Abstract
Magnetic Resonance Imaging (MRI) is a valuable tool for medical diagnosis, because of the excellent soft tissue contrast and the absence of ionizing radiation. However, MRI is a slow imaging modality. For areas of the body that can be immobilized (e.g., brain), a total examination time of 30 ~ 60 minutes will be performed, which largely reduces patient comfort and cooperation. For areas of the body subject to physiological motion (e.g., breathing motion), any involuntary movements during a scan will degrade the image quality. Thus, a major challenge for MRI is to reduce the long scan time while obtaining images with clinically acceptable quality.
Rapid MRI involves a rich collection of techniques that improve the speed of MRI data acquisition (e.g., non-Cartesian trajectories, parallel imaging). Speed improvements are desirable in many clinical applications. This dissertation will cover two applications: T2-weighted imaging and imaging of cardiac function. The overall goal of this dissertation is to provide rapid data acquisition approaches using spiral k-space trajectories with advanced image reconstruction methods, as well as strategies for compensation of system imperfections (e.g., B0 inhomogeneity).
A new approach to 2D turbo spin-echo imaging using annular spiral rings with a retraced in/out trajectory, dubbed “SPRING-RIO TSE”, was developed for fast T2-weighted brain imaging at 3T. A detailed procedure of spiral rings implementation was presented, as well as effective correction methods for gradient infidelity and B0 inhomogeneity. Volunteer data showed that the proposed method achieves high-quality 2D T2-weighted brain imaging with a higher scan efficiency (0:45 min/14 slices versus 1:31 min/14 slices), improved image contrast, and reduced specific absorption rate (SAR) (~ 86% reduction) compared to conventional 2D Cartesian TSE.
For scanning at 0.55 T and 1.5 T, strategies of sequence modifications were implemented in SPRING-RIO TSE for compensation of concomitant gradient terms at the echo time and across echo spacings, along with reconstruction-based corrections to simultaneously compensate for the residual concomitant- and B0-field induced phase accruals along the readout. Volunteer data showed that after full correction, SPRING-RIO TSE achieves high image quality with improved SNR efficiency (15% ~ 20% increase) and reduced RF SAR (~ 50% reduction) compared to standard Cartesian TSE, presenting potential benefits, especially in regaining SNR at low-field. The compensation principles can be extended to correct for other trajectory types that are time-varying and asymmetric along the echo train in TSE-based imaging.
A 3D spiral-in/out SPACE pulse sequence that incorporates variable-flip-angle refocusing RF pulses with an echo-reordering strategy, concomitant gradient compensation, and variable-density undersampling, was proposed for 1 mm isotropic whole brain T2-weighted imaging at 0.55 T. Volunteer data showed increased apparent SNR values when using spiral SPACE over Cartesian SPACE (17.1±2.3% gain) for similar scan times, providing a potential to mitigate the intrinsic lower SNR of 0.55 T via the improved SNR efficiency of prolonged spiral sampling.
In cardiac imaging, both spiral-out and -in/out bSSFP pulse sequences were developed for accelerated ungated, free-breathing real-time cine at 1.5 T. Volunteer data showed that the two spiral cine techniques showed clinically diagnostic images (score > 3). Compared to standard cine, there were significant differences in global image quality and edge sharpness for spiral techniques, while there was significant difference in image contrast for the spiral-out cine but no significant difference for the spiral-in/out cine. There was good agreement in left ventricular ejection fraction for both the spiral-out cine (-1.6±3.1%) and spiral-in/out cine (-1.5±2.8%) against standard cine.
We demonstrated all proposed techniques lead to improved sampling efficiency and scan comfort, and provide promising alternatives to standard Cartesian acquisition counterparts.
