INTRODUCTION Intracranial aneurysms (IAs) are relatively common life-threatening diseases with a prevalence of 3%-5% in adults¹. Several imaging markers for unruptured IAs instability have been explored and aneurysm wall enhancement (AWE) as identified by contrast-enhanced vessel wall MRI (VW-MRI) has also been reported as a marker of unstable IAs^2,3. Although conventional imaging is limited in its ability to detect inflammatory wall changes, AWE has been accepted as s surrogate marker of aneurysm wall inflammation⁴. However, evaluation and stratification of aneurysm rupture risk is still clinical challenging. One potential novel imaging marker of aneurysm vulnerability is irregular pulsation detected by four-dimensional computed tomography angiography (4D-CTA), which may represent a focal weakening of the aneurysm wall⁵. Our previous studies showed IAs with irregular pulsation are associated with larger size, irregular-shape, and non-internal carotid artery origin⁶. But the association between AWE and irregular pulsation of unruptured IAs is still unknown and our study aims to investigate this open question.
METHODS This retrospective study included consecutive patients with IAs who underwent 4D-CTA and VW-MRI between January 2017 to July 2021. 4D-CTA was performed on a 320-detector row CT scanner (Aquilion ONE VISION, Canon Medical System Corporation, Otawara, Japan). Vessel wall MRI was performed on a 3.0T MR scanner (Prisma, Siemens Healthcare or Ingenia, Philips Healthcare or Prisma). The imaging protocols for MR scanning on Ingenia and Prisma were as follows respectively: 1) T1WI volume isotropic turbo spin-echo acquisition (T1-VISTA) sequence with following parameters: TR/TE 800/20msec, number of slices 80, field of view 250mm×162mm, voxel size 0.6mm×0.6mm×0.6mm, acquisition matrix 416×269, and scan time 5 minutes 2 seconds. 2) T1-weighted 3D black-blood fast spin echo with variable flip angle trains (SPACE) sequence with following parameters: TR/TE 1000/15msec, number of slices 240, field of view 193mm× 193mm, voxel size 0.6mm× 0.6mm× 0.6 mm, acquisition matrix 320× 320, and scan time 9 minutes 13 seconds. Post-contrast VW-MRI sequence was acquired 5 minutes after Gadolinium-based contrast agent (Magnevist, Bayer Healthcare, Berlin, German) injection (with a dose of 0.1 mmol/kg, at a rate of 1.5ml/s). Geometric and morphologic parameters were measured and the presence of irregular pulsation (defined as a temporary focal protuberance ≥1 mm on more than three successive frames) was identified on 4D-CTA movies⁶. AWE was estimated on VW-MRI and classified as 4 grades (grade 0 for none or questionable focal trace enhancement, grade 1 for focal thick [<1mm] enhancement, grade 2 for thin [maximum thickness, ≤1 mm] circumferential wall enhancement, or grade 3 for thick [maximum thickness, ≥1 mm] circumferential wall enhancement)⁷. The association between AWE and irregular pulsation was analyzed.
RESULTS One hundred and fifty-three patients with 176 saccular IAs were included in this study. Irregular pulsation was observed in 35.8% (63/176) of IAs. Among 176 aneurysms, 64 (36.4%) were enhancing; 23(13.1%) were grade 1, 26 (14.8%) were grade 2, and 15 (8.5%) were grade 3. Larger size, aspect ratio and irregular shape were positively associated with irregular pulsation (p < 0.05). IAs with irregular pulsation had much less AWE grade 0 (27.0% versus 84.1%, p<0.05) and more AWE grade 1, 2 and 3 (25.4%, 30.2% and 17.5%, respectively) compared with IAs without irregular pulsation (6.2%, 6.2% and 3.5%, respectively) (all p<0.05). There was a significantly strong correlation between AWE and irregular pulsation (phi coefficient=0.569, p<0.001).
DISCUSSION Recent studies have showed AWE was promising for identification of unstable aneurysms and it is gaining increasing attention^7,8. This study demonstrated a strong positive correlation between AWE and irregular pulsation, indicating that irregular pulsation might have the same potential for predicting aneurysm rupture risk. Wall enhancement possibly indicates vasa vasorum which is an indirect marker of inflammation⁹. Persistent inflammation of aneurysm wall involves increased elastic lamina breakdown and may thereby cause focal weakening of the wall, perhaps explaining the formation of irregular pulsation. Further research is still needed, however, to study the association between irregular pulsation and AWE, wall thinning and inflammation. We think irregular pulsation and AWE might be two different concepts of the aneurysm instability, but should be mutually complementary. We are currently running a prospective study and scanning patients with both 4D CTA and black blood MRI, and we hope such multi-modality imaging study will help determine the best predictors of aneurysm instability.
CONCLUSION The strong correlation between AWE and irregular pulsation further confirmed the important role of irregular pulsation in predicting aneurysm instability. These two imaging markers should be validated in future longitudinal studies for their values in predicting aneurysm growth and rupture.

Acknowledgements

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