UCLA

Magnetic resonance multitasking (MR Multitasking) is a multi-dimensional imaging framework that was developed recently. With low-rank tensor modeling, signal correlation among images at different time dimensions are exploited in MR Multitasking to resolve motion, accelerate image acquisition, and enhance image quality. Though initially developed for cardiovascular imaging, it has also been extended to many other applications, such as whole-brain multi-parametric mapping, free-breathing abdominal dynamic contrast enhanced imaging, etc. The primary focus of this dissertation is to improve two important MR tissue characterization techniques using MR Multitasking: (1) Electrocardiogram (ECG)-less myocardial T1 and extracellular volume fraction (ECV) mapping in small animals at 9.4 T, and (2) Fast 3D chemical exchange saturation transfer (CEST) imaging for human studies at 3.0 T.

ECV quantification with cardiovascular magnetic resonance T1 mapping is a powerful tool for the characterization of focal or diffuse myocardial fibrosis. However, it is technically challenging to acquire high-quality T1 and ECV maps in small animals for preclinical research because of high heart rates and high respiration rates. An ECG-less, free-breathing ECV mapping method using MR Multitasking was developed on a 9.4 T small animal MR system. The feasibility of characterizing diffuse myocardial fibrosis was tested in a rat heart failure model with preserved ejection fraction (HFpEF). A 25-min exam, including two 4-min T1 Multitasking scans before and after gadolinium injection, were performed on each rat. It allows a cardiac temporal resolution of 20 ms for a heart rate of ~300 bpm. Elevated ECV found in the HFpEF group is consistent with previous human studies and well correlated with histological data. This technique has the potential to be a viable imaging tool for myocardial tissue characterization in small animal models.

CEST imaging is a non-contrast MRI technique that indirectly detects exchangeable protons in the water pool. It is achieved by performing frequency selective saturation at those protons before acquiring water signal readout. CEST MRI provides a novel contrast mechanism to image important physiological information, such as pH and metabolite concentration. However, long scan time is still a crucial problem in many CEST imaging applications, which makes it difficult to translate current CEST techniques into clinical practice. A novel 3D steady-state CEST method using MR Multitasking was developed in the brain at 3.0 T. This allows the Z-spectrum of 55 frequency offsets to be acquired with whole-brain coverage at 1.7 � 1.7 � 3.0 mm3 spatial resolution in 5.5 min. Quantitative CEST maps from multi-pool fitting showed consistent image quality across the volume.

Motion handling in moving organs is another challenge for practical CEST imaging. For instance, breath-holding is currently needed in liver CEST imaging to reduce motion artifacts, which limits not only spatial resolution, but also scan volume coverage. Following the whole-brain CEST protocol, a respiration-resolved 3D abdominal CEST imaging technique using MR Multitasking was developed, which enables whole-liver coverage with free-breathing acquisition. CEST images of 55 frequency offsets with entire-liver coverage and 2.0 x 2.0 x 6.0 mm3 spatial resolution were generated within 9 min. Both APTw and glycoCEST signals showed high sensitivity between post-fasting and post-meal acquisitions.