ASSESSMENT OF MYOCARDIAL PERFUSION BY MAGNETIC-RESONANCE-IMAGING

Magnetic resonance imaging (MRI) has proven useful for anatomic and fu nctional evaluation of the heart. However, until recently assessment o f myocardial perfusion has not been possible by MRI. Using newly devel oped ultrafast imaging sequences, images can be acquired rapidly with a high temporal resolution, which is a prerequisite for imaging the in itial passage of a bolus of MR-contrast medium through the myocardium. Only gadolinium chelates, which rapidly diffuse out of vascular space , are currently approved for clinical use. The first pass of a bolus o f one of these agents through hypoperfused myocardium distal to a coro nary artery stenosis enhances this area less as compared to normally p erfused areas. This different myocardial enhancement is often visible when looking at the series of MR images. However, intensity difference s are rapidly decreasing as MR-contrast media are diluted in the syste mic circulation after the first pass and diffuse to the interstitium. Therefore, only the first pass is of interest for MR-perfusion imaging . Additional and often more precise information can be derived by meas uring parameters of the signal intensity time curve such as mean trans it time, maximum signal intensity increase, upslope, downslope, and de lay before reaching maximum signal intensity. Temporal resolution is t he crucial factor in MR-perfusion imaging because it takes only 20 to 60 seconds for the contrast medium to pass through the myocardium. The refore, this dynamic process must be imaged with a high temporal resol ution. Moreover, image acquisition must be fast enough to minimize mot ion artefacts and to maximize the spatial coverage of the ventricle. U ltrafast gradient echo techniques and echo planar imaging are in princ iple capable to fulfill these demands. While ultrafast gradient echo s equences enable one to acquire a maximum of 2 slices per heartbeat, ec ho planar sequences need only 30 to 50 msec to completely acquire one image and are thus able to image the entire ventricle within one heart beat. However, they are also more susceptible to image artefacts. As g radients capable of producing high quality echo planar images are not widely available, ultrafast gradient echo techniques are commonly used for MR-perfusion imaging. A good correlation between quantitative est imates of myocardial perfusion by MRI after injection of an intravascu lar contrast agent and microsphere measurements has been shown in anim al experiments but quantitative MR perfusion measurements have not yet been performed in humans. Clinical studies have until now focused on visual and parametric analysis of signal intensity time curves. From t hese studies, sensitivities and specifities in the range of 60 to 90% as compared to x-ray coronary angiography and scintigraphy were report ed despite the fact that only parts of the left ventricular myocardium could be assessed. However, a generally accepted method of acquiring and analysing MR perfusion images does not yet exist. Therefore, futur e improvements of hardware and pulse-sequences as well as the developm ent of new blood pool contrast agents are necessary before MR-perfusio n imaging will become a widely accepted and clinically useful diagnost ic procedure.