Skull Factors Affecting Outcomes of Magnetic Resonance-Guided Focused Ultrasound for Patients with Essential Tremor

Gender disparity in access to advanced therapies for patients with Parkinson's disease: a retrospective real-word study

Background

Gender differences in the access to advanced therapies for Parkinson's disease (PD) are poorly investigated.

Objective

The objective of this study was to investigate the presence of any gender disparity in the access to advanced therapies for PD.

Design

Retrospective study.

Methods

Data from patients with consistent access to the Parkinson's and Movement Disorder Center of L'Aquila over the last 10-year period were screened. Patients selected for advanced therapies were included.

Results

Out of 1,252 patients, 200 (mean age ± SD 71.02 ± 9.70; 72% males; median Hoen Yahr level: 3, minimum 1 maximum 5) were selected for advanced therapies: 133 for Magnetic Resonance guided Focused Ultrasound (MRgFUS) thalamotomy (mean age ± SD 70.0 ± 8.9; 77% males), 49 for Levodopa/Carbidopa Intestinal Gel (LCIG) infusion (mean age ± SD 74.3 ± 11.4; 59% males), 12 for Deep Brain Stimulation (DBS) (mean age ± SD 71.2 ± 6.3; 75% males), and 7 for Continuous Subcutaneous Apomorphine Infusion (CSAI) (mean age ± SD 69.7 ± 5.5; 43% males). No sex differences were found in relation to age (MRgFUS group: males vs. females 70.2 ± 8.9 vs. 70.8 ± 8.9, p-value = 0.809; LCIG group: males vs. females 73.5 ± 13.0 vs. 75.5 ± 8.5, p-value = 0.557; DBS group: males vs. females 77.2 ± 8.1 vs. 67.3 ± 8.6, p-value = 0.843; CSAI group: males vs. females 73.3 ± 4.0 vs. 67.0 ± 5.2, p-value = 0.144) and disease duration (MRgFUS group: males vs. females 8.3 ± 4.4 vs. 9.6 ± 6.7, p-value = 0.419; LCIG group: males vs. females 14.5 ± 5.81 vs. 17.3 ± 5.5; p-value = 0.205; DBS group: males vs. females 15.0 ± 9.6 vs. 15.5 ± 7.7, p-value = 0.796; CSAI group: males vs. females 11.7 ± 3.7 vs. 10.3 ± 3.7, p-value = 0.505).

Conclusion

The predominance of males is higher than that expected based on the higher prevalence of PD in men. Women are less confident in selecting advanced therapies during the natural progression of their disease. Factors accounting for this discrepancy deserve further investigation.

Magnetic resonance-guided focused ultrasound thalamotomy for essential tremor patients with low skull density ratio: a case-matched analysis

Introduction

Skull density ratio (SDR) is the ratio between the mean Hounsfield units of marrow and cortical bone, impacting energy transmission through the skull. Low SDR has been used as an exclusion criterion in major trials of magnetic resonance-guided focused ultrasound (MRgFUS) thalamotomy for medication-refractory essential tremor (ET). However, some studies have suggested that patients with low SDR can safely undergo MRgFUS with favorable outcomes. In this case-matched study, we aim to compare the characteristics, sonication parameters, lesion sizes, and clinical outcomes of patients with low SDR vs. patients with high SDR who underwent unilateral MRgFUS thalamotomy for medication-refractory ET.

Methods

Between March 2016 and April 2023, all patients (n = 270) who underwent unilateral MRgFUS thalamotomy for medication-refractory ET at a single institution were classified as low SDR (<0.40) and high SDR (≥0.40). All clinical and radiological data was prospectively collected and retrospectively analyzed using non-case-matched and 1:1 case-matched methodology.

Results

Thirty-one patients had low SDR, and 239 patients had high SDR. Fifty-six patients (28 in each cohort) were included in 1:1 case-matched analysis. There were no significant differences in baseline characteristics between the two groups in both non-case-matched and 1:1 case-matched analyses. In both analyses, compared to patients with high SDR, patients with low SDR required a significantly higher maximum sonication power, energy, and duration, and reached a lower maximum temperature with smaller lesion volumes. In the non-case-matched and case-matched analyses, low SDR patients did not have significantly less tremor control at any postoperative timepoints. However, there was a higher chance of procedure failure in the low SDR group with three patients not obtaining an appropriately sized lesion. In both analyses, imbalance was observed more often in high SDR patients on postoperative day 1 and month 3.

Discussion

ET patients with SDR <0.40 can be safely and effectively treated with MRgFUS, though there may be higher rates of treatment failure and intraoperative discomfort.

The Role of Treatment-Related Parameters and Brain Morphology in the Lesion Volume of Magnetic-Resonance-Guided Focused Ultrasound Thalamotomy in Patients with Tremor-Dominant Neurological Conditions

PURPOSE

To determine the best predictor of lesion volume induced by magnetic resonance (MR)-guided focused ultrasound (MRgFUS) thalamotomy in patients with tremor-dominant symptoms in Parkinson's disease (PD) and essential tremor (ET) patients.

METHODS

Thirty-six neurological patients with medication-refractory tremor (n°19 PD; n°17 ET) were treated using a commercial MRgFUS brain system (Exablate Neuro 4000, Insightec) integrated with a 1.5 T MRI unit (Sigma HDxt; GE Medical System). Linear regression analysis was used to determine how the demographic, clinical, radiological (Fazekas scale), volumetric (total GM/WM/CSF volume, cortical thickness), and MRgFUS-related parameters [Skull Density Ratio (SDR), n° of transducer elements, n° of sonications, skull area, maximal energy delivered (watt), maximal power delivered (joule), maximal sonication time delivered, maximal mean temperature reached (T°C_max), accumulated thermal dose (ATD)] impact on ventral intermediate (VIM)-thalamotomy-related 3D volumetric lesions of necrosis and edema.

RESULTS

The VIM thalamotomy was clinically efficacious in improving the tremor symptoms of all the patients as measured at 1 week after treatment. Multiple regression analysis revealed that T°C_max and n° of transducer elements were the best predictors of the necrosis and edema volumes. Moreover, total WM volume also predicted the size of necrosis.

CONCLUSIONS

Our study provides new insights into the clinical MRgFUS procedures that can be used to forecast brain lesion size and improve treatment outcomes.

Patient-Reported Outcomes After Focused Ultrasound Thalamotomy for Tremor-Predominant Parkinson's Disease

BACKGROUND

Magnetic resonance-guided focused ultrasound (MRgFUS) has emerged as a precise, incisionless approach to cerebral lesioning and an alternative to neuromodulation in movement disorders. Despite rigorous clinical trials, long-term patient-centered outcome data after MRgFUS for tremor-predominant Parkinson's Disease (TPPD) are relatively lacking.

OBJECTIVE

To report long-term data on patient satisfaction and quality of life after MRgFUS thalamotomy for TPPD.

METHODS

In a retrospective study of patients who underwent MRgFUS thalamotomy for TPPD at our institution between 2015 and 2022, a patient survey was administered to collect self-reported measures of tremor improvement, recurrence, Patients' Global Impression of Change (PGIC), and side effects. Patient demographics, FUS parameters, and lesion characteristics were analyzed.

RESULTS

A total of 29 patients were included with a median follow-up of 16 months. Immediate tremor improvement was achieved in 96% of patients. Sustained improvement was achieved in 63% of patients at last follow-up. Complete tremor recurrence to baseline occurred for 17% of patients. Life quality improvement denoted by a PGIC of 1 to 2 was reported by 69% of patients. Long-term side effects were reported by 38% of patients and were mostly mild. Performing a secondary anteromedial lesion to target the ventralis oralis anterior/posterior nucleus was associated with higher rates of speech-related side effects (56% vs 12%), without significant improvement in tremor outcomes.

CONCLUSION

Patient satisfaction with FUS thalamotomy for tremor-predominant PD was very high, even at longer term. Extended lesioning to target the motor thalamus did not improve tremor control and may contribute to greater frequency of postoperative motor- and speech-related side effects.

A simulation study on the sensitivity of transcranial ray-tracing ultrasound modeling to skull properties

In transcranial focused ultrasound therapies, such as treating essential tremor via thermal ablation in the thalamus, acoustic energy is focused through the skull using a phased-array transducer. Ray tracing is a computationally efficient method that can correct skull-induced phase aberrations via per-element phase delay calculations using patient-specific computed tomography (CT) data. However, recent studies show that variations in CT-derived Hounsfield unit may account for only 50% of the speed of sound variability in human skull specimens, potentially limiting clinical transcranial ultrasound applications. Therefore, understanding the sensitivity of treatment planning methods to material parameter variations is essential. The present work uses a ray-tracing simulation model to explore how imprecision in model inputs, arising from clinically significant uncertainties in skull properties or considerations of acoustic phenomena, affects acoustic focusing quality through the skull. We propose and validate new methods to optimize ray-tracing skull simulations for clinical treatment planning, relevant for predicting intracranial target's thermal rise, using experimental data from ex-vivo human skulls.

Magnetic Resonance-Guided focused ultrasound surgery for Parkinson's disease: A mini-review and comparison between deep brain stimulation

Magnetic resonance-guided focused ultrasound (MRgFUS) is a new surgical treatment for Parkinson's disease (PD). Previous experience with radiofrequency lesionectomy and deep brain stimulation (DBS) has identified several candidate targets for MRgFUS intended to alleviate the motor symptoms of PD. The main advantage of MRgFUS is that it is incisionless. MRgFUS has certain limitations and is associated with adverse effects. The present study reviews the literature on conventional surgical interventions for PD, discusses recent studies on MRgFUS, and the comparison between DBS and MRgFUS for PD. The reviews aims to provide an essential reference for neurologists to select the appropriate treatments for patients with PD.

Comparison of the impact of skull density ratio with alternative skull metrics on magnetic resonance-guided focused ultrasound thalamotomy for tremor

OBJECTIVE

One of the key metrics that is used to predict the likelihood of success of MR-guided focused ultrasound (MRgFUS) thalamotomy is the overall calvarial skull density ratio (SDR). However, this measure does not fully predict the sonication parameters that would be required or the technical success rates. The authors aimed to assess other skull characteristics that may also contribute to technical success.

METHODS

The authors retrospectively studied consecutive patients with essential tremor who were treated by MRgFUS at their center between 2017 and 2021. They evaluated the correlation between the different treatment parameters, particularly maximum power and energy delivered, with a range of patients' skull metrics and demographics. Machine learning algorithms were applied to investigate whether sonication parameters could be predicted from skull density metrics alone and whether including combined local transducer SDRs with overall calvarial SDR would increase model accuracy.

RESULTS

A total of 62 patients were included in the study. The mean age was 77.1 (SD 9.2) years, and 78% of treatments (49/63) were performed in males. The mean SDR was 0.51 (SD 0.10). Among the evaluated metrics, SDR had the highest correlation with the maximum power used in treatment (ρ = -0.626, p < 0.001; proportion of local SDR values ≤ 0.8 group also had ρ = +0.626, p < 0.001) and maximum energy delivered (ρ = -0.680, p < 0.001). Machine learning algorithms achieved a moderate ability to predict maximum power and energy required from the local and overall SDRs (accuracy of approximately 80% for maximum power and approximately 55% for maximum energy), and high ability to predict average maximum temperature reached from the local and overall SDRs (approximately 95% accuracy).

CONCLUSIONS

The authors compared a number of skull metrics against SDR and showed that SDR was one of the best indicators of treatment parameters when used alone. In addition, a number of other machine learning algorithms are proposed that may be explored to improve its accuracy when additional data are obtained. Additional metrics related to eventual sonication parameters should also be identified and explored.

Magnetic resonance-guided focused ultrasound for the treatment of tremor

INTRODUCTION

Magnetic resonance-guided focused ultrasound (MRgFUS) is an emerging treatment for tremor and other movement disorders. An incisionless therapy, it is becoming increasingly common worldwide. However, given MRgFUS' relative novelty, there remain limited data on its benefits and adverse effects.

AREAS COVERED

We review the current state of evidence of MRgFUS for tremor, highlight its challenges, and discuss future perspectives.

EXPERT OPINION

Essential tremor (ET) has been the major indication for MRgFUS since a milestone randomized controlled trial (RCT) in 2016, with substantial evidence attesting to the efficacy and acceptable safety profile of this treatment. Patients with other tremor etiologies are also being treated with MRgFUS, with studies - including an RCT - suggesting parkinsonian tremor in particular responds well to this intervention. Additionally, targets other than the ventral intermediate nucleus, such as the subthalamic nucleus and internal segment of the globus pallidus, have been reported to improve parkinsonian symptoms beyond tremor, including rigidity and bradykinesia. Although MRgFUS is encumbered by certain unique technical challenges, it nevertheless offers significant advantages compared to alternative neurosurgical interventions for tremor. The fast-growing interest in this treatment modality will likely lead to further scientific and technological advancements that could optimize and expand its therapeutic potential.

Clinical Intervention Using Focused Ultrasound (FUS) Stimulation of the Brain in Diverse Neurological Disorders

Various surgical techniques and pharmaceutical treatments have been developed to improve the current technologies of treating brain diseases. Focused ultrasound (FUS) is a new brain stimulation modality that can exert a therapeutic effect on diseased brain cells, with this effect ranging from permanent ablation of the pathological neural circuit to transient excitatory/inhibitory modulation of the neural activity depending on the acoustic energy of choice. With the development of intraoperative imaging technology, FUS has become a clinically available noninvasive neurosurgical option with visual feedback. Over the past 10 years, FUS has shown enormous potential. It can deliver acoustic energy through the physical barrier of the brain and eliminate abnormal brain cells to treat patients with Parkinson's disease and essential tremor. In addition, FUS can help introduce potentially beneficial therapeutics at the exact brain region where they need to be, bypassing the brain's function barrier, which can be applied for a wide range of central nervous system disorders. In this review, we introduce the current FDA-approved clinical applications of FUS, ranging from thermal ablation to blood barrier opening, as well as the emerging applications of FUS in the context of pain control, epilepsy, and neuromodulation. We also discuss the expansion of future applications and challenges. Broadening FUS technologies requires a deep understanding of the effect of ultrasound when targeting various brain structures in diverse disease conditions in the context of skull interface, anatomical structure inside the brain, and pathology.

Cutoff values for the best management strategy for magnetic resonance-guided focused ultrasound ablation for essential tremor

OBJECTIVE

The efficacy of magnetic resonance-guided focused ultrasound (MRgFUS) ablation for essential tremor (ET) is well known; however, no prognostic factors have been established. The authors aimed to retrospectively investigate MRgFUS ablation outcomes and associated factors and to define the cutoff values for each prognostic factor.

METHODS

Sixty-four Japanese patients who underwent unilateral ventral intermediate nucleus thalamotomy with MRgFUS for ET were included. Follow-up evaluations were performed at 1 week and 1, 3, 6, 12, and 24 months postoperatively. Tremor suppression was evaluated using the Clinical Rating Scale for Tremor (CRST), and adverse effects were recorded postoperatively. Outcome-associated factors were examined preoperatively, intraoperatively, and postoperatively using multivariate analyses. The cutoff values for the prognostic factors were calculated using receiver operating characteristics.

RESULTS

Percentage improvements in the CRST scores of the affected upper limb were 82.4%, 72.2%, 68.6%, and 65.9% at 1, 3, 6, and 12 months, respectively. Preoperatively, a high skull density ratio (SDR) (p ≤ 0.047), low CRST part B score (used to assess tremors during several tasks) (cutoff value 25, p ≤ 0.041), and nonoccurrence of resting tremors (p = 0.027) were significantly associated with improved tremor control. An intraoperatively high maximum mean temperature (cutoff value 52.5°C, p ≤ 0.047), postoperatively large lesion (cutoff value 3.9 mm in the anterior-posterior direction, p ≤ 0.002; cutoff value 5.0-5.55 mm in the superior-inferior direction, p ≤ 0.026), and small transducer focus correction (p ≤ 0.015) were also associated with improved tremor control. No valid cutoff value was found for SDR. Adverse effects (limb weakness, sensory disturbance, ataxia/walking disturbance, dysgeusia, dysarthria, and facial swelling) occurred transiently and were associated with high SDR, high temperature, high number of sonication sessions, large lesion, and occurrence of resting tremor. Patients who developed leg weakness experienced greater percentage improvement in tremors at 3 months postoperatively than those who did not.

CONCLUSIONS

MRgFUS ablation could be used to achieve good tremor control with acceptable adverse effects in Japanese patients with ET. The relatively low SDR in Asian ethnic groups as compared with that of Western populations makes treatment difficult; however, the cutoff values obtained in this study may be useful for achieving good treatment outcomes even in such patients. Clinical trial registration no.: UMIN000026952 (University Hospital Medical Information Network).

ABBREVIATIONS

ACPC = anterior commissure-posterior commissure; AP = anterior to posterior; CRST = Clinical Rating Scale for Tremor; ET = essential tremor; MRgFUS = magnetic resonance-guided focused ultrasound; PC = posterior commissure; PSA = posterior subthalamic area; RL = right to left; ROC = receiver operating characteristic; SDR = skull density ratio; SI = superior to inferior; T2WI = T2-weighted imaging; VIM = ventral intermediate nucleus.

Predictors of thermal increase in magnetic resonance-guided focused ultrasound treatment for essential tremor: histogram analysis of skull density ratio values for 1024 elements

OBJECTIVE

Sufficient thermal increase capable of generating thermocoagulation is indispensable for an effective clinical outcome in patients undergoing magnetic resonance-guided focused ultrasound (MRgFUS). The skull density ratio (SDR) is one of the most dominant predictors of thermal increase prior to treatment. However, users currently rely only on the average SDR value (SDRmean) as a screening criterion, although some patients with low SDRmean values can achieve sufficient thermal increase. The present study aimed to examine the numerical distribution of SDR values across 1024 elements to identify more precise predictors of thermal increase during MRgFUS.

METHODS

The authors retrospectively analyzed the correlations between the skull parameters and the maximum temperature achieved during unilateral ventral intermediate nucleus thalamotomy with MRgFUS in a cohort of 55 patients. In addition, the numerical distribution of SDR values was quantified across 1024 elements by using the skewness, kurtosis, entropy, and uniformity of the SDR histogram. Next, the authors evaluated the correlation between the aforementioned indices and a peak temperature > 55°C by using univariate and multivariate logistic regression analyses. Receiver operating characteristic curve analysis was performed to compare the predictive ability of the indices. The diagnostic performance of significant factors was also assessed.

RESULTS

The SDR skewness (SDRskewness) was identified as a significant predictor of thermal increase in the univariate and multivariate logistic regression analyses (p < 0.001, p = 0.013). Moreover, the receiver operating characteristic curve analysis indicated that the SDRskewness exhibited a better predictive ability than the SDRmean, with area under the curve values of 0.847 and 0.784, respectively.

CONCLUSIONS

The SDRskewness is a more accurate predictor of thermal increase than the conventional SDRmean. The authors suggest setting the SDRskewness cutoff value to 0.68. SDRskewness may allow for the inclusion of treatable patients with essential tremor who would have been screened out based on the SDRmean exclusion criterion.

Factors Associated with Energy Efficiency of Focused Ultrasound Through the Skull: A Study of 3D-Printed Skull Phantoms and Its Comparison with Clinical Experiences

While focused ultrasound (FUS) is non-invasive, the ultrasound energy is attenuated by the skull which results in differences in energy efficiency among patients. In this study, we investigated the effect of skull variables on the energy efficiency of FUS. The thickness and density of the skull and proportion of the trabecular bone were selected as factors that could affect ultrasound energy transmittance. Sixteen 3D-printed skull models were designed and fabricated to reflect the three factors. The energy of each phantom was measured using an ultrasonic sound field energy measurement system. The thickness and proportion of trabecular bone affected the attenuation of transmitted energy. There was no difference in the density of the trabecular bone. In clinical data, the trabecular bone ratio showed a significantly greater correlation with dose/delivered energy than that of thickness and the skull density ratio. Currently, for clinical non-thermal FUS, the data are not sufficient, but we believe that the results of this study will be helpful in selecting patients and appropriate parameters for FUS treatment.

Technical and operative factors affecting magnetic resonance imaging-guided focused ultrasound thalamotomy for essential tremor: experience from 250 treatments

OBJECTIVE

Magnetic resonance imaging-guided focused ultrasound (MRgFUS) provides real-time monitoring of patients to assess tremor control and document any adverse effects. MRgFUS of the ventral intermediate nucleus (VIM) of the thalamus has become an effective treatment option for medically intractable essential tremor (ET). The aim of this study was to analyze the correlations of clinical and technical parameters with 12-month outcomes after unilateral MRgFUS thalamotomy for ET to help guide future clinical treatments.

METHODS

From October 2013 to January 2019, data on unilateral MRgFUS thalamotomy from the original pivotal study and continued-access studies from three different geographic regions were collected. Authors of the present study retrospectively reviewed those data and evaluated the efficacy of the procedure on the basis of improvement in the Clinical Rating Scale for Tremor (CRST) subscore at 1 year posttreatment. Safety was based on the rates of moderate and severe thalamotomy-related adverse events. Treatment outcomes in relation to various patient- and sonication-related parameters were analyzed in a large cohort of patients with ET.

RESULTS

In total, 250 patients were included in the present analysis. Improvement was sustained throughout the 12-month follow-up period, and 184 (73.6%) of 250 patients had minimal or no disability due to tremor (CRST subscore < 10) at the 12-month follow-up. Younger age and higher focal temperature (Tmax) correlated with tremor improvement in the multivariate analysis (OR 0.948, p = 0.013; OR 1.188, p = 0.025; respectively). However, no single statistically significant factor correlated with Tmax in the multivariate analysis. The cutoff value of Tmax in predicting a CRST subscore < 10 was 55.8°C. Skull density ratio (SDR) was positively correlated with heating efficiency (β = 0.005, p < 0.001), but no significant relationship with tremor improvement was observed. In the low-temperature group, 1-3 repetitions to the right target with 52°C ≤ Tmax ≤ 54°C was sufficient to generate sustained tremor suppression within the investigated follow-up period. The high-temperature group had a higher rate of balance disturbances than the low-temperature group (p = 0.04).

CONCLUSIONS

The authors analyzed the data of 250 patients with the aim of improving practices for patient screening and determining treatment endpoints. These results may improve the safety, efficacy, and efficiency of MRgFUS thalamotomy for ET.

Correction of the skull density ratio for transcranial MRI-guided focused ultrasound thalamotomy: clinical significance of predicting therapeutic temperature

OBJECTIVE

In transcranial magnetic resonance imaging-guided focused ultrasound (TcMRgFUS), a high skull density ratio (SDR) is advantageous to achieve a sufficiently high temperature at the target. However, it is not easy to estimate the temperature rise because the SDR shows different values depending on the reconstruction filter used. The resolution characteristic of a computed tomography (CT) image depends on a modulation transfer function (MTF) defined by the reconstruction filter. Differences in MTF induce unstable SDRs. The purpose of this study was both to standardize SDR by developing a method to correct the MTF and to enable effective patient screening prior to TcMRgFUS treatment and more accurate predictions of focal temperature.

METHODS

CT images of a skull phantom and five subjects were obtained using eight different reconstruction filters. A frequency filter (FF) was calculated using the MTF of each reconstruction filter, and the validity of SDR standardization was evaluated by comparing the variation in SDR before and after FF correction. Subsequently, FF processing was similarly performed using the CT images of 18 patients who had undergone TcMRgFUS, and statistical analyses were performed comparing the relationship between the SDRs before and after correction and the maximum temperature in the target during TcMRgFUS treatment.

RESULTS

The FF was calculated for each reconstruction filter based on one manufacturer's BONE filter. In the CT images of the skull phantom, the SDR before FF correction with five of the other seven reconstruction filters was significantly smaller than that with the BONE filter (p < 0.01). After FF correction, however, a significant difference was recognized under only one condition. In the CT images of the five subjects, variation of the SDR due to imaging conditions was significantly improved after the FF correction. In 18 cases treated with TcMRgFUS, there was no correlation between SDR before FF correction and maximum temperature (rs = 0.31, p > 0.05); however, a strong positive correlation was observed after FF correction (rs = 0.71, p < 0.01).

CONCLUSIONS

After FF correction, the difference in SDR due to the reconstruction filter used is smaller, and the correlation with temperature is stronger. Therefore, the SDR can be standardized by applying the FF, and the maximum temperature during treatment may be predicted more accurately.

Prognostic value of brain tissues' volumes in patients with essential tremor treated with MRgFUS thalamotomy

MRgFUS Vim thalamotomy is a novel, effective, minimally invasive therapeutic option for patients with essential tremor (ET). Among the selection criteria, some parameters related to the patient's anatomy, such as the skull density ratio (SDR), are well recognized. The role of brain tissue interposed between the target and the ultrasound transducers has never been explored. Therefore, the purpose of our study was to evaluate the correlation and the possible predictive value between brain tissue volumes (grey matter - GM, white matter - WM, and cerebrospinal fluid - CSF) and several treatment-related variables (periprocedural parameters, MRI imaging findings, and the clinical outcome). We analysed data from thirty ET patients previously submitted to MRgFUS thalamotomy. Pre-treatment images were automatically segmented in sopra-tentorial (ST) WM, GM, and CSF using SPM 12. The most significant findings were a positive correlation of the ST-GM with the Accumulated Thermal Dose (ATD) (p < 0,001) and a negative correlation of the ATD temperature with ST-CSF and ST-TIV (p < 0,001). Ultrasound propagation speed is lower in fluids than brain tissues. Also, WM has an attenuation rate of 1.5 higher than the GM. Therefore, the difference in the ATD may be explained by the different acoustic properties of normal brain tissues interposed between the transducers and the VIM.

Comprehensive Evaluation of Factors Affecting Tremor Relapse after MRgFUS Thalamotomy: A Case-Control Study

OBJECTIVE

To identify possible relevant factors contributing to tremor relapse after MRgFUS thalamotomy in patients with essential tremor (ET) and Parkinson's disease (PD).

METHODS

We identified patients with tremor relapse from a series of 79 treatments in a single institution. The demographic and clinical characteristics of the study group patients were compared to those of patients who did not relapse in the same follow-up period. Imaging and procedural factors were compared using a control group matched for clinical and demographic characteristics.

RESULTS

Concerning clinical and demographic characteristics, we did not find statistically significant differences in gender and age. Seventy-three percent of patients with tremor relapse were Parkinson's disease patients. Using MRI, we found larger thalamotomy lesions at the 1-year follow-up in the control group with stable outcomes, compared to patients with tremor relapse. In the tractography evaluation, we found a more frequent eccentric position of the DRTt in patients with tremor relapse.

CONCLUSIONS

The most relevant determining factors for tremor relapse after MRgFUS thalamotomy appear to be tremor from Parkinson's disease and inaccurate thalamic targeting. Size of the thalamotomy lesion can also influence the outcome of treatment.

Focused ultrasound for functional neurosurgery

INTRODUCTION

Brain lesioning is a fundamental technique in the functional neurosurgery world. It has been investigated for decades and presented promising results long before novel pharmacological agents were introduced to treat movement disorders, psychiatric disorders, pain, and epilepsy. Ablative procedures were replaced by effective drugs during the 1950s and by Deep Brain Stimulation (DBS) in the 1990s as a reversible neuromodulation technique. In the last decade, however, the popularity of brain lesioning has increased again with the introduction of magnetic resonance-guided focused ultrasound (MRgFUS).

OBJECTIVE

In this review, we will cover the current and emerging role of MRgFUS in functional neurosurgery.

METHODS

Literature review from PubMed and compilation.

RESULTS

Investigated since 1930, MRgFUS is a technology enabling targeted energy delivery at the convergence of mechanical sound waves. Based on technological advancements in phased array ultrasound transducers, algorithms accounting for skull penetration by sound waves, and MR imaging for targeting and thermometry, MRgFUS is capable of brain lesioning with sub-millimeter precision and can be used in a variety of clinical indications.

CONCLUSION

MRgFUS is a promising technology evolving as a dominant tool in different functional neurosurgery procedures in movement disorders, psychiatric disorders, epilepsy, among others.

Focused Ultrasound Thalamotomy for the Treatment of Essential Tremor: A 2-Year Outcome Study of Chinese People

Background: Essential tremor (ET) is a common movement disorder among elderly individuals worldwide and is occasionally associated with a high risk for mild cognitive impairment and dementia. This retrospective study aimed to determine the clinical outcome of unilateral magnetic resonance-guided focused ultrasound (MRgFUS) thalamotomy in Chinese patients with ET. Methods: In total, 31 male and 17 female patients with drug-refractory ET were enrolled in this research study from January 2017 to September 2019. The severity of tremor and disability were assessed using the Clinical Rating Scale for Tremor (CRST) within a 2-year follow-up period. Results: The mean age of the participants was 59.14 ± 13.5 years. The mean skull density ratio (SDR) was 0.5 ± 0.1. The mean highest temperature was 57.0 ± 2.4°C. The mean number of sonications was 10.0 ± 2.6. The average maximum energy was 19,710.5 ± 8,624.9 J. The total CRST scores and sub-scores after MRgFUS thalamotomy significantly reduced during each follow-up (p < 0.001). All but four (8.3%) of the patients had reversible adverse events (AEs) after the procedure. Conclusions: MRgFUS had sustained clinical efficacy 2 years after treatment for intractable ET. Only few patients presented with thalamotomy-related AEs including numbness, weakness, and ataxia for an extended period. Most Chinese patients were treated safely and effectively despite their low SDR.

Device profile of exAblate Neuro 4000, the leading system for brain magnetic resonance guided focused ultrasound technology: an overview of its safety and efficacy in the treatment of medically refractory essential tremor

Introduction: Magnetic Resonance guided Focused UltraSound (MRgFUS) is an emerging technique that utilizes multiple high-energy low-frequency ultrasound beams generated from a multi-element transducer focused onto a single site to cause thermal ablation of the target tissue. The ExAblate Neuro 4000 system is the leading MRgFUS brain system, performing targeted thermal ablation on specific nuclei in the brain. Its precision targeting opens up new and exciting possibilities for future treatments of a wide range of neurological diseases.  Areas covered: This article aims to introduce the non-expert reader (clinician and non-clinicians) to the role of the ExAblate Neuro 4000 System in brain MRgFUS. The current clinical uses of the ExAblate system in the brain are explored with a particular focus on Essential Tremor, where internationally there is most experience, this includes reference to current literature. The safety and efficacy of MRgFUS treatments are explored and the challenges the ExAblate system must overcome to balance these juxtaposed outcomes.Expert opinion: We describe the hopes for future clinical uses of the ExAblate Neuro 4000 system to treat neurological disease and consider further advancements in MRgFUS transducer technology that may open up new exciting frontiers within the brain.

Magnetic resonance-guided focused ultrasound treatment for essential tremor shows sustained efficacy: a meta-analysis

Although magnetic resonance-guided focused ultrasound (MRgFUS) is a viable treatment option for essential tremor, some studies note a diminished treatment benefit over time. A PubMed search was performed adhering to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Studies were included if hand tremor scores (HTS), total Clinical Rating Scale for Tremor (CRST) scores, or Quality of Life in Essential Tremor Questionnaire (QUEST) scores at regular intervals following MRgFUS treatment for essential tremor were documented. Data analyses included a random effects model of meta-analysis and mixed-effects model of meta-regression. Twenty-one articles reporting HTS for 395 patients were included. Mean pre-operative HTS was 19.2 ± 5.0. Mean HTS at 3 months post-treatment was 7.4 ± 5.0 (61.5% improvement, p < 0.001). Treatment effect was mildly decreased at 36 months at 9.1 ± 5.4 (8.8% reduction). Meta-regression of time since treatment as a modifier of HTS revealed a downward trend in effect size, though this was not statistically significant (p = 0.208). Only 4 studies included follow-up ≥ 24 months. Thirteen included articles reported total CRST scores with standardized follow-up for 250 patients. Mean pre-operative total CRST score decreased by 46.2% at 3 months post-treatment (p < 0.001). Additionally, mean QUEST scores at 3 months post-treatment significantly improved compared to baseline (p < 0.001). HTS is significantly improved from baseline ≥ 24 months post-treatment and possibly ≥ 48 months post-treatment. There is a current paucity of long-term CRST and QUEST score reporting in the literature.

Outcome and Complications of MR Guided Focused Ultrasound for Essential Tremor: A Systematic Review and Meta-Analysis

Background: Magnetic resonance guided focused ultrasound (MRgFUS) is a relatively novel technique to treat essential tremor (ET). The objective of this review was to analyze the efficacy and the safety profile of MRgFUS for ET.

Methods: A systematic literature review was done. The post procedure changes in the Clinical Rating Scale for Tremor (CRST) score, hand score, disability and quality of life scores were analyzed.

Results: We found 29 studies evaluating 617 patients. DTI based targeting was utilized in six cohorts. A significant difference was observed in the pooled standard mean difference between the pre and postoperative total CRST score (p-value < 0.001 and 0.0002), hand score (p-value 0.03 and 0.02); and the disability at 12 months (p-value 0.01). Head pain and dizziness were the most in procedure complications. The immediate pooled proportion of ataxia was 50%, while it was 20% for sensory complications, which, respectively, declined to 31 and 13% on long term follow up. A significant reduction (p = 0.03) in immediate ataxia related complications was seen with DTI targeting.

Conclusion: MRgFUS for ET seems to be an effective procedure for relieving unilateral tremor. Use of DTI based targeting revealed a significant reduction in post procedure ataxia related complications as compared to traditional targeting techniques. Analysis of other complications further revealed a decreasing trend on follow up.

Safety and Efficacy of Magnetic Resonance-Guided Focused Ultrasound Surgery With Autofocusing Echo Imaging

Objective

Magnetic resonance-guided focused ultrasound surgery (MRgFUS) lesioning is a new treatment for brain disorders. However, the skull is a major barrier of ultrasound sonication in MRgFUS because it has an irregular surface and varies its size and shape among individuals. We recently developed the concept of skull density ratio (SDR) to select candidates for MRgFUS from among patients with essential tremor (ET). However, SDR is not the only factor contributing to successful MRgFUS lesioning treatment-refining the target through exact measurement of the ultrasonic echo in the transducer also improves treatment efficacy. In the present study, we carried out MRgFUS lesioning using an autofocusing echo imaging technique. We aimed to evaluate the safety and efficacy of this new approach, especially in patients with low SDR in whom previous focusing methods have failed.

Methods

From December 2019 to March 2020, we recruited 10 patients with ET or Parkinson's disease (PD) who had a low SDR. Two patients dropped out of the trial due to the screening failure of other medical diseases. In total, eight patients were included: six with ET who underwent MRgFUS thalamotomy and two with PD who underwent MRgFUS pallidotomy. The autofocusing echo imaging technique was used in all cases.

Results

The mean SDR of the patients with ET was 0.34 (range: 0.29-0.39), while that of the patients with PD was 0.41 (range: 0.38-0.44). The mean skull volume of patients with ET was 280.57 cm³ (range: 227-319 cm³), while that of the patients with PD was 287.13 cm³ (range: 271-303 cm³). During MRgFUS, a mean of 15 sonications were performed, among which a mean of 5.63 used the autofocusing technique. The mean maximal temperature (Tmax) achieved was 55.88°C (range: 52-59°C), while the mean energy delivered was 34.75 kJ (range: 20-42 kJ) among all patients. No serious adverse events occurred during or after treatment. Tmax or sonication factors (skull volume, SDR, sonication number, autofocusing score, similarity score, energy range, and power) were not correlated with autofocusing technique (p > 0.05, autofocusing score showed a p-value of 0.071).

Conclusion

Using autofocusing echo imaging lesioning, a safe and efficient MRgFUS treatment, is available even for patients with a low SDR. Therefore, the indications for MRgFUS lesioning could be expanded to include patients with ET who have an SDR < 0.4 and those with PD who have an SDR < 0.45.

Clinical Trial Registration

clinicaltrials.gov, identifier: NCT03935581.

Magnetic Resonance-Guided Focused Ultrasound Surgery for Obsessive-Compulsive Disorders: Potential for use as a Novel Ablative Surgical Technique

Surgical treatment for psychiatric disorders, such as obsessive-compulsive disorder (OCD) and depression, using ablative techniques, such as cingulotomy and capsulotomy, have historically been controversial for a number of scientific, social, and ethical reasons. Recently, with the elucidation of anatomical and neurochemical substrates of brain function in healthy controls and patients with such disorders using various functional neuroimaging techniques, these criticisms are becoming less valid. Furthermore, by using new techniques, such as deep brain stimulation (DBS), and identifying more precise targets, beneficial effects and the lack of serious complications have been demonstrated in patients with psychiatric disorders. However, DBS also has many disadvantages. Currently, magnetic resonance-guided focused ultrasound surgery (MRgFUS) is used as a minimal-invasive surgical method for generating precisely placed focal thermal lesions in the brain. Here, we review surgical techniques and their potential complications, along with anterior limb of the internal capsule (ALIC) capsulotomy by radiofrequency lesioning and gamma knife radiosurgery, for the treatment of OCD and depression. We also discuss the limitations and technical issues related to ALIC capsulotomy with MRgFUS for medically refractory OCD and depression. Through this review we hope MRgFUS could be considered as a new treatment choice for refractory OCD.

Factors Associated with Heating Efficiency in Transcranial Focused Ultrasound Therapy

Transcranial magnetic resonance-guided focused ultrasound (FUS) therapy is a less invasive stereotactic treatment for tremor and other movement disorders. A sufficiently high temperature in the target brain tissue is crucial during ablation procedures for good outcomes. Therefore, maximizing the heating efficiency is critical in cases where high temperature cannot be achieved because of patient-related characteristics. However, a strategy to achieve the desired therapeutic temperature with FUS has not yet been established. This study aimed to investigate the procedural factors associated with heating efficiency in FUS. We retrospectively reviewed and analyzed data from patients who underwent FUS for ventralis intermedius (VIM) nucleus thalamotomy. In all, 30 consecutive patients were enrolled. 18 with essential tremor (ET), 11 with tremor-dominant Parkinson’s disease (TDPD), and 1 with Holmes tremor. Multivariate regression analysis showed that decline in heating efficiency was associated with lower skull density ratio (SDR) and a greater subtotal rise in temperature until the previous sonication. To maximize heating efficiency, the temperature increase should be set to the least value in the target alignment and verification phases, and subsequently should be increased sufficiently in the treatment phase. This strategy may be particularly beneficial in cases where high ablation temperatures cannot be achieved because of patient-related characteristics. Importantly, a broad patient population would benefit from this strategy as it could reduce the need for high energy to achieve therapeutic temperatures, thereby decreasing the risks of adverse events.

Attenuation of the human skull at broadband frequencies by using a carbon nanotube composite photoacoustic transducer

The shockwave generated from a focused carbon nanotube (CNT) composite photoacoustic transducer has a wide frequency band that reaches several MHz in a single pulse. The objective of this study was to measure the transmission characteristics of a shockwave generated by a CNT composite photoacoustic transducer through Asian skulls and compare the results with numerical simulation ones. Three Korean cadaver skulls were used, and five sites were measured for each skull. The average densities and sound speeds of the three skulls were calculated from computed tomography images. The sound pressure after skull penetration was about 11% of the one before skull penetration. High-frequency energy was mostly attenuated. The average attenuation coefficients measured at the five sites of the three skulls were 3.59 ± 0.29, 5.99 ± 1.07, and 3.90 ± 0.86 np/cm/MHz. These values were higher than those previously measured at 270, 836, and 1402 kHz from other groups. The attenuation coefficients simulated by Sim4life were slightly smaller than the experimental values, with similar trends at most sites. The attenuation coefficients varied with measurement sites, skull shape, and thickness. These results may provide important data for future applications of shockwaves in noninvasive neurological treatments.