The objective of this study was to investigate if dynamic contrast-enhanced magnetic resonance imaging (MRI) can be used to quantitate the distribution volume (ve) in regions of normal and infarcted myocardium. ve reflects the volume of the extra-cellular, extra-vascular space within the myocardial tissue. In regions of the heart where an infarct has occurred, the loss of viable cardiac cells results in an elevated ve compared to normal regions. A quantitative estimate of the magnitude and spatial distribution of ve is significant because it may provide information complementary to delayed enhancement MRI alone.
Using a hybrid gradient echo-echo planar imaging (GRE-EPI) pulse sequence on a 1.5T MRI scanner, 12 normal subjects and four infarct patients were imaged dynamically, during the injection of a contrast agent, to measure the regional blood and tissue enhancement in the left ventricular (LV) myocardium. Seven of the normal subjects and all of the infarct patients were also imaged at steady-state contrast enhancement to estimate the steady-state ratio of contrast agent in the tissue and blood (Ct/Cb)—a validated measure of ve. Normal and infarct regions of the LV were manually selected and the blood and tissue enhancement curves were fit to a compartment model to estimate ve. Also, the effect of the vascular blood signal on estimates of ve was evaluated using simulations and in the dynamic and steady-state studies.
Aggregate estimates of ve were 23.6% ± 6.3% in normal myocardium and 45.7% ± 3.4% in regions of infarct. These results were not significantly different from the reference standards of Ct/Cb (22.9% ± 6.8% and 42.6% ± 6.3%, p=0.073). From the dynamic contrast-enhanced studies, approximately one minute of scan time was necessary to estimate ve in the normal myocardium to within 10% of the steady-state estimate. In regions of infarct, up to three minutes of dynamic data was required to estimate ve to within 10% of the steady-state ve value.
By measuring the kinetics of blood and tissue enhancement in the myocardium during an extended dynamic contrast enhanced MRI study, ve may be estimated using compartment modeling.