Balanced Steady-State Free Precession Imaging of the Temporal Bone and Paranasal Sinuses at 0.5T
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Magnetic resonance imaging (MRI) performed with main magnetic field strengths below the conventional 1.5T or 3T has typically been considered inferior, due, primarily, to the proportional decrease in raw signal available. However, recent advancements in gradient systems and radiofrequency receive chains have opened the door to new image acquisition strategies that benefit from use at low field. Hence, this thesis investigates balanced steady-state free precession (bSSFP) protocols designed for a 0.5T system with fast, strong gradients. The protocols are used to image regions that demonstrate susceptibility-induced off-resonance effects, which require phase-cycling techniques to mitigate banding artifacts at conventional field strengths. The work presented herein consists of three studies. The first, titled "Artifact-resistant balanced steady-state free precession imaging of the temporal bone and paranasal sinuses without phase-cycling at 0.5T", illustrates the high artifact tolerance and signal-to-noise ratio attained, at clinical resolutions, by the 0.5T bSSFP protocols. While these metrics are informative, the true value of an image is based on a radiologist’s ability to use it to answer a clinical question. To that end, the second study, titled `"Low-field vs. conventional field balanced steady-state free precession imaging of the temporal bone: radiologist rating of anatomical visualization", examines radiologists’ ability to visualize structures of the temporal bone when viewing images acquired with bSSFP at 0.5T or phase-cycled bSSFP at 3T. Analyses revealed no significant difference, overall, in radiologists’ ratings, indicating the images were of similar quality. Finally, the third study, titled "Super resolution allows for the benefits of low resolution balanced steady-state free precession imaging without degrading image quality", applies a machine learning pipeline to low resolution images resulting from a faster 0.5T bSSFP temporal bone acquisition, bringing them back to high resolution. Evaluation of image ratings shows that radiologists’ ability to visualize temporal bone structures is not significantly reduced, thereby permitting a reduction in scan time. Together, these works illustrate the new opportunities afforded by low-field bSSFP imaging with high-performance gradient systems. These advantages, combined with super resolution techniques, have the potential to make MRI of challenging regions fast and artifact-free, thereby improving patient experience without sacrificing image utility.