Concepts of myocardial perfusion imaging in magnetic resonance imaging.

Based on the major innovations in ultrafast magnetic resonance (MR) imaging in recent years, myocardial perfusion imaging with MR has become the focus of many investigators. Two major approaches to myocardial perfusion imaging involve either exogenous or endogenous contrast agents. For the first category of perfusion experiments, we review the characteristics of the common contrast agents and MR techniques for experimental and clinical first-pass studies and in particular address the question of extracting quantitative estimates for myocardial blood flow (milliliters per minute per gram) and volume (milliliters per gram). We demonstrated quantitative perfusion analysis using intravascular relaxation agents and heavily T1-weighted ultrafast gradient echo sequences. Signal time curves need to be transformed to content time curves and the resulting residue functions were analyzed with a multiple-pathway, axially distributed perfusion model. These preliminary results suggest that quantitative assessment of myocardial perfusion is feasible, but additional studies should provide further confidence for this novel MR approach. The exact sensitivity and specificity of MR first-pass imaging in conjunction with extracellular contrast agents in patient studies and its diagnostic accuracy as judged against coronary angiography and scintigraphic perfusion imaging remain yet undefined. The second category of perfusion experiments does not require exogenous contrast agents and has not yet been tested in patient studies. Progress is reported on several MR perfusion-sensitive methods that use the tissue water as an endogenous contrast agent in combination with magnetization transfer techniques as well as paramagnetic deoxyhemoglobin for measuring tissue oxygenation using heavily T2*-weighted sequences for blood oxygen-level-dependent contrast. Possible future directions and developments toward further improvements for MR myocardial perfusion measurements and contraction-perfusion matching are also addressed.