Cardiologists often need to know how much blood is making its way through heart tissue to gauge heart health and the risk of heart attacks. For patients with kidney failure awaiting transplants, however, the dye added to the bloodstream to make such measurements can be fatal.
Blockage in the left anterior descending coronary artery of a patient as seen by ASL-MRI (top) and X-ray angiography (bottom).
In an article that appeared in the December 2011 issue of the Journal of the American College of Cardiology (JACC): Cardiovascular Imaging, Professor Krishna Nayak and recent Ph.D. graduate Zungho Zun, both of the Viterbi School Ming Hsieh Department of Electrical Engineering, report successful cardiac perfusion tests of a method called Arterial Spin Labeling (ASL) that uses basic fluid properties of blood to measure flows.
ASL-MRI is quite different from standard perfusion MRI, in which a timed intravenous injection of chemical contrast agents into the patient’s bloodstream creates a ‘bolus’ of dye that cardiologists then follow through the heart tissue.
The MRI process involves some interpretive challenges because it solely monitors the first pass of the chemicals through the heart, and that first pass can sometimes contain artifacts that make the flow measurements and interpretation difficult. Also, the dye chemicals are not tolerated by patients with end-stage kidney disease, creating a potentially fatal toxic syndrome.
ASL is an alternative process that has been used extensively for brain perfusion imaging. Nayak and other investigators have been developing improvements so that it may be applied to the heart. ASL uses the fact that blood is, beneath its cargo of cells, hormones, and all the rest, water.
Instead of a dye, a measured pulse of radio waves is administered to arteries carrying blood to the area of interest – for heart imaging, the aorta and coronary arteries. The radio waves energize the hydrogen in blood water molecules in a measurable way, so that these energized molecules can be detected and used as proxies for total blood flow through the tissue in question. The article is entitled “Arterial Spin Labeled Cardiac Magnetic Resonance Detects Clinically Relevant Increase in Myocardial Blood Flow with Vasodilation.” In the study, to test the effect of the blood-vessel dilating drug adenosine on the perfusion of blood through cardiac tissue, the arteries were pulsed without administering the drug, and an ASL-MRI test recorded the volume of energized fluid detected in heart tissue. The process was then repeated with the administration of the drug.
Comparative studies such as this "demonstrate that ASL is able to detect a clinically relevant increase in myocardial blood perfusion (MBF) with vasodilation.”
Zungho Zun, Ph.D. 2011 (now a postdoc at Stanford)
Working with Nayak and Zun, are doctors Padmini Varadarajan and Ramdas G. Pai of Loma Linda University Medical Center and physicist Eric C. Wong of the UC San Diego School of Medicine. The Wallace H. Coulter Foundation supports the research.