MRI and fMRI for Neuromodulation

Introduction

From the first reported applications in pain ( ), the indications for neuromodulation have broadened over the years to movement disorders, epilepsy, spasticity, and psychiatric disorders ( ). Neurostimulation systems consist of metallic electrode contacts (either paddle or cylindrical electrodes) connected to an implantable pulse generator (IPG) via subcutaneous insulated wires. The IPG is usually implanted in a subcutaneous or subfascial pocket away from the stimulation site. Various systems with this configuration are now routinely implanted for deep brain stimulation [DBS; US Food and Drug Administration (FDA) approved for essential tremor, Parkinson disease (PD), dystonia, obsessive–compulsive disorder; ], peripheral nerve field stimulation (for chronic pain and migraine; ), vagus nerve stimulation (VNS; for intractable epilepsy; ), and spinal cord stimulation (SCS; postlaminectomy syndrome and complex regional pain syndrome; ). The number of patients with implanted neurostimulation systems has steadily increased; e.g., approximately 140,000 patients have received deep brain stimulators worldwide ( http://professional.medtronic.com/pt/neuro/dbs-md/prod/- .WGQ4k4c8BHi ). SCS implantations significantly outnumber deep brain stimulators, with some estimates suggesting nearly 14,000 new implants every year around the world ( ). This increase in the number of patients with implanted neurostimulation devices parallels the larger availability and need for magnetic resonance imaging (MRI) ( ). MRI has also been leveraged to understand the mechanisms underlying the efficacy of neuromodulation systems and more recently to guide therapy. Therefore, it is imperative that clinicians in the field of neuromodulation understand the interactions between the implanted neuromodulation devices and the magnetic field during MRI, especially at higher strengths (e.g., 3 T). Potential interactions include the effects on imaging processes, such as distortions, that affect the image quality (e.g., an in situ neurostimulator device can induce susceptibility artifacts at the tissue–electrode interface), but more importantly, the potential loss of efficacy and even thermal injury to patients. Safety concerns have led a number of centers to avoid performing MRI in patients with indwelling neurostimulators. In a survey of 42 National Parkinson Foundation Centers of Excellence, 17 were not performing postoperative MRI in DBS patients, mainly to adhere to industry-proposed warnings ( ). Sometimes patients are faced with the proposition of explanting their neurostimulator to obtain a diagnostic MRI study ( ). Therefore it is important to critically evaluate the need for MRI in patients with implanted neurostimulation systems, understand the safety concerns, and monitor critical safety parameters. The safety of MRI in patients with implanted neurostimulators is discussed elsewhere in this book. Here we briefly discuss the current indications of diagnostic MRI in this population, and review in detail the role of MRI, particularly functional neuroimaging, in advancing the field of neuromodulation, both for improved diagnostic studies and for MRI-guided therapy.