Fast magnetic resonance imaging techniques for cardiovascular disease
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Stanford University
Abstract
Cardiovascular disease, which can manifest as heart attack and stroke,
is the leading cause of death in industrialized nations. Magnetic resonance
im aging (MRI) allows minimally-invasive detection of the effects of
cardiovascular disease such as narrowing of blood vessels by atherosclerotic
plaque and the resulting changes in blood velocity. MRI is routinely
employed for imaging problems of the central nervous system and the
musculoskeletal system, such as brain tumors and knee-ligament tears.
However, MRI is not yet widely accepted for imaging the cardiovascular
system because image quality can be severely compromised by a number of
motion sources such as patient fidgeting, breathing and cardiac motion.
Fast MRI, which can acquire a two-dimensional image in less than one
second, can avoid these motion problems. However, fast MRI can be crippled
by artifacts from blood flow and imperfections in the magnetic fields used for
imaging. In this thesis, a linear-systems approach to MRI was used to obtain
novel fast-MRI techniques for which these artifacts are negligible. This
linear-system s approach is based in the frequency dom ain since the
underlying physical phenomenon is a resonant frequency of the hydrogen nucleus which depends on the magnetic-field strength, allowing us to say that
MRI data is acquired in the Fourier domain.
This thesis offers three contributions to cardiovascular MRI: (i) Echo planar imaging (EPI) is a popular fast MRI technique that suffers from signal
dropouts and ghosting in the presence of blood flow. By exploiting the
relative importance of the low spatial frequencies in the image, EPI was
modified to dramatically reduce these artifacts. In particular, partial-flyback
EPI reduces sensitivity to flow in one direction and inside-out EPI reduces
sensitivity to flow in the orthogonal direction, (ii) Velocity imaging can have
long scan times because three or more dimensions are required: two spatial
dimensions and one or more velocity dimensions. Previous incarnations of
a fast technique that acquires velocity images in real time, with typical frame
rates of 30/s, suffer from excessing blurring caused by magnetic-field
imperfections. This technique was modified so that field imperfections cause
a simple shift of the image instead of blurring, (iii) Atherosclerotic-plaque
imaging currently uses techniques that focus on the degree of narrowing of
the blood vessel. However, it is believed that the internal structure of the
lesion can be a more useful predictor of which lesions will rupture and
thereby cause sudden events such as stroke. Imaging the internal structure of
plaque lesions is challenging because they have irregular geometries and
widths of only a few millimeters. Moreover, signal from flowing blood can
obscure signal from the lesion since it is typically much brighter. A novel
imaging technique was designed, combining signal-excitation methods and
fast-im aging m ethods to produce three-dim ensional images w ith sub millimeter resolution and flow suppression, all in a reasonable scan time.
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Citation
Luk-Pat, G. T. (1998), Fast magnetic resonance imaging techniques for cardiovascular disease. Retrieved from ProQuest Digital Dissertations (AAT 9924460)