasibility of using MRI-guided focused ultrasound to perform a third ventriculostomy as a possible noninvasive alternative to endoscopic third ventriculostomy for hydrocephalus.

Methods

A craniectomy was performed in male pigs weighing 13–19 kg to expose the supratentorial brain, leaving the dura mater intact. Seven pigs were treated through the craniectomy, while 2 pigs were treated through ex vivo human skulls placed in the beam path. Registration and targeting was done using T2-weighted MRI sequences. For transcranial treatments a CT scan was used to correct the beam from aberrations due to the skull and maintain a small, high-intensity focus. Sonications were performed at both 650 kHz and 230 kHz at a range of intensities, and the in situ pressures were estimated both from simulations and experimental data to establish a threshold for tissue fractionation in the brain.

Results

In craniectomized animals at 650 kHz, a peak pressure ≥ 22.7 MPa for 1 second was needed to reliably create a ventriculostomy. Transcranially at this frequency the ExAblate 4000 was unable to generate the required intensity to fractionate tissue, although cavitation was initiated. At 230 kHz, ventriculostomy was successful through the skull with a peak pressure of 8.8 MPa.

Conclusions

This is the first study to suggest that it is possible to perform a completely noninvasive third ventriculostomy using ultrasound. This may pave the way for future studies and eventually provide an alternative means for the creation of CSF communications in the brain, including perforation of the septum pellucidum or intraventricular membranes.

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Ryan M. Jones, Shona Kamps, Yuexi Huang, Nadia Scantlebury, Nir Lipsman, Michael L. Schwartz and Kullervo Hynynen

OBJECTIVE

The object of this study was to correlate lesion size with accumulated thermal dose (ATD) in transcranial MRI-guided focused ultrasound (MRgFUS) treatments of essential tremor with focal temperatures limited to 50°C–54°C.

METHODS

Seventy-five patients with medically refractory essential tremor underwent MRgFUS thalamotomy at the authors’ institution. Intraoperative MR thermometry was performed to measure the induced temperature and thermal dose distributions (proton resonance frequency shift coefficient = −0.00909 ppm/°C). In 19 patients, it was not possible to raise the focal temperature above 54°C because of unfavorable skull characteristics and/or the pain associated with cranial heating. In this patient subset, sonications with focal temperatures between 50°C and 54°C were repeated (5.1 ± 1.5, mean ± standard deviation) to accumulate a sufficient thermal dose for lesion formation. The ATD profile sizes (17, 40, 100, 200, and 240 cumulative equivalent minutes at 43°C [CEM43]) calculated by combining axial MR thermometry data from individual sonications were correlated with the corresponding lesion sizes measured on axial T1-weighted (T1w) and T2-weighted (T2w) MR images acquired 1 day posttreatment. Manual corrections were applied to the MR thermometry data prior to thermal dose accumulation to compensate for off-resonance–induced spatial-shifting artifacts.

RESULTS

Mean lesion sizes measured on T2w MRI (5.0 ± 1.4 mm) were, on average, 28% larger than those measured on T1w MRI (3.9 ± 1.4 mm). The ATD thresholds found to provide the best correlation with lesion sizes measured on T2w and T1w MRI were 100 CEM43 (regression slope = 0.97, R2 = 0.66) and 200 CEM43 (regression slope = 0.98, R2 = 0.89), respectively, consistent with data from a previous study of MRgFUS thalamotomy via repeated sonications at higher focal temperatures (≥ 55°C). Two-way linear mixed-effects analysis revealed that dominant tremor subscores on the Fahn-Tolosa-Marin Clinical Rating Scale for Tremor (CRST) were statistically different from baseline at 3 months and 1 year posttreatment in both low-temperature (50°C–54°C) and high-temperature (≥ 55°C) patient cohorts. No significant fixed effect on the dominant tremor scores was found for the temperature cohort factor.

CONCLUSIONS

In transcranial MRgFUS thalamotomy for essential tremor, repeated sonications with focal temperatures between 50°C and 54°C can accumulate a sufficient thermal dose to generate lesions for clinically relevant tremor suppression up to 1 year posttreatment, and the ATD can be used to predict the size of the resulting ablation zones measured on MRI. These data will serve to guide future clinical MRgFUS brain procedures, particularly those in which focal temperatures are limited to below 55°C.

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Michael L. Schwartz, Robert Yeung, Yuexi Huang, Nir Lipsman, Vibhor Krishna, Jennifer D. Jain, Martin G. Chapman, Andres M. Lozano and Kullervo Hynynen

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.

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Benjamin Davidson, Karim Mithani, Yuexi Huang, Ryan M. Jones, Maged Goubran, Ying Meng, John Snell, Kullervo Hynynen, Clement Hamani and Nir Lipsman

OBJECTIVE

Magnetic resonance imaging–guided focused ultrasound (MRgFUS) is an emerging treatment modality that enables incisionless ablative neurosurgical procedures. Bilateral MRgFUS capsulotomy has recently been demonstrated to be safe and effective in treating obsessive-compulsive disorder (OCD) and major depressive disorder (MDD). Preliminary evidence has suggested that bilateral MRgFUS capsulotomy can present increased difficulties in reaching lesional temperatures as compared to unilateral thalamotomy. The authors of this article aimed to study the parameters associated with successful MRgFUS capsulotomy lesioning and to present longitudinal radiographic findings following MRgFUS capsulotomy.

METHODS

Using data from 22 attempted MRgFUS capsulotomy treatments, the authors investigated the relationship between various sonication parameters and the maximal temperature achieved at the intracranial target. Lesion volume and morphology were analyzed longitudinally using structural and diffusion tensor imaging. A retreatment procedure was attempted in one patient, and their postoperative imaging is presented.

RESULTS

Skull density ratio (SDR), skull thickness, and angle of incidence were significantly correlated with the maximal temperature achieved. MRgFUS capsulotomy lesions appeared similar to those following MRgFUS thalamotomy, with three concentric zones observed on MRI. Lesion volumes regressed substantially over time following MRgFUS. Fractional anisotropy analysis revealed a disruption in white matter integrity, followed by a gradual return to near-baseline levels concurrent with lesion regression. In the patient who underwent retreatment, successful bilateral lesioning was achieved, and there were no adverse clinical or radiographic events.

CONCLUSIONS

With the current iteration of MRgFUS technology, skull-related parameters such as SDR, skull thickness, and angle of incidence should be considered when selecting patients suitable for MRgFUS capsulotomy. Lesions appear to follow morphological patterns similar to what is seen following MRgFUS thalamotomy. Retreatment appears to be safe, although additional cases will be necessary to further evaluate the associated safety profile.

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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

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.