Vessel wall imaging
MR vessel wall imaging refers to MRI techniques used to evaluate for disease within the walls of arteries, beyond the luminal abnormalities depicted on angiographic imaging. This can be used anywhere in the body but is particularly important intracranially in distinguishing between various causes of luminal stenosis such as intracranial atherosclerotic disease versus vasculitis.
Vessel wall MRI requires high spatial and contrast resolution to depict thin arterial walls discrete from their surrounding tissues. Several techniques to achieve this resolution are applied to sequences weighted towards various tissue contrasts (T1-weighted images before and after contrast medium most commonly, T2-weighted images, or proton density-weighted images) .
First, due to the small size of the target vessels, high spatial resolution is imperative. Higher field strength (3.0 T rather than 1.5 T) is preferred due to the improved signal-to-noise ratio and acquisition times when selecting smaller voxel dimensions. For intracranial imaging at 3 T, a 3D acquisition with isotropic voxel sizes in the 0.4-0.7 mm range is commonly used . If 2D sequences are used, because of the poor spatial resolution in the slice-select direction (at least 2 mm slice thickness), multiplanar imaging is required to optimally depict vessels in both short and long axes .
Second, the signal from blood must be kept low to distinguish the vessel wall from the lumen. These so-called black/dark blood techniques usually exploit the fact that blood flows while vessel walls remain stationary. Turbo/fast spin-echo sequences intrinsically have low arterial blood signal (flow voids) due to time of flight effects (spins move out of the imaged slice before the 180° refocusing pulse) and/or intravoxel dephasing (spins moving with different velocities due to turbulent or laminar flow acquire different phases) . Both mechanisms contribute when 2D techniques are employed but only the latter is important in 3D techniques in which a large slab is imaged at once.
The most commonly used 3D sequences for intracranial vessel wall imaging use turbo/fast spin-echo with variable low refocusing flip angles, which maintain high spatial resolution during long echo trains. These sequences are known by brand names such as SPACE, Cube, and VISTA .
Other methods of blood suppression are available :
- spatial presaturation (saturation band): in 2D techniques, the spins that start in a slab adjacent to the imaged slice can be selectively dephased with a spoiling gradient and cannot regain signal when they flow into the imaged slice
- double inversion recovery: a non-selective inversion pulse with a TI near the null point of blood followed by a slice selective inversion pulse recovers signal in stationary tissue but not in flowing blood
- delay alternating with nutation for tailored excitation (DANTE)
Finally, to depict the vessel wall discretely, signal must also be suppressed from the surrounding tissue. For vessel wall imaging of the extracranial head and neck, for instance, this means suppressing fat signal. For intracranial vessel wall imaging, this means suppressing CSF signal. Fortunately, because CSF flows, many techniques used to suppress signal from flowing blood also suppress CSF signal. Depending on the contrast weighting (CSF signal is low on T1-weighted images but not T2- or proton density-weighted images), dedicated methods to suppress CSF may be needed, such as inversion recovery (FLAIR), antidriven equilibrium, or DANTE .
Correlative luminal imaging
Vessel wall imaging is usually performed in addition to a bright blood MRA sequence, such as the gradient echo-based time of flight MRA, to assess the lumen caliber and localize abnormalities for further vessel wall assessment.
Clinical uses and interpretation
Several indications exist, in varying stages of clinical versus research application:
- intracranial arterial steno-occlusive diseases: distinguish between common causes of stenosis and/or identify potentially symptomatic lesions without significant stenosis due to remodeling
- atherosclerotic plaque: eccentric wall thickening, often heterogeneously enhancing
- vasculitis: concentric wall thickening and homogeneous enhancement
- reversible cerebral vasoconstriction syndrome (RCVS): non-enhancing or minimally enhancing wall thickening
- arterial dissection: intimal flap on T2-weighted images, intramural hematoma on T1-weighted images
- moyamoya disease: non-enhancing intracranial carotid stenosis
- intracranial saccular aneurysms: lack of wall enhancement predicts aneurysm stability
- giant cell (temporal) arteritis: site selection for temporal artery biopsy and predicting results of temporal artery biopsy
- extracranial carotid artery atherosclerotic plaque: detect features associated with high risk for ischemic events, such as thin/ruptured fibrous cap, large lipid-rich necrotic core, or intraplaque hemorrhage
Several common interpretive pitfalls exist :
- slow flow: in laminar flow or intra-aneurysmal circulation, slower blood flow adjacent to the vessel wall may have incompletely suppressed signal, mimicking wall thickening and enhancement; it is helpful to correlate with lumen caliber on MRA or the precontrast medium vessel wall sequence
- veins: enhancing veins, which have slower flow, mimic arteries with wall enhancement; it is helpful to correlate with time-of-flight MRA to distinguish arteries and veins
- vasa vasorum: the extracranial carotid and vertebral arteries and sometimes proximal intracranial arteries as well (especially in elderly with severe atherosclerosis), are supplied by the vasa vasorum that appears as diffuse smooth concentric enhancement, which mimics wall enhancement from vasculitis; the distribution and clinical picture helps distinguish