Technical Comparison of Treatment Efficiency of Magnetic Resonance-Guided Focused Ultrasound Thalamotomy and Pallidotomy in Skull Density Ratio-Matched Patient Cohorts.

Abdul-Kareem Ahmed,Howard M Eisenberg,Elias R Melhem,Ryan Clanton,Alexander Ksendzovsky,Sijia Guo,Rao P Gullapalli,Timothy R Miller,Dheeraj Gandhi,Nathaniel Kelm

doi:10.3389/fneur.2021.808810

Abdul-Kareem Ahmed, Howard M Eisenberg + Show 8 more

Open Access

https://doi.org/10.3389/fneur.2021.808810

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Abstract

ObjectiveMR-guided focused ultrasound (MRgFUS) is increasingly being used to treat patients with essential tremor (ET) and Parkinson's disease (PD) with thalamotomy and pallidotomy, respectively. Pallidotomy is performed off-center within the cranium compared to thalamotomy and may present challenges to therapeutic lesioning due to this location. However, the impact of target location on treatment efficiency and ability to create therapeutic lesions has not been studied. This study aimed to compare the physical efficiency of MRgFUS thalamotomy and pallidotomy.MethodsTreatment characteristics were compared between patients treated with thalamotomy (n = 20) or pallidotomy (n = 20), matched by skull density ratios (SDR). Aspects of treatment efficiency were compared between these groups. Demographic and comparative statistics were conducted to assess these differences. Acoustic field simulations were performed to compare and validate the simulated temperature profile for VIM and GPi ablation.ResultsLower SDR values were associated with greater energy requirement for thalamotomy (R2 = 0.197, p = 0.049) and pallidotomy (R2 = 0.342, p = 0.007). The impact of low SDR on efficiency reduction was greater for pallidotomy, approaching significance (p = 0.061). A nearly two-fold increase in energy was needed to reach 50°C in pallidotomy (10.9kJ) than in thalamotomy (5.7kJ), (p = 0.002). Despite lower energy requirement, the maximum average temperature reached was higher in thalamotomy (56.7°C) than in pallidotomy (55.0°C), (p = 0.017). Mean incident angle of acoustic beams was lesser in thalamotomy (12.7°) than in pallidotomy (18.6°), (p < 0.001). For all patients, a lesser mean incident angle correlated with a higher maximum average temperature reached (R2 = 0.124, p = 0.026), and less energy needed to reach 50°C (R2=0.134, p = 0.020). Greater skull thickness was associated with a higher maximum energy for a single sonication for thalamotomy (R2 = 0.206, p = 0.045) and pallidotomy (R2 = 0.403, p = 0.003). An acoustic and temperature field simulation validated similar findings for thalamotomy and pallidotomy in a single patient.ConclusionThe centrally located VIM offers a more efficient location for therapeutic lesioning compared to GPi pallidotomy in SDR matched cohort of patients. The impact on therapeutic lesioning with lower SDR may be greater for pallidotomy patients. As newer off-center targets are investigated, these findings can inform patient selection and treatment requirements for lesion production.

Highlights

Magnetic resonance imaging-guided focused ultrasound (MRgFUS) is a promising, non-invasive technology that is being increasingly applied to treat various neurological disorders, including essential tremor and Parkinson’s disease [1]
20 patients treated with MRgFUS unilateral ventral intermediate (VIM) thalamotomy for essential tremor (ET) and 20 patients treated with unilateral globus pallidus internus (GPi) pallidotomy for Parkinson’s disease (PD) were selected in pairs with matching skull density ratio (SDR), defined as values within 0.02 of each other
Our results demonstrate that MRgFUS ablation of the GPi is less efficient than ablation of the thalamic VIM nucleus in patient populations with matched skull density ratios

Summary

Introduction

Magnetic resonance imaging-guided focused ultrasound (MRgFUS) is a promising, non-invasive technology that is being increasingly applied to treat various neurological disorders, including essential tremor and Parkinson’s disease [1]. In 2002, Clement and Hynynen introduced an approach utilizing computer tomography data of the subject’s calvarium to focus individual ultrasound beams through the intact skull [5] This transcranial technique works by registering CT data with MR imaging to predict the phase aberration and beam attenuation occurring at the calvarial-soft tissue interface, which is corrected for by steering of individual transducer elements [6]. The efficiency of transcranial ablation using a hemispherical array of transducers is known to vary by target location, with a small treatment envelope being present in the center of the brain In this envelope, the incident angles of individual transducer elements at the calvarium are optimal for current mid-frequency systems [7]. This variation could have implications for new targets and for patients with low SDRs

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