Arterial Spin Labeling (ASL) Imaging of Cerebral Perfusion at High Magnetic Fields

  • Author / Creator
    Mora Álvarez, María G.
  • Arterial spin labeling (ASL) is a magnetic resonance imaging (MRI) noninvasive method, capable of measuring perfusion, i.e. blood flow, with blood as an intrinsic contrast. ASL measurements of cerebral perfusion are usually acquired at very low spatial resolution (> 60mm3 voxels), for several reasons (scan time, brain coverage, signal-to-noise ratio— SNR). While this may be effective for the detection of large lesions, it is inadequate for smaller areas of perfusion deficit. Stronger magnetic fields can help to increase this resolution due to higher SNR and longer lasting arterial tag. Two ASL sequences (continuous ASL— CASL and pseudo-continuous ASL— pCASL) were developed on a 4.7T Varian MRI. High-resolution CASL (7 mm3 voxels) is shown here to be feasible at the stronger field of 4.7T in a clinically acceptable time of 6~min. PCASL implementation is challenging at high magnetic fields due to specific absorption rate (SAR) limitations and field inhomogeneities, which degrades the efficiency of the method. Correction strategies to minimize off- resonance effects in pCASL have yet to be developed on the 4.7T Varian. In this thesis, a CASL study with different resolutions (95, 60, 45, 27, and 7 mm3 voxels) demonstrated that cerebral blood flow (CBF) in the cortex is better depicted in high-resolution scans (7 mm3 voxels). Whole brain coverage is limited by loss of the arterial tag with time in the current context of 2D slice acquisition, but this can be overcome with 3D acquisition methods. A comparison between CASL 2D single-shot GE-EPI at 4.7T and pulsed ASL (PASL) 3D GRASE at 3T with different resolutions shows that high resolution ASL can also be implemented in clinical cutting-edge 3T scanners. By taking advantage of high magnetic fields, high resolution ASL images could aid in the detection of smaller perfusion deficits associated with transient ischemic attack or minor stroke.

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  • Degree
    Master of Science
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    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.