Multi-sequence whole-brain intracranial vessel wall imaging at 7.0 tesla

Vessel Wall Characterization Using Quantitative MR Imaging

MR imaging's exceptional capabilities in vascular imaging stem from its ability to visualize and quantify vessel wall features, such as plaque burden, composition, and biomechanical properties. The application of advanced MR imaging techniques, including two-dimensional and three-dimensional black-blood MR imaging, T1 and T2 relaxometry, diffusion-weighted imaging, and dynamic contrast-enhanced MR imaging, wall shear stress, and arterial stiffness, empowers clinicians and researchers to explore the intricacies of vascular diseases. This array of techniques provides comprehensive insights into the development and progression of vascular pathologies, facilitating earlier diagnosis, targeted treatment, and improved patient outcomes in the management of vascular health.

Imaging of intracranial arterial disease: a comparison between MRI and unenhanced CT

Background and purpose

Arterial calcifications on unenhanced CT scans and vessel wall lesions on MRI are often used interchangeably to portray intracranial arterial disease. However, the extent of pathology depicted with each technique is unclear. We investigated the presence and distribution of these two imaging findings in patients with a history of cerebrovascular disease.

Materials and methods

We analyzed CT and MRI data from 78 patients admitted for stroke or TIA at our institution. Vessel wall lesions were assessed on 7 T MRI sequences, while arterial calcifications were assessed on CT scans. The number of vessel wall lesions, severity of intracranial internal carotid artery (iICA) calcifications, and overall presence and distribution of the two imaging findings were visually assessed in the intracranial arteries.

Results

At least one vessel wall lesion or arterial calcification was assessed in 69 (88%) patients. Only the iICA and vertebral arteries (VA) showed a substantial number of both calcifications and vessel wall lesions. The other vessels showed almost exclusively vessel wall lesions. The number of vessel wall lesions was associated with the severity of iICA calcification (p = 0.013).

Conclusions

The number of vessel wall lesions increases with the severity of iICA calcifications. Nonetheless, the distribution of vessel wall lesions on MRI and arterial calcifications on CT shows remarkable differences. These findings support the need for a combined approach to examine intracranial arterial disease.

Visualisation of lenticulostriate arteries using contrast-enhanced time-of-flight magnetic resonance angiography at 7 Tesla

7 Tesla-field-strength (7 T) Magnetic Resonance Imaging allows the small perforating arteries in the brain to be visualised, and this modality may allow visualisation of the arterial pathology in cerebral small vessel disease. Most studies have used standard Time-of-Flight (ToF) Magnetic Resonance Angiography (MRA). Whether the use of contrast enhancement improves perforating artery visualisation at 7 T remains unclear. In a prospective study, we compared standard ToF MRA with contrast-enhanced (CE) ToF MRA at 7 T for the visualisation of the lenticulostriate arteries (LSAs). Ten patients with symptomatic lacunar stroke were recruited (mean age, SD, 64 ± 9.9 years). Visualisation was assessed using a visual rating scale administered by two independent expert readers and length of the LSAs visible. Visualisation of the LSAs was improved with CE ToF MRA. The mean Visibility and Sharpness Score was higher for CE ToF MRA over standard ToF MRA (2.55 ± 0.64 vs. 1.75 ± 0.68; P = 0.0008). The mean length of LSA visualised was significantly longer with CE ToF MRA compared to standard ToF MRA (24.4 ± 4.5 vs. 21.9 ± 4.0 mm; P = 0.01). CE ToF MRA offers improved visualisation of the LSAs over standard ToF MRA. The addition of contrast may improve the ability to visualise cerebral small vessel disease arterial pathology.

T2-Weighted Whole-Brain Intracranial Vessel Wall Imaging at 3 Tesla With Cerebrospinal Fluid Suppression

Background

T₂-weighted (T₂w) intracranial vessel wall imaging (IVWI) provides good contrast to differentiate intracranial vasculopathies and discriminate various important plaque components. However, the strong cerebrospinal fluid (CSF) signal in T₂w images interferes with depicting the intracranial vessel wall. In this study, we propose a T₂-prepared sequence for whole-brain IVWI at 3T with CSF suppression.

Methods

A preparation module that combines T₂ preparation and inversion recovery (T₂IR) was used to suppress the CSF signal and was incorporated into the commercial three-dimensional (3D) turbo spin echo sequence-Sampling Perfection with Application optimized Contrast using different flip angle Evolution (SPACE). This new technique (hereafter called T₂IR-SPACE) was evaluated on nine healthy volunteers and compared with two other commonly used 3D T₂-weighted sequences: T₂w-SPACE and FLAIR-SPACE (FLAIR: fluid-attenuated inversion recovery). The signal-to-noise ratios (SNRs) of the vessel wall (VW) and CSF and contrast-to-noise ratios (CNRs) between them were measured and compared among these three T₂-weighted sequences. Subjective wall visualization of the three T₂-weighted sequences was scored blindly and independently by two radiologists using a four-point scale followed by inter-rater reproducibility analysis. A pilot study of four stroke patients was performed to preliminarily evaluate the diagnostic value of this new sequence, which was compared with two conventional T₂-weighted sequences.

Results

T₂IR-SPACE had the highest CNR (11.01 ± 6.75) compared with FLAIR-SPACE (4.49 ± 3.15; p < 0.001) and T₂w-SPACE (-56.16 ± 18.58; p < 0.001). The subjective wall visualization score of T₂IR-SPACE was higher than those of FLAIR-SPACE and T₂w-SPACE (T₂IR-SPACE: 2.35 ± 0.59; FLAIR-SPACE: 0.52 ± 0.54; T₂w-SPACE: 1.67 ± 0.58); the two radiologists' scores showed excellent agreement (ICC = 0.883).

Conclusion

The T₂IR preparation module markedly suppressed the CSF signal without much SNR loss of the other tissues (i.e., vessel wall, white matter, and gray matter) compared with the IR pulse. Our results suggest that T₂IR-SPACE is a potential alternative T₂-weighted sequence for assessing intracranial vascular diseases.

Intracranial vessel wall lesions on 7T MRI and MRI features of cerebral small vessel disease: The SMART-MR study

The etiology of cerebral small vessel disease (CSVD) is the subject of ongoing research. Although intracranial atherosclerosis (ICAS) has been proposed as a possible cause, studies on their relationship remain sparse. We used 7 T vessel wall magnetic resonance imaging (MRI) to study the association between intracranial vessel wall lesions-a neuroimaging marker of ICAS-and MRI features of CSVD. Within the SMART-MR study, cross-sectional analyses were performed in 130 patients (68 ± 9 years; 88% male). ICAS burden-defined as the number of vessel wall lesions-was determined on 7 T vessel wall MRI. CSVD features were determined on 1.5 T and 7 T MRI. Associations between ICAS burden and CSVD features were estimated with linear or modified Poisson regression, adjusted for age, sex, vascular risk factors, and medication use. In 125 patients, ≥1 vessel wall lesions were found (mean 8.5 ± 5.7 lesions). ICAS burden (per + 1 SD) was associated with presence of large subcortical and/or cortical infarcts (RR = 1.65; 95%CI: 1.12-2.43), lacunes (RR = 1.45; 95% CI: 1.14-1.86), cortical microinfarcts (RR = 1.48; 95%CI: 1.13-1.94), and total white matter hyperintensity volume (b = 0.24; 95%CI: 0.02-0.46). Concluding, patients with a higher ICAS burden had more CSVD features, although no evidence of co-location was observed. Further longitudinal studies are required to determine if ICAS precedes development of CSVD.

Intracranial Atherosclerotic Burden and Cerebral Parenchymal Changes at 7T MRI in Patients With Transient Ischemic Attack or Ischemic Stroke

The relevance of intracranial vessel wall lesions detected with MRI is not fully established. In this study (trial identification number: NTR2119; www.trialregister.nl), 7T MRI was used to investigate if a higher vessel wall lesion burden is associated with more cerebral parenchymal changes in patients with ischemic stroke or transient ischemic attack (TIA). MR images of 82 patients were assessed for the number of vessel wall lesions of the large intracranial arteries and for cerebral parenchymal changes, including the presence and number of cortical, small subcortical, and deep gray matter infarcts; lacunes of presumed vascular origin; cortical microinfarcts; and periventricular and deep white matter hyperintensities (WMHs). Regression analyses showed that a higher vessel wall lesion burden was associated with the presence of small subcortical infarcts, lacunes of presumed vascular origin, and deep gray matter infarcts (relative risk 1.18; 95% CI, 1.03–1.35) and presence of moderate-to-severe periventricular WMHs (1.21; 95% CI, 1.03–1.42), which are all manifestations of small vessel disease (SVD). The burden of enhancing vessel wall lesions was associated with the number of cortical microinfarcts only (1.48; 95% CI, 1.04–2.11). These results suggest an interrelationship between large vessel wall lesion burden and cerebral parenchymal manifestations often linked to SVD or, alternatively, that vascular changes occur in both large and small intracranial arteries simultaneously.

Black-Blood Contrast in Cardiovascular MRI

MRI is a versatile technique that offers many different options for tissue contrast, including suppressing the blood signal, so‐called black‐blood contrast. This contrast mechanism is extremely useful to visualize the vessel wall with high conspicuity or for characterization of tissue adjacent to the blood pool. In this review we cover the physics of black‐blood contrast and different techniques to achieve blood suppression, from methods intrinsic to the imaging readout to magnetization preparation pulses that can be combined with arbitrary readouts, including flow‐dependent and flow‐independent techniques. We emphasize the technical challenges of black‐blood contrast that can depend on flow and motion conditions, additional contrast weighting mechanisms (T₁, T₂, etc.), magnetic properties of the tissue, and spatial coverage. Finally, we describe specific implementations of black‐blood contrast for different vascular beds.

Imaging of intracranial atherosclerotic plaques using 3.0 T and 7.0 T magnetic resonance imaging-current trends and future perspectives

Intracranial atherosclerotic disease (ICAD) is one of the most common causes of ischemic stroke and carries a relatively high risk of stroke recurrence. Advances in high-resolution magnetic resonance imaging (HRMRI) techniques of intracranial arteries now have made it possible to directly visualize atherosclerotic plaque itself, allowing detailed assessments of plaque morphology and components. Currently available intracranial HRMRI could be performed with 2-dimensional (2D) and 3D acquisitions, and multicontrast weightings in clinically reasonable scan times. Until now, HRMRI research of ICAD has focused on the identification of plaque vulnerability, and the relationship between plaque characteristics and ischemic stroke. HRMRI at ultra-high-field strength (7.0 T) holds promise in better visualizing intracranial vessel walls, as well as identifying early lesions and total burden of ICAD. As a result, intracranial HRMRI provides great insights into pathology of intracranial atherosclerotic plaques, stroke mechanisms, and future stroke risk. In this article, we will review the technical implementation, preclinical research, clinical applications, and future directions of HRMRI for the evaluation of ICAD at 3.0 T and 7.0 T.

Intracranial atherosclerosis on 7T MRI and cognitive functioning: The SMART-MR study

OBJECTIVE

To investigate the association between intracranial atherosclerosis (ICAS) and cognitive functioning in patients with a history of vascular disease.

METHODS

Within the Second Manifestations of Arterial Disease-Magnetic Resonance (SMART-MR) study, cross-sectional analyses were performed in 130 patients (mean ± SD age 68 ± 9 years) with 7T vessel wall MRI data. Vessel wall lesions were rated according to established criteria and summed into a circulatory and artery-specific ICAS burden. Associations between ICAS burden and Z scores of memory, executive functioning, working memory, and processing speed were estimated using linear regression analyses adjusted for age, sex, education, reading ability, and vascular risk factors.

RESULTS

A total of 125 patients (96%) had ≥1 vessel wall lesion; the mean ICAS burden was 8.5 ± 5.7. A statistically nonsignificant association was found between total ICAS burden and memory (b = -0.03 per +1 lesion; 95% confidence interval [CI] -0.05 to 0.00). No associations were found for the other domains. A statistically significant association was found for ICAS burden of the posterior cerebral artery (PCA) and memory (b = -0.12 per +1 lesion; 95% CI -0.23 to -0.01) and executive functioning (b = -0.10 per +1 lesion; 95% CI -0.19 to -0.01). Statistically nonsignificant associations were found for the anterior cerebral artery (ACA) burden and memory (b = -0.13 per +1 lesion; 95% CI -0.26 to 0.01) and executive functioning (b = -0.11 per +1 lesion; 95% CI -0.22 to 0.01). Additional adjustments for large infarcts, white matter hyperintensities, lacunes, and ≥50% carotid stenosis produced similar results.

CONCLUSIONS

Our results suggest an artery-specific vulnerability of memory and executive functioning to ICAS, possibly due to strategic brain regions involved with these cognitive domains, which are located in the arterial territory of the PCA and ACA.

MR Intracranial Vessel Wall Imaging: A Systematic Review

The purpose of this systematic review is to identify trends and extent of variability in intracranial vessel wall MR imaging (VWI) techniques and protocols. Although variability in selection of protocol design and pulse sequence type is known, data on what and how protocols vary are unknown. Three databases were searched to identify publications using intracranial VWI. Publications were screened by predetermined inclusion/exclusion criteria. Technical development publications were scored for completeness of reporting using a modified Nature Reporting Summary Guideline to assess reproducibility. From 2,431 articles, 122 met the inclusion criteria. Trends over the last 23 years (1995-2018) show increased use of 3-Tesla MR (P < .001) and 3D volumetric T1-weighted acquisitions (P < .001). Most (65%) clinical VWI publications report achieving a noninterpolated in-plane spatial resolution of ≤.55 mm. In the last decade, an increasing number of technical development (n = 20) and 7 Tesla (n = 12) publications have been published, focused on pulse sequence development, improving cerebrospinal fluid suppression, scan efficiency, and imaging ex vivo specimen for histologic validation. Mean Reporting Summary Score for the technical development publications was high (.87, range: .63-1.0) indicating strong scientific technical reproducibility. Innovative work continues to emerge to address implementation challenges. Gradual adoption into the research and scientific community was suggested by a shift in the name in the literature from "high-resolution MR" to "vessel wall imaging," specifying diagnostic intent. Insight into current practices and identifying the extent of technical variability in the literature will help to direct future clinical and technical efforts to address needs for implementation.

MRI Vessel Wall Imaging after Intra-Arterial Treatment for Acute Ischemic Stroke

BACKGROUND AND PURPOSE

Vessel wall imaging is increasingly performed in the diagnostic work-up of patients with ischemic stroke. The aim of this study was to compare vessel wall enhancement after intra-arterial thrombosuction with that in patients not treated with thrombosuction.

MATERIALS AND METHODS

From 2009 to 2017, forty-nine patients with an ischemic stroke underwent 7T MR imaging within 3 months after symptom onset as part of a prospective intracranial vessel wall imaging study. Fourteen of these patients underwent intra-arterial treatment using thrombosuction (intra-arterial treatment group). In the intra-arterial treatment group, vessel walls were evaluated for major vessel wall changes. All patients underwent pre- and postcontrast vessel wall imaging to assess enhancing foci of the vessel wall using coregistered subtraction images. A Wilcoxon signed rank test was performed to test for differences.

RESULTS

In the intra-arterial treatment group, 11 of 14 patients (79%) showed vessel wall enhancement compared with 17 of 35 patients without intra-arterial treatment (49%). In the intra-arterial treatment group, more enhancing foci were detected on the ipsilateral side (n = 18.5) compared with the contralateral side (n = 3, P = .005). Enhancement was more often concentric on the ipsilateral side (n = 8) compared with contralateral side (n = 0, P = .01). No differences were found in the group without intra-arterial treatment between the number and configuration of ipsilateral and contralateral enhancing foci.

CONCLUSIONS

Patients with intra-arterial treatment by means of thrombosuction showed more (concentric) enhancing foci of the vessel wall ipsilateral compared with contralateral to the treated artery than the patients without intra-arterial treatment, suggesting reactive changes of the vessel wall. This finding should be taken into account when assessing vessel wall MR images in patients with stroke.

Emerging Use of Ultra-High-Field 7T MRI in the Study of Intracranial Vascularity: State of the Field and Future Directions

Cerebrovascular disease is a major source of mortality that commonly requires neurosurgical intervention. MR imaging is the preferred technique for imaging cerebrovascular structures, as well as regions of pathology that include microbleeds and ischemia. Advanced MR imaging sequences such as time-of-flight, susceptibility-weighted imaging, and 3D T2-weighted sequences have demonstrated excellent depiction of arterial and venous structures with and without contrast administration. While the advantages of 3T compared with 1.5T have been described, the role of ultra-high-field (7T) MR imaging in neurovascular imaging remains poorly understood. In the present review, we examine emerging neurosurgical applications of 7T MR imaging in vascular imaging of diverse conditions and discuss current limitations and future directions for this technique.

Focal irregularities in 7-Tesla MRI of unruptured intracranial aneurysms as an indicator for areas of altered blood-flow parameters

OBJECTIVE

In the last several decades, various factors have been studied for a better evaluation of the risk of rupture in incidentally discovered intracranial aneurysms (IAs). With advanced MRI, attempts were made to delineate the wall of IAs to identify weak areas prone to rupture. However, the field strength of the MRI investigations was insufficient for reasonable image resolution in many of these studies. Therefore, the aim of this study was to analyze findings of IAs in ultra-high field MRI at 7 Tesla (7 T).

METHODS

Patients with incidentally found IAs of at least 5 mm in diameter were included in this study and underwent MRI investigations at 7 T. At this field strength a hyperintense intravascular signal can be observed on nonenhanced images with a brighter "rim effect" along the vessel wall. Properties of this rim effect were evaluated and compared with computational fluid dynamics (CFD) analyses.

RESULTS

Overall, 23 aneurysms showed sufficient image quality for further evaluation. In 22 aneurysms focal irregularities were identified within this rim effect. Areas of such irregularities showed significantly higher values in wall shear stress and vorticity compared to areas with a clearly visible rim effect (p = 0.043 in both).

CONCLUSIONS

A hyperintense rim effect along the vessel wall was observed in most cases. Focal irregularities within this rim effect showed higher values of the mean wall shear stress and vorticity when compared by CFD analyses. Therefore, these findings indicate alterations in blood flow in IAs within these areas.

Intracranial Atherosclerotic Burden on 7T MRI Is Associated with Markers of Extracranial Atherosclerosis: The SMART-MR Study

BACKGROUND AND PURPOSE

Intracranial atherosclerosis, a major risk factor for ischemic stroke, is thought to have different atherogenic mechanisms than extracranial atherosclerosis. Studies investigating their relationship in vivo are sparse and report inconsistent results. We studied the relationship between intracranial atherosclerosis and extracranial atherosclerosis in a cohort of patients with a history of vascular disease.

MATERIALS AND METHODS

Within the Second Manifestations of ARTerial disease-Magnetic Resonance (SMART) study, cross-sectional analyses were performed in 130 patients (mean age, 68 ± 9 years) with a history of vascular disease and with assessable 7T intracranial vessel wall MR imaging data. Intracranial atherosclerosis burden was defined as the number of intracranial vessel wall lesions in the circle of Willis and its major branches. Age- and sex-adjusted unstandardized regression coefficients (b-value) were calculated with intracranial atherosclerosis burden as the dependent variable and extracranial atherosclerosis markers as independent variables.

RESULTS

Ninety-six percent of patients had ≥1 vessel wall lesion, with a mean intracranial atherosclerosis burden of 8.5 ± 5.7 lesions. Significant associations were observed between higher intracranial atherosclerosis burden and carotid intima-media thickness (b = 0.53 lesions per +0.1  mm; 95% CI, 0.1-1.0 lesions), 50%-100% carotid stenosis versus no stenosis (b = 6.6 lesions; 95% CI, 2.3-10.9 lesions), ankle-brachial index ≤ 0.9 versus >0.9 (b = 4.9 lesions; 95% CI, 1.7-8.0 lesions), and estimated glomerular filtration rate (b = -0.77 lesions per +10 mL/min; 95% CI, -1.50 to -0.03 lesions). No significant differences in intracranial atherosclerosis burden were found among different categories of vascular disease.

CONCLUSIONS

Intracranial atherosclerosis was associated with various extracranial markers of atherosclerosis, not supporting a different etiology.

Subacute vessel wall imaging at 7-T MRI in post-thrombectomy stroke patients

PURPOSE

Reports from 3-T vessel wall MRI imaging have shown contrast enhancement following thrombectomy for acute stroke, suggesting potential intimal damage. Comparisons have shown higher SNR and more lesions detected by vessel wall imaging when using 7 T compared with 3 T. The aim of this study was to investigate the vessel walls after stent retriever thrombectomy using high-resolution vessel wall imaging at 7 T.

METHODS

Seven patients with acute stroke caused by occlusion of the distal internal carotid artery (T-occlusion), or proximal medial cerebral artery, and treated by stent retriever thrombectomy with complete recanalization were included and examined by 7-T MRI within 2 days. The MRI protocol included a high-resolution black blood sequence with prospective motion correction (iMOCO), acquired before and after contrast injection. Flow measurements were performed in the treated and untreated M1 segments.

RESULTS

All subjects completed the MRI examination. Image quality was independently rated as excellent by two neuroradiologists for all cases, and the level of motion artifacts did not impair diagnostic quality, despite severe motion in some cases. Contrast enhancement correlated with the deployment location of the stent retrievers. Flow data showed complete restoration of flow after treatment.

CONCLUSION

Vessel wall imaging with prospective motion correction can be performed in patients following thrombectomy with excellent imaging quality at 7 T. We show that vessel wall contrast enhancement is the normal post-operative state and corresponds to the deployment location of the stent retriever.

High-resolution contrast-enhanced vessel wall imaging in patients with suspected cerebral vasculitis: Prospective comparison of whole-brain 3D T1 SPACE versus 2D T1 black blood MRI at 3 Tesla

Purpose

Vessel wall imaging (VWI) using T1 dark blood MRI can depict inflammation of intracranial arteries in patients with cerebral vasculitis. Recently, 3D VWI sequences were introduced at 3 Tesla. We aimed to compare 2D and 3D VWI for detection of intracranial vessel wall enhancement (VWE) in patients suspected of cerebral vasculitis.

Methods

44 MRI scans of 39 patients were assessed that included bi-planar 2D T1 and whole-brain 3D T1 SPACE dark blood VWI pre and post contrast. Visibility and VWE were analyzed in 31 pre-specified intracranial artery segments. Additionally, leptomeningeal and parenchymal contrast enhancement was assessed.

Results

Overall, more arterial segments were visualized with 3D VWI (p<0.0001). Detection of VWE showed fair agreement between 2D and 3D VWI (κ = 0.583). On segmental level, more VWE was detected in intradural ICA by 2D VWI (p<0.001) and in VA V4 segment by 3D VWI (p<0.05). 3D VWI showed more leptomeningeal (p<0.05) and parenchymal (p<0.01) contrast enhancement. In patients with positive diagnosis of cerebral vasculitis, sensitivity was of 67% (2D and 3D VWI) and specificity was 44% (2D VWI) and 48% (3D VWI); more VWE was seen in arteries distal to VA and ICA compared to non-vasculitic patients.

Conclusion

2D and 3D VWI differed in the ability to detect VWE. Whole brain coverage with better evaluability of VAs and distal intracranial artery segments, and depiction of more parenchymal and leptomeningeal enhancement make 3D VWI more favorable. As VWE in arteries distal to VA and ICA may be used for discrimination of vasculitic and non-vasculitic patients, future increase in spatial resolution of 3D VWI sequences may be beneficial.

Accelerated multi-contrast high isotropic resolution 3D intracranial vessel wall MRI using a tailored k-space undersampling and partially parallel reconstruction strategy

OBJECTIVE

To develop a 3D multi-contrast IVW protocol with 0.5-mm isotropic resolution and a scan time of 5 min per sequence.

MATERIALS AND METHODS

Pre-contrast T1w VISTA, DANTE prepared PDw VISTA, SNAP, and post-contrast T1w VISTA were accelerated using cartesian undersampling with target ordering method (CUSTOM) and self-supporting tailored k-space estimation for parallel imaging reconstruction (STEP). CUSTOM + STEP IVW was compared to full-sample IVW, SENSE-accelerated IVW, and CUSTOM + zero-filled Fourier reconstruction in normal volunteers and subjects with intracranial atherosclerotic disease (ICAD). Image quality, vessel delineation, CSF suppression, and blood suppression were compared.

RESULTS

CUSTOM + STEP vessel wall delineation was comparable to full-sample IVW and better than SENSE IVW for vessel wall delineation on T1w VISTA and luminal contrast on SNAP. Average image quality and wall depiction were significantly improved using STEP reconstruction compared with zero-filled Fourier reconstruction, with no significant difference in CSF or blood suppression.

CONCLUSIONS

CUSTOM + STEP allowed multi-contrast 3D 0.5-mm isotropic IVW within 30 min. Although some quantitative and qualitative scores for CUSTOM - STEP were lower than fully sampled IVW, CUSTOM + STEP provided comparable vessel wall delineation as full-sample IVW and was superior to SENSE. CUSTOM + STEP IVW was well tolerated by patients and showed good delineation of ICAD plaque.

Variable impact of CSF flow suppression on quantitative 3.0T intracranial vessel wall measurements

BACKGROUND

Flow suppression techniques have been developed for intracranial (IC) vessel wall imaging (VWI) and optimized using simulations; however, simulation results may not translate in vivo.

PURPOSE

To evaluate experimentally how IC vessel wall and lumen measurements change in identical subjects when evaluated using the most commonly available blood and cerebrospinal fluid (CSF) flow suppression modules and VWI sequences.

STUDY TYPE

Prospective.

POPULATION/SUBJECTS

Healthy adults (n = 13; age = 37 ± 15 years) were enrolled.

FIELD STRENGTH/SEQUENCE

A 3.0T 3D T1 /proton density (PD)-weighted turbo-spin-echo (TSE) acquisition with post-readout anti-driven equilibrium module, with and without Delay-Alternating-with-Nutation-for-Tailored-Excitation (DANTE) was applied. DANTE flip angle (8-12°) and TSE refocusing angle (sweep = 40-120° or 50-120°) were varied.

ASSESSMENT

Basilar artery and internal carotid artery (ICA) wall thicknesses, CSF signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and signal ratio (SR) were assessed. Measurements were made by two readers (radiology resident and board-certified neuroradiologist).

STATISTICAL TESTS

A Wilcoxon signed-rank test was applied with corrected two-sided P < 0.05 required for significance (critical P = 0.008, 0.005, and 0.05 for SNR/CNR, SR, and wall thickness, respectively).

RESULTS

A TSE pulse sweep = 40-120° and sweep = 50-120° provided similar (P = 0.55) CSF suppression. Addition of the DANTE preparation reduced CSF SNR from 17.4 to 6.7, thereby providing significant (P < 0.008) improvement in CSF suppression. The DANTE preparation also resulted in a significant (P < 0.008) reduction in vessel wall SNR, but variable vessel wall to CSF CNR improvement (P = 0.87). There was a trend for a difference in blood SNR with vs. without DANTE (P = 0.05). The outer vessel wall diameter and wall thickness values were lower (P < 0.05) with (basilar artery 4.45 mm, 0.81 mm, respectively) vs. without (basilar artery 4.88 mm, 0.97 mm, respectively) DANTE 8°.

DATA CONCLUSION

IC VWI with TSE sweep = 40-120° and with DANTE flip angle = 8° provides the best CSF suppression and CNR of the approaches evaluated. However, improvements are heterogeneous, likely owing to intersubject vessel pulsatility and CSF flow variations, which can lead to variable flow suppression efficacy in these velocity-dependent modules.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;48:1120-1128.

3D Black-Blood Luminal Angiography Derived from High-Resolution MR Vessel Wall Imaging in Detecting MCA Stenosis: A Preliminary Study

BACKGROUND AND PURPOSE

3D high-resolution vessel wall imaging is increasingly used for intracranial arterial diseases. This study compared the diagnostic performance of black-blood luminal angiography derived from 3D vessel wall imaging with source images of vessel wall imaging and TOF-MRA in detecting middle cerebral artery stenosis.

MATERIALS AND METHODS

Sixty-two patients with suspected MCA atherosclerosis underwent TOF-MRA, vessel wall imaging, and CTA. Intracranial black-blood luminal angiography was created from source images of vessel wall imaging using minimum intensity projection. The degree and length of MCA stenosis were measured on source images of vessel wall imaging, TOF-MRA, and black-blood luminal angiography and compared using CTA as a reference standard.

RESULTS

The image quality of black-blood luminal angiography was diagnostic in most patients. The intra- and interobserver agreement for both stenosis degree and length measurements was excellent for black-blood luminal angiography. It was comparable with that of source images of vessel wall imaging in grading stenosis. Compared with TOF-MRA, black-blood luminal angiography showed significantly higher sensitivity for the detection of severe stenosis (89.3% versus 64.3%, P = .039) and higher specificity for the detection of occlusion (95.4% versus 84.6%, P = .039). Lesion length estimated on source images of vessel wall imaging was significantly greater than that measured by CTA and black-blood luminal angiography (P < .001 and P = .010).

CONCLUSIONS

Black-blood luminal angiography is better than TOF-MRA in detecting severe stenosis and occlusion of the MCA. Compared with source images of vessel wall imaging, it is more accurate in evaluating stenosis length. Black-blood luminal angiography can be produced as a derivative from vessel wall imaging and implemented as an adjunct to vessel wall imaging and TOF-MRA without extra acquisition time.

3D whole-brain vessel wall cardiovascular magnetic resonance imaging: a study on the reliability in the quantification of intracranial vessel dimensions

BACKGROUND

One of the potentially important applications of three-dimensional (3D) intracranial vessel wall (IVW) cardiovascular magnetic resonance (CMR) is to monitor disease progression and regression via quantitative measurement of IVW morphology during medical management or drug development. However, a prerequisite for this application is to validate that IVW morphologic measurements based on the modality are reliable. In this study we performed comprehensive reliability analysis for the recently proposed whole-brain IVW CMR technique.

METHODS

Thirty-four healthy subjects and 10 patients with known intracranial atherosclerotic disease underwent repeat whole-brain IVW CMR scans. In 19 of the 34 subjects, two-dimensional (2D) turbo spin-echo (TSE) scan was performed to serve as a reference for the assessment of vessel dimensions. Lumen and wall volume, normalized wall index, mean and maximum wall thickness were measured in both 3D and 2D IVW CMR images. Scan-rescan, intra-observer, and inter-observer reproducibility of 3D IVW CMR in the quantification of IVW or plaque dimensions were respectively assessed in volunteers and patients as well as for different healthy subjectsub-groups (i.e. < 50 and ≥ 50 years). The agreement in vessel wall and lumen measurements between the 3D technique and the 2D TSE method was also investigated. In addition, the sample size required for future longitudinal clinical studies was calculated.

RESULTS

The intra-class correlation coefficient (ICC) and Bland-Altman plots indicated excellent reproducibility and inter-method agreement for all morphologic measurements (All ICCs > 0.75). In addition, all ICCs of patients were equal to or higher than that of healthy subjects except maximum wall thickness. In volunteers, all ICCs of the age group of ≥50 years were equal to or higher than that of the age group of < 50 years. Normalized wall index and mean and maximum wall thickness were significantly larger in the age group of ≥50 years. To detect 5% - 20% difference between placebo and treatment groups, normalized wall index requires the smallest sample size while lumen volume requires the highest sample size.

CONCLUSIONS

Whole-brain 3D IVW CMR is a reliable imaging method for the quantification of intracranial vessel dimensions and could potentially be useful for monitoring plaque progression and regression.

Intracranial Large Artery Disease of Non-Atherosclerotic Origin: Recent Progress and Clinical Implications

Intracranial large artery disease (ILAD) is the major cause of stroke worldwide. With the application of recently introduced diagnostic techniques, the prevalence of non-atherosclerotic ILAD is expected to increase. Herein, we reviewed recent reports and summarized progress in the diagnosis and clinical impact of differentiation between ILAD of atherosclerotic and non-atherosclerotic origin. Our review of the literature suggests that more careful consideration of non-atherosclerotic causes and the application of appropriate diagnostic techniques in patients with ILAD may not only provide better results in the treatment of patients, but it may also lead to more successful clinical trials for the treatment of intracranial atherosclerosis.

Comparison of 3T Intracranial Vessel Wall MRI Sequences

BACKGROUND AND PURPOSE

Intracranial vessel wall MR imaging plays an increasing role in diagnosing intracranial vascular diseases. For a complete assessment, pre- and postcontrast sequences are required, and including other sequences, these result in a long scan duration. Ideally, the scan time of the vessel wall sequence should be reduced. The purpose of this study was to evaluate different intracranial vessel wall sequence variants to reduce scan duration, provided an acceptable image quality can be maintained.

MATERIALS AND METHODS

Starting from the vessel wall sequence that we use clinically (6:42 minutes), 6 scan variants were tested (scan duration ranging between 4:39 and 8:24 minutes), creating various trade-offs among spatial resolution, SNR, and contrast-to-noise ratio. In total, 15 subjects were scanned on a 3T MR imaging scanner: In 5 subjects, all 7 variants were performed precontrast-only, and in 10 other subjects, the fastest variant (4:39 minutes) and our clinically used variant (6:42 minutes) were performed pre- and postcontrast.

RESULTS

The fastest variant (4:39 minutes) had higher or comparable SNRs/contrast-to-noise ratios of the intracranial vessel walls compared with the reference sequence (6:42 minutes). Qualitative assessment showed that the contrast-to-noise ratio was most suppressed in the fastest variant of 4:39 minutes and the variant of 6:42 minutes pre- and postcontrast. SNRs/contrast-to-noise ratios of the fastest variant were all, except one, higher compared with the variant of 6:42 minutes (P < .008). Furthermore, the fastest variant (4:39 minutes) detected all vessel wall lesions identified on the 6:42-minute variant.

CONCLUSIONS

A 30% faster vessel wall sequence was developed with high SNRs/contrast-to-noise ratios that resulted in good visibility of the intracranial vessel wall.

Clinical vascular imaging in the brain at 7T

Stroke and related cerebrovascular diseases are a major cause of mortality and disability. Even at standard-field-strengths (1.5T), MRI is by far the most sensitive imaging technique to detect acute brain infarctions and to characterize incidental cerebrovascular lesions, such as white matter hyperintensities, lacunes and microbleeds. Arterial time-of-flight (TOF) MR angiography (MRA) can depict luminal narrowing or occlusion of the major brain feeding arteries, and this without the need for contrast administration. Compared to 1.5T MRA, the use of high-field strength (3T) and even more so ultra-high-field strengths (7T), enables the visualization of the lumen of much smaller intracranial vessels, while adding a contrast agent to TOF MRA at 7T may enable the visualization of even more distal arteries in addition to veins and venules. Moreover, with 3T and 7T, the arterial vessel walls beyond the circle of Willis become visible with high-resolution vessel wall imaging. In addition, with 7T MRI, the brain parenchyma can now be visualized on a submillimeter scale. As a result, high-resolution imaging studies of the brain and its blood supply at 7T have generated new concepts of different cerebrovascular diseases. In the current article, we will discuss emerging clinical applications and future directions of vascular imaging in the brain at 7T MRI.

Vessel wall characterization using quantitative MRI: what's in a number?

The past decade has witnessed the rapid development of new MRI technology for vessel wall imaging. Today, with advances in MRI hardware and pulse sequences, quantitative MRI of the vessel wall represents a real alternative to conventional qualitative imaging, which is hindered by significant intra- and inter-observer variability. Quantitative MRI can measure several important morphological and functional characteristics of the vessel wall. This review provides a detailed introduction to novel quantitative MRI methods for measuring vessel wall dimensions, plaque composition and permeability, endothelial shear stress and wall stiffness. Together, these methods show the versatility of non-invasive quantitative MRI for probing vascular disease at several stages. These quantitative MRI biomarkers can play an important role in the context of both treatment response monitoring and risk prediction. Given the rapid developments in scan acceleration techniques and novel image reconstruction, we foresee the possibility of integrating the acquisition of multiple quantitative vessel wall parameters within a single scan session.

Detecting Intracranial Vessel Wall Lesions With 7T-Magnetic Resonance Imaging: Patients With Posterior Circulation Ischemia Versus Healthy Controls

BACKGROUND AND PURPOSE

Vessel wall magnetic resonance imaging sequences have been developed to directly visualize the intracranial vessel wall, enabling detection of vessel wall changes, including those that have not yet caused luminal narrowing. In this study, vessel wall lesion burden was assessed in patients with recent posterior circulation ischemia using 7T-magnetic resonance imaging and compared with matched healthy controls.

METHODS

Fifty subjects (25 patients and 25 matched healthy controls) underwent 7T-magnetic resonance imaging with an intracranial vessel wall sequence before and after contrast administration. Two raters scored the presence and contrast enhancement of arterial wall lesions in individual segments of the circle of Willis and its primary branches. Total burden and distribution of vessel wall lesions and lesion characteristics (configuration, thickening pattern, and contrast enhancement) were compared both between and within both groups.

RESULTS

Overall, vessel wall lesion burden and distribution were comparable between patients and controls. Regarding individual arterial segments, only vessel wall lesions in the posterior cerebral artery were more frequently observed in patients (18.0%) than in controls (5.4%; P=0.003). Many of these lesions showed enhancement, both in patients (48.9%) and in controls (43.5%; P=0.41). In patients, the proportion of enhancing lesions was higher in the posterior circulation (53.3%) than in the anterior circulation (20.6%; P=0.008).

CONCLUSIONS

Although overall intracranial vessel wall lesion burden and contrast enhancement were comparable between patients with recent posterior circulation ischemia and healthy controls, this study also revealed significant differences between the 2 groups, suggesting an association between posterior circulation lesion burden/enhancement and ischemic events.

CLINICAL TRIAL REGISTRATION

URL: http://www.trialregister.nl. Unique identifier: NTR5688.

High-resolution Magnetic Resonance Vessel Wall Imaging for Intracranial Arterial Stenosis

OBJECTIVE

To discuss the feasibility and clinical value of high-resolution magnetic resonance vessel wall imaging (HRMR VWI) for intracranial arterial stenosis.

DATE SOURCES

We retrieved information from PubMed database up to December 2015, using various search terms including vessel wall imaging (VWI), high-resolution magnetic resonance imaging, intracranial arterial stenosis, black blood, and intracranial atherosclerosis.

STUDY SELECTION

We reviewed peer-reviewed articles printed in English on imaging technique of VWI and characteristic findings of various intracranial vasculopathies on VWI. We organized this data to explain the value of VWI in clinical application.

RESULTS

VWI with black blood technique could provide high-quality images with submillimeter voxel size, and display both the vessel wall and lumen of intracranial artery simultaneously. Various intracranial vasculopathies (atherosclerotic or nonatherosclerotic) had differentiating features including pattern of wall thickening, enhancement, and vessel remodeling on VWI. This technique could be used for determining causes of stenosis, identification of stroke mechanism, risk-stratifying patients, and directing therapeutic management in clinical practice. In addition, a new morphological classification based on VWI could be established for predicting the efficacy of endovascular therapy.

CONCLUSIONS

This review highlights the value of HRMR VWI for discrimination of different intracranial vasculopathies and directing therapeutic management.

7-T MRI in Cerebrovascular Diseases: Challenges to Overcome and Initial Results

Magnetic resonance imaging (MRI) plays a key role in the investigation of cerebrovascular diseases. Compared with computed tomography (CT) and digital subtraction angiography (DSA), its advantages in diagnosing cerebrovascular pathology include its superior tissue contrast, its ability to visualize blood vessels without the use of a contrast agent, and its use of magnetic fields and radiofrequency pulses instead of ionizing radiation. In recent years, ultrahigh field MRI at 7 tesla (7 T) has shown promise in the diagnosis of many cerebrovascular diseases. The increased signal-to-noise ratio (SNR; 2.3x and 4.7x increase compared with 3 and 1.5 T, respectively) and contrast-to-noise ratio (CNR) at this higher field strength can be exploited to obtain a higher spatial resolution and higher lesion conspicuousness, enabling assessment of smaller brain structures and lesions. Cerebrovascular diseases can be assessed at different tissue levels; for instance, changes of the arteries feeding the brain can be visualized to determine the cause of ischemic stroke, regional changes in brain perfusion can be mapped to predict outcome after revascularization, and tissue damage, including old and recent ischemic infarcts, can be evaluated as a marker of ischemic burden. For the purpose of this review, we will discriminate 3 levels of assessment of cerebrovascular diseases using MRI: Pipes, Perfusion, and Parenchyma (3 Ps). The term Pipes refers to the brain-feeding arteries from the heart and aortic arch, upwards to the carotid arteries, vertebral arteries, circle of Willis, and smaller intracranial arterial branches. Perfusion is the amount of blood arriving at the brain tissue level, and includes the vascular reserve and perfusion territories. Parenchyma refers to the acute and chronic burden of brain tissue damage, which includes larger infarcts, smaller microinfarcts, and small vessel disease manifestations such as white matter lesions, lacunar infarcts, and microbleeds. In this review, we will describe the key developments in the last decade of 7-T MRI of cerebrovascular diseases, subdivided for these 3 levels of assessment.

Whole-brain vessel wall MRI: A parameter tune-up solution to improve the scan efficiency of three-dimensional variable flip-angle turbo spin-echo

PURPOSE

To propose and evaluate a parameter tune-up solution to expedite a three-dimensional (3D) variable-flip-angle turbo spin-echo (TSE) sequence for whole-brain intracranial vessel wall (IVW) imaging.

MATERIALS AND METHODS

Elliptical k-space sampling and prolonged echo train length (ETL), were used to expedite a 3D variable-flip-angle TSE-based sequence. To compensate for the potential loss in vessel wall signal, optimal combination of prescribed T₂ and ETL was experimentally investigated on 22 healthy volunteers at 3 Tesla. The optimized protocol (7-8 min) was then compared with a previous protocol (reference protocol, 11-12 min) in terms of signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), vessel wall sharpness, and wall delineation quality on a 4-point scale (0:poor; 3:excellent) in 10 healthy volunteers. A pilot study of five patients was performed and lesion delineation score was used to demonstrate the diagnostic quality.

RESULTS

A protocol with ETL = 52 and prescribed T₂  = 170 ms was deemed an optimized one, which, compared with the reference protocol, provided significantly improved wall SNR (12.0 ± 1.3 versus 10.0 ± 1.1; P = 0.002), wall-lumen CNR (9.7 ± 1.2 versus 8.0 ± 0.9; P = 0.002), wall-CSF CNR (2.8 ± 1.0 versus 1.7 ± 1.0; P = 0.026), similar vessel wall sharpness at both inner (1.59 ± 0.18 versus 1.58 ± 0.14, P = 0.87) and outer (1.71 ± 0.25 versus 1.83 ± 0.30; P = 0.18) boundaries, and comparable vessel wall delineation score for individual segments (1.95-3; P > 0.06). In all patients, atherosclerotic plaques (10) or wall dissection (5) were identified with a delineation score of 3 or 2.

CONCLUSION

A parameter tune-up solution can accelerate 3D variable-flip-angle TSE acquisitions, particularly allowed for expedited whole-brain IVW imaging with preserved wall delineation quality.

LEVEL OF EVIDENCE

2. Technical Efficacy: Stage 1 J. MAGN. RESON. IMAGING 2017;46:751-757.

Qualitative Evaluation of a High-Resolution 3D Multi-Sequence Intracranial Vessel Wall Protocol at 3 Tesla MRI

Background and Purpose

Intracranial vessel wall imaging using MRI has great potential as a clinical method for assessing intracranial atherosclerosis. The purpose of the current study was to compare three 3T MRI vessel wall sequences with different contrast weightings (T₁w, PD, T₂w) and dedicated sagittal orientation perpendicular to the middle cerebral artery, to the reconstructed sagittal image from a transverse 3D T₁w volumetric isotropically reconstructed turbo spin-echo acquisition (VIRTA), and provide a clinical recommendation.

Materials and Methods

The above-mentioned sequences were acquired in 10 consecutive Chinese ischemic stroke or TIA patients (age: 68 years, sex: 4 females) with angiographic-confirmed MCA stenosis at 3T. Institutional review board approval was obtained. Two raters qualitatively scored all images on overall image quality, presence of artifacts, and visibility of plaques. Data were compared using Repeated measures ANOVA and Sidak’s adjusted post hoc tests.

Results

All sequences except the T₂w sequence were able to depict the walls of the large vessels of the Circle of Willis (p<0.05). T₁w sagittal oblique VIRTA showed significantly more artifacts (p<0.01). Peripherally located plaques were sometimes missed on the sagittal sequences, but could be appreciated on the transverse T₁w VIRTA.

Conclusion

With the 3T multi-sequence vessel wall protocol we were able to assess the intracranial plaque with two different image contrast weightings. The sequence of preference to include in a clinical protocol would be the transverse 3D T₁w VIRTA based on absence of artifacts, larger coverage including the whole Circle of Willis, and excellent lesion depiction.

Intracranial Vessel Wall MRI: Principles and Expert Consensus Recommendations of the American Society of Neuroradiology

Intracranial vessel wall MR imaging is an adjunct to conventional angiographic imaging with CTA, MRA, or DSA. The technique has multiple potential uses in the context of ischemic stroke and intracranial hemorrhage. There remain gaps in our understanding of intracranial vessel wall MR imaging findings and research is ongoing, but the technique is already used on a clinical basis at many centers. This article, on behalf of the Vessel Wall Imaging Study Group of the American Society of Neuroradiology, provides expert consensus recommendations for current clinical practice.

High-resolution intracranial vessel wall MRI in an elderly asymptomatic population: comparison of 3T and 7T

Objectives

Several intracranial vessel wall sequences have been described in recent literature, with either 3-T or 7-T magnetic resonance imaging (MRI). In the current study, we compared 3-T and 7-T MRI in visualising both the intracranial arterial vessel wall and vessel wall lesions.

Methods

Twenty-one elderly asymptomatic volunteers were scanned by 3-T and 7-T MRI with an intracranial vessel wall sequence, both before and after contrast administration. Two raters scored image quality, and presence and characteristics of vessel wall lesions.

Results

Vessel wall visibility was equal or significantly better at 7 T for the studied arterial segments, even though there were more artefacts hampering assessment. The better visualisation of the vessel wall at 7 T was most prominent in the proximal anterior cerebral circulation and the posterior cerebral artery. In the studied elderly asymptomatic population, 48 vessel-wall lesions were identified at 3 T, of which 7 showed enhancement. At 7 T, 79 lesions were identified, of which 29 showed enhancement. Seventy-one percent of all 3-T lesions and 59 % of all 7-T lesions were also seen at the other field strength.

Conclusions

Despite the large variability in detected lesions at both field strengths, we believe 7-T MRI has the highest potential to identify the total burden of intracranial vessel wall lesions.

Key Points

• Intracranial vessel wall visibility was equal or significantly better at 7-T MRI

• Most vessel wall lesions in the cerebral arteries were found at 7-T MRI

• Many intracranial vessel wall lesions showed enhancement after contrast administration

• Large variability in detected intracranial vessel wall lesions at both field strengths

• Seven-tesla MRI has the highest potential to identify total burden of intracranial atherosclerosis

Electronic supplementary material

The online version of this article (doi:10.1007/s00330-016-4483-3) contains supplementary material, which is available to authorized users.

Relations between location and type of intracranial atherosclerosis and parenchymal damage

The aim of this study was to assess the relation between location and type of intracranial atherosclerosis (ICAS) and cortical microinfarcts (CMIs) and macroinfarcts in 18 patients presenting with ischemic stroke (n = 12) or transient ischemic attack (TIA) (n = 6) using 7 tesla MR imaging. The protocol included: 3D T2-weighted FLAIR and 3D T1-weighted Magnetization-Preparation Inversion Recovery Turbo Spin Echo sequence. ICAS lesions and infarcts were scored by two raters. The relation between ICAS lesions, calculated ratios of ICAS lesion characteristics, location, and infarcts were examined using linear regression analyses. A total number of 75 ICAS lesions (all patients), 101 CMIs (78% of patients), and 31 macroinfarcts (67% of patients) were found. Seventy-six and sixty-five percent of the CMIs and macroinfarcts, respectively, were found in the same vascular territory as the ICAS lesions (p = 0.977, p = 0.167, respectively). A positive correlation existed between the number of macroinfarcts and CMIs (p < 0.05). In patients with macroinfarcts, we found more concentric (p < 0.01) and diffuse (p < 0.05) type of ICAS lesions. A high prevalence of brain tissue lesions, both macroinfarcts and CMIs, were found in patients with ICAS. Macroinfarcts were found to be related to specific ICAS lesion types. The type of ICAS lesion seems to be promising as a marker for ICAS patients at higher risk of future infarcts.

High-resolution intracranial vessel wall imaging: imaging beyond the lumen

Accurate and timely diagnosis of intracranial vasculopathies is important due to significant risk of morbidity with delayed and/or incorrect diagnosis both from the disease process as well as inappropriate therapies. Conventional vascular imaging techniques for analysis of intracranial vascular disease provide limited information since they only identify changes to the vessel lumen. New advanced MR intracranial vessel wall imaging (IVW) techniques can allow direct characterisation of the vessel wall. These techniques can advance diagnostic accuracy and may potentially improve patient outcomes by better guided treatment decisions in comparison to previously available invasive and non-invasive techniques. While neuroradiological expertise is invaluable in accurate examination interpretation, clinician familiarity with the application and findings of the various vasculopathies on IVW can help guide diagnostic and therapeutic decision-making. This review article provides a brief overview of the technical aspects of IVW and discusses the IVW findings of various intracranial vasculopathies, differentiating characteristics and indications for when this technique can be beneficial in patient management.

Ex-vivo imaging and plaque type classification of intracranial atherosclerotic plaque using high resolution MRI

BACKGROUND AND AIMS

Recent development of high resolution MRI techniques have enabled imaging of intracranial atherosclerotic plaque in vivo. However, identifying plaque composition remains challenging given the small size and the lack of histological validation. This study aims to quantify the relaxation times of intracranial plaque components ex vivo at 3 T and to determine whether multi-contrast MRI could classify intracranial plaque according to the American Heart Association classification with histological validation.

METHODS

A total of 53 intracranial arteries with atherosclerotic plaques from 20 cadavers (11 male, age 73.8 ± 10.9) were excised. Quantitative T1/T2/T2* mapping sequences and multi-contrast fast-spin echo sequences (T1, T2, proton-density weighted and short time inversion recovery) were acquired. Plaque components including: fibrous cap, lipid core, fibrous tissue, calcification, and healthy wall were segmented on histology, and their relaxation times were derived from quantitative images. Two radiologists independently classified plaque type blinded to the histology results.

RESULTS

Relaxation times of plaque components are distinct and different. T2 and T2* values of lipid core are lower than fibrous cap (p = 0.026 & p < 0.0001), but are comparable with fibrous tissue and healthy wall (p = 0.76 & p = 0.42). MRI reliably classified plaque type compared with histology (κ = 0.69) with an overall accuracy of 80.7%. The sensitivity and specificity using MRI to identify fibro-lipid atheroma (type IV-V) was 94.8% and 77.1%, respectively. Inter-observer agreement was excellent (κ = 0.77).

CONCLUSION

Intracranial plaque components have distinct and different relaxation times at 3 T. High-resolution MRI is able to characterize intracranial plaque composition and classify plaque types ex vivo at 3 T.

Whole-brain intracranial vessel wall imaging at 3 Tesla using cerebrospinal fluid-attenuated T1-weighted 3D turbo spin echo

PURPOSE

Although three-dimensional (3D) turbo spin echo (TSE) with variable flip angles has proven to be useful for intracranial vessel wall imaging, it is associated with inadequate suppression of cerebrospinal fluid (CSF) signals and limited spatial coverage at 3 Tesla (T). This work aimed to modify the sequence and develop a protocol to achieve whole-brain, CSF-attenuated T₁ -weighted vessel wall imaging.

METHODS

Nonselective excitation and a flip-down radiofrequency pulse module were incorporated into a commercial 3D TSE sequence. A protocol based on the sequence was designed to achieve T₁ -weighted vessel wall imaging with whole-brain spatial coverage, enhanced CSF-signal suppression, and isotropic 0.5-mm resolution. Human volunteer and pilot patient studies were performed to qualitatively and quantitatively demonstrate the advantages of the sequence.

RESULTS

Compared with the original sequence, the modified sequence significantly improved the T₁ -weighted image contrast score (2.07 ± 0.19 versus 3.00 ± 0.00, P = 0.011), vessel wall-to-CSF contrast ratio (0.14 ± 0.16 versus 0.52 ± 0.30, P = 0.007) and contrast-to-noise ratio (1.69 ± 2.18 versus 4.26 ± 2.30, P = 0.022). Significant improvement in vessel wall outer boundary sharpness was observed in several major arterial segments.

CONCLUSIONS

The new 3D TSE sequence allows for high-quality T₁ -weighted intracranial vessel wall imaging at 3 T. It may potentially aid in depicting small arteries and revealing T₁ -mediated high-signal wall abnormalities. Magn Reson Med 77:1142-1150, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Thinner Regions of Intracranial Aneurysm Wall Correlate with Regions of Higher Wall Shear Stress: A 7T MRI Study

BACKGROUND AND PURPOSE

Both hemodynamics and aneurysm wall thickness are important parameters in aneurysm pathophysiology. Our aim was to develop a method for semi-quantitative wall thickness assessment on in vivo 7T MR images of intracranial aneurysms for studying the relation between apparent aneurysm wall thickness and wall shear stress.

MATERIALS AND METHODS

Wall thickness was analyzed in 11 unruptured aneurysms in 9 patients who underwent 7T MR imaging with a TSE-based vessel wall sequence (0.8-mm isotropic resolution). A custom analysis program determined the in vivo aneurysm wall intensities, which were normalized to the signal of nearby brain tissue and were used as measures of apparent wall thickness. Spatial wall thickness variation was determined as the interquartile range in apparent wall thickness (the middle 50% of the apparent wall thickness range). Wall shear stress was determined by using phase-contrast MR imaging (0.5-mm isotropic resolution). We performed visual and statistical comparisons (Pearson correlation) to study the relation between wall thickness and wall shear stress.

RESULTS

3D colored apparent wall thickness maps of the aneurysms showed spatial apparent wall thickness variation, which ranged from 0.07 to 0.53, with a mean variation of 0.22 (a variation of 1.0 roughly means a wall thickness variation of 1 voxel [0.8 mm]). In all aneurysms, apparent wall thickness was inversely related to wall shear stress (mean correlation coefficient, -0.35; P < .05).

CONCLUSIONS

A method was developed to measure the wall thickness semi-quantitatively, by using 7T MR imaging. An inverse correlation between wall shear stress and apparent wall thickness was determined. In future studies, this noninvasive method can be used to assess spatial wall thickness variation in relation to pathophysiologic processes such as aneurysm growth and rupture.

Imaging Inflammation in Cerebrovascular Disease

Imaging inflammation in large intracranial artery pathology may play an important role in the diagnosis of and risk stratification for a variety of cerebrovascular diseases. Looking beyond the lumen has already generated widespread excitement in the stroke community, and the potential to unveil molecular processes in the vessel wall is a natural evolution to develop a more comprehensive understanding of the pathogenesis of diseases, such as ICAD and brain aneurysms.

Multicontrast High-Resolution Vessel Wall Magnetic Resonance Imaging and Its Value in Differentiating Intracranial Vasculopathic Processes

Background and Purpose—

Although studies have attempted to differentiate intracranial vascular disease using vessel wall magnetic resonance imaging (VWI), none have incorporated multicontrast imaging. This study uses T1- and T2-weighted VWI to differentiate intracranial vasculopathies.

Methods—

We retrospectively reviewed patients with clinically defined intracranial vasculopathies causing luminal stenosis/irregularity who underwent VWI studies. Two blinded experts evaluated T1 precontrast and postcontrast and T2-weighted VWI characteristics, including the pattern of wall thickening; presence, pattern, and intensity of postcontrast enhancement; and T2 signal characteristics.

Results—

Twenty-one cases of atherosclerosis (intracranial atherosclerotic disease [ICAD]), 4 of reversible cerebral vasoconstriction syndrome, and 4 of vasculitis were identified, with a total of 118 stenotic lesions (81 ICAD, 22 reversible cerebral vasoconstriction syndrome, and 15 vasculitic lesions). There was substantial to excellent inter-reader agreement for the assessment of lesional T2 hyperintensity ( κ =0.80), pattern of wall thickening ( κ =0.87), presence ( κ =0.90), pattern ( κ =0.73), and intensity ( κ =0.77) of enhancement. ICAD lesions were significantly more likely to have eccentric wall involvement (90.1%) than reversible cerebral vasoconstriction syndrome (8.2%; P <0.001) and vasculitic lesions (6.7%; P <0.001) and were also more likely to have T2 hyperintensity present than the other 2 vasculopathies (79% versus 0%; P <0.001). There were also significant differences in the presence, intensity, and pattern of enhancement between all lesion types. Combining T1 and T2 VWI increased the sensitivity of VWI in differentiating ICAD from other vasculopathies from 90.1% to 96.3%.

Conclusions—

Multicontrast VWI can be a complementary tool for intracranial vasculopathy differentiation, which often leads to more invasive workups when reversible cerebral vasoconstriction syndrome and vasculitis are in the differential diagnosis.

7 Tesla MRI in Cerebral Small Vessel Disease

Cerebral small vessel disease (SVD) is a major cause of stroke and cognitive decline. Magnetic resonance imaging (MRI) currently plays a central role in diagnosis, and advanced MRI techniques are widely used in research but are limited by spatial resolution. Human 7 Tesla (7T) MRI has recently become available offering the ability to image at higher spatial resolution. This may provide additional insights into both the vascular pathology itself as well as parenchymal markers which could only previously be examined post mortem. In this review we cover the advantages and limitations of 7T MRI, review studies in SVD performed to date, and discuss potential future insights into SVD which 7T MRI may provide.

[Ultrahigh field MRI in context of neurological diseases]

Ultrahigh field magnetic resonance imaging (UHF-MRI) has recently gained substantial scientific interest. At field strengths of 7 Tesla (T) and higher UHF-MRI provides unprecedented spatial resolution due to an increased signal-to-noise ratio (SNR). The UHF-MRI method has been successfully applied in various neurological disorders. In neuroinflammatory diseases UHF-MRI has already provided a detailed insight into individual pathological disease processes and elucidated differential diagnoses of several disease entities, e.g. multiple sclerosis (MS), neuromyelitis optica (NMO) and Susac's syndrome. The excellent depiction of normal blood vessels, vessel abnormalities and infarct morphology by UHF-MRI can be utilized in vascular diseases. Detailed imaging of the hippocampus in Alzheimer's disease and the substantia nigra in Parkinson's disease as well as sensitivity to iron depositions could be valuable in neurodegenerative diseases. Current UHF-MRI studies still suffer from small sample sizes, selection bias or propensity to image artefacts. In addition, the increasing clinical relevance of 3T-MRI has not been sufficiently appreciated in previous studies. Although UHF-MRI is only available at a small number of medical research centers it could provide a high-end diagnostic tool for healthcare optimization in the foreseeable future. The potential of UHF-MRI still has to be carefully validated by profound prospective research to define its place in future medicine.