- Neurosurgery Research Group, Biomedicum Helsinki, Helsinki, Finland
- Experimental MRI Laboratory, Department of Neurology, Helsinki, Finland
- Department of Neurosurgery, Helsinki University Central Hospital, Helsinki, Finland
Juhana K Frösen
Neurosurgery Research Group, Biomedicum Helsinki, Helsinki, Finland
Department of Neurosurgery, Helsinki University Central Hospital, Helsinki, Finland
DOI:10.4103/2152-7806.132960Copyright: © 2014 Honkanen P This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
How to cite this article: Honkanen P, Juhana K Frösen, Abo-Ramadan U, Hernesniemi JA, Mika R Niemelä. Visualization of luminal thrombosis and mural Iron accumulation in giant aneurysms with Ex vivo 4.7T magnetic resonance imaging. Surg Neurol Int 21-May-2014;5:74
How to cite this URL: Honkanen P, Juhana K Frösen, Abo-Ramadan U, Hernesniemi JA, Mika R Niemelä. Visualization of luminal thrombosis and mural Iron accumulation in giant aneurysms with Ex vivo 4.7T magnetic resonance imaging. Surg Neurol Int 21-May-2014;5:74. Available from: http://sni.wpengine.com/surgicalint_articles/visualization-of-luminal-thrombosis-and-mural-iron-accumulation-in-giant-aneurysms-with-ex-vivo-4-7t-magnetic-resonance-imaging/
Background:Better diagnostic tools to identify rupture-prone saccular intracranial aneurysms (sIA) are needed. Inflammation and luminal thrombus associate with degeneration and rupture of the sIA wall. Iron-uptake has been detected in the inflammatory cells of the sIA wall and thrombus is the likely source of this iron. We investigated ex vivo the use of magnetic resonance imaging (MRI) to detect iron accumulation and luminal thrombus in giant sIAs.
Methods:Giant sIAs (n = 3) were acquired from microsurgical operations, fixed with formalin, embedded in agar and imaged at 4.7T. Samples were sectioned maintaining the orientation of the axial plane of MRI scans, and stained (hematoxylin-eosin and Prussian blue).
Results:All three giant sIAs showed a degenerated hypocellular wall with both mural and adventitial iron accumulation and displayed different degrees of luminal thrombus formation and thrombus organization. Signal intensity varied within the same sIA wall and associated with iron accumulation in all tested sequences. Wall areas with iron accumulation had significantly lower signal to noise ratio (SNR) compared with areas without iron accumulation (P = 0.002). Fresh and organizing thrombus differed in their MRI presentation and differed in signal intensity of the aneurysm wall (P = 0.027).
Conclusion:MRI can detect ex vivo the accumulation of iron in giant sIA wall, as well as fresh and organizing luminal thrombus. These features have been previously associated with fragile, rupture-prone aneurysm wall. Further studies of iron accumulation as a marker of rupture-prone aneurysm wall are needed.
Keywords: Giant aneurysm, iron, magnetic resonance imaging, rupture risk, thrombus
Despite advances in microsurgical and endovascular therapies, preventive treatment of saccular intracranial aneurysms (sIA) is not without risk of morbidity and mortality. Since unruptured sIAs are relatively common in the middle-aged population (approximately 3.5%),[
Histopathological studies have demonstrated structural differences in ruptured and unruptured sIA walls,[
Iron accumulates in the aneurysm walls also without any iron-oxide contrast injection.[
The macrophages that take up the iron-oxide given as a contrast agent are likely to take up also the iron that accumulates in the aneurysm wall spontaneously, making it therefore possible that one might detect iron-loaded macrophages in degenerated sIA walls even without contrast. This could potentially be used to identify sIAs with a rupture-prone wall.
The iron that accumulated in the aneurysm wall may be derived from microhemorrhages in the aneurysm wall or from luminal thrombus, and possibly even from iron in the plasma. Microhemorrhages in the aneurysm wall are not frequently observed,[
We investigated with ex vivo MRI and histopathology the wall of giant sIAs that often contain a lot of luminal thrombus of different ages, and therefore are likely to also have iron accumulation in their walls. Our aim was to test the concept that MRI without any iron-oxide contrast could be sufficient to detect iron accumulation and thrombus in the sIA wall, which might help in the detection of degenerated, rupture-prone sIA wall.
Three giant sIAs were collected during microsurgical treatment. Samples were fixed in 10% formalin. Patient and sIA characteristics were obtained from medical records [
Samples were embedded in 1% agar gel and imaged with a 4.7T MRI scanner (Bruker Pharmascan, Bruker, Germany) in a linear birdcage radiofrequency coil with inner diameter of 60 mm. After a pilot scan, the whole aneurysm was imaged with rapid acquisition with relaxation enhancement (RARE) sequence, with RARE factors 4 for T1-weighted (TR 600 ms, TEeff 10 ms, number of excitations (NEX) 5, matrix size (MTX) 256 × 256), and with RARE factors 8 for T2-weighted (TR 2600 ms, TEeff 40 ms, NEX 10, MTX 256 × 256) and with RARE factors 4 for proton density weighted (TR 3000 ms, TEeff 10 ms, NEX 3, MTX 256 × 256) imaging. Additionally, fast low-angle shot sequence (FLASH) was used for T2* weighted images (TR 500 ms, TE 8 ms, NEX 4, MTX 256 × 256 or 512 × 512 depending on aneurysm size) and fast imaging in steady-state precession (FISP) T1/T2* weighted images (TR 14.226 ms, TE 7.113 ms, NEX 20, MTX 512 × 512) scans were acquired.
After imaging, while still embedded in agar, samples were cut parallel to the axial plane of MR images and reembedded in OCT Tissue Tek compound (Sakura, Netherlands). The distance of the plane of cut from the apex of the aneurysm dome was measured. Tissues were serial sectioned at 10 μm and the cut surface was photographed at 100 μm intervals. Each section and its respective distance from the initial cut was recorded. Sections were stained using either hematoxylin and eosin or prussian blue and nuclear fast red. MR images, photographs taken during sectioning, distance data and stained sections were compared to verify the relationship between MR images and histological sections. Examples are provided in
Representative T1/T2*-weighted MR image (a), histological section stained with Prussian blue (b) and the corresponding photograph taken during sectioning (c). FT = fresh thrombus, OT = organizing thrombus, arrow heads = wall with iron accumulation, arrows = wall without iron accumulation. Microphotographs D and E demonstrate accumulation of iron (blue, marked with arrows, Prussian blue staining) into giant aneurysm wall without (d) luminal thrombus or with (e) luminal thrombus
Analysis of MRI scans
MR images were reviewed on a Macintosh workstation using OsiriX software (OsiriX version 5.5.1, open source,
SDnoise was the standard deviation of the signal measured from a ROI drawn in the background air (free of ghosting artefacts).
Differences in SNR distributions were compared using related samples nonparametric Wilcoxon signed rank test. The level of significance was set at P < 0.05. All statistical analysis was performed with IBM SPSS Statistics software (IBM SPSS Statistics Version 19, IBM Corporation, New York, USA).
All three sIAs had luminal thrombus formation and thrombus organization and showed a degenerated hypocellular wall with focal luminal, intramural, and adventitial iron accumulation [
Aneurysm walls displayed varying SI in all the sequences and wall SI changed within a single sIA [
Histological view of sIA wall segment (a) and corresponding T1-weighted MR image (b) show a thin aneurysm wall with intramural iron accumulation (Arrows) and luminal organizing (OT) and fresh thrombus (FT). Scale bars = 1 mm. SNR analysis shows differences in median SNR and 95% confidence intervals of thrombi of different age (c) and wall segments with and without iron accumulation (d)
In histological analysis, organized thrombus was found in all 3/3 sIAs and fresh thrombus was found in 2/3 of the aneurysms. In T1-weighted images, fresh thrombus had high and homogenous SI, whereas organizing thrombus had low and homogenous SI. In T2 and T2* weighted images, fresh thrombus had mostly low and heterogenous SI, whereas organizing thrombus had high (although also heterogenous) SI [
This is a proof of concept study that demonstrates that MRI without contrast can be used to detect iron accumulation in degenerated, hypocellular giant sIA walls with histological structure similar to rupture-prone sIA walls.[
Luminal thrombus was detectable in all samples in all the MRI sequences we used. In addition, we found a visually clear difference in the SI of organized or fresh luminal thrombus in MR images [Figures
MRI with T2* sensitive and other sequences can be used to detect spontaneous iron accumulation in the aneurysm wall. This finding is of potential importance for the detection of rupture-prone, aneurysm walls, as well as for imaging studies that use ferumoxytol or other iron-based contrast agents to detect inflammation in aneurysms, and merits further investigation in larger series.
The work was supported by grants from the special governmental subsidy for health sciences research (EVO) and The Sigrid Juselius Foundation.
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