Improving tremor response to focused ultrasound thalamotomy

Tremor Severity and Operative Parameters Predict Imbalance in Patients Undergoing Focused Ultrasound Thalamotomy

BACKGROUND

Imbalance is the most commonly reported side effect following focused ultrasound (FUS) thalamotomy for essential tremor (ET). It remains unknown which patients are more likely to develop imbalance following FUS treatment.

OBJECTIVE

To identify preoperative and treatment-related sonication parameters that are predictive of imbalance following FUS treatment.

METHODS

We retrospectively collected demographic data, preoperative Fahn-Tolosa-Marin Clinical Rating Scale for Tremor (FTM) scores and FUS treatment parameters in patients undergoing FUS thalamotomy for treatment of ET. The presence of imbalance was evaluated at several discrete time-points with up to 4 years of follow-up. Multiple machine learning classifiers were built and evaluated, aiming to maximize accuracy while minimizing feature set.

RESULTS

Of the 297 patients identified, the presence of imbalance peaked at 1 week following operation at 79%. This declined rapidly with 29% reporting imbalance at 3 months, and only 15% at 4 years. At 1 week, total preoperative FTM scores and Maximum Energy delivered in FUS could predict the presence of imbalance at 92.8% accuracy. At 3 months, the total preoperative FTM scores and maximum power delivered could predict the presence of imbalance with 90.6% accuracy. Post-operative lesion size and extent into thalamic nuclei, internal capsule, and subthalamic regions were identified as likely key underlying drivers of these predictors.

CONCLUSIONS

A machine learning model based on preoperative tremor scores and maximum energy/power delivered predicted the development of short-term imbalance and long-term imbalance following FUS thalamotomy.

Quantitative Tractography-Based Evaluations in Essential Tremor Patients after MRgFUS Thalamotomy

BACKGROUND

Magnetic resonance-guided focused ultrasound (MRgFUS) targeting the thalamic ventral intermediate nucleus (VIM) is an innovative treatment for drug-refractory essential tremor (ET). The relationship between lesion characteristics, dentate-rubro-thalamic-tract (DRTT) involvement and clinical benefit remains unclear.

OBJECTIVES

To investigate whether clinical outcome is related to lesion volume and/or its overlap with the DRTT. To compare the reliability of probabilistic versus deterministic tractography in reconstructing the DRTT and improving VIM targeting.

METHODS

Forty ET patients who underwent MRgFUS thalamotomy between 2019 and 2022 were retrospectively analyzed. Clinical outcomes and adverse effects were recorded at 1/6/12 months after the procedure. The DRTT was generated using deterministic and probabilistic tractography on preoperative diffusion-tensor 3 T-images and location and volume of the lesion were calculated.

RESULTS

Probabilistic tractography identified both decussating (d-DRTT) and non-decussating (nd-DRTT) components of the DRTT, whereas the deterministic approach only identified one component overlapping with the nd-DRTT. Despite the lesions predominantly intersecting the medial portion of the d-DRTT, with a significantly greater overlap in responder patients, we observed only a non-significant correlation between tremor improvement and increased d-DRTT-lesion overlap (r = 0.22, P = 0.20). The lesion volume demonstrated a significant positive correlation with clinical improvement at 1-day MRI (r = 0.42, P < 0.01).

CONCLUSION

Variability in the reconstructed DRTT position relative to the lesion center of mass, even among good responders, suggests that this fiber bundle is unlikely to be considered the sole target for a successful MRgFUS thalamotomy in ET. Indirect individualized targeting allows for more precise and reproducible identification of actual treatment coordinates than the direct method.

Four-tract tractography: multiparametric direct targeting of the dentatorubrothalamic tract

This video article presents a case study of a 70-year-old male with medically refractory essential tremor treated with magnetic resonance-guided focused ultrasound (MRgFUS). Following an initial successful ablation of the right thalamus, the patient underwent left-sided thalamotomy. After two tractography-guided sonications, the authors observed a significant reduction in his right-hand tremor with no immediate side effects. Postprocedure evaluation revealed sustained tremor reduction with minimal side effects, showcasing bilateral MRgFUS as an effective, noninvasive option for essential tremor management. The video can be found here: https://stream.cadmore.media/r10.3171/2024.7.FOCVID2483.

2D ultrasound thermometry during thermal ablation with high-intensity focused ultrasound

The clinical use of high intensity focused ultrasound (HIFU) therapy for noninvasive tissue ablation has recently gained momentum. Guidance is provided by either magnetic resonance imaging (MRI) or conventional B-mode ultrasound imaging, each with its own advantages and disadvantages. The main limitation of ultrasound imaging is its inability to provide temperature measurements over the ranges corresponding to the target temperatures during ablative thermal therapies (between 55 °C and 70 °C). Here, variations in ultrasound backscattered energy (ΔBSE) were used to monitor temperature increases in liver tissue up to an absolute value of 90 °C during and after HIFU treatment. In vitro experimental measurements were performed in 47 bovine liver samples using a toroidal HIFU transducer operating at 2.5 MHz to increase the temperature of tissues. An ultrasound imaging probe working at 7.5 MHz was placed in the center of the HIFU transducer to monitor the backscattered signals. The free-field acoustic power was set to 9 W, 12 W or 16 W in the different experiments. HIFU sonications were performed for 240 s using a duty cycle of 83 % to allow ultrasound imaging and raw radiofrequency data acquisition during exposures. Measurements showed a linear relationship between ΔBSE (in dB) and temperature (r = 0.94, p < 0.001) over a temperature range from 37 °C to 90 °C, with a high reliability of temperature measurements below 75 °C. Monitoring can be performed at the frame rate of ultrasound imaging scanners with an accuracy within an acceptable threshold of 5 °C, given the temperatures targeted during thermal ablations. If the maximum temperature reached is below 70 °C, ΔBSE is also a reliable approach for estimating the temperature during cooling. Histological analysis shown the impact of the treatment on the spatial arrangement of cells that can explain the observed variation of ΔBSE. These results demonstrate the ability of ΔBSE measurements to estimate temperature in ultrasound images within an effective therapeutic range. This method can be implemented clinically and potentially applied to other thermal-based therapies.

Magnetic Resonance-Guided Focused Ultrasound for Treatment of Essential Tremor: Ventral Intermediate Nucleus Ablation Alone or Additional Posterior Subthalamic Area Lesioning?

BACKGROUND

Magnetic resonance-guided focused ultrasound (MRgFUS) for treatment of essential tremor (ET) traditionally targets the ventral intermediate (Vim) nucleus. Recent strategies include a secondary lesion to the posterior subthalamic area (PSA).

OBJECTIVE

The aim was to compare lesion characteristics, tremor improvement, and adverse events (AE) between patients in whom satisfactory tremor suppression was achieved with lesioning of the Vim alone and patients who required additional lesioning of the PSA.

METHODS

Retrospective analysis of data collected from ET patients treated with MRgFUS at St Vincent's Hospital Sydney was performed. Clinical Rating Scale for Tremor (CRST), hand tremor score (HTS), and Quality of Life in Essential Tremor Questionnaire (QUEST) were collected pre- and posttreatment in addition to the prevalence of AEs. The lesion coordinates and overlap with the dentatorubrothalamic tract (DRTT) were evaluated using magnetic resonance imaging.

RESULTS

Twenty-one patients were treated in Vim only, and 14 were treated with dual Vim-PSA lesions. Clinical data were available for 29 of the 35 patients (19 single target and 10 dual target). At follow-up (mean: 18.80 months) HTS, CRST, and QUEST in single-target patients improved by 57.97% (P < 0.001), 36.71% (P < 0.001), and 58.26% (P < 0.001), whereas dual-target patients improved by 68.34% (P < 0.001), 35.37% (P < 0.003), and 46.97% (P < 0.005), respectively. The Vim lesion of dual-target patients was further anterior relative to the posterior commissure (PC) (7.84 mm), compared with single-target patients (6.92 mm), with less DRTT involvement (14.85% vs. 23.21%). Dual-target patients exhibited a greater proportion of patients with acute motor AEs (100% vs. 58%); however, motor AE prevalence was similar in both groups at long-term follow-up (33% vs. 38%).

CONCLUSION

Posterior placement of lesions targeting the Vim may confer greater tremor suppression. The addition of a PSA lesion, in patients with inadequate tremor control despite Vim lesioning, had a trend toward better long-term tremor suppression; however, this approach was associated with greater prevalence of gait disturbance in the short term.

MR-guided focused ultrasound in movement disorders and beyond: Lessons learned and new frontiers

The development of MR-guided focused ultrasound (MRgFUS) has provided a new therapeutic tool for neuropsychiatric disorders. In contrast to previously available neurosurgical techniques, MRgFUS allows precise impact on deep brain structures without the need for incision and yields an immediate effect. In its high-intensity modality (MRgHIFU), it produces accurate therapeutic thermoablation in previously selected brain targets. Importantly, the production of the lesion is progressive and highly controlled in real-time by both neuroimaging and clinical means. MRgHIFU ablation is already an accepted and widely used treatment for medically-refractory Parkinson's disease and essential tremor. Notably, other neurological disorders and diverse brain targets, including bilateral treatments, are currently under examination. Conversely, the low-intensity modality (MRgLIFU) shows promising prospects in neuromodulation and transient blood-brain barrier opening (BBBO). In the former circumstance, MRgLIFU could serve as a powerful clinical and research tool for non-invasively modulating brain activity and function. BBBO, on the other hand, emerges as a potentially impactful method to influence disease pathogenesis and progression by increasing brain target engagement of putative therapeutic agents. While promising, these applications remain experimental. As a recently developed technology, MRgFUS is not without challenges and questions to be addressed. Further developments and broader experience are necessary to enhance MRgFUS capabilities in both research and clinical practice, as well as to define device constraints. This clinical mini-review aims to provide an overview of the main evidence of MRgFUS application and to highlight unmet needs and future potentialities of the technique.