RF Pulse Design for Parallel Excitation in Magnetic Resonance Imaging

Parallel excitation is an emerging technique to improve or accelerate multi-dimensional spatially selective excitations in magnetic resonance imaging (MRI) using multi-channel transmit arrays. The technique has potential in many applications, such as accelerating imaging speed, mitigating field inhomogeneity in high-field MRI, and alleviating the susceptibility artifact in functional MRI (fMRI). In these applications, controlling radiofrequency (RF) power deposition (quantified by Specific Absorption Rate, or SAR) under safe limit is a critical issue, particularly in high-field MRI. This \dissertation will start with a review of multidimensional spatially selective excitation in MRI and current parallel excitation techniques. Then it will present two new RF pulse design methods to achieve reduced local/global SAR for parallel excitation while preserving the time duration and excitation pattern quality. Simulations incorporating human-model based tissue density and dielectric property were performed. Results have show that the proposed methods can achieve significant SAR reductions without enlonging the pulse duration at high-fields

Novel Methods for the Prevention of Neurodegeneration around Neural Implants

The recent development of neural prosthetic technology has demonstrated a therapeutic potential for restoring lost sensory or motor functions via a brain-machine interface. Often the devices require direct contact between neural tissue and implanted electrodes to function properly by electrically stimulating or recording neurons on the scale of micro-volts. It is thus critical that the interface between the tissue and the electrode is seamless as well as stable for durations relevant to clinical applications. Unfortunately, the formation of an abiotic/biotic interface is often riddled with host tissue responses that interfere with device function. Neuronal loss due to insertion injury and chronic inflammation and exclusion of recordable cells by an encapsulated glial sheath have all been implicated as potential sources for chronic neural recording failure. Failure to establish a stable interface severely limits the function of implanted neural devices. In this work we rely upon interdisciplinary techniques in chemistry, biology, and physics to develop methods for the formation of a healthy brain-machine interface as well as new tools for the diagnostics of the tissue response. Through the creation of new biomaterials we have improved the neural interface, reduced local neurodegeneration around chronic implants, and demonstrated in vivo improvement of neural recording. In addition, novel conductive and magnetic nanomaterials have been developed for electrochemically detecting reactive oxygen species associated with implant induced tissue damage as well as treatment strategies using on demand drug release. By establishing a method for the formation of healthy and stable interfaces between neural tissue and electrode recording devices, the development of neural prosthetic devices can continue to progress towards clinical acceptance