Monitoring serial change in the lumen and outer wall of vertebrobasilar aneurysms

Abstract

Background and purpose: Estimation of the stability of fusiform aneurysms of the basilar artery requires precise monitoring of the luminal and outer wall volumes. In this report we describe the use of MR imaging and 3D postprocessing methods to study the evolution of those aneurysms.

Materials and methods: Nine patients with fusiform basilar artery aneurysms underwent MR imaging studies covering at least 2 different time points (mean delay between studies, 7.1 +/- 4.6 months). Imaging included multisection 2D T1-weighted fast spin-echo and/or 3D steady-state imaging to assess the outer wall and contrast-enhanced MR angiography to study the lumen. The outer and inner walls were extracted using, respectively, a manual delineation (made by 2 observers) and a thresholding technique. The 2 studies were subsequently coregistered at each time point, as well as between differing time points. Volumes of each vessel component were calculated.

Results: Mean volume was 6760 +/- 6620 mm(3) for the outer wall and 2060 +/- 1200 mm(3) for the lumen. Evolution of the lumen and outer wall was highly variable from 1 patient to another, with a trend toward increase of the vessel wall for the largest aneurysms. Interobserver reproducibility for outer wall delineation was on the order of 90%.

Conclusion: Combining MR imaging methods to study both the outer wall and lumen with 3D registration tools provides a powerful method for progression of fusiform basilar aneurysmal disease.

Fig 1.

MRA study from a patient with a large aneurysm (arrow) of the basilar trunk. Left, Maximum intensity projection in a paracoronal plane. Right, Axial multiplanar reconstruction through the basilar artery (arrow). Also noted are the carotid arteries (arrowheads).

Fig 2.

Steady-state bFFE axial image obtained from the same patient and same plane as on the right of Fig 1. Left, Full FOV image. Right, Magnified image of the aneurysm showing the manual contour used to delineate the outer wall.

Fig 3.

Lumenal and outer wall surfaces for another patient with aneurysmal basilar disease. Left, Surface shaded depiction of the lumen obtained from CE-MRA (blue for the extra-aneurysmal part of lumen and green for intra-aneurysmal part). Right, Surface shaded depiction of the outer wall obtained from bFFE images (red).

Fig 4.

Patient with a fusiform aneurysm of the distal basilar artery. Left, Registration of the lumen of the baseline study (blue) and the most recent study (red). Consistency of threshold across studies is ensured by requiring volume preservation in a reference segment of the carotid artery (black arrow). A detail of this registration by using a mesh display is shown on the right side showing the asymmetrical increase of the lumen volume of the aneurysm.

Fig 5.

The difference between outer wall and inner wall volumes is plotted as a function of outer wall volume. The volume of the aneurysm region that is not circulating increases with increasing aneurysm size.

Fig 6.

Changes of the inner and outer walls of aneurysms over time. Left, Plot of change in lumen shows that the lumenal wall of the aneurysm can either increase or decrease. The substantial decrease in lumen size for patient 3 is related to a partial thrombosis of the lumen between the 2 first time points. Right, Plot of changes in outer wall over time shows that changes are highly variable between individuals. Outer wall volumes either remain essentially unchanged (±5%) or increase over time.

Fig 7.

Plot of the ratio of lumen-to-outer-wall volume over time. Relative evolution of the 2 components (lumen and thrombus) of the aneurysms is variable from 1 patient to another.

Fig 8.

Bland-Altman plot representation of outer wall volume measured from bFFE and T1-weighted TSE sequence. Solid line shows the mean difference and dotted lines the 95% confidence interval of the mean.