Patient-specific effects on sonication heating efficiency during magnetic resonance-guided focused ultrasound thalamotomy

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.

Longitudinal MR imaging after unilateral MR-guided focused ultrasound thalamotomy: clinical and radiological correlation

Introduction

Magnetic-resonance-guided focused ultrasound (MRgFUS) thalamotomy uses multiple converging high-energy ultrasonic beams to produce thermal lesions in the thalamus. Early postoperative MR imaging demonstrates the location and extent of the lesion, but there is no consensus on the utility or frequency of postoperative imaging. We aimed to evaluate the evolution of MRgFUS lesions and describe the incidence, predictors, and clinical effects of lesion persistence in a large patient cohort.

Methods

A total of 215 unilateral MRgFUS thalamotomy procedures for essential tremor (ET) by a single surgeon were retrospectively analyzed. All patients had MR imaging 1 day postoperatively; 106 had imaging at 3 months and 32 had imaging at 1 year. Thin cut (2 mm) axial and coronal T2-weighted MRIs at these timepoints were analyzed visually on a binary scale for lesion presence and when visible, lesion volumes were measured. SWI and DWI sequences were also analyzed when available. Clinical outcomes including tremor scores and side effects were recorded at these same time points. We analyzed if patient characteristics (age, skull density ratio), preoperative tremor score, and sonication parameters influenced lesion evolution and if imaging characteristics correlated with clinical outcomes.

Results

Visible lesions were present in all patients 1 day post- MRgFUS and measured 307.4 ± 128.7 mm3. At 3 months, residual lesions (excluding patients where lesions were not visible) were 83.6% smaller and detectable in only 54.7% of patients (n = 58). At 1 year, residual lesions were detected in 50.0% of patients (n = 16) and were 90.7% smaller than 24 h and 46.5% smaller than 3 months. Lesions were more frequently visible on SWI (100%, n = 17), DWI (n = 38, 97.4%) and ADC (n = 36, 92.3%). At 3 months, fewer treatment sonications, higher maximum power, and greater distance between individual sonications led to larger lesion volumes. Volume at 24 h did not predict if a lesion was visible later. Lesion visibility at 3 months predicted sensory side effects but was not correlated with tremor outcomes.

Discussion

Overall, lesions are visible on T2-weighted MRI in about half of patients at both 3 months and 1 year post-MRgFUS thalamotomy. Certain sonication parameters significantly predicted persistent volume, but residual lesions did not correlate with tremor outcomes.

Update in the clinical application of focused ultrasound

PURPOSE OF REVIEW

To review the most recent evidence about the clinical applicability of transcranial MRI-guided focused ultrasound (MRgFUS), including clinical evidence and indications, recent technical developments for its use and future prospects.

RECENT FINDINGS

Unilateral MRgFUS thalamotomy for both essential and parkinsonian tremors is an approved and well established therapy. Recent studies have focused on its long-term safety and efficacy as well as technical advances for refining the approach. Moreover, ultrasound has expanded its application in Parkinson's disease, with clinical trials successfully targeting other brain regions like the subthalamic nucleus, the globus pallidus and the pallidothalamic tract, providing benefits for features that thalamotomy neglects. New indications, such as focal dystonia or neuropsychiatric conditions (namely obsessive-compulsive disorder and depression) have also been explored, with encouraging preliminary results. Finally, the application of ultrasound in low-intensity modality allows other approaches like focal blood-brain barrier opening and neuromodulation, which promise to be highly relevant in translational research.

SUMMARY

MRgFUS is a growing emergent technique. Its application in clinical routine is becoming widely accepted as a therapeutic option. Novel approaches and new potential applications are anticipated.

Neuropathology of Parkinson's disease after focused ultrasound thalamotomy

Focused ultrasound (FUS) thalamotomy is an emerging treatment for tremor-dominant Parkinson's disease (PD). We report the first postmortem neuropathologic study of FUS thalamotomy in a 68-year-old man with tremor-dominant PD, which was performed seven months before he died. Although the peak voxel temperature at the target was <54 °C, his tremor improved on intraoperative and postoperative assessments. Additionally, postoperative MRI demonstrated a thalamic lesion. Lewy body-related pathology consistent with PD was detected. There was also a 5-mm lesion in the ventral lateral thalamus characterized by demyelination and neuropil loss, with many lipid-laden macrophages, but no lymphocytic infiltrates and relatively preserved neurons and axons. Additional pathological assessments after FUS thalamotomy are needed to determine if the observed brain changes are typical of this procedure.