Introduction

Intracranial aneurysms have a prevalence of up to 3% and generally harbor a low risk of rupture [1, 2]. Still, subarachnoid hemorrhage following rupture of an intradural aneurysm is associated with considerable mortality and morbidity [3]. Therefore, risk stratification of patients diagnosed with an unruptured intradural aneurysm is crucial, but optimal management remains controversial. Recently, published data suggest an association of wall enhancement in intracranial saccular aneurysms on MR vessel wall imaging with a higher risk for rupture [4,5,6,7]. Specifically, thick circumferential wall enhancement has been linked to a higher risk of rupture [4]. Moreover, wall enhancement probably identifies ruptured aneurysms in patients with subarachnoid hemorrhage and multiple aneurysms [8], and in AVMs, respectively [9]. In a prior study, inflammatory and degenerative wall changes were detected in a histologic analysis of unruptured aneurysms showing wall enhancement [10].

Focal enhancement has been reported to be associated with ruptured aneurysms and the rupture site [11, 12]. While low-flow conditions near the aneurysm wall have been described to coincide with focal enhancement [MR vessel wall imaging

MR vessel wall imaging was acquired on a 3T MR scanner (Achieva, Philips Healthcare, Best, The Netherlands) equipped with a 32-channel head coil. The protocol comprised a T1-weighted black blood 3D variable refocusing flip angle sequence (VISTA) (TE/TR, 27/700 ms; matrix, 268 × 332; field of view, 200 × 250 × 160 mm; voxel size, 0.75 × 0.75 × 0.75 mm; acquisition time, 4 min 45 s) and a TOF-angiography; both sequences were acquired before and after administration of 0.1 mmol/kg gadoterate meglumine (Dotarem, Guerbet, Villepinte, France).

Contrast enhancement of the aneurysm wall was visually assessed on multiplanar reformatted postcontrast 3D T1 black blood images.

Focal wall enhancement was defined as a hyperintense signal adjacent to the black blood lumen or wall-adherent thrombus with at least one clearly discernible unenhanced wall segment of any extent. The presence of a double-layer appearance indicative of thrombus, defined as a non-enhancing, wall-adjacent structure bordered by an enhancing layer on both the luminal and the wall-adjacent surface, was documented.

Two neuroradiologists (N.L., F.W.), each with at least 9 years of experience in neurovascular imaging, who were blinded for clinical and hemodynamic data, independently assessed the aneurysm wall. Disagreement between the readers was solved by consensus.

Digital subtraction angiography

Digital subtraction angiography (DSA) was performed under local anesthesia on a biplane flat panel DSA unit (Allura Xper FD 20/10, Philips, Best, The Netherlands) via a femoral access. 3D-RA was performed with a selective contrast injection of 20 ml Imeron 300 (Iomeprol, Bracco Imaging, Milan, Italy) in the ipsilateral internal carotid artery at a flow rate of 2 ml/s. The dataset was transferred to a dedicated workstation (XtraVision, Philips, Best, The Netherlands) and reconstructed with a voxel size of 0.27 × 0.27 × 0.27 mm3.

Vessel segmentation and image co-registration

Segmentation of vessel surfaces from 3D-RA datasets and co-registration to MRI was carried out using MeVisLab 2.7 (MeVis Medical Solutions AG, Bremen, Germany). A threshold-based segmentation approach was applied to the 3D-RA images, followed by marching cubes to obtain a triangulated surface mesh. Enhanced regions were manually delineated in black blood MR images using the contour segmentation objects library of MeVisLab. The enhancement mask was converted into a triangle surface mesh as well. Finally, co-registration utilizing a rigid multi-resolution registration [25] with a Quasi-Newton optimizer within MeVisLab yielded transformation matrices for each patient’s dataset. Applying the transformation matrices to the manually segmented enhancement regions allowed for the combination with the vessel surface (see Fig. 1). The percentage of the aneurysm surface area covered by enhanced regions was determined for each aneurysm (enhancement area EA) and utilized as a quantitative measure for the extent of enhancement.

Fig. 1
figure 1

A Illustration of a 3D rotational angiography (3D-RA) dataset of a middle cerebral artery aneurysm (arrow). B Corresponding MRI dataset with yellow contours delineating enhanced regions. C 3D model of the enhanced regions. D Co-registered 3D-RA aneurysm surface mesh and 3D MRI enhancement surface mesh

Full size image

Hemodynamic simulations

Based on the segmented aneurysm surfaces, hemodynamic simulations derived from 3D-RA datasets were carried out in all cases using STAR-CCM+ 13.04 (Siemens PLM Software Inc., Plano, TX, USA). For spatial discretization a base size of Δx = 0.08 mm was chosen leading to 1.2 to 3.1 million polyhedral and prism elements in total depending on the considered vasculature. At each inlet, a time-dependent flow waveform was applied obtained from PC-MRI measurements in a healthy volunteer [26]. Flow-splitting at all outlet cross-sections was defined according to the recent model by Chnafa et al. [27]. Blood was treated as an incompressible (ρ = 1055 kg/m3) and Newtonian (η = 4 mPa s) fluid, while the flow was assumed to be laminar. Three cardiac cycles were simulated, while only the last one was considered for analysis.

Quantification of shear-related parameters was conducted for enhanced and non-enhanced wall segments separately, including time-averaged wall shear stress (AWSS), normalized wall shear stress (nWSS), maximum oscillatory shear index (maxOSI), and low shear area (LSA). Mean neck inflow rate (meanNIR), inflow concentration index (ICI), and shear concentration index (SCI) were determined for entire aneurysms [28].

Semi-automatic extraction of morphological parameters

In each aneurysm, a semiautomatic neck curve reconstruction was performed. First, four points in the neck region of the aneurysm were determined following a manual one-click selection of the aneurysm. Based on these points, a neck curve was created and the ostium plane derived. For a detailed description of the computational process, see Saalfeld et al. [29]. Utilizing the neck curve and ostium plane, the following morphological parameters were automatically extracted: aneurysm surface area (A), aneurysm volume (V), ostium area1 (OA1), ostium area2 (OA2), maximum diameter (Dmax), maximum height (Hmax), maximum width perpendicular to Hmax (Wmax), maximum height perpendicular to the ostium plane (Hortho), maximum width parallel to the ostium plane (Wortho), maximum neck curve diameter (Nmax), average neck curve diameter (Navg), aspect ratio 1 (AR1), aspect ratio 2 (AR2), volume of the convex hull (Vch), surface area of the convex hull (Ach), ellipticity index (EI), non-sphericity index (NSI), undulation index (UI), and aneurysm tilt angle γ [29].

Aneurysm clip** and histologic analysis

Microsurgical clip** of the aneurysms was performed via a pterional-transsylvian approach.

Preoperative, postoperative, and intraoperative management of the patients was performed according to institutional standards, including preoperative and postoperative DSA of the intracranial vessels, and, in most cases, intraoperative angiography with indocyanine green. After clip** of the aneurysm, the aneurysm sac was removed.

Histologic analysis was performed as described before [30]. In brief, formalin-fixed and paraffin-embedded resection specimens were cut into 2.5-μm-thin tissue sections. Slides were stained either with hematoxylin and eosin, or using rabbit anti-MPO polyclonal antibodies (1:1000, DAKO, Glostrup, Denmark) after antigen retrieval with ER2 (EDTA-buffer bond pH 8.9), or a monoclonal antibody against CD34 (Beckman Coulter, Clone QBEnd10, 1:700) without antigen retrieval using the autostainer Bond Max System (Leica Microsystems GmbH, Wetzlar, Germany).

Statistical analysis

Statistical analysis was performed in RStudio (RStudio v1.2.1335 with R v3.5.1, RStudio Inc., Boston, USA). Normally distributed variables are presented as mean (range), non-normally distributed continuous variables as median (range) or median (standard deviation), and categorical variables in the form of percentage distributions. A Shapiro–Wilk test was employed to determine, if a continuous variable was normally distributed. Non-normally distributed data were nonlinearly transformed before they were included in univariate analysis. Hemodynamic parameters were analyzed using the Wilcoxon signed-rank test for comparison of enhanced regions and the entire aneurysm surface, and the Mann–Whitney U test for comparison of the enhancement area EA in aneurysms with and without histologic markers of inflammation and wall degeneration. Pearson’s correlation test was used to assess the relationship of hemodynamic and morphological parameters with the enhancement area EA. Only parameters that were significantly associated with EA in univariate analysis were entered into a linear regression model in order to restrict the number of variables included in multivariate analysis. Furthermore, since some of the morphological variables have been reported to show a strong degree of correlation [29], only the variable with the lowest P value was selected for multivariate analysis. A P < 0.05 was considered significant in univariate and multivariate analyses.

Results

Twenty-one patients with 22 aneurysms were included; 71% were located on the right side. Clinical data were available for all included patients. The interrater agreement concerning focal enhancement was rated as substantial (Cohen’s κ = 0.742). Demographic information of the patients is summarized in Table 1. The PHASES score was positively correlated with EA (P = 0.0275). An association of gender, age, hypertension, diabetes, ASA and statin intake, smoking, and aneurysm location with EA could not be demonstrated.

Table 1 Demographic characteristics
Full size table

Microsurgical clip** with subsequent histologic analysis of the resected aneurysm sac was conducted in 9 patients. The surgical indication was based on the patient’s preference in 5 cases, and on aneurysm morphology (wide-necked aneurysm) in 4. Features of all aneurysms alongside histologic data are listed in Table 2.

Table 2 Size, enhancement area, and histologic features of all aneurysms
Full size table

Wall enhancement and blood flow evaluation

Flow simulation and calculation of hemodynamic variables were successful in all patients. A qualitative observation of the blood flow revealed that flow structures perpendicular to the main flow direction occurred in the vicinity of the enhancement zones. In cases with an increased aspect ratio, it was observed that these flows move sideward through the aneurysm sac instead of following the global flow direction. Additionally, it is important to highlight that enhancement appeared predominantly at the sides of the wall and was only present at the aneurysm dome in one case. Thus, the entering flow-jet and the corresponding im**ement zone (which causes increased shear stresses on the luminal surface) were not associated with the enhanced wall segments.

To further quantify the flow behavior along the aneurysmal lumen comparing enhanced and non-enhanced regions, three shear-related parameters were assessed.

AWSS was significantly lower in enhanced wall segments (3.74 Pa (3.91 Pa)) compared with the entire aneurysm wall (7.76 Pa (4.78 Pa)) (P < 0.0001), while maxOSI decreased from 0.45 (0.05) to 0.37 (0.14) (P < 0.0001). In the analysis of LSA, values were significantly higher in enhanced segments (68.09% (34.36%)) compared with the entire aneurysm (56.91% (30.34%)) (P < 0.0001).

There was no significant correlation of inflow-related hemodynamic parameters or SCI with EA. There was a trend towards higher LSA in aneurysms with a larger extent of enhancement (P = 0.08222). For a summary of the analysis of hemodynamic and morphologic parameters, see Table 3.

Table 3 P values from univariate analyses of the relationship of enhancement (enhancement area EA) with hemodynamic and morphologic parameters
Full size table

Wall enhancement and morphologic parameters

OA2, Dmax, Hmax, Wortho, Vch, Ach, EI, NSI, and UI were significantly correlated with EA in univariate analysis (Table 3). EI showed the lowest P value and was entered alongside Dmax into multiple regression analysis, the remaining variables were not considered due to collinearity as described before [29]. EI remained significantly associated with EA (P = 0.0395).

Histologic features

Four aneurysms stained positive for MPO-expressing cells. In three aneurysms, vasa vasorum were present. CD34-positive neovessel endothelium could not be detected in any of the aneurysms. Representative cases are illustrated in Figs. 2 and 3. In this subset of patients, the presence of one or two of these histologic markers was significantly associated with a larger extent of wall enhancement (P = 0.03175).

Fig. 2
figure 2

Post-contrast 3D T1 black blood images showing A aneurysm with a low extent of wall enhancement and B aneurysm with substantial enhancement of the wall. C Double layer appearance in an aneurysm with wall-adjacent thrombus that was histologically confirmed

Full size image
Fig. 3
figure 3

Representative photomicrographs showing A vasa vasorum (arrow) in the adventitial layer (hematoxylin and eosin stain, original magnification × 10), and B numerous myeloperoxidase-expressing cells staining brown (myeloperoxidase stain, × 10)

Full size image

Wall-adherent thrombus was detected on MR vessel wall imaging in two aneurysms, and intraluminal wall adherent thrombus was confirmed by histologic analysis. Enhancement was visible both on the luminal surface of the thrombus and the adjacent wall, leading to a double-layer appearance (Fig. 2c).

Extracellular lipid deposition was detected in two aneurysms. Hyalinosis, fibrosis, and calcification were common features in most aneurysms.

Discussion

Focal enhancement, hemodynamic parameters, and morphology

In this study, focal enhancement was colocalized with low AWSS, low maxOSI, and high LSA in agreement with results recently published by **, possibly inducing bias towards aneurysms with morphologies unfavorable for endovascular therapy. Histological data were only available in nine aneurysms. Microsurgical clip** provides only specimens of the aneurysm dome; neck segments under the clip cannot be retrieved. The small sample size precluded subgroup analysis of histologic features with adequate power. We chose to restrict our analysis to middle cerebral artery aneurysms in an effort to exclude a possible heterogeneity in aneurysm phenotype caused by different pathogenetic mechanisms potentially underlying the evolution and progression of aneurysms in different locations [43]. Additionally, an overestimation of the neck size using 3D imaging techniques can occur compared with 2D images, but careful segmentation using advanced algorithms was applied [44, 45]. Finally, no patient-specific inflow conditions were available for the blood flow simulations. However, a comparison between enhanced regions and the whole aneurysm remains feasible.

Conclusion

Focal wall enhancement in unruptured intracranial aneurysms is colocalized with hemodynamic factors that have been related to a higher risk of rupture. It is correlated with morphological factors linked to rupture risk, a higher PHASES score, and histologic markers of wall destabilization. The results support the hypothesis that focal enhancement could serve as a surrogate marker for aneurysm instability.