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Stephen Monteith, John Snell, Mathew Eames, Neal F. Kassell, Edward Kelly and Ryder Gwinn

OBJECTIVE

In appropriate candidates, the treatment of medication-refractory mesial temporal lobe epilepsy (MTLE) is primarily surgical. Traditional anterior temporal lobectomy yields seizure-free rates of 60%–70% and possibly higher. The field of magnetic resonance–guided focused ultrasound (MRgFUS) is an evolving field in neurosurgery. There is potential to treat MTLE with MRgFUS; however, it has appeared that the temporal lobe structures were beyond the existing treatment envelope of currently available clinical systems. The purpose of this study was to determine whether lesional temperatures can be achieved in the target tissue and to assess potential safety concerns.

METHODS

Cadaveric skulls with tissue-mimicking gels were used as phantom targets. An ablative volume was then mapped out for a “virtual temporal lobectomy.” These data were then used to create a target volume on the InSightec ExAblate Neuro system. The target was the amygdala, uncus, anterior 20 mm of hippocampus, and adjacent parahippocampal gyrus. This volume was approximately 5cm3. Thermocouples were placed on critical skull base structures to monitor skull base heating.

RESULTS

Adequate focusing of the ultrasound energy was possible in the temporal lobe structures. Using clinically relevant ultrasound parameters (power 900 W, duration 10 sec, frequency 650 kHz), ablative temperatures were not achieved (maximum temperature 46.1°C). Increasing sonication duration to 30 sec demonstrated lesional temperatures in the mesial temporal lobe structures of interest (up to 60.5°C). Heating of the skull base of up to 24.7°C occurred with 30-sec sonications.

CONCLUSIONS

MRgFUS thermal ablation of the mesial temporal lobe structures relevant in temporal lobe epilepsy is feasible in a laboratory model. Longer sonications were required to achieve temperatures that would create permanent lesions in brain tissue. Heating of the skull base occurred with longer sonications. Blocking algorithms would be required to restrict ultrasound beams causing skull base heating. In the future, MRgFUS may present a minimally invasive, non-ionizing treatment of MTLE.

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Zhiyuan Xu, Carissa Carlson, John Snell, Matt Eames, Arik Hananel, M. Beatriz Lopes, Prashant Raghavan, Cheng-Chia Lee, Chun-Po Yen, David Schlesinger, Neal F. Kassell, Jean-Francois Aubry and Jason Sheehan

OBJECT

In biological tissues, it is known that the creation of gas bubbles (cavitation) during ultrasound exposure is more likely to occur at lower rather than higher frequencies. Upon collapsing, such bubbles can induce hemorrhage. Thus, acoustic inertial cavitation secondary to a 220-kHz MRI-guided focused ultrasound (MRgFUS) surgery is a serious safety issue, and animal studies are mandatory for laying the groundwork for the use of low-frequency systems in future clinical trials. The authors investigate here the in vivo potential thresholds of MRgFUS-induced inertial cavitation and MRgFUS-induced thermal coagulation using MRI, acoustic spectroscopy, and histology.

METHODS

Ten female piglets that had undergone a craniectomy were sonicated using a 220-kHz transcranial MRgFUS system over an acoustic energy range of 5600–14,000 J. For each piglet, a long-duration sonication (40-second duration) was performed on the right thalamus, and a short sonication (20-second duration) was performed on the left thalamus. An acoustic power range of 140–300 W was used for long-duration sonications and 300–700 W for short-duration sonications. Signals collected by 2 passive cavitation detectors were stored in memory during each sonication, and any subsequent cavitation activity was integrated within the bandwidth of the detectors. Real-time 2D MR thermometry was performed during the sonications. T1-weighted, T2-weighted, gradient-recalled echo, and diffusion-weighted imaging MRI was performed after treatment to assess the lesions. The piglets were killed immediately after the last series of posttreatment MR images were obtained. Their brains were harvested, and histological examinations were then performed to further evaluate the lesions.

RESULTS

Two types of lesions were induced: thermal ablation lesions, as evidenced by an acute ischemic infarction on MRI and histology, and hemorrhagic lesions, associated with inertial cavitation. Passive cavitation signals exhibited 3 main patterns identified as follows: no cavitation, stable cavitation, and inertial cavitation. Low-power and longer sonications induced only thermal lesions, with a peak temperature threshold for lesioning of 53°C. Hemorrhagic lesions occurred only with high-power and shorter sonications. The sizes of the hemorrhages measured on macroscopic histological examinations correlated with the intensity of the cavitation activity (R2 = 0.74). The acoustic cavitation activity detected by the passive cavitation detectors exhibited a threshold of 0.09 V·Hz for the occurrence of hemorrhages.

CONCLUSIONS

This work demonstrates that 220-kHz ultrasound is capable of inducing a thermal lesion in the brain of living swines without hemorrhage. Although the same acoustic energy can induce either a hemorrhage or a thermal lesion, it seems that low-power, long-duration sonication is less likely to cause hemorrhage and may be safer. Although further study is needed to decrease the likelihood of ischemic infarction associated with the 220-kHz ultrasound, the threshold established in this work may allow for the detection and prevention of deleterious cavitations.

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Edward H. Oldfield, Johanna J. Loomba, Stephen J. Monteith, R. Webster Crowley, Ricky Medel, Daryl R. Gress, Neal F. Kassell, Aaron S. Dumont and Craig Sherman

Object

Intravenous sodium nitrite has been shown to prevent and reverse cerebral vasospasm in a primate model of subarachnoid hemorrhage (SAH). The present Phase IIA dose-escalation study of sodium nitrite was conducted to determine the compound's safety in humans with aneurysmal SAH and to establish its pharmacokinetics during a 14-day infusion.

Methods

In 18 patients (3 cohorts of 6 patients each) with SAH from a ruptured cerebral aneurysm, nitrite (3 patients) or saline (3 patients) was infused. Sodium nitrite and saline were delivered intravenously for 14 days, and a dose-escalation scheme was used for the nitrite, with a maximum dose of 64 nmol/kg/min. Sodium nitrite blood levels were frequently sampled and measured using mass spectroscopy, and blood methemoglobin levels were continuously monitored using a pulse oximeter.

Results

In the 14-day infusions in critically ill patients with SAH, there was no toxicity or systemic hypotension, and blood methemoglobin levels remained at 3.3% or less in all patients. Nitrite levels increased rapidly during intravenous infusion and reached steady-state levels by 12 hours after the start of infusion on Day 1. The nitrite plasma half-life was less than 1 hour across all dose levels evaluated after stopping nitrite infusions on Day 14.

Conclusions

Previous preclinical investigations of sodium nitrite for the prevention and reversal of vasospasm in a primate model of SAH were effective using doses similar to the highest dose examined in the current study (64 nmol/kg/min). Results of the current study suggest that safe and potentially therapeutic levels of nitrite can be achieved and sustained in critically ill patients after SAH from a ruptured cerebral aneurysm. Clinical trial registration no.: NCT00873015 (ClinicalTrials.gov).

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Neal F. Kassell and G. Frederick Wooten

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Stephen J. Monteith, Sagi Harnof, Ricky Medel, Britney Popp, Max Wintermark, M. Beatriz S. Lopes, Neal F. Kassell, W. Jeff Elias, John Snell, Matthew Eames, Eyal Zadicario, Krisztina Moldovan and Jason Sheehan

Object

Intracerebral hemorrhage (ICH) is a major cause of death and disability throughout the world. Surgical techniques are limited by their invasive nature and the associated disability caused during clot removal. Preliminary data have shown promise for the feasibility of transcranial MR-guided focused ultrasound (MRgFUS) sonothrombolysis in liquefying the clotted blood in ICH and thereby facilitating minimally invasive evacuation of the clot via a twist-drill craniostomy and aspiration tube.

Methods and Results

In an in vitro model, the following optimum transcranial sonothrombolysis parameters were determined: transducer center frequency 230 kHz, power 3950 W, pulse repetition rate 1 kHz, duty cycle 10%, and sonication duration 30 seconds. Safety studies were performed in swine (n = 20). In a swine model of ICH, MRgFUS sonothrombolysis of 4 ml ICH was performed. Magnetic resonance imaging and histological examination demonstrated complete lysis of the ICH without additional brain injury, blood-brain barrier breakdown, or thermal necrosis due to sonothrombolysis. A novel cadaveric model of ICH was developed with 40-ml clots implanted into fresh cadaveric brains (n = 10). Intracerebral hemorrhages were successfully liquefied (> 95%) with transcranial MRgFUS in a highly accurate fashion, permitting minimally invasive aspiration of the lysate under MRI guidance.

Conclusions

The feasibility of transcranial MRgFUS sonothrombolysis was demonstrated in in vitro and cadaveric models of ICH. Initial in vivo safety data in a swine model of ICH suggest the process to be safe. Minimally invasive treatment of ICH with MRgFUS warrants evaluation in the setting of a clinical trial.

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Stephen J. Monteith, Neal F. Kassell, Oded Goren and Sagi Harnof

Intracerebral hemorrhage remains a significant cause of morbidity and mortality. Current surgical therapies aim to use a minimally invasive approach to remove as much of the clot as possible without causing undue disruption to surrounding neural structures. Transcranial MR-guided focused ultrasound (MRgFUS) surgery is an emerging technology that permits a highly concentrated focal point of ultrasound energy to be deposited to a target deep within the brain without an incision or craniotomy. With appropriate ultrasound parameters it has been shown that MRgFUS can effectively liquefy large-volume blood clots through the human calvaria. In this review the authors discuss the rationale for using MRgFUS to noninvasively liquefy intracerebral hemorrhage (ICH), thereby permitting minimally invasive aspiration of the liquefied clot via a small drainage tube. The mechanism of action of MRgFUS sonothrombolysis; current investigational work with in vitro, in vivo, and cadaveric models of ICH; and the potential clinical application of this disruptive technology for the treatment of ICH are discussed.

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Stephen Monteith, Jason Sheehan, Ricky Medel, Max Wintermark, Matthew Eames, John Snell, Neal F. Kassell and W. Jeff Elias

Magnetic resonance–guided focused ultrasound surgery (MRgFUS) has the potential to create a shift in the treatment paradigm of several intracranial disorders. High-resolution MRI guidance combined with an accurate method of delivering high doses of transcranial ultrasound energy to a discrete focal point has led to the exploration of noninvasive treatments for diseases traditionally treated by invasive surgical procedures. In this review, the authors examine the current intracranial applications under investigation and explore other potential uses for MRgFUS in the intracranial space based on their initial cadaveric studies.

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Stephen J. Monteith, Ricky Medel, Neal F. Kassell, Max Wintermark, Matthew Eames, John Snell, Eyal Zadicario, Javier Grinfeld, Jason P. Sheehan and W. Jeff Elias

Object

Transcranial MR-guided focused ultrasound surgery (MRgFUS) is evolving as a treatment modality in neurosurgery. Until now, the trigeminal nerve was believed to be beyond the treatment envelope of existing high-frequency transcranial MRgFUS systems. In this study, the authors explore the feasibility of targeting the trigeminal nerve in a cadaveric model with temperature assessments using computer simulations and an in vitro skull phantom model fitted with thermocouples.

Methods

Six trigeminal nerves from 4 unpreserved cadavers were targeted in the first experiment. Preprocedural CT scanning of the head was performed to allow for a skull correction algorithm. Three-Tesla, volumetric, FIESTA MRI sequences were performed to delineate the trigeminal nerve and any vascular structures of the cisternal segment. The cadaver was positioned in a focused ultrasound transducer (650-kHz system, ExAblate Neuro, InSightec) so that the focus of the transducer was centered at the proximal trigeminal nerve, allowing for targeting of the root entry zone (REZ) and the cisternal segment. Real-time, 2D thermometry was performed during the 10- to 30-second sonication procedures. Post hoc MR thermometry was performed on a computer workstation at the conclusion of the procedure to analyze temperature effects at neuroanatomical areas of interest. Finally, the region of the trigeminal nerve was targeted in a gel phantom encased within a human cranium, and temperature changes in regions of interest in the skull base were measured using thermocouples.

Results

The trigeminal nerves were clearly identified in all cadavers for accurate targeting. Sequential sonications of 25–1500 W for 10–30 seconds were successfully performed along the length of the trigeminal nerve starting at the REZ. Real-time MR thermometry confirmed the temperature increase as a narrow focus of heating by a mean of 10°C. Postprocedural thermometry calculations and thermocouple experiments in a phantom skull were performed and confirmed minimal heating of adjacent structures including the skull base, cranial nerves, and cerebral vessels. For targeting, inclusion of no-pass regions through the petrous bone decreased collateral heating in the internal acoustic canal from 16.7°C without blocking to 5.7°C with blocking. Temperature at the REZ target decreased by 3.7°C with blocking. Similarly, for midcisternal targeting, collateral heating at the internal acoustic canal was improved from a 16.3°C increase to a 4.9°C increase. Blocking decreased the target temperature increase by 4.4°C for the same power settings.

Conclusions

This study demonstrates focal heating of up to 18°C in a cadaveric trigeminal nerve at the REZ and along the cisternal segment with transcranial MRgFUS. Significant heating of the skull base and surrounding neural structures did not occur with implementation of no-pass regions. However, in vivo studies are necessary to confirm the safety and efficacy of this potentially new, noninvasive treatment.

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Don Ilodigwe, M. Stat., Gordon D. Murray, Neal F. Kassell, James Torner, Richard S. C. Kerr, Andrew J. Molyneux and R. Loch Macdonald

Object

In randomized clinical trials of subarachnoid hemorrhage (SAH) in which the primary clinical outcomes are ordinal, it has been common practice to dichotomize the ordinal outcome scale into favorable versus unfavorable outcome. Using this strategy may increase sample sizes by reducing statistical power. Authors of the present study used SAH clinical trial data to determine if a sliding dichotomy would improve statistical power.

Methods

Available individual patient data from tirilazad (3552 patients), clazosentan (the Clazosentan to Overcome Neurological Ischemia and Infarction Occurring After Subarachnoid Hemorrhage trial [CONSCIOUS-1], 413 patients), and subarachnoid aneurysm trials (the International Subarachnoid Aneurysm Trial [ISAT], 2089 patients) were analyzed. Treatment effect sizes were examined using conventional fixed dichotomy, sliding dichotomy (logical or median split methods), or proportional odds modeling. Whether sliding dichotomy affected the difference in outcomes between the several age and neurological grade groups was also evaluated.

Results

In the tirilazad data, there was no significant effect of treatment on outcome (fixed dichotomy: OR = 0.92, 95% CI 0.80–1.07; and sliding dichotomy: OR = 1.02, 95% CI 0.87–1.19). Sliding dichotomy reversed and increased the difference in outcome in favor of the placebo over clazosentan (fixed dichotomy: OR = 1.06, 95% CI 0.65–1.74; and sliding dichotomy: OR = 0.85, 95% CI 0.52–1.39). In the ISAT data, sliding dichotomy produced identical odds ratios compared with fixed dichotomy (fixed dichotomy vs sliding dichotomy, respectively: OR = 0.67, 95% CI 0.55–0.82 vs OR = 0.67, 95% CI 0.53–0.85). When considering the tirilazad and CONSCIOUS-1 groups based on age or World Federation of Neurosurgical Societies grade, no consistent effects of sliding dichotomy compared with fixed dichotomy were observed.

Conclusions

There were differences among fixed dichotomy, sliding dichotomy, and proportional odds models in the magnitude and precision of odds ratios, but these differences were not as substantial as those seen when these methods were used in other conditions such as head injury. This finding suggests the need for different outcome scales for SAH.

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Introduction

Focused ultrasound surgery

W. Jeffrey Elias and Neal F. Kassell