Quantitative regional blood volume studies in rat myocardium in Vivo

Many pathophysiological processes in the myocardium are in close relation to changes of the regional blood volume and regional myocardial blood flow or perfusion. Only few methods exist to obtain quantitative values for these parameters. Quantitative regional blood volume (RBV) studies in rat myocardium are presented using snapshot fast low angle shot (FLASH) inversion recovery T₁ measurements with two different blood pool contrast agents, gadolinium diethylenetriamin-opentaacetic acid (Gd-DTPA) albumin and Gd-DTPA polyly-sine. In contrast to previous attempts, each snapshot FLASH image acquisition was ECG-triggered under breathhold conditions. To measure relaxation times shorter than a heart cycle, each T₁ sequence was repeated two times with different delays between inversion pulse and first image acquisition. The experiments were performed on a Bruker Biospec 70/21 using a homogeneous transmitter coil and a circularly polarized surface receiver coil, a special ECG trigger unit, and a respirator that is controlled by the pulse program. Based on a fast exchange model RBV_m maps were calculated from the relaxation time maps for different concentrations of the two blood pool contrast agents. A significant dependence of the RBV, values on blood T₁ was found. This is in accordance with a model that has been developed recently relating the dependence of RBV_m on T₁ of blood to perfusion. For Gd-DTPA albumin, the application of the model to the experimental data yields realistic values for RBV and perfusion. The values, which are in accordance with literature data, were obtained at highest contrast agent concentrations i.e., lowest relaxation times of blood (ca. 200 ms).