Assessment of myocardial infarction using magnetic resonance imaging

Coronary artery disease is a major cause of mortality and morbidity in Western society. Development of magnetic resonance imaging (MRI) techniques can potentially provide a non-invasive assessment of myocardial ischemia and infarction. However, more quantitative MRI techniques still need to be developed and validated. In MR imaging of cardiac function, cardiac and respiratory motion can cause blurring and ghost artifacts in the images. The conventional fast imaging techniques during breath-hold have limitations since cardiac patients often have difficulty to hold their breath. In this study, a dual cardiac-respiratory gating device has been constructed to acquire high quality tagged cardiac images without requiring suspension of respiration. Moreover, it allows high spatial and temporal resolution image acquisition, as well as provides a better tag-myocardium contrast in the image. Magnetization transfer has offered a novel contrast mechanism and a unique quantitative method of tissue characterization. A cardiac- and respiratory-gated fast gradient-echo imaging sequence with magnetization transfer preparation has been developed and applied to cardiac imaging. The pilot study in normal volunteers has shown a uniform MT saturation across the ventricular wall and the cardiac cycle, whereas a decreased MT suppression was observed in chronic infarct compared to normal myocardium in an in vivo ovine model. Theoretical analysis of the magnetization transfer Z spectra using a non-exchange two-spin system model revealed that changes of the lineshape due to myocardial infarction were the results of changes in free and macromolecule-bound water fraction, as well as the T1/T2 value. In the quantification of myocardial perfusion using first-pass fast gradient-echo echo-planar imaging, a calibration procedure was developed to measure dynamic extracellular contrast agent uptake in the blood and myocardium, as well as to correct for the inhomogeneous sensitivity profile of the surface coils utilized. Using the same ovine model, absolute myocardial blood flow was quantified with MR perfusion imaging; and MRI findings were validated with the microsphere technique. The further development of quantitative MRI techniques for study of myocardial function, tissue characterization, and perfusion should provide us a comprehensive means for assessment of myocardial infarction.