MR-IMAGING OF CONTRAST-ENHANCED PORCINE MYOCARDLAL INFARCTION - ASSESSMENT OF REPERFUSION AND TISSUE VIABILITY

To assess the usefulness of Dy-DTPA-BMA-induced signal reduction, as an indicator of myocar dial viability, myocardial infarction was induced in 17 domestic pigs by ligating a diagonal branch of the left anterior descending coronary artery (LAD). In 6 pigs, Dy-DTPA-BMA (1 mmol/kg b.w.) was administered 3 hours after induction of ischaemia. In 5 additional pigs, Gd-DTPA-BMA (0.3 mmol/kg b.w.) and Dy-DTPA-BMA (1 mmol/kg b.w.) were simultaneously injected after 4 hours of ischaemia to ascertain whether Dy-DTPA-BMA counteracted the signal enhancement effect of Gd DTPA-BMA. A further 6 pigs with infarctions, not administered contrast medium, served as controls. All pigs were sacrificed after 6 hours of ischaemia, and the extirpated hearts were investigated with MR (ex vivo). The concentrations of Dy and Gd were determined in tissue samples from infarcted and non-ischaemic myocardium. The extracellular concentrations of both contrast media were monitored over time during 2 hours in the double-contrast group (in vivo), using a microdialysis technique and analysed by inductively coupled plasma atomic emission spectrometry (ICP-AES). The infarctions demonstrated a high SI in the proton density- and T2-weighted sequences, in both the Dy-DTPA-BMA and control groups, although the former group demonstrated a 3-fold greater concentration of Dy in infarcted compared with non-ischaemic myocardium. Dy-DTPA-BMA did not counteract the Gd-DTPA-BMA-induced enhancement of the infarcted tissue despite a 3-fold higher concentration. This lack of detectable susceptibility effects of Dy may be caused by a loss of cell membrane integrity in the infarcts, resulting in a homogeneous intra- and extracellular distribution of the contrast agent. This hypothesis of an expanded volume of distribution in infarcted tissue was further supported by the microdialysis data, demonstrating a similar extracellular concentration of contrast agents in infarcted and non-ischaemic myocardium, despite a proven 3-fold greater concentration in infarcted tissue samples. To investigate whether Gd-DTPA-BMA-enhanced MR imaging (ex vivo) permits differentiation between reperfused and non-reperfused myocardial infarction, and whether Dy-DTPA-BMA-enhanced MR imaging enables a differentiation between reversible and irreversible myocardial injury following reperfusion, myocardial infarction was induced in 24 domestic pigs (divided into 4 groups) by placing a patched ligature around a diagonal branch of the LAD. Four additional hearts were reperfused after 2 min of brief occlusion, not long enough to cause irreversible injury. Myocardial infarction was also induced in 12 pigs to ascertain whether a double-contrast method, combining a positive and a negative MR contrast agent, could improve the visualization of reperfused myocardial infarctions, compared with a single-contrast technique using Gd-DTPA-BMA alone. Radiolabelled microspheres were used to confirm ischaemia and to verify reperfusion in infarcted myocardium. The concentrations of Dy and Gd in tissue samples from infarcted, reversibly injured and non-ischaemic myocardium were determined by ICP-AES. The reperfused infarctions were strongly enhanced in the T1-weighted images, corresponding to a 5-fold higher Gd concentration compared with non-ischaemic myocardium. Hearts subjected to occlusion without reperfusion demonstrated only a peripheral rim enhancement of the infarctions, while the central parts demonstrated a SI and Gd content similar to that of non-ischaemic myocardium. In the proton density- and T2-weighted sequences, reperfused, infarcted myocardium demonstrated a high SI despite a 5-fold higher Dy-concentration compared with both non-reperfused infarcted and non-ischaemic myocardium. Reversibly injured myocardium could not be distinguished from adjacent non-ischaemic myocardium, probably due to a preserved cell membrane integrity, preventing Dy-DTPA-BMA from entering the cells. Dy-DTPA-BMA-induced signal reduction may thus serve as an indicator of myocardial viability. The double-contrast technique, whereby Dy-DTPA-BMA was injected in addition to Gd-DTPA-BMA, yielded a superior infarct visualization in the T2-weighted sequences, compared with the single-contrast technique using Gd-DTPA-BMA alone. This improvement of infarct visualization could be attributed to Dy-induced loss of SI, due to susceptibility effect in non-ischaemic myocardium. The double-contrast technique thus resulted in an excellent infarct visualization in both T1- and T2-weighted sequences.