QUANTITATIVE BEURTEILUNG DER MYOKARDPERFUSION MIT DER ULTRASCHNELLEN MAGNETRESONANZTOMOGRAPHIE

With the advent of ultrafast Magnetic Resonance Imaging (MRI), it is now possible to produce images with high temporal resolution. This gives the opportunity to record the passage of the paramagnetic contrast material Gadolinium-DTPA through the tissue of the heart muscle, yielding information on regional myocardial perfusion. We assessed the accuracy of MRI to detect and quantify reductions in coronary flow secondary to stenosis in dogs and patients. Regional blood flow was measured in dogs by left atrial injection of microspheres labeled with different radioactive isotopes. Signal intensity (SI) curves were generated in regions of interest over the myocardium and the cavum of the left ventricle. A newly developed two-compartment model based on the indicator-dilution method was used for interpretation of the SI-curves. In an optimization process the free parameters of the model equation were fitted to the measured SI-curves. The following flow parameters were determined: model parameter Q*, time to peak intensity (T), maximum signal intensity (SI(max)) and mean transit time (MTT) as calculated from a gamma variate fit. Absolute blood flow values were calculated for the parameters MTT and Q* assuming that the intravascular volume represents 10% of the total myocardial tissue volume. Measurements were performed on a 1.5 T Magnetom SP (Siemens AG, Erlangen) using a Turbo Flash sequence (TR=6.5 ms, TE=3 ms, Tl=100 ms, Flip Winkel=9°). Endsystolic images (voxel size=1.8, 2.7, 15 mm³) were taken with an 18-cm Helmholtz surface coil in the short-axis view. A Gd-DTPA bolus (0.05 mmol/kg) was injected into the left atrium of 3 anesthetized closed-chest dogs. From the myocardial SI-curves the different parameters of myocardial perfusion were compared with flow assessed by microsphere injection over a wide range of myocardial blood flows (from 0.04 ml/min/g to 7.6 ml/min/g). A third-order polynominal fit showed a good correlation for the parameter Q* and MTT whereas T and SI(max) were found to have a poor correlation. The linear regression analysis for a limited range of <2 ml/min/g showed a superior estimation of myocardial perfusion for the parameter Q* than MTT. Blood flow >2 ml/min/g was significantly underestimated by the MRT-measurements, but the parameter Q* showed the smallest amount of the divergent changes. In our study of 10 healthy human subjects and 10 patients the Gd-DTPA bolus was delivered through an intravenous catheter placed in the right subclavian vein. Patients with a significant proximal coronary artery stenosis identified by selective coronary angiography were found to have significantly lower blood flow values in regional myocardium with wall motion impairment compared to the non-diseased myocardium. It is concluded that ultrafast MRI can measure myocardial blood flow over a limited but clinically relevant range of flows as defined as slightly above normal to ischemic.