We will review the current recommended imaging parameters to achieve consistent high-quality head/neck contrast-enhanced MRA at both 1.5T and 3.0T. Discuss the utilization of clinical MPRAGE and 3D FSE T1W sequences to detect intraplaque hemorrhage and possibly necrotic core at the carotid bifurcation. Review how this additional vessel wall imaging affords better risk stratification of future stroke than carotid stenosis measurements. Discuss the rapidly evolving intracranial vessel wall imaging (IC VWI) techniques which are currently obtained with works-in-progress investigational sequences. Early implement has documented the potential added value of IC VWI to identify specific etiologies of proximal intracranial arterial narrowing.
Specialty area: Cardiovascular MRI: Vascular
Speaker name: J. Kevin DeMarco, M.D.
Highlights: ·
* New insights into improved image quality of head and neck contrast-enhanced (CE) MRA with specific recommendations of imaging parameters and tailored gadolinium bolus administration. ·
* Provide specific protocols to image the cervical arterial vessel wall at 3T and how to interpret the various plaque components. Review how carotid plaque MRI can better depict risk than carotid stenosis measurements and how this information can be used clinically.
* Review current protocol recommendations for intracranial vessel wall imaging. Review how these images are likely to be a useful adjunct to differentiate between intracranial plaque, vasculitis, reversible cerebral vasoconstriction, and dissection.
SUPRA-AORTIC AND INTRACRANIAL MR VASCULAR IMAGING
TARGET AUDIENCE
* Radiologists, clinicians, researchers interested in head and neck MRA as well as vessel wall imaging ·
OBJECTIVES
* Understand current resolution recommendations for head and neck CE MRA with specific protocols to implement high quality CE MRA
* Be able to discuss the merits of a tailored gadolinium bolus injection for head and neck CE MRA
* Review key vulnerable carotid bifurcation plaque components and how to detect them with in vivo MRI. Discuss potential clinical utility of this information.
* Understand the technical implementation of intracranial vessel wall imaging (IC VWI)
* Discuss how IC VWI is likely a useful adjunct to differentiate between plaque, vasculitis, reversible cerebral vasoconstriction and dissection ·
PURPOSE
* To provide specific recommendations for spatial resolution of head/neck CE MRA based for many MR vendors at 1.5 and 3.0T and review optimal gadolinium bolus injection schemes
* To illustrate how to detect vulnerable plaque features with clinical sequences and coils and discuss how this additional information improves risk stratification
* To discuss technical implementation of intracranial vessel wall imaging and how this additional information may be able to differentiate between various etiologies of proximal intracranial arterial narrowing ·
METHODS/RESULTS
* Despite widespread clinical availability of head/neck CE MRA for greater than 15 years, there is still marked variability in image quality that limits accurate stenosis measurement. Two sources of variability have been identified.
- First, insufficient resolution results in a CE MRA that can’t be accurately measured. The user needs to understand the difference between acquired resolution and interpolated resolution. Most MR manufacturers only report the interpolated resolution. Careful prescription of spatial resolution and matrix size is necessary to maintain submillimeter resolution CE MRA. Judicious use of partial or rectangular FOV, partial ky, partial kz, percent scan which cuts the corners of 3D k-space, and parallel imaging are all methods to decrease scan time usually at the price of lower SNR. Specific protocols optimizing these parameters are available for multiple MR manufacturers at 1.5 and 3.0 T including screen shots from each MR manufacturer to make implementation consistent.(1)
- Suboptimal gadolinium-based contrast agent injection can be a second source of variable image quality on head/neck CE MRA. Recent work has demonstrated that the increased sensitivity to injected GBCA at 3.0T can support the use of lower GBCA dose.(2) The smaller dose of GBCA was diluted with saline to maintain a total volume of 25 mL injected over 15 seconds. Subsequent modeling has demonstrated that slower/longer bolus injections are beneficial to minimize T2* shortening and blurring.(3) There may be a benefit to matching the GBCA plateau with the length of the CE MRA acquisition.
* Carotid bifurcation plaque MRI has identified a variety of vulnerable plaque features.
- Extensive literature demonstrates that intraplaque hemorrhage (IPH) can be identified as a focal region of signal intensity greater than the adjacent muscle on black-blood 3D T1W series (3D IR FSGPR aka 3D MPRAGE). In a recent meta-analysis, IPH was shown to correlate with future ipsilateral stroke/TIA equally well if the images were obtained with dedicated carotid coils or large FOV clinical neurovascular (NV) coils.(4)
- Lipid-rich necrotic core (LRNC) is well depicted on research carotid coils and research 2D black-blood T1 and T2 sequences as a focal region of T2 hypointensity and non-enhancement. In a recent meta-analysis the hazard ratio of LRNC as a predictor of subsequent stroke/TIA is 3.00 (95% CI 1.51-5.95).(4) One of the included studies also demonstrated a dose response with large LRNC having a larger incidence of future stroke/TIA.(5) Recent work suggests that large non-enhancing LRNC can be identified using clinical 3D FSE T1W and large FOV NV coils.(6)
- The presence of IPH and/or large LRNC does a better job of predicting future stroke/TIA than carotid stenosis.(7) As such, the presence of these vulnerable plaque features suggest these patients should be carefully monitored and considered for intensive medical therapy including intensive lipid lowering therapy and BP control.
* Intracranial vessel wall imaging (IC VWI) has rapidly evolved. Isotropic 0.5 mm 3D T1/PDW is now possible relying on intravoxel dephasing from laminar flow and/or diffusion-sensitizing gradient preparation to achieve black-blood with works-in-progress investigational sequences that are being rapidly made into clinical sequences.
- IC VWI has been applied in a multi-center trial to measure intracranial atherosclerotic disease in a prospective, population-based ARIC study.(8)
- Recent position paper by the Vessel Wall Imaging Study Group of the ASNR detailed how IC VWI may be a useful adjunct to differentiate between various causes of proximal intracranial arterial narrowing that are difficult to distinguish clinically and have different treatment paradigms.(9)
- Plaque is best seen as an eccentric, heterogeneous wall thickening. May see internal structures such as necrotic core. Plaque is treated with lipid lowering medication.
- Vasculitis is best seen as a concentric, homogeneous wall thickening with enhancement. Vasculitis can occasionally be eccentric, but more homogeneous compared with plaque. Vasculitis is treated with steroids and immunosuppressive drugs.
- Reversible cerebral vasoconstriction (RCVS) is best seen as a concentric, homogeneous wall thickening without enhancement or minimal enhancement. Occasionally RCVS can mimic vasculitis by showing strong enhancement. RCVS is treated with observation and/or calcium channel blockers.
- Dissection is best seen as an eccentric T1 hyperintense wall thickening with a laminar appearance on CE T1W. Intracranial dissection can occasionally be seen as an isolated lesion.
DISCUSSION
* Consistent high quality head/neck CE MRA is possible with careful attention to imaging parameters, judicious use of techniques to accelerate the acquisition, and optimal bolus injection of GBCA
* Additional clinical 3D MRPAGE and 3D FSE T1W sequences can detect IPH and perhaps LRNC which offer improved risk stratification of future stroke/TIA over carotid stenosis measurements alone
* Recent advances have made intracranial vessel wall imaging possible. Recent consensus statement highlight how IC VWI may add benefit to MRA by differentiating between plaque, vasculitis, RCVS, and dissection
CONCLUSION
High quality head/neck CE MRA requires careful attention to imaging parameters and GBCA administration. Additional clinical 3D MR sequences can identify vulnerable plaque features that improve risk stratification over stenosis measurements alone. Intracranial vessel wall imaging is possible and may add value by identifying specific etiologies of proximal intracranial arterial narrowing.
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