Innovative designs of RF transmit array coils and RF heating analysis of patients with implanted DBS

Abstract

A safe and efficient magnetic resonance imaging (MRI) test would rely on informed specification, design, implementation, assessment, and application of appropriately selected radiofrequency (RF) coils. Towards these goals, this dissertation comprises three contributions to novel RF transmit array (TxArray) coil design techniques and two contributions to RF heating reduction of deep brain stimulation (DBS) implants in RF transmit coils. TxArray coils with multiple transmit elements provide the additional degrees of freedom that can be used to enhance field uniformity, accelerate acquisition time, enable RF shimming while intending to mitigate specific absorption rate (SAR) hotspots, and increase power efficiency. How a TxArray coil is designed can have a significant impact on its gain from parallel transmission technology. Thus, the first contribution of this dissertation is on the eigenmode analysis of the scattering matrix for the design of TxArray coils to obtain their efficient operation modes and achieve an efficient RF shimming in terms of power consumption. The algorithm is tested for the design of four 3T TxArray coils with 8 to 32 channels, and it is shown that it can enlarge the dimension of the excitation space by up to 50% compared with the commonly used design techniques. The next contribution is to establish a fast finetuning procedure to precisely design an imperfectly manufactured TxArray coil using its corresponding equivalent circuit model. By fitting the measured scattering parameters to a lumped circuit model, all inductances/resistances of an 8-channel 3T TxArray coil are estimated. The manufactured coil is then appropriately tuned only in a single iteration. As another contribution, a theoretical coil size optimization is introduced to minimize the magnetic coupling between non-adjacent transmit channels of a TxArray coil. By calculating all self/mutual-inductances of a 12-channel 3T TxArray coil and minimizing mutual-inductances, its sizes are determined. The finite element simulations are performed to demonstrate the feasibility of this approach. One of the safety considerations of RF transmit coils is the localized SAR amplification due to the interaction of metallic implants with the coil's electric fields. Therefore, the fourth contribution is on evaluating SAR mitigation performance at tips of patient-derived realistic DBS implants using a 3T patient-adjustable transmit coil, which uses a mechanically rotating linearly polarized birdcage resonator. The reconfigurable coil system decreases the SAR on average by 83% for unilateral leads and by 59% for bilateral leads in comparison to a conventional quadrature birdcage coil. In the final study, the local SAR amplification surrounding the tips of a large cohort of DBS implants with realistic lead trajectories in a commercially available vertical open-bore 1.2T coil and a standard horizontal closed-bore 1.5T birdcage coil is presented. On average, SAR is decreased by 31-fold in the 1.2T vertical coil compared to the 1.5T horizontal coil. Overall, this dissertation proposes innovative approaches for designing TxArray coils and heating assessment and SAR reduction of DBS patients, which mainly contribute to improving the performance of RF transmit coils in terms of power efficiency and patient safety.