Coronary imaging using MRI.

Abstract

Without use of ionizing radiation and injection of contrast material magnetic resonance imaging (MRI) can be applied to generate signal from flowing blood and create tomographic images of the bloodstream in coronary arteries, which resemble conventional contrast enhanced X-ray angiograms. The tortuosity, small diameter and motion of the coronary arteries provided technically demanding problems, which had to be solved before MR coronary angiography became realistic. Faster pulse sequences, dedicated radiofrequency receiver coils, cardiac and respiratory gating techniques were introduced and are still in the process of constant development to improve the quality of the images. To date, most clinical experience has been obtained using 2D approaches necessitating repetitive breath-holds to encompass the coronary artery tree. A substantial part of the proximal and middle parts of the coronary arteries can be visualized, which has proven to be accurate in identifying anomalous coronary anatomy and patency of proximal coronary artery bypass grafts. However, the technique is currently not sensitive enough to reliably detect coronary artery stenoses. Another approach is a single acquisition respiratory gated 3D technique which is less operator and patient dependent, requires less imaging time for an entire coronary protocol and is more comfortable for the patient than the 2D breath-hold approach. Initial experience demonstrates the capability to identify the major epicardial coronary vessels. But here too, further development is required to demonstrate coronary stenoses. A unique feature of MR imaging is the feasibility to non-invasively quantitate flow in the coronary arteries. This offers the potential to selectively determine the flow reserve of coronary arteries and to assess the functional and physiological implications of moderately severe stenoses. It can be envisaged that, although currently not apt to replace conventional coronary angiography, MR coronary angiography and flow measurement will become of use in the evaluation of specific, well defined clinical issues in coronary artery disease.