Multimodal validation of focal enhancement in intracranial aneurysms as a surrogate marker for aneurysm instability

MR Imaging of the Cerebral Aneurysmal Wall for Assessment of Rupture Risk

The evaluation of unruptured intracranial aneurysms requires a comprehensive and multifaceted approach. The comprehensive analysis of aneurysm wall enhancement through high-resolution MRI, in tandem with advanced processing techniques like finite element analysis, quantitative susceptibility mapping, and computational fluid dynamics, has begun to unveil insights into the intricate biology of aneurysms. This enhanced understanding of the etiology, progression, and eventual rupture of aneurysms holds the potential to be used as a tool to triage patients to intervention versus observation. Emerging tools such as radiomics and machine learning are poised to contribute significantly to this evolving landscape of diagnostic refinement.

The relationship between symptomatic status and aneurysm wall enhancement characteristics of single unruptured intracranial aneurysm

OBJECTIVE

We aimed to analyze the aneurysm wall enhancement (AWE) characteristics of a single unruptured intracranial aneurysm (UIA) and observe the relationship between the symptoms of a single UIA and the aneurysm wall.

METHODS

In our hospital, 85 patients diagnosed with a single UIA using computed tomography angiography (CTA) were retrospectively analyzed. The patients were divided into symptomatic and asymptomatic groups, including 46 asymptomatic and 39 symptomatic aneurysms. High-resolution magnetic resonance imaging of the vascular wall (HR-MR-VWI) was utilized to ascertain the presence, degree, and extent of AWE and thick-wall enhancement. In addition to AWE characteristics, morphological parameters of aneurysms, such as maximal size, shape, height, neck width, aspect ratio (AR), and size ratio (SR), were scanned using CTA. The differences in the parameters of a single UIA between the two groups were compared. An investigation explored the correlation between the symptom status of a single UIA and AWE.

RESULTS

We observed a correlation between symptom status and maximal size, height, and neck width for a single UIA, the presence or absence of AWE, and the levels and boundaries of AWE and thick-wall reinforcement. This study found that the AWE range was independently correlated with symptom status in the multivariate regression analysis.

CONCLUSION

A larger AWE range was an independent risk factor for the onset of symptoms in a single UIA.

A Systematic Review and Meta-Analysis of the Pathology Underlying Aneurysm Enhancement on Vessel Wall Imaging

Intracranial aneurysms are common, but only a minority rupture and cause subarachnoid haemorrhage, presenting a dilemma regarding which to treat. Vessel wall imaging (VWI) is a contrast-enhanced magnetic resonance imaging (MRI) technique used to identify unstable aneurysms. The pathological basis of MR enhancement of aneurysms is the subject of debate. This review synthesises the literature to determine the pathological basis of VWI enhancement. PubMed and Embase searches were performed for studies reporting VWI of intracranial aneurysms and their correlated histological analysis. The risk of bias was assessed. Calculations of interdependence, univariate and multivariate analysis were performed. Of 228 publications identified, 7 met the eligibility criteria. Individual aneurysm data were extracted for 72 out of a total of 81 aneurysms. Univariate analysis showed macrophage markers (CD68 and MPO, p = 0.001 and p = 0.002), endothelial cell markers (CD34 and CD31, p = 0.007 and p = 0.003), glycans (Alcian blue, p = 0.003) and wall thickness (p = 0.030) were positively associated with enhancement. Aneurysm enhancement therefore appears to be associated with inflammatory infiltrate and neovascularisation. However, all these markers are correlated with each other, and the literature is limited in terms of the numbers of aneurysms analysed and the parameters considered. The data are therefore insufficient to determine if these associations are independent of each other or of aneurysm size, wall thickness and rupture status. Thus, the cause of aneurysm-wall enhancement currently remains unknown.

[Analysis of intracranial saccular aneurysm wall: neuroimaging and histopathological correlates]

BACKGROUND

Contrast enhancement of intracranial aneurysm wall during MRI with targeted visualization of vascular wall correlates with previous aneurysm rupture and, according to some data, may be a predictor of further rupture of unruptured aneurysms.

OBJECTIVE

To analyze possible causes of aneurysm contrast enhancement considering morphological data of aneurysm walls.

MATERIAL AND METHODS

The study included 44 patients with intracranial aneurysms who underwent preoperative MRI between November 2020 and September 2022. Each aneurysm was assessed regarding contrast enhancement pattern. Microsurgical treatment of aneurysm was accompanied by resection of its wall for subsequent histological and immunohistochemical analysis regarding thrombosis, inflammation and neovascularization. Specimens were subjected to histological and immunochemical analysis. Immunohistochemical analysis was valuable to estimate inflammatory markers CD68 and CD3, as well as neurovascularization marker SD31.

RESULTS

Aneurysms with contrast-enhanced walls were characterized by higher number of CD3+, CD68+, CD31+ cells and parietal clots. Intensity of contrast enhancement correlated with aneurysm wall abnormalities.

CONCLUSION

Contrast enhancement of aneurysm wall can characterize various morphological abnormalities.

Wall enhancement as a biomarker of intracranial aneurysm instability: a histo-radiological study

BACKGROUND

The aim of this is to explore the histological basis of vessel wall enhancement (WE) on magnetic resonance imaging (MRI), which is a strong radiological biomarker of aneurysmal prone to rupture compared to other classical risk predictors (e.g., PHASES score, size, morphology).

METHODS

A prospective observational study was performed including all consecutive patients presenting with a saccular intracranial aneurysm at Vall d'Hebron University Hospital between October 2017 and May 2019. The patients underwent high-resolution 3 T MRI, and their aneurysms were classified into asymptomatic, symptomatic, and ruptured. A histological and immunohistochemical study was performed in a subgroup of patients (n = 20, of which 15 presented with WE). Multiple regression analyses were performed to identify predictors of rupture and aneurysm symptoms.

RESULTS

A total of 132 patients were enrolled in the study. WE was present in 36.5% of aneurysms: 22.9% asymptomatic, 76.9% symptomatic, and 100% ruptured. Immunohistochemical markers associated with WE were CD3 T cell receptor (p = 0.05) and CD45 leukocyte common antigen (p = 0.05). Moreover, WE is an independent predictor of symptomatic and ruptured aneurysms (p < 0.001).

CONCLUSIONS

Aneurysms with WE present multiple histopathological changes that may contribute to wall disruption and represent the pathophysiological basis of radiological WE. Moreover, WE is an independent diagnostic predictor of aneurysm symptoms and rupture.

3D aneurysm wall enhancement is associated with symptomatic presentation

BACKGROUND

Aneurysm wall enhancement (AWE) is a potential surrogate biomarker for aneurysm instability. Previous studies have assessed AWE using 2D multiplanar methods, most of which were conducted qualitatively.

OBJECTIVE

To use a new quantitative tool to analyze a large cohort of saccular aneurysms with 3D-AWE maps METHODS: Saccular aneurysms were imaged prospectively with 3T high resolution MRI. 3D-AWE maps of symptomatic (defined as ruptured or presentation with sentinel headache/cranial nerve neuropathy) and asymptomatic aneurysms were created by extending orthogonal probes from the aneurysm lumen into the wall. Three metrics were used to characterize enhancement: 3D circumferential AWE (3D-CAWE), aneurysm-specific contrast uptake (SAWE), and focal AWE (FAWE). Aneurysms with a circumferential AWE higher than the corpus callosum (3D-CAWE ≥1) were classified as 3D-CAWE+. Symptomatic presentation was analyzed with univariate and multivariate logistic models. Aneurysm size, size ratio, aspect ratio, irregular morphology, and PHASES and ELAPSS scores were compared with the new AWE metrics. Bleb and microhemorrhage analyses were also performed.

RESULTS

Ninety-three aneurysms were analyzed. 3D-CAWE, SAWE, and FAWE were associated with symptomatic status (OR=1.34, 1.25, and 1.08, respectively). A multivariate model including aneurysm size, 3D-CAWE+, age, female gender, and FAWE detected symptomatic status with 80% specificity and 90% sensitivity (area under the curve=0.914, =0.967). FAWE was also associated with irregular morphology and high-risk location (p=0.043 and p=0.001, respectively). In general, blebs enhanced 56% more than the aneurysm body. Areas of microhemorrhage co-localized with areas of increased SAWE (p=0.047).

CONCLUSIONS

3D-AWE mapping provides a new set of metrics that could potentially improve the identification of symptomatic aneurysms.

Aneurysm wall enhancement, hemodynamics, and morphology of intracranial fusiform aneurysms

Background and objective

Intracranial fusiform aneurysms (IFAs) are considered to have a complex pathophysiology process and poor natural history. The purpose of this study was to investigate the pathophysiological mechanisms of IFAs based on the characteristics of aneurysm wall enhancement (AWE), hemodynamics, and morphology.

Methods

A total of 21 patients with 21 IFAs (seven fusiform types, seven dolichoectatic types, and seven transitional types) were included in this study. Morphological parameters of IFAs were measured from the vascular model, including the maximum diameter (Dmax), maximum length (Lmax), and centerline curvature and torsion of fusiform aneurysms. The three-dimensional (3D) distribution of AWE in IFAs was obtained based on high-resolution magnetic resonance imaging (HR-MRI). Hemodynamic parameters including time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), gradient oscillatory number (GON), and relative residence time (RRT) were extracted by computational fluid dynamics (CFD) analysis of the vascular model, and the relationship between these parameters and AWE was investigated.

Results

The results showed that Dmax (p = 0.007), Lmax (p = 0.022), enhancement area (p = 0.002), and proportion of enhancement area (p = 0.006) were significantly different among three IFA types, and the transitional type had the largest Dmax, Lmax, and enhancement area. Compared with the non-enhanced regions of IFAs, the enhanced regions had lower TAWSS but higher OSI, GON, and RRT (p &lt; 0.001). Furthermore, Spearman’s correlation analysis showed that AWE was negatively correlated with TAWSS, but positively correlated with OSI, GON, and RRT.

Conclusion

There were significant differences in AWE distributions and morphological features among the three IFA types. Additionally, AWE was positively associated with the aneurysm size, OSI, GON, and RRT, while negatively correlated with TAWSS. However, the underlying pathological mechanism of the three fusiform aneurysm types needs to be further studied.

Comprehensive morphomechanical analysis of brain aneurysms

OBJECTIVE

Brain aneurysms comprise different compartments that undergo unique biological processes. A detailed multimodal analysis incorporating 3D aneurysm wall enhancement (AWE), computational fluid dynamics (CFD), and finite element analysis (FEA) data can provide insights into the aneurysm wall biology.

METHODS

Unruptured aneurysms were prospectively imaged with 7 T high-resolution MRI (HR-MRI). 3D AWE color maps of the entire aneurysm wall were generated and co-registered with contour plots of morphomechanical parameters derived from CFD and FEA. A multimodal analysis of the entire aneurysm was performed using 3D circumferential AWE (3D-CAWE), wall tension (WT), time-averaged wall shear stress (TAWSS), wall shear stress gradient (WSSG), and oscillatory shear index (OSI). A detailed compartmental analysis of each aneurysm's dome, bleb, and neck was also performed.

RESULTS

Twenty-six aneurysms were analyzed. 3D-CAWE + aneurysms had higher WT (p = 0.03) and higher TAWSS (p = 0.045) than 3D-CAWE- aneurysms. WT, TAWSS, and WSSG were lower in areas of focal AWE in the aneurysm dome compared to the neck (p = 0.009, p = 0.049, and p = 0.040, respectively), whereas OSI was higher in areas of focal AWE compared to the neck (p = 0.020). When compared to areas of no AWE of the aneurysm sac (AWE = 0.92 vs. 0.49, p = 0.001), blebs exhibited lower WT (1.6 vs. 2.45, p = 0.010), lower TAWSS (2.6 vs. 6.34), lower OSI (0.0007 vs. 0.0010), and lower WSSG (2900 vs. 5306). Fusiform aneurysms had a higher 3D-CAWE and WT than saccular aneurysms (p = 0.046 and p = 0.003, respectively).

CONCLUSIONS

Areas of focal high AWE in the sac and blebs are associated with low wall tension, low wall shear stress, and low flow conditions (TAWSS and WSSG). Conversely, the neck had average AWE, high wall tension, high wall shear stress, and high flow conditions. The aneurysm dome and the aneurysm neck have different morphomechanical environments, with increased mechanical load at the neck.

Association of circumferential aneurysm wall enhancement with recurrence after coiling of unruptured intracranial aneurysms: a preliminary vessel wall imaging study

OBJECTIVE

Recent histopathological studies of unruptured intracranial aneurysms (UIAs) have confirmed that aneurysm wall enhancement (AWE) on MR vessel wall imaging (VWI) is related to wall degeneration with in vivo inflammatory cell infiltration. Therefore, pretreatment aneurysm wall status on VWI may be associated with recurrence after endovascular treatment.

METHODS

VWI with gadolinium was performed on 67 consecutive saccular UIAs before endovascular treatment between April 2017 and June 2021. The mean (range) follow-up period after treatment was 24.4 (6-54) months. AWE patterns were classified as circumferential AWE (CAWE), focal AWE (FAWE), and negative AWE (NAWE). The authors retrospectively investigated the relationship between aneurysm recurrence and AWE patterns, as well as conventional risk factors.

RESULTS

Sixty-seven patients with 67 saccular UIAs were eligible for the present study. AWE patterns were as follows: 10 CAWE (14.9%), 20 FAWE (29.9%), and 37 NAWE (55.2%). Follow-up MRA detected aneurysm recurrence in 18 of 69 cases (26.1%). Univariate analysis identified maximum diameter (mean ± SD 5.8 ± 2.2 mm in patients with stable aneurysms vs 7.7 ± 3.8 mm in those with unstable aneurysms, p = 0.02), aspect ratio (1.4 ± 0.5 vs 1.1 ± 0.4, p < 0.01), aneurysm location in posterior circulation (4.1% vs 27.8%, p < 0.01), volume embolization ratio (29.6% ± 7.8% vs 25.2% ± 6.1%, p = 0.02), and AWE pattern (p = 0.04) as significant predictive factors of recurrence. Among the 3 AWE patterns, CAWE was significantly more frequent in the unstable group, but no significant differences in stability of the treated aneurysms were observed with the FAWE and NAWE patterns. In multivariate logistic regression analysis, CAWE pattern (OR 14.2, 95% CI 1.8-110.8, p = 0.01) and volume embolization ratio ≥ 25% (OR 8.6, 95% CI 2.1-34.3, p < 0.01) remained as significant factors associated with aneurysm stability after coiling.

CONCLUSIONS

VWI before coiling provides novel insights into the stability of treated aneurysms. Aneurysms with the CAWE pattern on VWI before coiling may be less stable after treatment.

Topographical Analysis of Aneurysm Wall Enhancement With 3-Dimensional Mapping

BACKGROUND

Aneurysm wall enhancement has been identified as a potential biomarker for aneurysm instability. Enhancement has been determined by different approaches on 2D multiplanar views. This study describes a new method to quantify enhancement through 3D heatmaps and histograms.

METHODS

A custom algorithm was developed using orthogonal probes extending from the aneurysm lumen into the wall to create 3D heatmaps and histograms of wall enhancement on 7T-MRI. Three quantitative metrics for general, specific, and focal wall enhancement were generated from the histograms.

RESULTS

Thirty-two aneurysms were analyzed and classified based on 3D heatmaps and histograms. Larger aneurysms were more enhancing (Spearman's r=0.472, p=0.006), and had more heterogeneous enhancement (Spearman's r=0.557, p<0.001) than smaller aneurysms. Patterns of enhancement differed between saccular, fusiform, and thrombosed aneurysms. Fusiform aneurysms were larger (p=0.015) and had more heterogenous enhancement compared to saccular aneurysms. Fusiform aneurysms had more areas of focal enhancement (p<0.001) and right skewed histograms (p=0.003).

CONCLUSIONS

The 3D analysis of aneurysm wall enhancement provides topographic data of the entire aneurysm wall. New metrics developed based on this method showed that large and fusiform aneurysms have heterogenous enhancement.

Hemodynamic Characteristic Analysis of Aneurysm Wall Enhancement in Unruptured Middle Cerebral Artery Aneurysm

Background and Purpose

Aneurysm wall enhancement (AWE) on vessel wall magnetic resonance imaging has been suggested as a marker of the unstable status of intracranial aneurysm (IA) and may predict IA rupture risk. However, the role of abnormal hemodynamics in unruptured IAs with AWE remains poorly understood. This study aimed to determine the association between abnormal hemodynamics and AWE in unruptured middle cerebral artery (MCA) aneurysms.

Methods

A total of 28 patients with 32 bifurcation aneurysms of the middle cerebral artery>3mm in size were retrospectively selected for this study. Vessel wall magnetic resonance images were reviewed, and the AWE pattern of each aneurysm was classified as no AWE, partial AWE, and circumferential AWE. Computational fluid dynamics were used to calculate the hemodynamic variables of each aneurysm. Univariate and multivariate analyses investigated the association between AWE and hemodynamic variables.

Results

AWE was present in 13 aneurysms (40.6%), with 7 (21.9%) showing partial AWE and 6 (18.7%) showing circumferential AWE. Kruskal-Wallis H analysis revealed that hemodynamic variables including wall shear stress (WSS), oscillatory shear index, aneurysm pressure (AP), relative residence time, and low shear area (LSA) were significantly associated with AWE (p < 0.05). Further ordinal logistic regression analysis found that WSS was the only factor with a significant association with AWE (p = 0.048); similar trends were identified for LSA (p = 0.055) and AP (p = 0.058). Spearman's correlation analysis showed that AWE was negatively correlated with WSS (rs = -0.622, p < 0.001) and AP (rs = -0.535, p = 0.002) but positively correlated with LSA (rs = 0.774, p < 0.001).

Conclusion

Low wall shear stress, low aneurysm pressure, and increased low shear area were associated with aneurysm wall enhancement on vessel wall magnetic resonance imaging in unruptured cerebral aneurysms. These abnormal hemodynamic parameters may induce inflammation and cause aneurysm wall enhancement. However, the association between these parameters and their underlying pathological mechanisms requires further investigation.

Intracranial aneurysm wall (in)stability-current state of knowledge and clinical perspectives

Intracranial aneurysm (IA), a local outpouching of cerebral arteries, is present in 3 to 5% of the population. Once formed, an IA can remain stable, grow, or rupture. Determining the evolution of IAs is almost impossible. Rupture of an IA leads to subarachnoid hemorrhage and affects mostly young people with heavy consequences in terms of death, disabilities, and socioeconomic burden. Even if the large majority of IAs will never rupture, it is critical to determine which IA might be at risk of rupture. IA (in)stability is dependent on the composition of its wall and on its ability to repair. The biology of the IA wall is complex and not completely understood. Nowadays, the risk of rupture of an IA is estimated in clinics by using scores based on the characteristics of the IA itself and on the anamnesis of the patient. Classification and prediction using these scores are not satisfying and decisions whether a patient should be observed or treated need to be better informed by more reliable biomarkers. In the present review, the effects of known risk factors for rupture, as well as the effects of biomechanical forces on the IA wall composition, will be summarized. Moreover, recent advances in high-resolution vessel wall magnetic resonance imaging, which are promising tools to discriminate between stable and unstable IAs, will be described. Common data elements recently defined to improve IA disease knowledge and disease management will be presented. Finally, recent findings in genetics will be introduced and future directions in the field of IA will be exposed.

Imaging Modalities for Intracranial Aneurysm: More Than Meets the Eye

Intracranial aneurysms (IA) are often asymptomatic and have a prevalence of 3 to 5% in the adult population. The risk of IA rupture is low, however when it occurs half of the patients dies from subarachnoid hemorrhage (SAH). To avoid this fatal evolution, the main treatment is an invasive surgical procedure, which is considered to be at high risk of rupture. This risk score of IA rupture is evaluated mainly according to its size and location. Therefore, angiography and anatomic imaging of the intracranial aneurysm are crucial for its diagnosis. Moreover, it has become obvious in recent years that several other factors are implied in this complication, such as the blood flow complexity or inflammation. These recent findings lead to the development of new IA imaging tools such as vessel wall imaging, 4D-MRI, or molecular MRI to visualize inflammation at the site of IA in human and animal models. In this review, we will summarize IA imaging techniques used for the patients and those currently in development.

High-Resolution Vessel Wall MRI of Endovascularly Treated Intracranial Aneurysms

(1) Background: The aim of this study was to determine the frequency and the pattern of post-procedural intracranial aneurysm contrast enhancement on high-resolution vessel wall magnetic resonance imaging (HR-VW MRI). We investigated the possible association between this imaging finding and factors such as time elapsed since embolization or aneurysm occlusion grade on baseline and follow-up imaging. (2) Methods: Consecutive patients presenting for follow-up after endovascular treatment of intracranial aneurysms were included. HR-VW MRI was acquired and interpreted independently by two radiologists. (3) Results: This study included 40 aneurysms in 39 patients. Contrast enhancement was detected in 30 (75%) aneurysms. It was peripheral in 12 (30.0%), central in 9 (22.5%), and both peripheral and central in 9 (22.5%) aneurysms. The statistical analysis did not reveal any relationship between follow-up period and the presence of contrast enhancement (p = 0.277). There were no statistically significant differences in the frequency of contrast enhancement between aneurysms with total occlusion and those with remnant flow on follow-up MR angiography (p = 0.850) nor between aneurysms with different interval changes in the aneurysm occlusion grade (p = 0.536). Multivariate analysis did not demonstrate aneurysm size, ruptured aneurysm status, nor initial complete aneurysm occlusion to be a predictor of contrast enhancement (p = 0.080). (4) Conclusions: Post-procedural aneurysm contrast enhancement is a common imaging finding on HR-VW MRI. The clinical utility of this imaging finding, especially in the prediction of aneurysm recurrence, seems limited. The results of our study do not support routine use of HR-VW MRI in the follow-up of patients after endovascular treatment of intracranial aneurysms.

Quantitative assessment of microstructural evolution of intracranial aneurysm wall by vessel wall imaging

PURPOSE

This study aimed to evaluate new quantitative parameters of aneurysm wall enhancement (AWE) on magnetic resonance vessel wall imaging (VWI) in differentiating between the stable and evolving unruptured intracranial aneurysms (UIAs).

METHODS

Thirty-eight consecutive patients with UIAs (27 stable and 11 evolving) underwent VWI with contrast-enhanced 3D T1 volume isotropic turbo spin echo acquisition. The voxel-based enhancement maps were created using pre- and post-contrast images. The aneurysmal lumen with signal suppression by black-blood method was segmented. Then, one voxel outer and inner layers of the lumen contour were automatically segmented. The shape features of the aneurysms and AWE of the two layers were compared between stable and evolving groups.

RESULTS

The shape features, including aneurysm volume, surface, and compacity were significantly different between the stable and evolving groups (P = 0.024, 0.028, and 0.033, respectively). Stable and evolving groups also differed significantly in the AWE at the union of outer and inner layers of the aneurysm wall (P = 0.0082) but not in that of the outer or inner layer alone. Multivariate logistic regression analysis revealed significant differences in aneurysm volume, surface, and AWE at the union of outer and inner layers between the two groups (P = 0.0029, 0.0092, and 0.0033, respectively). Receiver operating characteristics curve analysis revealed that the area under the curve of the logistic regression model was 0.89.

CONCLUSION

Quantitative combined analysis of aneurysm shape features and AWE of the union of outer and inner layers were effective for differentiating between stable and evolving UIAs.

Analysis of Cerebral Aneurysm Wall Tension and Enhancement Using Finite Element Analysis and High-Resolution Vessel Wall Imaging

Biomechanical computational simulation of intracranial aneurysms has become a promising method for predicting features of instability leading to aneurysm growth and rupture. Hemodynamic analysis of aneurysm behavior has helped investigate the complex relationship between features of aneurysm shape, morphology, flow patterns, and the proliferation or degradation of the aneurysm wall. Finite element analysis paired with high-resolution vessel wall imaging can provide more insight into how exactly aneurysm morphology relates to wall behavior, and whether wall enhancement can describe this phenomenon. In a retrospective analysis of 23 unruptured aneurysms, finite element analysis was conducted using an isotropic, homogenous third order polynomial material model. Aneurysm wall enhancement was quantified on 2D multiplanar views, with 14 aneurysms classified as enhancing (CR_stalk≥0.6) and nine classified as non-enhancing. Enhancing aneurysms had a significantly higher 95th percentile wall tension (μ = 0.77 N/cm) compared to non-enhancing aneurysms (μ = 0.42 N/cm, p < 0.001). Wall enhancement remained a significant predictor of wall tension while accounting for the effects of aneurysm size (p = 0.046). In a qualitative comparison, low wall tension areas concentrated around aneurysm blebs. Aneurysms with irregular morphologies may show increased areas of low wall tension. The biological implications of finite element analysis in intracranial aneurysms are still unclear but may provide further insights into the complex process of bleb formation and aneurysm rupture.

Aneurysm risk metrics and hemodynamics are associated with greater vessel wall enhancement in intracranial aneurysms

Vessel wall enhancement (VWE) in contrast-enhanced magnetic resonance imaging (MRI) is a potential biomarker for intracranial aneurysm (IA) risk stratification. In this study, we investigated the relationship between VWE features, risk metrics, morphology and hemodynamics in 41 unruptured aneurysms. We reconstructed the IA geometries from MR angiography and mapped pituitary stalk-normalized MRI intensity on the aneurysm surface using an in-house tool. For each case, we calculated the maximum intensity (CRstalk) and IA risk (via size and the rupture resemblance score (RRS)). We performed correlation analysis to assess relationships between CRstalk and IA risk metrics (size and RRS), as well as each parameter encompassed in RRS, i.e. aneurysmal size ratio (SR), normalized wall shear stress (WSS) and oscillatory shear index. We found that CRstalk had a strong correlation (Pearson correlation coefficient, PCC = 0.630) with size and a moderate correlation (PCC = 0.472) with RRS, indicating an association between VWE and IA risk. Furthermore, CRstalk had a weak negative correlation with normalized WSS (PCC = -0.320) and a weak positive correlation with SR (PCC = 0.390). Local voxel-based analysis showed only a weak negative correlation between normalized WSS and contrast-enhanced MRI signal intensity (PCC = -0.240), suggesting that if low-normalized WSS induces enhancement-associated pathobiology, the effect is not localized.

An Image-Based Workflow for Objective Vessel Wall Enhancement Quantification in Intracranial Aneurysms

BACKGROUND

VWE in contrast-enhanced magnetic resonance imaging (MRI) is a potential biomarker for the evaluation of IA. The common practice to identify IAs with VWE is mainly based on a visual inspection of MR images, which is subject to errors and inconsistencies. Here, we develop and validate a tool for the visualization, quantification and objective identification of regions with VWE.

METHODS

N = 41 3D T1-MRI and 3D TOF-MRA IA images from 38 patients were obtained and co-registered. A contrast-enhanced MRI was normalized by the enhancement intensity of the pituitary stalk and signal intensities were mapped onto the surface of IA models generated from segmented MRA. N = 30 IAs were used to identify the optimal signal intensity value to distinguish the enhancing and non-enhancing regions (marked by an experienced neuroradiologist). The remaining IAs (n = 11) were used to validate the threshold. We tested if the enhancement area ratio (EAR-ratio of the enhancing area to the IA surface-area) could identify high risk aneurysms as identified by the ISUIA clinical score.

RESULTS

A normalized intensity of 0.276 was the optimal threshold to delineate enhancing regions, with a validation accuracy of 81.7%. In comparing the overlap between the identified enhancement regions against those marked by the neuroradiologist, our method had a dice coefficient of 71.1%. An EAR of 23% was able to discriminate high-risk cases with an AUC of 0.7.

CONCLUSIONS

We developed and validated a pipeline for the visualization and objective identification of VWE regions that could potentially help evaluation of IAs become more reliable and consistent.

Current Clinical Applications of Intracranial Vessel Wall MR Imaging

Intracranial vessel wall MR imaging (VWI) is increasingly being used as a valuable adjunct to conventional angiographic imaging techniques. This article will provide an updated review on intracranial VWI protocols and image interpretation. We review VWI technical considerations, describe common VWI imaging features of different intracranial vasculopathies and show illustrative cases. We review the role of VWI for differentiating among steno-occlusive vasculopathies, such as intracranial atherosclerotic plaque, dissections and Moyamoya disease. We also highlight how VWI may be used for the diagnostic work-up and surveillance of patients with vasculitis of the central nervous system and cerebral aneurysms.

Aneurysm Wall Enhancement Is Associated With Decreased Intrasaccular IL-10 and Morphological Features of Instability

BACKGROUND

High-resolution vessel wall imaging plays an increasingly important role in assessing the risk of aneurysm rupture.

OBJECTIVE

To introduce an approach toward the validation of the wall enhancement as a direct surrogate parameter for aneurysm stability.

METHODS

A total of 19 patients harboring 22 incidental intracranial aneurysms were enrolled in this study. The aneurysms were dichotomized according to their aneurysm-to-pituitary stalk contrast ratio using a cutoff value of 0.5 (nonenhancing < 0.5; enhancing ≥ 0.5). We evaluated the association of aneurysm wall enhancement with morphological characteristics, hemodynamic features, and inflammatory chemokines directly measured inside the aneurysm.

RESULTS

Differences in plasma concentration of chemokines and inflammatory molecules, morphological, and hemodynamic parameters were analyzed using the Welch test or Mann-Whitney U test. The concentration ΔIL-10 in the lumen of intracranial aneurysms with low wall enhancement was significantly increased compared to aneurysms with strong aneurysm wall enhancement (P = .014). The analysis of morphological and hemodynamic parameters showed significantly increased values for aneurysm volume (P = .03), aneurysm area (P = .044), maximal diameter (P = .049), and nonsphericity index (P = .021) for intracranial aneurysms with strong aneurysm wall enhancement. None of the hemodynamic parameters reached statistical significance; however, the total viscous shear force computed over the region of low wall shear stress showed a strong tendency toward significance (P = .053).

CONCLUSION

Aneurysmal wall enhancement shows strong associations with decreased intrasaccular IL-10 and established morphological indicators of aneurysm instability.

Luminal enhancement in intracranial aneurysms: fact or feature?-A quantitative multimodal flow analysis

Purpose

Intracranial aneurysm (IA) wall enhancement on post-contrast vessel wall magnetic resonance imaging (VW-MRI) is assumed to be a biomarker for vessel wall inflammation and aneurysm instability. However, the exact factors contributing to enhancement are not yet clarified. This study investigates the relationship between luminal enhancement and intra-aneurysmal flow behaviour to assess the suitability of VW-MRI as a surrogate method for determining quantitative and qualitative flow behaviour in the aneurysm sac.

Methods

VW-MRI signal is measured in the lumen of three patient-specific IA flow models and compared with the intra-aneurysmal flow fields obtained using phase-contrast magnetic resonance imaging (PC-MRI) and computational fluid dynamics (CFD). The IA flow models were supplied with two different time-varying flow regimes.

Results

Overall, the velocity fields acquired using PC-MRI or CFD were in good agreement with the VW-MRI enhancement patterns. Generally, the regions with slow-flowing blood show higher VW-MRI signal intensities, whereas high flow leads to a suppression of the signal. For all aneurysm models, a signal value above three was associated with velocity values below three cm/s.

Conclusion

Regions with lower enhancements have been correlated with the slow and high flow at the same time. Thus, further factors like flow complexity and stability can contribute to flow suppression in addition to the flow magnitude. Nevertheless, VW-MRI can qualitatively assess intra-aneurysmal flow phenomena and estimate the velocity range present in the corresponding region.

Semiautomated 3D mapping of aneurysmal wall enhancement with 7T-MRI

Aneurysm wall enhancement (AWE) after the administration of contrast gadolinium is a potential biomarker of unstable intracranial aneurysms. While most studies determine AWE subjectively, this study comprehensively quantified AWE in 3D imaging using a semi-automated method. Thirty patients with 33 unruptured intracranial aneurysms prospectively underwent high-resolution imaging with 7T-MRI. The signal intensity (SI) of the aneurysm wall was mapped and normalized to the pituitary stalk (PS) and corpus callosum (CC). The CC proved to be a more reliable normalizing structure in detecting contrast enhancement (p < 0.0001). 3D-heatmaps and histogram analysis of AWE were used to generate the following metrics: specific aneurysm wall enhancement (SAWE), general aneurysm wall enhancement (GAWE) and focal aneurysm wall enhancement (FAWE). GAWE was more accurate in detecting known morphological determinants of aneurysm instability such as size ≥ 7 mm (p = 0.049), size ratio (p = 0.01) and aspect ratio (p = 0.002). SAWE and FAWE were aneurysm specific metrics used to characterize enhancement patterns within the aneurysm wall and the distribution of enhancement along the aneurysm. Blebs were easily identified on 3D-heatmaps and were more enhancing than aneurysm sacs (p = 0.0017). 3D-AWE mapping may be a powerful objective tool in characterizing different biological processes of the aneurysm wall.

Pseudo-Enhancement in Intracranial Aneurysms on Black-Blood MRI: Effects of Flow Rate, Spatial Resolution, and Additional Flow Suppression

BACKGROUND

Vessel-wall enhancement (VWE) on black-blood MRI (BB MRI) has been proposed as an imaging marker for a higher risk of rupture and associated with wall inflammation. Whether VWE is causally linked to inflammation or rather induced by flow phenomena has been a subject of debate.

PURPOSE

To study the effects of slow flow, spatial resolution, and motion-sensitized driven equilibrium (MSDE) preparation on signal intensities in BB MRI of patient-specific aneurysm flow models.

STUDY TYPE

Prospective.

SUBJECTS/FLOW ANEURYSM MODEL/VIRTUAL VESSELS

Aneurysm flow models based on 3D rotational angiography datasets of three patients with intracranial aneurysms were 3D printed and perfused at two different flow rates, with and without Gd-containing contrast agent.

FIELD STRENGTH/SEQUENCE

Variable refocusing flip angle 3D fast-spin echo sequence at 3 T with and without MSDE with three voxel sizes ((0.5 mm)³ , (0.7 mm)³ , and (0.9 mm)³ ); time-resolved with phase-contrast velocity-encoding 3D spoiled gradient echo sequence (4D flow MRI).

ASSESSMENT

Three independent observers performed a qualitative visual assessment of flow patterns and signal enhancement. Quantitative analysis included voxel-wise evaluation of signal intensities and magnitude velocity distributions in the aneurysm.

STATISTICAL TESTS

Kruskal-Wallis test, potential regressions.

RESULTS

A hyperintense signal in the lumen and adjacent to the aneurysm walls on BB MRI was colocalized with slow flow. Signal intensities increased by a factor of 2.56 ± 0.68 (P < 0.01) after administering Gd contrast. After Gd contrast administration, the signal was suppressed most in conjunction with high flows and with MSDE (2.41 ± 2.07 for slow flow without MSDE, and 0.87 ± 0.99 for high flow with MSDE). A clear result was not achieved by modifying the spatial resolution .

DATA CONCLUSIONS

Slow-flow phenomena contribute substantially to aneurysm enhancement and vary with MRI parameters. This should be considered in the clinical setting when assessing VWE in patients with an unruptured aneurysm.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 2.

Intracranial aneurysm wall enhancement as an indicator of instability: a systematic review and meta-analysis

BACKGROUND AND PURPOSE

Aneurysm wall enhancement (AWE) of intracranial aneurysms on magnetic resonance imaging has been described in previous studies as a surrogate marker of instability. With this study, an updated literature overview and summary risk estimates of the association between AWE and different specific outcomes (i.e., rupture, growth or symptomatic presentation) for both cross-sectional and longitudinal studies are provided.

METHODS

The PRISMA guideline was followed and a search was performed of PubMed and Embase to 1 January 2021 for studies that reported on AWE and aneurysm instability. In cross-sectional studies, AWE was compared between patients with stable and unstable aneurysms. In longitudinal studies, AWE of stable aneurysms was assessed at baseline after which patients were followed longitudinally. Risk ratios were calculated for longitudinal studies, prevalence ratios for cross-sectional studies and then the ratios were pooled in a random-effects meta-analysis. Also, the performance of AWE to differentiate between stable and unstable aneurysms was evaluated.

RESULTS

Twelve studies were included with a total of 1761 aneurysms. In cross-sectional studies, AWE was positively associated with rupture (prevalence ratio 11.47, 95% confidence interval [CI] 4.05-32.46) and growth or symptomatic presentation (prevalence ratio 4.62, 95% CI 2.85-7.49). Longitudinal studies demonstrated a positive association between AWE and growth or rupture (risk ratio 8.00, 95% CI 2.14-29.88). Assessment of the performance of AWE showed high sensitivities, mixed specificities, low positive predictive values and high negative predictive values.

CONCLUSIONS

Although AWE is positively associated with aneurysm instability, current evidence mostly supports the use of its absence as a surrogate marker of aneurysm stability.

Segmentation of aneurysm wall enhancement in evolving unruptured intracranial aneurysms

OBJECTIVE

Morphological changes in unruptured intracranial aneurysms (UIAs) are an imaging marker of aneurysm instability. Recent studies have indicated the ability of MR vessel wall imaging (VWI) to stratify unstable UIAs based on a correlation with histopathological aneurysm wall inflammation. In the present study the authors investigated the relationships between aneurysm growth patterns and the segmentation of aneurysm wall enhancement (AWE) in VWI.

METHODS

A total of 120 aneurysms with serial angiography from a follow-up period of at least 2 years (mean 65 months, range 24-215 months) were assessed by VWI. Two readers independently evaluated the patterns of morphological changes (stable, whole sac expansion, and secondary aneurysm formation) and the segmentation of AWE (no, focal, and circumferential AWE). The contrast enhancement ratio of the aneurysm wall versus the pituitary stalk (CRstalk) was calculated for the quantitative assessment of AWE. Statistical analyses were performed to investigate the relationships between AWE patterns and patient baseline profiles, aneurysm characteristics, and morphological modifications.

RESULTS

Forty-one of 120 UIAs (34%) exhibited aneurysm growth (whole sac expansion in 19 and secondary aneurysm formation in 22). AWE was detected in 35 of 120 UIAs (focal AWE in 25 and circumferential AWE in 10). The maximum diameter of, irregularities in, and morphological modifications in aneurysms were associated with the segmentation of AWE. Focal AWE correlated with secondary aneurysm formation, and circumferential AWE correlated with whole sac expansion. In focal AWE, CRstalk was significantly higher in secondary aneurysm formation than in stable UIAs. UIAs without AWE (categorized as no AWE) correlated with aneurysm stability.

CONCLUSIONS

The segmentation of AWE was associated with aneurysm growth scenarios and may provide a novel insight into the evaluation of unstable UIAs.

Intracranial vessel wall imaging

PURPOSE OF REVIEW

To give an overview regarding the potential usefulness of vessel wall imaging (VWI) in distinguishing various intracranial vascular diseases, their common imaging features, and potential pitfalls.

RECENT FINDINGS

VWI provides direct visualization of the vessel wall and allows the discrimination of different diseases such as vasculitis, atherosclerosis, dissection, Moyamoya disease, and reversible cerebral vasoconstriction syndrome. Recent studies showed that concentric and eccentric involvement in the vessel wall, as well as the enhancement pattern were found important for the distinguishing these diseases and evaluating their activity.

SUMMARY

Most of the imaging techniques currently used are based on luminal imaging. However, these imaging methods are not adequate to distinguish different diseases that can demonstrate similar radiological findings. VWI is being increasingly used as a noninvasive imaging method to offset this limitation.

The association between hemodynamics and wall characteristics in human intracranial aneurysms: a review

Hemodynamics plays a key role in the natural history of intracranial aneurysms (IAs). However, studies exploring the association between aneurysmal hemodynamics and the biological and mechanical characteristics of the IA wall in humans are sparse. In this review, we survey the current body of literature, summarize the studies' methodologies and findings, and assess the degree of consensus among them. We used PubMed to perform a systematic review of studies that explored the association between hemodynamics and human IA wall features using different sources. We identified 28 publications characterizing aneurysmal flow and the IA wall: 4 using resected tissues, 17 using intraoperative images, and 7 using vessel wall magnetic resonance imaging (MRI). Based on correlation to IA tissue, higher flow conditions, such as high wall shear stress (WSS) with complex pattern and elevated pressure, were associated with degenerated walls and collagens with unphysiological orientation and faster synthesis. MRI studies strongly supported that low flow, characterized by low WSS and high blood residence time, was associated with thicker walls and post-contrast enhancement. While significant discrepancies were found among those utilized intraoperative images, they generally supported that thicker walls coexist at regions with prolonged residence time and that thinner regions are mainly exposed to higher pressure with complex WSS patterns. The current body of literature supports a theory of two general hemodynamic-biologic mechanisms for IA development. One, where low flow conditions are associated with thickening and atherosclerotic-like remodeling, and the other where high and impinging flow conditions are related to wall degeneration, thinning, and collagen remodeling.

High-Resolution Vessel Wall Magnetic Resonance Imaging of Small Unruptured Intracranial Aneurysms

Background: We decided to investigate whether aneurysm wall enhancement (AWE) on high-resolution vessel wall magnetic resonance imaging (HR VW-MRI) coexists with the conventional risk factors for aneurysm rupture. Methods: We performed HR VW-MRI in 46 patients with 64 unruptured small intracranial aneurysms. Patient demographics and clinical characteristics were recorded. The PHASES score was calculated for each aneurysm. Results: Of the 64 aneurysms, 15 (23.4%) showed wall enhancement on post-contrast HR VW-MRI. Aneurysms with wall enhancement had significantly larger size (p = 0.001), higher dome-to-neck ratio (p = 0.024), and a more irregular shape (p = 0.003) than aneurysms without wall enhancement. The proportion of aneurysms with wall enhancement was significantly higher in older patients (p = 0.011), and those with a history of prior aneurysmal SAH. The mean PHASES score was significantly higher in aneurysms with wall enhancement (p < 0.000). The multivariate logistic regression analysis revealed that aneurysm irregularity and the PHASES score are independently associated with the presence of AWE. Conclusions: Aneurysm wall enhancement on HR VW-MRI coexists with the conventional risk factors for aneurysm rupture.

Imaging Inflammation - From Whole Body Imaging to Cellular Resolution

Imaging techniques have evolved impressively lately, allowing whole new concepts like multimodal imaging, personal medicine, theranostic therapies, and molecular imaging to increase general awareness of possiblities of imaging to medicine field. Here, we have collected the selected (3D) imaging modalities and evaluated the recent findings on preclinical and clinical inflammation imaging. The focus has been on the feasibility of imaging to aid in inflammation precision medicine, and the key challenges and opportunities of the imaging modalities are presented. Some examples of the current usage in clinics/close to clinics have been brought out as an example. This review evaluates the future prospects of the imaging technologies for clinical applications in precision medicine from the pre-clinical development point of view.