MRI manifestations of persistent microvascular obstruction and acute left ventricular remodeling in an experimental reperfused myocardial infarction

Abstract

Purpose: To investigate varied manifestations of persistent microvascular obstruction (PMO) and acute left ventricular (LV) remodeling in an experimental reperfused myocardial infarction (MI) using MRI.

Methods: In eleven Yorkshire pigs an acute MI was produced through a 90-minute balloon occlusion of the middle left anterior descending coronary artery, followed by reperfusion. All animals underwent MRI examinations on a 1.5T system including a SSFP functional study, first pass myocardial perfusion (FPMP), T1 preparation Look-Locker and delayed contrast-enhanced MRI (DE-MRI). Imaging was performed immediately post-intervention (day 0) and at days 7-9. In four animals a repeat MRI examination was performed at day 2 as well. Upon study completion, animals underwent histological analysis including infarct assessment with triphenyltetrazolium chloride (TTC).

Results: Following reperfusion, Thrombolysis In Myocardial Infarction (TIMI) Flow grade 3 was achieved in all animals, demonstrated by repeat angiography following balloon deflation (day 0). Various MR appearances of PMO were noticed including predominance in the subendocardial region, a central core within the infarcted tissue and also multiple separate clusters. In ten of eleven animals PMO was demonstrated as a persistent hypo-enhanced area in FPMP and DE-MRI, and identified as bright regions in later T1 difference images. In one animal PMO was identified only at day 2. At day 7-9 PMO could be identified on early DE-MRI at 5-15 minutes post Gd injection but not on late DE-MRI and T1 difference images after 45-60 minutes post-contrast. A larger volume of PMO and MI at day 2 was noted in comparison to data from day 0 but the difference was not statistically significant. An increased end-diastolic LV volume (EDV) without changes in end-systolic LV volume (ESV) and LV mass at end-diastolic phase (LVM) was observed at day 7-9 in comparison to data from day 0. There was good correlation between the relative extent of persistent MO in the infarcted myocardium (% MO/MI) and EDV at day 7-9 (r=0.83, n=10, P=0.003). MI was confirmed in all animals by TTC staining and/or histology.

Conclusion: A variable MR appearance of persistent microvascular obstruction is observed during a short time course MRI study of reperfused acute MI. Acute negative LV remodeling was closely related to the relative extent of persistent microvascular obstruction within the infarct myocardium.

Keywords: Microvascular obstruction; T1 difference image; contrast-enhanced MRI; left ventricular remodeling.

Figure 1.. Angiography and TIMI Flow Grade.

A: Pre-occlusion angiogram of middle LAD showing the patency of epicardial coronary artery; B: Balloon occlusion of middle LAD (arrow); C: Post-balloon release showing re-establishment of TIMI flow grade 3 (i.e. complete patency of epicardial coronary artery).

Figure 2.. Varied MR appearances and progression of persistent MO (MO indicated by arrows; animal #1: A-D; animal #2: E-F).

A: DE-MRI at day 0: MO predominantly in subendomyocardial region; B (DE-MRI at day 2): MO as a core within the MI region and with an increased volume; C: DE-MRI at day 7: MO is no longer seen; D. TTC staining confirming the presence of myocardial infarction; E-F: Multiple foci of MO on DE-MRI (E) and T1 Preparation difference image (F) at day 0. Note that MO appears as a hypo-enhanced region on DE-MRI and as a positive contrast on T1 difference image.

Figure 3.. Correlation between the relative extent of persistent MO in the infarct myocardium (% MO/MI) at day 0 and end-diastolic volume at day 7-9.