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

Transcranial Focused Ultrasound: A History of Our Future

The history of focused ultrasound is a parallel history of neuroradiology, functional neurosurgery, and physics and engineering. Multiple pioneers collaborated as ultrasound transitioned from a wartime technology to a therapeutic one, particularly in using it to ablate the brain to treat movement disorders. Several competing technologies ensured that this "ultrasonic neurosurgery" remained in a lull. An algorithm and other advancements that obviated a craniectomy for ultrasonic neurosurgery allowed magnetic resonance-guided focused ultrasound to flourish to its modern phase.

A Wearable, Steerable, Transcranial Low-Intensity Focused Ultrasound System

OBJECTIVES

Transcranial low-intensity focused ultrasound (LIFU) offers unique opportunities for precisely neuromodulating small and/or deep targets within the human brain, which may be useful for treating psychiatric and neurological disorders. This article presents a novel ultrasound system that delivers focused ultrasound through the forehead to anterior brain targets and evaluates its safety and usability in a volunteer study.

METHODS

The ultrasound system and workflow are described, including neuronavigation, LIFU planning, and ultrasound delivery components. Its capabilities are analyzed through simulations and experiments in water to establish its safe steering range. A cohort of 20 healthy volunteers received a LIFU protocol aimed at the anterior medial prefrontal cortex (amPFC), using imaging and questionnaires to screen for adverse effects. Additional development after the study also analyzes the effect of the skull and sinus cavities on delivered ultrasound energy.

RESULTS

Simulations and hydrophone readings agreed with <5% error, and the safe steering range was found to encompass a 1.8 cm × 2.5 cm × 2 cm volume centered at a depth 5 cm from the surface of the skin. There were no adverse effects evident on qualitative assessments, nor any signs of damage in susceptibility-weighted imaging scans. All participants tolerated the treatment well. The interface effectively enabled the users to complete the workflow with all participants. In particular, the amPFC of every participant was within the steering limits of the system. A post hoc analysis showed that "virtual fitting" could aid in steering the beams around subjects' sinuses.

CONCLUSIONS

The presented system safely delivered LIFU through the forehead while targeting the amPFC in all volunteers, and was well-tolerated. With the capabilities validated here and positive results of the study, this technology appears well-suited to explore LIFU's efficacy in clinical neuromodulation contexts.

Focused ultrasound blood-brain barrier disruption in high-grade gliomas: Scoping review of clinical studies

BACKGROUND

This scoping review aims to comprehensively review the available literature on the safety and efficacy of focused ultrasound (FUS) for blood-brain barrier disruption (BBBD) in patients with high-grade gliomas, including glioblastoma (GBM). High-grade gliomas pose significant challenges in neuro-oncology due to their aggressiveness and intricate location, often limiting the efficacy of traditional treatments. FUS offers a promising approach by transiently disrupting the blood-brain barrier, thereby facilitating enhanced drug delivery to tumor cells while minimizing systemic side effects.

METHODS

A scoping review adhering to PRISMA guidelines was conducted to explore the literature on FUS-induced BBBD in glioma patients. PubMed and Embase databases were searched from inception to April 2024 using defined keywords. Original clinical studies focusing on FUS for BBBD in gliomas were included. Two reviewers independently screened records, with conflicts resolved by a third reviewer. Data extraction and quality assessment were performed accordingly.

RESULTS

A total of 1,310 studies were initially identified, resulting in nine eligible studies after screening and selection. These studies, published between 2016 and 2024, included 106 patients (39.6 % female) with ages ranging from 29 to 80 years. Recurrent GBM was the most common diagnosis (100 patients), with other diagnoses including anaplastic astrocytoma, diffuse infiltrating glioma, and oligodendroglioma. Various FUS devices and microbubble contrast agents were employed across the studies. Safety and efficacy were assessed in both experimental and clinical settings, with no significant adverse events reported during BBBD procedures. Notably, BBBD facilitated enhanced drug delivery to tumor tissue, demonstrating potential therapeutic benefits.

CONCLUSION

Studies investigating BBBD using FUS demonstrate promising outcomes in experimental and clinical settings. BBBD procedures in patients with malignant gliomas and recurrent GBM show safety and successful enhancement of drug delivery potential. Overall, FUS-mediated BBBD emerges as a safe and feasible approach for improving therapeutic outcomes in brain tumor patients, warranting further clinical exploration and optimization.

Translating ultrasound-mediated drug delivery technologies for CNS applications

Ultrasound enhances drug delivery into the central nervous system (CNS) by opening barriers between the blood and CNS and by triggering release of drugs from carriers. A key challenge in translating setups from in vitro to in vivo settings is achieving equivalent acoustic energy delivery. Multiple devices have now been demonstrated to focus ultrasound to the brain, with concepts emerging to also target the spinal cord. Clinical trials to date have used ultrasound to facilitate the opening of the blood-brain barrier. While most have focused on feasibility and safety considerations, therapeutic benefits are beginning to emerge. To advance translation of these technologies for CNS applications, researchers should standardise exposure protocol and fine-tune ultrasound parameters. Computational modelling should be increasingly used as a core component to develop both in vitro and in vivo setups for delivering accurate and reproducible ultrasound to the CNS. This field holds promise for transformative advancements in the management and pharmacological treatment of complex and challenging CNS disorders.

Magnetically navigated microbubbles coated with albumin/polyarginine and superparamagnetic iron oxide nanoparticles

While microbubbles (MB) are routinely used for ultrasound (US) imaging, magnetic MB are increasingly explored as they can be guided to specific sites of interest by applied magnetic field gradient. This requires the MB shell composition tuning to prolong MB stability and provide functionalization capabilities with magnetic nanoparticles. Hence, we developed air-filled MB stabilized by a protein-polymer complex of bovine serum albumin (BSA) and poly-L-arginine (pArg) of different molecular weights, showing that pArg of moderate molecular weight distribution (15-70 kDa) enabled MB with greater stability and acoustic response while preserving MB narrow diameters and the relative viability of THP-1 cells after 48 h of incubation. After MB functionalization with superparamagnetic iron oxide nanoparticles (SPION), magnetic moment values provided by single MB confirmed the sufficient SPION deposition onto BSA + pArg MB shells. During MB magnetic navigation in a blood vessel mimicking phantom with magnetic tweezers and in a Petri dish with adherent mouse renal carcinoma cell line, we demonstrated the effectiveness of magnetic MB localization in the desired area by magnetic field gradient. Magnetic MB co-localization with cells was further exploited for effective doxorubicin delivery with drug-loaded MB. Taken together, these findings open new avenues in control over albumin MB properties and magnetic navigation of SPION-loaded MB, which can envisage their applications in diagnostic and therapeutic needs.

The evolution of ventral intermediate nucleus targeting in MRI-guided focused ultrasound thalamotomy for essential tremor: an international multi-center evaluation

Background

The ventral intermediate nucleus (VIM) is the premiere target in magnetic resonance-guided focused ultrasound (MRgFUS) thalamotomy for tremor; however, there is no consensus on the optimal coordinates for ablation. This study aims to ascertain the various international VIM targeting approaches (VIM-TA) and any evolution in practice.

Methods

International MRgFUS centers were invited to share VIM-TAs in 2019 and 2021. Analyses of any modification in practice and of anatomical markers and/or tractography in use were carried out. Each VIM-TA was mapped in relation to the mid-commissural point onto a 3D thalamic nucleus model created from the Schaltenbrand-Wahren atlas.

Results

Of the 39 centers invited, 30 participated across the study period, providing VIM-TAs from 26 centers in 2019 and 23 in 2021. The results are reported as percentages of the number of participating centers in that year. In 2019 and 2021, respectively, 96.2% (n = 25) and 95.7% (n = 22) of centers based their targeting on anatomical landmarks rather than tractography. Increased adoption of tractography in clinical practice and/or for research was noted, changing from 34.6% to 78.3%. There was a statistically significant change in VIM-TAs in the superior-inferior plane across the study period; the percentage of VIM-TAs positioned 2 mm above the intercommissural line (ICL) increased from 16.0% in 2019 to 40.9% in 2021 (WRST, p < 0.05). This position is mapped at the center of VIM on the 3D thalamic model created based on the Schaltenbrand-Wahren atlas. In contrast, the VIM-TA medial-lateral and anterior-posterior positions remained stable. In 2022, 63.3% of participating centers provided the rationale for their VIM-TAs and key demographics. The centers were more likely to target 2 mm above the ICL if they had increased experience (more than 100 treatments) and/or if they were North American.

Conclusion

Across the study period, FUS centers have evolved their VIM targeting superiorly to target the center of the VIM (2 mm above the ICL) and increased the adoption of tractography to aid VIM localization. This phenomenon is observed across autonomous international centers, suggesting that it is a more optimal site for FUS thalamotomy in tremors.

'Am I fixed, am I better now?': undergoing MR-guided focused ultrasound for essential tremor: an interpretative phenomenological analysis

Introduction

Essential tremor (ET) is characterised by postural and intentional tremor typically affecting the upper limbs, which can negatively impact functionality and quality of life. Magnetic Resonance-guided Focused Ultrasound (MRgFUS) is a novel and promising non-invasive treatment for ET which offers instantaneous results.

Methods

Using interpretative phenomenological analysis we explored the experience of undergoing MRgFUS in six ET patients as well as their experiences pre- and post-procedure.

Results

One-time, retrospective semi-structured interviews were conducted and six themes emerged: Life pre-treatment: "It's everyday tasks that get you down" and "Most people who understand, they are okay. Some people aren't"; MRgFUS: Treatment day: "Going into the unknown" and "There's no way I was going to press that button"; and Life post-treatment: "One is good. Two is better" and "Am I fixed, am I better now?."

Discussion

The findings point to a significant period of adjustment associated with living with ET and the effects of undergoing ET MRgFUS treatment. As ET progressed, participants struggled to cope with increasing symptoms and had to develop coping strategies to manage life with ET. The procedure itself was perceived as strange and extraordinary and despite some immediate adverse effects participants were determined to go through with it. Post procedure, all participants reported tremor suppression which was life changing. While some participants still felt burdened by ET, others expressed it took them a while to psychologically adjust to what essentially was their new body. This study has highlighted the need for patients to be supported at all stages of their ET journey.

Targeting diffuse midline gliomas: The promise of focused ultrasound-mediated blood-brain barrier opening

Diffuse midline gliomas (DMGs), including diffuse intrinsic pontine glioma, have among the highest mortality rates of all childhood cancers, despite recent advancements in cancer therapeutics. This is partly because, unlike some CNS tumors, the blood-brain barrier (BBB) of DMG tumor vessels remains intact. The BBB prevents the permeation of many molecular therapies into the brain parenchyma, where the cancer cells reside. Focused ultrasound (FUS) with microbubbles has recently emerged as an innovative and exciting technology that non-invasively permeabilizes the BBB in a small focal region with millimeter precision. In this review, current treatment methods and biological barriers to treating DMGs are discussed. State-of-the-art FUS-mediated BBB opening is then examined, with a focus on the effects of various ultrasound parameters and the treatment of DMGs.

Sonodynamic Therapy and Sonosensitizers for Glioma Treatment: A Systematic Qualitative Review

Sonodynamic therapy (SDT) has emerged as an encouraging noninvasive technique that uses ultrasound to activate targeted agents to induce antitumor effects for the treatment of glioma. With extensive variation in the types of sonosensitizers, protocols for sonication, and model systems, a comprehensive overview of existing preclinical data on the efficacy of SDT in glioma treatment is warranted. Here, we conduct a systematic review of preclinical and early clinical literature on implementing SDT to treat in vitro and in vivo models of glioma. Our findings suggest that coupling sonosensitizers such as 5-aminolevulinic acid, hematoporphyrin monomethyl ether, and sinoporphyrin sodium with focused ultrasound induces robust cytotoxic activity in tumor cells (in vitro and in vivo). These effects are likely mediated by the oxidative stress induced by reactive oxygen species production, apoptotic signaling cascades, and intracellular calcium overload. Future research is needed to better understand the biochemical and mechanistic properties of SDT, and ongoing trials may help elucidate the clinical feasibility of glioma treatment with optimized sonically activated treatments.

Influence of focused ultrasound on locoregional drug delivery to the brain: Potential implications for brain tumor therapy

INTRODUCTION

Efficient delivery of therapeutics across the blood-brain barrier (BBB) for the treatment of central nervous system (CNS) tumors is a major challenge to the development of safe and efficacious therapies. Locoregional drug delivery platforms offer an improved therapeutic index by achieving high drug concentrations in the target tissue with negligible systemic exposure. Intrathecal (intraventricular) [IT] and convection-enhanced delivery [CED] are two clinically relevant methods being employed for various CNS malignancies. Both of these standalone platforms suffer from passive post-administration distribution forces, sometimes limiting the desired distribution for tumor therapy. Focused ultrasound and microbubble-mediated blood-brain barrier opening (FUS-BBBO) is a recent modality used for enhanced drug delivery. It is postulated that coupling of FUS with these alternative delivery routes may provide benefits. Multimodality FUS may provide the desired ability to increase the depth of parenchymal delivery following IT administration and provide a means for contour directionality with CED. Further, the transient enhanced permeability achieved with FUS-BBBO is well established, but drug residence and transit times, important to clinical dose scheduling, have not yet been defined. The present investigation comprises two discrete studies: 1. Conduct a comprehensive quantitative evaluation to elucidate the effect of FUS-BBBO as it relates to varying routes of administration (IT and IV) in its capacity to facilitate drug penetration within the striatal-thalamic region. 2. Investigate the impact of combining FUS-BBBO with CED on drug distribution, with a specific focus on the temporal dynamics of drug retention within the target region.

METHODS

Firstly, we quantitatively assessed how FUS-BBBO coupled with IT and IV altered fluorescent dye (Dextran 2000 kDa and 70 kDa) distribution and concentration in a predetermined striatal-thalamic region in naïve mice. Secondly, we analyzed the pharmacokinetic effects of using FUS mediated BBB disruption coupled with CED by measuring the volume of distribution and time-dependent concentration of the dye.

RESULTS

Our results indicate that IV administration coupled with FUS-BBBO successfully enhances delivery of dye into the pre-defined sonication targets. Conversely, measurable dye in the sonication target was consistently less after IT administration. FUS enhances the distribution volume of dye after CED. Furthermore, a shorter time of residence was observed when CED was coupled with FUS-BBBO application when compared to CED alone.

CONCLUSION

1. Based on our findings, IV delivery coupled with FUS-BBBO is a more efficient means for delivery to deep targets (i.e. striatal-thalamic region) within a predefined spatial conformation compared to IT administration. 2. FUS-BBBO increases the volume of distribution (Vd) of dye after CED administration, but results in a shorter time of residence. Whether this finding is reproducible with other classes of agents (e.g., cytotoxic agents, antibodies, viral particles, cellular therapies) needs to be studied.

First-in-human prospective trial of sonobiopsy in high-grade glioma patients using neuronavigation-guided focused ultrasound

Sonobiopsy is an emerging technology that combines focused ultrasound (FUS) with microbubbles to enrich circulating brain disease-specific biomarkers for noninvasive molecular diagnosis of brain diseases. Here, we report the first-in-human prospective trial of sonobiopsy in high-grade glioma patients to evaluate its feasibility and safety in enriching plasma circulating tumor biomarkers. A nimble FUS device integrated with a clinical neuronavigation system was used to perform sonobiopsy following an established clinical workflow for neuronavigation. Analysis of blood samples collected before and after FUS sonication showed that sonobiopsy enriched plasma circulating tumor DNA (ctDNA), including a maximum increase of 1.6-fold for the mononucleosome cell-free DNA (cfDNA) fragments (120-280 bp), 1.9-fold for the patient-specific tumor variant ctDNA level, and 5.6-fold for the TERT mutation ctDNA level. Histological analysis of surgically resected tumors confirmed the safety of the procedure. Transcriptome analysis of sonicated and nonsonicated tumor tissues found that FUS sonication modulated cell physical structure-related genes. Only 2 out of 17,982 total detected genes related to the immune pathways were upregulated. These feasibility and safety data support the continued investigation of sonobiopsy for noninvasive molecular diagnosis of brain diseases.

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.

Loading Intracranial Drug-Eluting Reservoirs Across the Blood-Brain Barrier With Focused Ultrasound

OBJECTIVE

Efficient, sustained and long-term delivery of therapeutics to the brain remains an important challenge to treatment of diseases such as brain cancer, stroke and neurodegenerative disease. Focused ultrasound can assist movement of drugs into the brain, but frequent and long-term use has remained impractical. Single-use intracranial drug-eluting depots show promise but are limited for the treatment of chronic diseases as they cannot be refilled non-invasively. Refillable drug-eluting depots could serve as a long-term solution, but refilling is hindered by the blood-brain barrier (BBB), which prevents drug refills from accessing the brain. In this article, we describe how focused ultrasound enables non-invasive loading of intracranial drug depots in mice.

METHODS

Female CD-1 mice (n = 6) were intracranially injected with click-reactive and fluorescent molecules that are capable of anchoring in the brain. After healing, animals were treated with high-intensity focused ultrasound and microbubbles to temporarily increase the permeability of the blood-brain barrier and deliver dibenzocyclooctyne (DBCO)-Cy7. The mice were perfused, and the brains were imaged via ex vivo fluorescence imaging.

RESULTS

Fluorescence imaging indicated small molecule refills are captured by intracranial depots as long as 4 wk after administration and are retained for up to 4 wk based on fluorescence imaging. Efficient loading was dependent on both focused ultrasound and the presence of refillable depots in the brain as absence of either prevented intracranial loading.

CONCLUSION

The ability to target and retain small molecules at predetermined intracranial sites with pinpoint accuracy provides opportunities to continuously deliver drugs to the brain over weeks and months without excessive BBB opening and with minimal off-target side effects.

Sonodynamic therapy and magnetic resonance-guided focused ultrasound: new therapeutic strategy in glioblastoma

Glioblastoma (GB) is one of the most aggressive and difficult-to-treat brain tumors, with a poor prognosis and limited treatment options. In recent years, sonodynamic therapy (SDT) and magnetic resonance focused ultrasound (MRgFUS) have emerged as promising approaches for the treatment of GB. SDT uses ultrasound waves in combination with a sonosensitizer to selectively damage cancer cells, while MRgFUS delivers high-intensity ultrasound waves to precisely target tumor tissue and disrupt the blood-brain barrier to enhance drug delivery. In this review, we explore the potential of SDT as a novel therapeutic strategy for GB. We discuss the principles of SDT, its mechanisms of action, and the preclinical and clinical studies that have investigated its use in Gliomas. We also highlight the challenges, the limitations, and the future perspectives of SDT. Overall, SDT and MRgFUS hold promise as novel and potentially complementary treatment modalities for GB. Further research is needed to optimize their parameters and determine their safety and efficacy in humans, but their potential for selective and targeted tumor destruction makes them an exciting area of investigation in the field of brain cancer therapy.

First-in-human prospective trial of sonobiopsy in glioblastoma patients using neuronavigation-guided focused ultrasound

Sonobiopsy is an emerging technology that combines focused ultrasound (FUS) with microbubbles to enrich circulating brain disease-specific biomarkers for noninvasive molecular diagnosis of brain diseases. Here, we report the first-in-human prospective trial of sonobiopsy in glioblastoma patients to evaluate its feasibility and safety in enriching circulating tumor biomarkers. A nimble FUS device integrated with a clinical neuronavigation system was used to perform sonobiopsy following an established clinical workflow for neuronavigation. Analysis of blood samples collected before and after FUS sonication showed enhanced plasma circulating tumor biomarker levels. Histological analysis of surgically resected tumors confirmed the safety of the procedure. Transcriptome analysis of sonicated and unsonicated tumor tissues found that FUS sonication modulated cell physical structure-related genes but evoked minimal inflammatory response. These feasibility and safety data support the continued investigation of sonobiopsy for noninvasive molecular diagnosis of brain diseases.

MRgFUS thalamotomy for the treatment of tremor: evaluation of learning curve and operator's experience impact on the procedural and clinical outcome

BACKGROUND

MRgFUS Vim ablation is increasingly used for the treatment of tremor in ET e PD patients but there is little published research on the importance of operator experience in this procedure. This study aims to evaluate the learning curve and the influence of the operator experience on the procedural and clinical outcomes.

METHODS

We retrospectively evaluated 90 patients (38 ET, 52 PD) submitted to MRgFUS unilateral thalamotomy in the period between February 2018 and July 2020. Clinical endpoints, procedural times, and technical parameters were recorded in all procedures. Based on the time of treatment, patients were divided into three groups of 30 units each, comparing all variables between each time period group.

RESULTS

In Group A, the average patient preparation time was 120.6 min, the treatment time was 105.2 min, the number of was sonications 14.1, and the mean target shifts 3.1. In Group B, the mean preparation time was 105.5 min, the treatment time was 89.5 min, the number of sonications was 13.2, and the target shifts 3.0. Group C showed inferior values of preparation time (101.9 min), treatment time (71.7 min), numbers of sonications (10.6), and shifts (1.7). Thalamotomy-related complications occurred in 9 patients of Group A, 2 of Group B, and 5 of Group C. Tremor relapse occurred in 7 patients of Group A, 3 of Group B, and 2 of Group C. The days of hospitalization were comparable in the three groups.

CONCLUSIONS

The operators experience is associated with the improvement of clinical and procedural outcome in MRgFUS thalatomy for the treatment of ET and PD tremor.

Evaluation of Cerebral Volume Changes in Patients with Tremor Treated by MRgFUS Thalamotomy

The purpose of the study is to quantify volumetric variations of cortical and subcortical brain structures after Vim ablation using MRgFUS, and correlate them with the patients’ clinical features and treatment outcomes. For this pilot retrospective study we enrolled 31 patients with a mean age of 70.86 years who were eligible for unilateral Vim thalamotomy. Clinical evaluation included tremor severity assessment using the FTM scale and cognitive assessment using the MoCA score. MRI data were acquired with a 3T scanner, using a dedicated 32-channel coil and acquiring a volumetric sequence of T1 3D IR FSPGR (BRAVO), before treatment and one year after MRgFUS thalamotomy. Image processing and volume data extraction were conducted with dedicated software. A volumetric analysis showed a significant reduction (p < 0.05) of the left thalamus 1 year after the treatment in patients with ET. Other significant results were found on the same side in the other nuclei of the basal ganglia and in the cerebellar cortex. In confronting the two groups (ET, PD), no significant differences were found in terms of age, FTM, MoCA scores, or brain volumes. Similarly, no significant correlations were found between the FTM and MoCA scores and the brain volumes before the treatment.

Medical Device Advances in the Treatment of Glioblastoma

Despite decades of research and the growing emergence of new treatment modalities, Glioblastoma (GBM) frustratingly remains an incurable brain cancer with largely stagnant 5-year survival outcomes of around 5%. Historically, a significant challenge has been the effective delivery of anti-cancer treatment. This review aims to summarize key innovations in the field of medical devices, developed either to improve the delivery of existing treatments, for example that of chemo-radiotherapy, or provide novel treatments using devices, such as sonodynamic therapy, thermotherapy and electric field therapy. It will highlight current as well as emerging device technologies, non-invasive versus invasive approaches, and by doing so provide a detailed summary of evidence from clinical studies and trials undertaken to date. Potential limitations and current challenges are discussed whilst also highlighting the exciting potential of this developing field. It is hoped that this review will serve as a useful primer for clinicians, scientists, and engineers in the field, united by a shared goal to translate medical device innovations to help improve treatment outcomes for patients with this devastating disease.

Design and Construction of a Low-Frequency Ultrasound Acquisition Device for 2-D Brain Imaging Using Full-Waveform Inversion

The main techniques used to image the brain and obtain structural data are magnetic resonance imaging and X-ray computed tomography. These techniques produce images with high spatial resolution, but with the disadvantage of requiring very large equipment with special installation needs. In addition, X-ray tomography uses ionizing radiation, which limits their use. Ultrasound imaging is a safe technology that is delivered using compact and mobile devices. However, conventional ultrasound reconstruction techniques have failed to obtain images of the brain because of, fundamentally, the presence of the skull and the distortion that it produces on ultrasound. Recent studies have indicated that full-waveform inversion, a computational technique originally from Earth science, has the potential to generate accurate 3-D images of the brain. This technology can overcome the limitations of conventional ultrasound imaging, but a prototype for transcranial applications does not yet exist. Here, we investigate different designs of an annular array of ultrasound transducers to optimize the number of elements and rotations needed to conduct transcranial imaging with full-waveform inversion. This device uses small-diameter, low-frequency transducers that readily propagate ultrasound through the skull with good signal-to-noise ratios. It also incorporates the use of rotations to produce a high-density coverage of the target and acquire redundant traces that are beneficial for full-waveform inversion. We have built a ring of 40 transducers to illustrate that this design is capable of reconstructing images of the brain, retrieving its anatomy and acoustic properties with millimeter resolution. Laboratory results reveal the ability of this device to successfully image a 2.5-D brain- and skull-mimicking phantom using full-waveform inversion. To our knowledge, this is the first prototype ever used for transcranial-like imaging. The importance of these findings and their implications for the design of a 3-D reconstruction system with possible clinical applications are discussed.

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.