T1-relaxation kinetics of extracellular, intracellular and intravascular MR contrast agents in normal and acutely reperfused infarcted myocardium using echo-planar MR imaging.

Abstract

The objective of this study was to determine and compare if MR contrast agents distributed into various compartments can provide estimation of fractional distribution volume (FDV) in normal and infarcted myocardium using inversion recovery echo-planar MR imaging (IR EPI). Three different types of MR agents were investigated: (a) an extracellular agent, GdDTPA-BMA (0.1 mmol/kg); (b) an intravascular agent, GdDTPA-albumin (0.025 mmol/kg); and (c) an intracellular agent, manganese chloride (0.025 mmol/kg). The null point was determined from a series of IR EPI images in which TI was varied. Temporal changes in DeltaR1 (DeltaR1 = 1/T1(post)-1/T1(pre)) were measured during the initial 29-59 min after administration. Rats (n = 24) were subjected to 1-h coronary artery occlusion/reperfusion. Histochemical staining confirmed the presence and location of infarction. GdDTPA-BMA caused increase in DeltaR1 of infarction < blood < < normal myocardium. DeltaR1 ratios were 1.55 +/- 0.08 for infarction and 0.33 +/- 0.03 for normal myocardium, consistent with FDV of 0.82 +/- 0.04 and 0.18 +/- 0.01. The fractional distribution of this agent in normal myocardium approximated the extracellular space of myocardium. GdDTPA-albumin caused increase in DeltaR1 of blood < < infarction < < normal myocardium. DeltaR1 ratio in normal, but not infarcted, myocardium was constant at 0.10 +/- 0.02 and approximated fractional blood volume. MnCl(2) caused equivalent increase in DeltaR1 of normal and infarcted myocardium. DeltaR1 of normal myocardium did not change overtime, whereas DeltaR1 of blood rapidly decreased, leading to overestimation of FDV in normal and infarcted myocardium. In conclusion, extracellular, intravascular and intracellular MR contrast agents exhibited different T1-relaxation kinetics in both normal and infarcted myocardium. Constant DeltaR1 ratio (myocardium/blood) after administration of MR contrast agent is a prerequisite for estimation of FDV of MR contrast agent in myocardium.