Parallel Radiofrequency Transmission for Safe Magnetic Resonance Imaging of Deep Brain Stimulation Patients at 3 Tesla

Abstract

Deep brain stimulation (DBS) improves the quality of life for patients suffering from neurological disorders such as Parkinson’s disease and, more recently, psychiatric/cognitive disorders such as depression and addiction. This treatment option involves the implantation of an implantable pulse generator (or neurostimulator) and leads (or electrodes) implanted deep within the human brain. Magnetic resonance imaging (MRI) is a powerful diagnostic tool that offers superior soft tissue contrast and is routinely used in clinics for neuroimaging applications. MRI is advantageous in DBS pre-surgical planning as precise lead placement within the brain is essential for optimal treatment outcomes. DBS patients can also benefit from post-surgery MRI, and studies have shown that DBS patients are more likely to require MRI within 5-10 years post-surgery. However, imaging DBS patients is restricted by substantial safety concerns that arise from localized electric charge accumulation along the implanted device during resonant radiofrequency (RF) excitation, which can potentially lead to tissue heating and bodily damage. With the technological advancement of ultra-high field (UHF) MRI systems and a growing DBS patient population, DBS MRI safety will become increasingly problematic in the future and needs to be addressed. Parallel RF transmission (pTx) is a promising technology that utilizes multiple transmit channels to generate a desired electromagnetic profile during MRI RF excitation. Several proof-of-concept studies successfully demonstrated its efficacy in creating a "safe mode" of imaging that minimizes the localized RF heating effects. However, pTx MRI systems are not easily accessible and are often custom-built and integrated onto existing MRI systems. Consequently, it adds system characterization and verification complexity to the DBS MRI safety problem. System channel count is also an important consideration as implementation costs can be very high, and the impact of system transmit channel count remains unexplored. Furthermore, in practice, DBS patients with motor-related disorders will impact the pTx MRI system’s ability to precisely generate these safe mode electromagnetic profiles. Commercial DBS devices (i.e., the neurostimulator and leads) are manufactured with fixed dimensions, and the caring surgeon typically manages the surgical orientation of the implanted DBS device and leads. Therefore, lead trajectories can vary hospital-to-hospital. As a result, standard phantoms, i.e., the ASTM International Standard, used in safety verification experiments may not be suitable for DBS MRI applications. To advance DBS patient safety in MRI, this thesis studied the implant heating effects of pTx system uncertainty, system channel count, patient motion on a novel pTx MRI research platform and its associated safe mode of imaging. It developed a new anthropomorphic heterogeneous phantom to improve safety verification experiments.