Coronary Microembolization with Normal Epicardial Coronary Arteries and No Visible Infarcts on Nitrobluetetrazolium Chloride-Stained Specimens: Evaluation with Cardiac Magnetic Resonance Imaging in a Swine Model

Abstract

Objective: To assess magnetic resonance imaging (MRI) features of coronary microembolization in a swine model induced by small-sized microemboli, which may cause microinfarcts invisible to the naked eye.

Materials and methods: Eleven pigs underwent intracoronary injection of small-sized microspheres (42 µm) and catheter coronary angiography was obtained before and after microembolization. Cardiac MRI and measurement of cardiac troponin T (cTnT) were performed at baseline, 6 hours, and 1 week after microembolization. Postmortem evaluation was performed after completion of the imaging studies.

Results: Coronary angiography pre- and post-microembolization revealed normal epicardial coronary arteries. Systolic wall thickening of the microembolized regions decreased significantly from 42.6 ± 2.0% at baseline to 20.3 ± 2.3% at 6 hours and 31.5 ± 2.1% at 1 week after coronary microembolization (p < 0.001 for both). First-pass perfusion defect was visualized at 6 hours but the extent was largely decreased at 1 week. Delayed contrast enhancement MRI (DE-MRI) demonstrated hyperenhancement within the target area at 6 hours but not at 1 week. The microinfarcts on gross specimen stained with nitrobluetetrazolium chloride were invisible to the naked eye and only detectable microscopically. Increased cTnT was observed at 6 hours and 1 week after microembolization.

Conclusion: Coronary microembolization induced by a certain load of small-sized microemboli may result in microinfarcts invisible to the naked eye with normal epicardial coronary arteries. MRI features of myocardial impairment secondary to such microembolization include the decline in left ventricular function and myocardial perfusion at cine and first-pass perfusion imaging, and transient hyperenhancement at DE-MRI.

Keywords: Cardiac MR imaging; Coronary angiography; Myocardial contraction; Myocardial microinfarct.

Fig. 1. Coronary angiograms pre- and post-coronary microembolization.

Coronary angiography before (A-D) and after coronary microembolization (E-H) demonstrates normal-appearing epicardial arteries with coronary flow of TIMI grade 3 after injection of microspheres. TIMI = Thrombolysis In Myocardial Infarction

Fig. 2. Examples of MR perfusion, DE-MRI, and NBT staining in one animal.

First-pass perfusion (A, C) and DE-MRI (B, D) at 6 hours (A, B) and 1 week (C, D) demonstrate effects of microembolization induced by small-sized microemboli. Perfusion defect was visualized in embolized region at 6 hours (A, arrowheads) but largely decreased at 1 week after microembolization. DE-MRI demonstrated patchy hyperenhanced regions at 6 hours (B, arrowheads) but not at 1 week (D). Microinfarcts were invisible to naked eye on NBT specimen (E) at 1 week after microembolization. DE-MRI = delayed contrast enhancement magnetic resonance imaging, NBT = nitrobluetetrazolium chloride

Fig. 3. Myocardial microinfarct demonstrated by microscopic examination.

Histologic slices (hematoxylin-eosin stain) obtained from target territory of coronary microembolization. Arrow (A) shows microsphere obstructing microvessel. I (B) indicates myocardial microinfarct and V (B) indicates viable myocardium. Scale bars = 50 µm.