Objective Quantification of Contrast Enhancement of Unruptured Intracranial Aneurysms: A High-Resolution Vessel Wall Imaging Validation Study

Abstract

Background

High-resolution vessel wall imaging (HR-VWI) has emerged as a valuable tool in assessing unruptured intracranial aneurysms (UIAs). There is no standardized method to quantify contrast enhancement of the aneurysmal wall. Contrast enhancement can be objectively measured as signal intensity (SI); or subjectively adjudicated. This study compares the different methods to quantify wall enhancement of UIAs and determines the sensitivity and specificity of each method as a surrogate of aneurysm instability. The study also compares SI quantification among different manufacturers.

Methods:

The University of Iowa HR-VWI Project database was analyzed. This database compiles patients with UIAs who prospectively underwent HR-VWI on a 3T Siemens MRI scanner. The mean and maximal SI values of the aneurysm wall, pituitary stalk and genu of the corpus callosum, were used to compare three different measurement methods: (1) aneurysm enhancement ratio (AER = SI_post − SI_pre/SI_pre); (2) aneurysm-to-pituitary stalk ratio (CR_stalk = SI_post/Pituitary stalk_post); and (3) aneurysm enhancement index (AEI = SI_post/SI_{brain post} − SI_pre/SI_{brain pre}/SI_pre/SI_{brain pre}). Size ≥7 mm was used as a surrogate of aneurysm instability for receiver-operating characteristic (ROC) curve analysis. To determine if the objective quantification of SI varies among scanner of different manufacturers, nine UIAs underwent the same HR-VWI protocol in a 3T General Electric (GE) and a 3T Siemens scanner. Three UIAs also underwent a third scan of different potency (7T GE).

Results:

Eighty patients with 102 UIAs were included in the study. Mean age was 64.5 ± 12.2 years-old, and 64 (80%) were women. UIAs ≥ 7mm had significantly higher SI when compared to smaller UIAs (< 7 mm): AER = 0.82 vs 0.49, P<0.001; CR_stalk = 0.84 vs 0.61, P<0.001 and AEI = 0.81 vs 0.48, P<0.001. ROC curves demonstrated optimal sensitivity of 81.5% for CR_stalk ≥ 0.60; 75.9% for AEI ≥ 0.50 and 74.1% for AER ≥ 0.49. Inter-manufacturer correlation between 3T GE and 3T Siemens MRI for CR_stalk using mean and maximal SI values was excellent (Pearson coefficients > 0.80, P < 0.001). A similar correlation was identified among the 3 UIAs that underwent 7T imaging.

Conclusion:

CR_stalk using maximal SI values was the most reliable objective method to quantify enhancement of UIAs on HR-VWI. The same ratios were obtained among different manufacturers and in scans of different potency.

INTRODUCTION

Unruptured intracranial aneurysms (UIAs) pose a therapeutic dilemma as the risk-benefit of therapeutic interventions has to be balanced against the natural history of the disease. Early recognition of brain aneurysms with a high risk of rupture is key when deciding treatment. Unfortunately, there is no biomarker of aneurysm instability that has been prospectively validated. High-resolution vessel wall imaging (HR-VWI) has emerged as a valuable tool in assessing unstable UIAs.³ There are promising observations in the characterization of aneurysm wall enhancement as a biomarker of aneurysm wall inflammation, growth and rupture.¹⁹

However, there is significant heterogeneity among the imaging protocols used to characterize UIAs with HR-VWI .²⁸ Furthermore, there is no consensus on the standard definition of wall enhancement.²⁴ Most studies classified wall enhancement subjectively into: ‘strong/avid’ vs ‘faint/no’ enhancement, or ‘focal’ vs ‘circumferential’ wall enhancement, based on the assessment by two or more adjudicators. Other studies have used objective data generated from the quantification of signal intensity (SI) in the aneurysm wall to define enhancement. Several formulas have been used to standardize enhancement measurements, including ratios generated through comparisons with the pituitary stalk,^15,16 and those considering SI measurements in pre- versus post-contrast sequences.^26,27

We aim to compare the different methods of aneurysm wall enhancement measurement to determine the most sensitive and specific. Moreover, we validate these measurements among scans of different manufacturers and potencies.

METHODS

Patient Population and Data Collection

After institutional review board approval (IRB), we analyzed the University of Iowa HR-VWI Project database. This is prospectively acquired database that includes patients with UIAs from January 2015 to August 2019. Every patient with an UIA undergoes a 3T HR-VWI. The IRB was amended to re-image a subset of patients in a second scan of a different manufacturer and sometimes with a third scan of different potency (7T). Demographics and clinical information, including age, sex and comorbidities, were obtained from electronic medical records.

Imaging Acquisition

Images are routinely acquired with a 3T Siemens MRI scanner (Siemens MAGNETOM Skyra, Munich, Germany). The HR-VWI protocol included a 3D T1-weighted SPACE fast-spin-echo (FSE) and a 3D T2-weighted sequence. Five minutes after intravenous injection of 0.1 mmol/kg gadolinium-based contrast agent (Gadavist, Bayer Pharmaceuticals, Whippany, NJ), a post-contrast 3D T1-weighted SPACE FSE sequence and a contrast-enhanced magnetic resonance angiography (CE-MRA) were obtained. The reproducibility of results was tested on a 3T GE MRI scanner (GE Healthcare, Chicago, IL) in 9 UIAs which underwent additional HR-VWI. Three UIAs underwent a third scan on a 7T GE MRI (GE Healthcare, Waukesha, WI). Technical parameters used for imaging acquisition on each scanner are described in the Supplementary Tables 1–3.

HR-VWI Assessment

All the images were analyzed with Picture Archiving Communication System, version 12.1.6.1005 (Carestream Vue PACS, Rochester, NY). Aneurysm size (diameter and neck) was measured on CE-MRA images. After six-fold magnification and auto-correction of viewer windowing, the aneurysm was manually co-registered in both pre- and post-contrast T1-weighted sequences in all 3 planes (axial, coronal and sagittal). A 2D region of interest (ROI) of the aneurysm wall was drawn at the level of maximal aneurysm diameter. A combination of CE-MRA and 3D T1 SPACE (pre-contrast) images were used as a reference to exclude the aneurysm lumen and delineate the inner surface of the aneurysm wall, while both 3D T2-weighted sequences and 3D T1-SPACE (post-contrast) were collectively used to distinguish artifact such as cerebrospinal fluid, meninges and veins. ROIs were visually and statistically analyzed to determine that they encompassed the same aneurysm wall area in pre- and post-contrast T1-weighted images (Figure 1).

Figure 1.

Co-registration of CE-MRA, pre- and post-contrast T1-weighted sequences in a basilar tip UIA using (A) axial, (B) coronal and (C) sagittal projections.

Three different methods of aneurysm wall enhancement measurement were compared: the aneurysm enhancement ratio (AER), the aneurysm enhancement index (AEI) and the aneurysm-to-pituitary stalk contrast ratio (CR_stalk). Table 1 summarizes the different formulas and SI measurements used for each method.

Table 1.

Ratios and indexes used to objectively quantify aneurysmal wall enhancement.

Method	Formula*	Reference
Aneurysm enhancement ratio (AER)	$\frac{({SI}_{\mathrm{wall\; post}}-{\mathrm{SI}}_{\mathrm{wall\; pre}})}{{\mathrm{SI}}_{\mathrm{wall\; pre}}}$	Wang et al.27
Aneurysm enhancement index (AEI)	$\frac{\left(\frac{{SI}_{\mathrm{wall\; post}}}{{SI}_{\mathrm{brain\; post}}}-\frac{{SI}_{\mathrm{wall\; pre}}}{{SI}_{\mathrm{brain\; pre}}}\right)}{\frac{{SI}_{\mathrm{wall\; pre}}}{{SI}_{\mathrm{brain\; pre}}}}$	Omodaka et al.16
Aneurysm-to-pituitary stalk contrast ratio (CRstalk)	$\frac{{SI}_{\mathrm{wall\; post}}}{{\mathrm{SI}}_{\mathrm{stalk\; post}}}$	Omodaka et al.15

SI: signal intensity.

^*Each formula was computed using mean and maximal SI_wall.

As proposed by Wang et al,²⁷ SI values measured in the aneurysmal wall from each projection (SI_wall) in pre- and post-contrast T1-weighted images were used to calculate the AER as follows: (SI_{wall post} − SI_{wall pre}) / SI_{wall pre}. Then, following a similar method described by Omodaka et al,¹⁶ SI was quantified in co-registered ROIs of 20 mm² drawn over the genu of the corpus callosum in all views (SI_brain). The AEI was calculated adjusting SI_wall to SI_brain measurements as follows: (SI_{wall post} / SI_{brain post}) − (SI_{wall pre} / SI_{brain pre}) / (SI_{wall pre} / SI_{brain pre}).

The third method analyzed was the CR_stalk. As described elsewhere, four different SI points were randomly sampled throughout the pituitary stalk in the sagittal post-contrast T1-weighted images (figure 2).¹⁵ The mean SI of the aneurysmal wall on the post-contrast T1-weighted sequence (SI_{wall post}) was divided by the SI of all sampled points over the pituitary stalk to calculate the CR_stalk as follows: (SI_{wall post} / SI_{stalk post}).

Figure 2.

Post-contrast T1-weighted sagittal projection of a basilar tip aneurysm showing the ROI of aneurysm wall (yellow), sampling of the genu of the corpus callosum (red) and of the pituitary stalk (black) for normalization. In this case, the maximal CR_stalk would be calculated as follows: 446 / 700 = 0.63. AR: area; AV: average; SD: standard deviation.

Some UIAs show a pattern of focal enhancement, while others demonstrate uniform circumferential wall enhancement (CAWE) in HR-VWI. To define whether the maximal SI on the aneurysm wall should be used instead of the mean SI, we calculated AER, AEI and CR_stalk using the maximal and mean SI in both pre- and post-contrast T1-weighted images. Consequently, a total of six different objective approaches to measure SI were analyzed (Table 1).

Statistical Analysis

Continuous variables are presented as mean ± SD, and categorical variables are presented as frequency and percentage. Differences in aneurysm enhancement were statistically assessed with Student t tests. A 2-sided P < 0.05 was considered significant. Based on results from the International Study of Unruptured Intracranial Aneurysms (ISUIA)³⁰ and other observational studies, UIAs ≥7 mm located in the anterior communicating (ACOM), posterior communicating (PCOM) and basilar arteries (BA) are more likely to rupture and were categorized as unstable. We used these variables (size and location) to perform the area under the receiver-operating characteristics (AUC/ROC) curve analysis.^2,12 All statistical analyses were performed with SPSS Statistics version 25.0 (IBM, Armonk, New York).

RESULTS

A total of 80 patients with 102 UIAs were included between January of 2015 and August of 2019. Mean age was 64.5 ± 12.2 years-old, and 64 (80%) were women (Table 2). Most aneurysms had saccular morphology (96 aneurysms, 94.1%). Correlation statistics demonstrated high agreement for areas covered by ROIs in the axial, coronal and sagittal projections for both pre- and post-contrast T1-weighted images (Pearson coefficients >0.92, P<.001).

Table 2.

Baseline characteristics.

Variable	Overall* Patients = 80 UIAs = 102	Enhancing UIAs† (n = 64)	Non-enhancing UIAs (n = 38)
Age (in years, mean ± SD)	64.5 ± 12.2	66.6 ± 11.9	59.9 ± 10.3
Women (%)	64 (80)	50 (78.1)	32 (84.2)
Current smoking (%)	42 (52.5)	33 (51.6)	22 (57.9)
Hypertension (%)	45 (56.3)	36 (56.3)	23 (60.5)
Diabetes (%)	12 (15)	10 (15.6)	4 (10.5)
Hyperlipidemia (%)	34 (42.5)	25 (39.1)	16 (42.1)
Family history of IAs (%)	7 (8.75)	5 (7.8)	5 (13.2)
Size (in mm, mean, range)	8.7 (3–31)	10.8 (3–31)	5.1 (3–11)
- ≥7 mm (%)	54 (52.9)	44 (68.8)	10 (26.3)
- <7 mm (%)	48 (47.1)	20 (31.3)	28 (73.7)
Location
- Internal carotid artery (%)	25 (24.5)	22 (34.4)	3 (7.9)
- Anterior communicating artery (%)	16 (15.7)	7 (10.9)	9 (23.7)
- Anterior cerebral artery (%)	5 (4.9)	3 (4.7)	2 (5.3)
- Middle cerebral artery (%)	27 (26.5)	14 (26.5)	13 (34.2)
- Posterior communicating artery (%)	5 (4.9)	3 (4.7)	2 (5.3)
- Basilar artery (%)	19 (18.6)	12 (18.8)	7 (18.4)
- Superior cerebellar (%)	3 (2.9)	1 (1.6)	2 (5.3)
- Vertebral artery (%)	2 (2.0)	2 (3.1)	0 (0)

CR_stalk: aneurysm-to-pituitary stalk ratio; mm: millimeters; UIAs: unruptured intracranial aneurysms; SD: standard deviation

^*In overall counts, frequencies for demographics are calculated over total number of patients (N=80), whereas size and location are presented over total number of UIAs (N=102).

^†According to CR_stalk ≥ 0.60 using maximal SI values.

Student t tests showed that aneurysms ≥ 7 mm had significantly higher maximal SI measurements for AER (0.82 vs 0.49, P<.001), CR_stalk (0.84 vs 0.61, P<.001) and AEI (0.81 vs 0.48, P<.001) when compared to smaller aneurysms. An analysis of aneurysms located in the ACOM, PCOM and BA, regardless of size, also reported increased maximal SI values for AER (0.71 vs 0.59, P=.154), CR_stalk (0.73 vs 0.72, P=.955) and AEI (0.71 vs 0.58, P=.135) compared to aneurysms in other locations. However, the differences for location were statistically non-significant.

The best AUC in subsequent ROC analyses was achieved by CR_stalk using maximal SI (0.776), followed by AER (0.738) and AEI (0.730) (Figure 3). Setting the specificity of all ratios to 60% or more, curve point coordinates demonstrated an optimal sensitivity of 81.5% for CR_stalk ≥ 0.60; 75.9% for AEI ≥ 0.50 and 74.1% for AER ≥ 0.49. The cut-offs for all the ratios using maximal and mean SI are summarized in the Supplementary Table 4.

Figure 3.

ROC curves for AER, CR_stalk and AEI using (A) maximal SI values and (B) mean SI values.

Finally, similar HR-VWI protocols were used in both 3T Siemens and 3T GE MRI scans in 9 UIAs (Table 3). Absolute SI values were significantly lower on the GE scanner when compared to the Siemens scanner (mean SI_pre = 121.3 vs 155.9; maximal SI_pre = 194.9 vs 287.7; mean SI_post = 183.9 vs 247.9; maximal SI_post = 288.3 vs 437.8, P<0.001). However, Pearson coefficients demonstrated an excellent correlation between the two scanners for CR_stalk using mean SI values (Pearson = 0.975, P < 0.001) and maximal SI values (Pearson = 0.814, P=0.008). A similar correlation pattern for CR_stalk was established in the 3 UIAs with 7T GE MRI scans (Pearson coefficients > 0.95). On the other hand, the inter-manufacturer correlation using AER and AEI was negligible and statistically non-significant (Pearson coefficients < 0.40 and P > 0.20).

Table 3.

SI measurements on pre- and post-contrast T1-weighted images in nine UIAs using different MRI manufacturers. CR_stalk also included for comparison.

MRI Scanner	Mean SIpre	Max SIpre	Mean SIpost	Max SIpost	CRstalk*
3T Siemens	155.9	287.7	247.9	437.8	0.71/0.42
3T GE	121.3	194.9	183.9	288.3	0.68/0.41
7T GE†	232.2	402.3	378.8	671.0	0.79/0.45

CR_stalk: aneurysm-to-piutitary stalk ratio; MRI: magnetic resonance imaging; SI: signal intensity; UIAs: unruptured intracranial aneurysms.

^*Maximal CR_stalk / Mean CR_stalk.

^†Only 3 UIAs underwent a 7T GE MRI.

DISCUSSION

Clinical and histological correlations have suggested that increased contrast enhancement of the aneurysm wall is a surrogate of aneurysm instability and increased risk of rupture.^24,28 Enhancement quantification has been performed mostly subjectively by experienced adjudicators.^2,12 Several objective methods have been described to quantify enhancement and SI. This study compared all the different objective methods and demonstrated that CR_stalk using maximal SI is the best predictor of aneurysms instability. Moreover, we demonstrated the reproducibility of this technique among different manufacturers, which would be pivotal in a multi-center prospective clinical trial.

Inflammation of the aneurysmal wall, usually initiated by a hemodynamic insult, may result in dysfunction of endothelial and vascular smooth muscle cells, local activation of cytokines, degradation of the extracellular matrix, aneurysm remodeling and rupture.⁴ Several studies have correlated enhancement of the aneurysmal wall on HR-VWI with inflammatory histopathological changes: increased inflammatory cells,^8,9,22 myeloperoxidase activity and vasa vasorum proliferation.^10,13,22,25 A histopathological study performed by our group demonstrated that UIAs with avid enhancement had increased macrophage infiltration and cellularity in comparison to aneurysms with mild- or no-enhancing wall.⁹ This suggests that weakened arterial walls of unstable UIAs exhibit increased contrast enhancement on HR-VWI, and these changes might be explained by an active inflammatory/vasculopathic reaction in the aneurysmal wall.¹⁹

Edjlali et al analyzed 108 UIAs with HR-VWI and introduced the concept of ‘CAWE’ as the presence of ‘circumferential aneurysmal wall enhacenment’.⁵ CAWE was most commonly seen in unstable than stable UIAs (27/31, 87% versus 22/77, 28.5%, respectively; P<0.0001). Omodaka et al proposed two different standardized tools to objectively assess wall enhancement: 1) CR_stalk on post-contrast imaging; and 2) AEI using matched volumes on pre- and post-contrast T1-weighted images in the right frontal lobe as reference.¹⁶ Later on, the same group compared CR_stalk in 69 stable UIAs, 26 evolving UIAs and 67 ruptured aneurysms, reporting significantly higher CR_stalk values in evolving UIAs compared to stable UIAs (0.54 vs 0.34, P<0.0001), but lower compared to ruptured aneurysms (0.54 vs 0.83, P<0.0002).¹⁵ Wang et al²⁷ showed significantly lower enhancement values in UIAs compared to ruptured aneurysms (0.63 vs 0.90, P<0.001) using AER.

In this study, we compared all these objective modalities for quantification of wall enhancement in UIAs. Omodaka et al¹⁵ reported an optimal cutoff value for CR_stalk ≥ 0.39 to distinguish evolving from stable UIAs (AUC 0.80), with sensitivity = 88% and specificity = 63%. Our analysis suggested the same cutoff value for CR_stalk using mean SI values. In a different publication, Omodaka et al¹⁶ used maximal SI ratios to distinguish ruptured (n=28) from UIAs (n=76). The authors reported that CR_stalk ≥ 0.64 achieved sensitivity = 75% and specificity = 83% (AUC = 0.84), whereas AEI ≥ 0.53 achieved sensitivity = 96% and specificity = 43% (AUC = 0.75). We also found similar cutoffs for CR_stalk (0.60) and AEI (0.50) using maximal SI values. Using a cutoff value for AER ≥ 0.615 to differentiate ruptured (n=19) from UIAs (n=87), Wang et al²⁷ achieved AUC = 0.798, sensitivity = 89.5% and specificity = 63.2%. Although we found lower cutoffs for AER (≥ 0.49), our predictive measurements were lower for sensitivity (74%) but similar for specificity (~60%). Overall, the cutoffs found in our study for each enhancement ratio/index correlate with those reported in previous studies. Minor differences in predictive measurements might be explained by the sample size, heterogeneity of the samples and statistical power of each study.

Our study determined that the best predictor of aneurysm instability is CR_stalk using maximal SI (0.60, sensitivity = 81.5%, specificity = 61%). HR-VWI is a relatively new biomarker of aneurysm instability, therefore there is no consensus about the best approach in quantifying aneurysm enhancement. Some studies use mean versus maximal SI.^13,15,16,27 Saccular aneurysms with focal enhancement will have lower mean SI as the ROI values average and dilute focal enhancement. Aneurysms with even CAWE will have higher mean SI. These variations may reflect an array of different morphological environments within the aneurysm wall and different stages of inflammation and/or vasculopathy.⁷ High SI values on areas of focal enhancement are associated with important structural changes of the aneurysmal wall, including blebs, daughter sacs, microbleeds, intraluminal thrombus, neovascularization and inflammatory cell infiltration.^13,20,22 Such focal abnormalities in the aneurysm wall, although recorded as ‘lower enhancement’ on HR-VWI when using mean SI values, are areas of instability that increase the risk of rupture. In this study, all the enhancement ratios/indexes using maximal SI values achieved higher AUC compared to those using mean SI values. We favor using the maximal SI regardless of the enhancement pattern (focal versus CAWE), when calculating enhancement ratios or indexes.

Clinical use of HR-VWI as a surrogate of aneurysm instability has been widely criticized for the lack of consensus regarding imaging protocols, heterogeneity in the definition of wall enhancement and inability to reproduce the same results.^11,24 Our study is the first to test the same HR-VWI protocol among scans of different manufacturers and different potencies. Although absolute SI values measured on 3T GE were significantly lower compared to those measured on 3T Siemens, inter-manufacturer correlation for CR_stalk using mean and maximal SI was almost perfect. On the other hand, AER and AEI results were not reproducible using different manufacturer MRI scanners. These findings reflect an important consideration for planning a multicenter prospective study of HR-VWI for characterization of UIAs.

MR studies assessing SI of the normal pituitary stalk are well described in the literature.²³ Contrast enhancement visualized in the pituitary stalk is attributed to the presence of hypothalamic axons directed to the neurohypophysis, and varies depending on the thickness of their myelin sheath.²¹ In healthy subjects, the level of enhancement of the pituitary stalk does not seem to change significantly over the years, whereas the pineal gland, habenula and ventricular choroid plexuses experience progressive physiological age-related calcification.^1,29 Presence of calcium deposits in these structures affects their SI measurements and precludes their use as a reliable reference of enhancement in HR-VWI.

Moreover, the AEI measured in our study (normalized to the genu of the corpus callosum) could not be reproduced among MRI scans of different manufacturers. The corpus callosum and other gray/white matter structures are contained inside the blood-brain barrier and it is unlikely that they will show significant post-contrast enhancement. On the other hand, the pituitary stalk largely resides outside the blood-brain barrier and exhibits stable gadolinium enhancement.²⁹ Therefore, the pituitary stalk may be the most appropriate intracranial structure to be used as reference when defining aneurysmal wall enhancement on post-contrast MRI. In this study of 102 UIAs, the pituitary stalk was the most reliable and reproducible reference for normalization of SI in the analysis of aneurysmal wall enhancement.

Future Directions

A previous trial that randomized UIAs to coiling versus conservative management failed due to slow recruitment.¹⁸ One of the major drawbacks of this trial was the lack of a biomarker of aneurysm instability. Patients and physicians faced the dichotomy of intervention versus observation. A new trial will have to include HR-VWI as a biomarker of aneurysm instability, and may incorporate the following parameters: SI, quantitative susceptibility mapping,¹⁴ wall thickness and advanced volumetric measurements. Patients and physicians will have to commit to participate in the study and follow-through with randomization, even if it involves serial imaging without intervention. Unfortunately, there is no prospective clinical data to determine if an UIA with a CR_stalk ≥ 0.60 should be treated. Patients with 3-7 mm UIAs may undergo HR-VWI at diagnosis and follow-up. Aneurysm growth, morphological changes, development of new symptoms or progression to rupture could be prospectively compared with parameters of aneurysm instability on HR-VWI.

Limitations

Our relatively small sample size limits the generalizability of the results. HR-VWI may be prone to more artifacts when analyzing smaller aneurysms located in the cavernous and para-clinoid segments of the internal carotid artery. The cavernous/sphenoid sinuses and dural folds of the skull base enhance with contrast and may be confounded with the aneurysmal wall. In this study, six (5.9%) aneurysms were in the cavernous portion of the internal carotid artery.

Additionally, our AUC/ROC curves analysis assumed that UIAs ≥7 mm located in the ACOM, PCOM and BA were unstable. Although such assumption is widely supported by the International Study of Unruptured Intracranial Aneurysms (ISUIA)²⁷ and the Unruptured Cerebral Aneurysms Study (UCAS)⁹, the gold standard would be to evaluate aneurysm instability prospectively. The PHASES score was not used to define aneurysm instability due to the known limitations of this scale: underrepresentation of patients with familial aneurysms and young smokers, limited long-term follow-up data, and exclusion of the risk of intervention or treatment.⁶ Moreover, the PHASES score did not account for aneurysm morphology as a risk factor of rupture.¹⁷ No UIAs ruptured during follow-up in our cohort. A total of 12 patients with 16 UIAs had follow-up HR-VWI (mean 7.2 months). These patients will be followed prospectively to determine if there are changes in the degree of aneurysmal wall enhancement, and whether UIAs grow or become symptomatic.

Another limitation when comparing SI across different scanners is the effect of manufacturer specific sequences/techniques for fat suppression/image acquisition, and the associated hardware differences between scanners (32-channel head coil for the GE scanner versus 20 channel-coil for the Siemens scanner). Nevertheless, these differences are more reflective of real world setting and likely to be encountered as HR-VWI is more frequently utilized.

CONCLUSION

CR_stalk using maximal SI values was the most reliable objective method to quantify aneurysm enhancement on HR-VWI. Aneurysms ≥ 7 mm located in the ACOM, PCOM and BA showed increased wall enhancement. Adjusting aneurysm enhancement for SI values measured in the pituitary stalk allows standardization and reproducibility of results among MRIs of different manufacturers and achieves higher sensitivity and specificity. These findings may help with standardization of quantifiable parameters of aneurysm enhancement and comparison of results across different manufacturer platforms.