Novel Chemically Selective Rapid Magnetic Resonance Imaging Techniques and the Clinical Applications
Magnetic Resonance Imaging (MRI) has become as powerful, non-invasive imaging modality for both diagnostic and research purposes. Nevertheless, the efficacy and efficiency of MRI can still be improved by the development of novel imaging methods (pulse sequences), as well as by exploring the everyday clinical and research applications of these new methods. This dissertation focuses on two novel chemically selective imaging methods for the generation of high quality fat-only or water-only images in short scan durations. High quality fat imaging is mainly used for human fat distribution measurements. The study of fat distribution is important for both clinical diagnosis and basic research. The abnormal distribution of fat can be a predictor of many diseases such as type 2 diabetes. A rapid fat imaging technique based on water saturated balanced steady-state free precession (WS b-SSFP) pulse sequences is proposed and theoretically and experimentally compared with traditional methods such as a water-suppressed T1-weighted turbo spin-echo (TSE) sequence. WS b-SSFP is shown to offer significantly improved image quality (higher signal-to- noise ratio, contrast-to-noise ratio, fewer artifacts and less blurring/distortion) in reduced scan duration. The high contrast between fat and water enables the use of a novel fat quantification method based on the image histogram, which includes the contribution of both full- and partial-volume fat pixels for fast and accurate fat quantification. Rapid fat suppression is critical in many applications of clinical MRI. Traditional fat suppression methods are generally time-consuming and/or not effective in generating high quality images, particularly in fast short-TR sequences. A novel rapid fat suppression strategy is introduced, which employs both a strong fat presaturation and a sustained water excitation in a short TR, turbo field echo (TFE) sequence with long echo train length. The sequence is validated to offer comparable fat suppression in much shorter scan duration than traditional presaturation-only fast field echo (FFE), or water-excitation only FFE approaches.