Left ventricular function under stress conditions by MRI

Summary

Cardiac patients with ischemic heart disease often show normal cardiac function at rest. Abnormalities may be discovered by these functional examinations only during increased cardiac work. Therefore, cardiac function assessment under physiological or drug induced stress is a key part of a comprehensive cardiac examination. The value of MR cardiac imaging is limited without the ability to study the effects of stress on patients. A number of ways to stress patients in MR scanners has been considered or tested, however, most of them are either not practical or useful. Promising techniques are dynamic bicycle exercise and pharmacological stress testing by injection of either dipyridamole or dobutamine. In the majority of studies published in literature stress was applied pharmacologically Commonly, short axis cine gradient echo images are acquired and regional wall motion abnormalities are studied. Generally, a good correlation between wall motion abnormalities and angiographic severity of coronary artery disease resulted. However, physical exercise would be a preferable form of stress. In this text, the feasibility of ergometer exercise in MR scanners is explored. The MR compatible equipment for supine bicycle exercise is described, and basic questions concerning patient safety are addressed. The MR sequences for imaging both for 5 seconds immediately after exercise, and for imaging during stress are described. They are based on gradient-echo echo-planar techniques (EPI). Imaging during stress can be done only with real time imaging using a new acquisition technique named turbo-field-echo-EPI, with an acquisition time of 75 ms per image. Short axis cine anatomical images suitable for qualitative wall motion assessment are presented, acquired within 5 seconds after exercise stop, as well as real time images acquired constantly during stress. Myocardial tagging is performed during exercise and within 5 seconds immediately after stress. The tagging data sets are suitable for a quantitative measurement of myocardialwall motion. Patient motion is the most severe problem to overcome. To this purpose, the acquisition needs to be extremely fast, in order to freeze motion. This is technically demanding, but with modern MR scanners it is possible to acquire cardiac images at a rate of approximately 10 images per second with a image resolution of approximately 2.5 mm.

From these preliminary experiments it is concluded that with some improvements in the imaging procedure and gradient performance of the MR system it is well feasible to perform left ventricular function assessment not only with pharmacological but also with physical stress using a bicycle ergometer.