Detection of Macrophage Localization in the Abdominal Aortic Aneurysm Wall Using Ex Vivo Superparamagnetic Iron Oxide–Enhanced Magnetic Resonance Imaging

Abstract

Macrophages in the aneurysmal wall play an important role in the pathogenesis of abdominal aortic aneurysms (AAAs). Superparamagnetic iron oxide (SPIO) is a macrophage-specific contrast agent that results in negative enhancement on magnetic resonance imaging (MRI). SPIO-enhanced MRI targeting the intraluminal thrombus of AAAs has been previously reported. However, macrophages in the media and adventitia of AAA wall have not been investigated in detail. This study aimed to evaluate macrophage localization using SPIO-enhanced MRI in the media and adventitia of AAA wall, as macrophages play a crucial role in AAA pathogenesis. Here, we included study and control patients planning to undergo open surgery for AAA. The study patients received SPIO injection 2days preoperatively (the SPIO group, n=7), whereas the control patients did not receive this injection (the control group). Exvivo MRI was performed on the harvested AAA wall in the SPIO group during the surgery. The concordance between the number of macrophages and berlin blue (BB)-stained areas was histologically evaluated in both groups. Moreover, the concordance between regions of interest in MR images and BB-stained areas was evaluated. The proportion of BB-stained macrophages was higher in the SPIO group (0.93; interquartile range [IQR], 0.83-0.95) than in the control group (0.03; IQR, 0.026-0.11) (P<0.05), indicating uptake of SPIO by macrophages in the AAA wall. A significant positive correlation was found between the number of BB-stained macrophages and BB-stained areas using Kendall rank correlation coefficient in the SPIO group (τ=0.58; P<0.05). Significant correlations were found in the distributions of the region of interest of SPIO-enhanced MRI and BB-stained areas in the media and adventitia in 5 of 7 patients. Macrophages present in the media and adventitia of the AAA wall showed an uptake of the SPIO contrast agent injected 2days prior, which were then detected by exvivo MRI. This suggests that SPIO-enhanced MR images help detect the localization of macrophages on the AAA wall, indicating its potential to serve as a novel index for AAA pathogenesis.