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Detection of Macrophage Localization in the Abdominal Aortic Aneurysm Wall Using Ex Vivo Superparamagnetic Iron
Michihisa Umetsu1, Hitoshi Goto1, Yasuhiro Nakamura2
1Division of Vascular Surgery, Department of Surgery, Tohoku University Hospital, Sendai, Miyagi, Japan.
Background:
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.
Methods:
Here, we included study and control patients planning to undergo open surgery for AAA. The study patients received SPIO injection 2 days preoperatively (the SPIO group, n = 7), whereas the control patients did not receive this injection (the control group). Ex vivo 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.
Results:
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.
Conclusions:
Macrophages present in the media and adventitia of the AAA wall showed an uptake of the SPIO contrast agent injected 2 days prior, which were then detected by ex vivo 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.
Insights
Superparamagnetic iron oxide (SPIO) enhanced MRI can detect macrophages in the abdominal aortic aneurysm (AAA) wall. This imaging technique shows potential for evaluating AAA pathogenesis by visualizing macrophage localization.
Area of Science:
- Biomedical Imaging
- Vascular Biology
- Immunology
Background:
- Macrophages are key players in abdominal aortic aneurysm (AAA) pathogenesis.
- Superparamagnetic iron oxide (SPIO) is a macrophage-specific MRI contrast agent.
- Previous studies focused on SPIO-enhanced MRI of AAA thrombus, not the vessel wall.
Purpose of the Study:
- To evaluate macrophage localization in the media and adventitia of the AAA wall using SPIO-enhanced MRI.
- To investigate the role of macrophages in AAA pathogenesis through detailed wall analysis.
Main Methods:
- Patients undergoing open AAA surgery were divided into SPIO-injected (n=7) and control groups.
- Ex vivo MRI of harvested AAA walls was performed on the SPIO group.
- Histological evaluation using Berlin blue (BB) staining assessed macrophage presence and SPIO uptake.
Main Results:
- A significantly higher proportion of BB-stained macrophages was observed in the SPIO group compared to controls (0.93 vs. 0.03, P < 0.05).
- A positive correlation was found between BB-stained macrophage counts and BB-stained areas in the SPIO group (τ = 0.58, P < 0.05).
- SPIO-enhanced MRI regions of interest correlated significantly with BB-stained areas in the media and adventitia of 5 out of 7 patients.
Conclusions:
- Macrophages in the AAA wall media and adventitia effectively uptake SPIO contrast agent.
- Ex vivo MRI successfully detected SPIO-labeled macrophages in the AAA wall.
- SPIO-enhanced MRI holds potential as a novel imaging biomarker for assessing AAA pathogenesis.
