VWMRI is a relatively new technique that allows neuroradiologists to further characterize vasculopathies by assessing and characterizing vessel wall lesions and optimize the differential diagnosis assessment and risk evaluation of future ischemic lesions.
Traditional methods to visualize intracranial and extracranial arteries are digital subtraction angiography (DSA), CT angiography, Doppler ultrasound and MR angiography (MRA). All of these techniques, except for ultrasound, focus on the lumen and not the wall itself.
Technique
The recommended technical criteria are:
- High spatial resolution: must be enough to distinguish the thin arterial wall. For this objective, a magnetic field of 3 Tesla (T) is recommended, with better signal to noise ratio, more spatial resolution and faster acquisition times with small voxel sizes.
- Use of multiplanar 2D and 3D acquisitions
- 2D sequences to focally evaluate vessels of interest with better image quality.
- Volumetric sequences to evaluate vessels in any plane, allowing more flexibility in assessment and reduced scan times.
- Multiple tissue weightings
- Time of flight (TOF) MRA to evaluate intraluminal alterations.
- T1 Black blood (BB) before and after gadolinium administration.
- T2 BB.
- Fat suppression (when assessing external carotid artery)
- Blood and cerebrospinal fluid signal suppression
Contrary to the TOF technique, in which blood flow shows a high signal, BB techniques use pulse settings that suppress the signal of blood flow, making it hypointense compared to stationary tissue. Some techniques can achieve the above.
- In spin echo, the nuclei receives at least two radiofrequency pulses, a 90º excitation pulse followed by a 180º rephasing pulse, that are applied to each slice of the study. This allows that only stationary tissues receive both pulses. Nuclei that are in movement, like the ones in blood, most likely won’t receive both of the pulses and therefore won’t produce a signal, showing a signal void in the image. This is called the outflow effect.
- BB preparation pulses are generally used to ensure full and homogeneous blood suppression.
- Turbo spin-echo sequences use a similar principle, but with a cycle of 90º pulses along with multiple rephasing 180º pulses that produce multiple signals (echo train), reducing acquisition time.
Volumetric isotropic sequences allow multiplanar reformations. They're based on rephasing pulses with a variable flip angle. General Electric’s Cube, for example, consists of a turbo spin-echo technique with a variable flip angle, long echo trains and ultra-short echo spacings. By using long echo trains, the signal is diminished in time, producing a blurred image. This is corrected by varying the flip angle along the echo train, which allows maintaining a stable signal in reasonable acquisition time.
Evaluation of vessel wall (Fig. 1)
In VWMRI, the normal arterial wall is visualized as a thin line that surrounds the vessel lumen and is isointense to brain parenchyma.
A vessel wall lesion is defined as
- Focal or diffuse thickening (> 50% compared to the adjacent normal wall)
- Focal or diffuse enhancement, established by comparing images before and after gadolinium contrast.
Lesions can also be further characterized by defining whether they’re:
- Concentric: affecting more than 50% of the vessel circumference
- Eccentric: affecting less than 50% of the vessel circumference.
A decrease or increase in external diameter must also be documented.
Fig. 1: Schematical representation of possible findings in VWMRI
As described above, for adequate evaluation of both intracranial and extracranial vessels, some authors have proposed that a magnetic field of at least 3T is required for adequate contrast to noise ratio and spatial resolution to assess the thin arterial wall and its lesions. In the following section, we present our local experience using a magnetic field strength of 1.5 T.