Fast magnetic resonance imaging techniques for cardiovascular disease

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.