Diffusion and functional MRI in surgical neuromodulation

Abstract

Surgical neuromodulation has witnessed significant progress in recent decades. Notably, deep brain stimulation (DBS), delivered precisely within therapeutic targets, has revolutionized the treatment of medication-refractory movement disorders and is now expanding for refractory psychiatric disorders, refractory epilepsy, and post-stroke motor recovery. In parallel, the advent of incisionless treatment with focused ultrasound ablation (FUSA) can offer patients life-changing symptomatic relief. Recent research has underscored the potential to further optimize DBS and FUSA outcomes by conceptualizing the therapeutic targets as critical nodes embedded within specific brain networks instead of strictly anatomical structures. This paradigm shift was facilitated by integrating two imaging modalities used regularly in brain connectomics research: diffusion MRI (dMRI) and functional MRI (fMRI). These advanced imaging techniques have helped optimize the targeting and programming techniques of surgical neuromodulation, all while holding immense promise for investigations into treating other neurological and psychiatric conditions. This review aims to provide a fundamental background of advanced imaging for clinicians and scientists, exploring the synergy between current and future approaches to neuromodulation as they relate to dMRI and fMRI capabilities. Focused research in this area is required to optimize existing, functional neurosurgical treatments while serving to build an investigative infrastructure to unlock novel targets to alleviate the burden of other neurological and psychiatric disorders.

Keywords: Deep brain nuclei; Deep brain stimulation; Diffusion MRI; Functional MRI; Neural circuitry; Neuromodulation.

Fig. 1

A. The implementation of deterministic tractography for personalized targeting for tremor surgery. Preoperative dMRI was acquired to precisely target the ventral intermediate nucleus and the adjacent white matter tracts using FDA-approved software (Brainlab Elements, Brainlab Inc., Munich, Germany). Shown here are the pyramidal tract (in red) and medial lemniscus (in blue) relative to the FUSA lesion (shown in orange) in axial, sagittal, and coronal projections. B. The use of dMRI to distinguish subregions of therapeutic interest within anatomically defined nuclei (adapted Sammartino et al. JNS 2021). Quantitative analysis of diffusion parameters revealed three distinct subregions within the globus pallidus (posterior, central, and anterior). The posterior and central subregion corresponded with the motor part of the globus pallidus. C. The functional segmentation of the subthalamic region is shown in a dMRI investigation (adapted from Sammartino et al. Brain Imaging and Behavior 2021). The stimulation volumes were modeled, and distinct stimulation-induced clinical effects were analyzed to reveal the symptom-specific tracts. The tracts associated with motor improvement (green) were lateral while those associated with non-motor side effects were medial with an anterior-to-posterior organization such that the tracts associated with stimulation-induced changes in mood (red) were organized most anteriorly. In contrast, tracts associated with dizziness (yellow) and sweating (blue) were the most posterior.

Fig. 2

3 Tesla MRI acquired in subjects with implanted DBS leads demonstrates the range of artifacts that can occur. Adapted from Sammartino et al. 2017: J Neurosurg 127:892–898.