Key points:
- To review the principles of intracranial VW-MR imaging and provides consensus recommendations for clinical practice.
Conventional techniques for imaging the intracranial arteries can fail to fully characterize disease that resides within the vessel wall. For this reason, there is growing interest in direct visualization of the vessel wall with high-resolution intracranial vessel wall MR imaging (VW-MR imaging).
The principal technical requirements for intracranial VW-MR imaging are the following:
High spatial resolution
The normal middle cerebral artery and basilar artery wall thickness is 0.2– 0.3 mm (smaller than the VW-MR imaging voxel dimensions currently achievable); for this reason it is visible suppressing MR signal of cerebrospinal fluid (CSF) and blood.
|
3T – 2D sequence |
3T – 3D sequence |
| voxel size (mm) |
2.0×0.4×0.4 |
0.5 isotropic |
| scan duration (min) |
5-7/2-4 cm-thick section |
7-10 |
Multiplanar 2D or 3D acquisitions
Accurate interpretation of VW-MR imaging requires visualization of the vessel wall in both short- and long-axis planes.
|
3T – 2D sequence |
3T – 3D sequence |
| Advantages |
better image quality |
reduced scan time and more flexibility |
| Disadvantages |
partial volume averaging effects |
lower quality |
Multiplanar 2D or 3D acquisitions
| Time-of-flight (TOF) |
used to characterize luminal abnormality |
| Time-of-flight (TOF) with gadolinium |
helpful to define optimal contour of the lumen |
| T1 and T1C+ |
PD-WI has higher SNR, but constrast enhancement is less cospicuous |
| T2 |
could be useful |
| fat-sat sequencies |
useful for the ECA branches (not needed for intracranial imaging) |
Suppression of signal in luminal blood and CSF
The most common methods of blood-signal suppression are the following:
| Spin-Echo |
| Spatial presaturation |
| Double inversion recovery preparation |
Clinical uses of VW-MR
SITUATIONS IN WHICH INTRACRANIAL VW-MR IMAGING IS LIKELY A USEFUL ADJUNCT TO CONVENTIONAL IMAGING
- Atherosclerotic Plaque. The VW-MR imaging has a layered appearance explicated by carotid endarterectomy specimens, with an enhancing layer adjacent to the lumen (fibrous cap), a non-enhancing layer (lipid core), and a peripheral thin rim of enhancement (increased vasa vasorum in the adventitia of the artery). The lipid core has an hypointensity-isointensity on T1- and T2-weighted images; this is different from the MR imaging appearance of lipids in other tissues, because in carotid plaque the main contributor to MR signal is water protons and the main component of the plaque is cholesterol and cholesteryl esters.
- Vasculitis. VW-MR imaging often demonstrates smooth, homogeneous, concentric arterial wall thickening and enhancement in patients with CNS vasculitis. Vasa vasorum–related contrast leakage is a potential cause of wall enhancement and dilated neovessels have been demonstrated within the extracranial arterial wall of patients with Takayasu arteritis.
- Reversible Cerebral Vasoconstriction Syndrome. VW-MR imaging may enable prospective differentiation between vasculitis and vasoconstriction; both disorders result in arterial wall thickening, but the vessel wall in RCVS is typically nonenhancing.
- Moya-Moya disease. VW-MR imaging have a lack of arterial wall thickening and enhancement, a smaller outer diameter of the vessel wall compared with patients with atherosclerotic plaques.
- Radiation_Induced Arteriopathy. A study37 of 5 patients with radiation-in- duced narrowing of the intracranial internal carotid arteries found circumferential arterial wall thickening and enhancement in all cases.
- Arterial Dissection. The VW-MR shows a curvilinear hyperintensity on T2-weighted images (in- timal flap) separating the true lumen from the false lumen and eccentric arterial wall thickening with the signal characteristics of blood (intramural hematoma).
Reference
- Mandell DM, Mossa-Basha M, Qiao Y, Hess CP, Hui F, Matouk C, Johnson MH, Daemen MJ, Vossough A, Edjlali M, Saloner D, Ansari SA, Wasserman BA, Mikulis DJ; Vessel Wall Imaging Study Group of the American Society of Neuroradiology. Intracranial Vessel Wall MRI: Principles and Expert Consensus Recommendations of the American Society of Neuroradiology. AJNR Am J Neuroradiol. 2017 Feb;38(2):218-229. doi: 10.3174/ajnr.A4893. Epub 2016 Jul 28.
