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“Assessment of Myocardial Blood Flow in Humans Using Arterial Spin Labeled MRI ”
by Zungho Zun
December 2010
Magnetic resonance imaging (MRI) is a powerful imaging modality that is both non-invasive and non-ionizing. MRI can be used to facilitate the evaluation of coronary artery disease (CAD), which is a leading cause of death worldwide. In particular, MRI-based first-pass techniques provide assessment of myocardial perfusion with high resolution in detection of CAD. Myocardial perfusion reflects the rate of blood delivery to tissue and is a powerful indicator of tissue health. However, these first-pass methods require the use of contrast agent which cannot be applied to the patients with end-stage renal disease (ESRD). This dissertation contributes a new method for measuring myocardial perfusion without contrast agent, using arterial spin labeled (ASL) MRI. Firstly, a graphical analysis of balanced steady-state free precession (SSFP) is presented. Balanced SSFP is an imaging sequence that provides high signal-to-noise ratio (SNR) efficiency. Because ASL-based perfusion imaging typically suffers from low intrinsic SNR, balanced SSFP was adopted as our proposed imaging sequence though this dissertation. The graphical approach provides an intuition for understanding balanced SSFP. Secondly, the feasibility of myocardial perfusion imaging using ASL (or myocardial ASL) is demonstrated in healthy volunteers. It is shown that myocardial ASL measurements are consistent with previous published literature values of perfusion using positron emission tomography (PET), are inflow-dependent, and increase with mild stress. In addition, analysis of noise is presented to assess its impact on ASL measurement error. Thirdly, the potential of myocardial ASL to detect angiographically significant CAD is demonstrated in patients. The perfusion reserve index is defined as the rate of perfusion during stress divided by that at rest. It is a measure of the severity of CAD. This study performed reststress myocardial ASL scans using vasodilator in patients with suspected CAD. Measured perfusion in normal myocardial segments increased by a factor of four during stress, matching literature values based on PET. There was also a statistically significant difference in perfusion reserve between normal and the most ischemic myocardial segments, which suggests that myocardial ASL may be capable of detecting CAD. Lastly, three attempts to improve measurement confidence in myocardial ASL are described. Three potential sources of measurement noise are identified and a relevant solution to each noise source is presented. Physiological noise is measured with and without the solution and statically significant reduction is examined.