Techniques and Applications in Rapid Spiral Magnetic Resonance Imaging

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Wang, Zhixing, Biomedical Engineering - School of Engineering and Applied Science, University of Virginia
Meyer, Craig, MD-BIOM Biomedical Eng, University of Virginia

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.

PHD (Doctor of Philosophy)
fast imaging, spiral imaging, turbo spin-echo imaging, real-time cardiac imaging, neuroimaging, 3D SPACE
Sponsoring Agency:
National Institutes of HealthSiemens Medical Solutions
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