Encyclopedia
Please note this is a comparison between Version 1 by Federico Mazzacane and Version 3 by Beatrix Zheng.
Cerebrovascular diseases are a leading cause of disability and death worldwide. The definition of stroke etiology is mandatory to predict outcome and guide therapeutic decisions. The diagnosis of pathological processes involving intracranial arteries is especially challenging, and the visualization of intracranial arteries’ vessel walls is not possible with routine imaging techniques. Vessel wall magnetic resonance imaging (VW-MRI) uses high-resolution, multiparametric MRI sequences to directly visualize intracranial arteries walls and their pathological alterations, allowing a better characterization of their pathology. VW-MRI demonstrated a wide range of clinical applications in acute cerebrovascular disease. Above all, it can be of great utility in the differential diagnosis of atherosclerotic and non-atherosclerotic intracranial vasculopathies. Additionally, it can be useful in the risk stratification of intracranial atherosclerotic lesions and to assess the risk of rupture of intracranial aneurysms. Recent advances in MRI technology made it more available, but larger studies are still needed to maximize its use in daily clinical practice.
- cerebrovascular disease
- vessel wall MRI
- intracranial vasculopathy
- stroke
- atherosclerosis
- vasculitis
- dissection
1. Introduction
Cerebrovascular diseases are a leading cause of disability and death worldwide. The definition of stroke etiology is mandatory to predict the prognosis and recurrence risk and guide therapeutic decisions [1][2][1,2]. The pathogenesis of cerebrovascular diseases often originates inside the vessel walls, therefore, standard luminal based imaging modalities lack the capability to visualize these pathological processes, unless they are associated with significant luminal changes [3]. This is particularly true for intracranial arteries, which have smaller diameter and thinner vessel walls, that are beyond the resolution of standard diagnostic techniques [4]. Vessel wall magnetic resonance imaging (VW-MRI) uses high-resolution, multiparametric MRI sequences to directly visualize intracranial arteries walls and their pathological alterations [5][6][5,6].
VW-MRI has demonstrated its utility in a wide range of clinical situations, including differential diagnosis of intracranial vasculopathies, identification and evaluation of intracranial atherosclerotic disease and also risk stratification in patients with intracranial aneurysms [7][8][7,8]. When applied to patients with cryptogenic stroke, it allows for the definition of the stroke etiology in a significant percentage of them, often revealing findings compatible with vasculitis or complicated atherosclerotic disease [9].
Therefore, VW-MRI has become an essential tool for clinicians in the field of cerebrovascular diseases, and continuous technological advances have made it more and more available and reliable for clinical applications.
2. Acquisition
VW-MRI requires sequences with multiple signal weightings, multiplanar 2D acquisitions or 3D acquisitions, high spatial resolution (HR) and suppression of luminal blood signal [10]. Additionally, sequences for intracranial VW-MRI need the suppression of signal of cerebrospinal fluid (CSF) and extended brain coverage because the site of cerebral vascular pathology is less predictable and defined than the extracranial compartment. The 3D acquisition with isotropic voxels allowing multiplanar reconstruction (MPR) is of utmost importance in the intracranial compartment, where vessels have higher tortuosity, to avoid partial volume artifacts [11].
Multi-contrast VW-MRI is needed for the differential diagnosis of vasculopathies and in the evaluation of plaque composition [12]. Therefore, the MR protocol should include HR 2D or 3D T1 weighted sequence, acquired pre- and post- gadolinium injection, and T2 weighted sequences. Proton density (PD) weighted imaging provides a higher signal-to-noise ratio (SNR) and can be a valid alternative to T1 weighted images. On the other hand, in PD sequences the CSF signal is similar to vessel wall signal, and the contrast enhancement is less evident [4]. The T2 weighted sequence is additional, and it is usually acquired in cases of suspected atherosclerosis [13]. Furthermore, a time-of-flight (TOF) magnetic resonance angiography (MRA) allows for the evaluation of luminal vessel contour and caliber in addition to the vessel wall. The TOF MRA helps the localization of the site of vessels affected and drives the positioning of the VW-MRI sequences. In patients with severe luminal narrowing, contrast enhanced MRA can help to carefully define the actual lumen as low flow velocity may cause flow artifacts within the lumen of the vessel [14]. The MR protocol for VW-MRI is summarized in Table 1.
Principal magnetic resonance sequences used for vessel wall magnetic resonance imaging (VW-MRI).
Table 1. Principal magnetic resonance sequences used for vessel wall magnetic resonance imaging (VW-MRI).
| MR Sequences | Technical Requirements | Contrast Medium |
Findings |
|---|---|---|---|
| T1-weighted (or PD) sequence | High spatial resolution; multiplanar 2D or 3D acquisition; blood and CSF signal suppression | Before and after Gd iv administration | Depiction of VW enhancement |
| T2-weighted sequence | High spatial resolution; multiplanar 2D acquisition | No need of Gd iv administration | Additional; usually acquired in cases of suspected atherosclerosis |
| MRA | Extended brain coverage; MIP reconstructions. | With or without Gd iv administration | Depiction of the site of vascular pathology; consider CEMRA in case of severe arterial narrowing or dilation |
PD: proton density; CSF: cerebrospinal fluid; Gd: gadolinium; i.v.: intravenous; VW: vessel wall; MRA: magnetic resonance angiography; MIP: maximum intensity projection; CEMRA: contrast enhanced MRA; T1-w: T1-weighted.
The use of 3D sequences with MPR reduces the overall MR examination time, avoiding multiple 2D acquisitions in different planes [4]. On the other hand, 2D sequences may provide higher in-plane resolution (up to 0.4 mm of voxel size) than 3D sequences and are of interest when targeting the VW-MRI study to a specific vessel. Therefore, the use of both 3D and 2D images should be considered in VW-MRI protocols. Furthermore, the advent of higher magnetic fields and the use of multichannel coils (32–64 channels) enabled an increased spatial resolution in reasonable acquisition time, even for peripheral vessels, due to the higher SNR [15][16][15,16].
In VW-MRI blood suppression techniques are needed to obtain black blood (BB) MR sequences with null signal within the vessel lumen and to avoid flow artifacts that may mimic vessel wall abnormalities. There are several methods to suppress the blood signal: spin echo sequences with a spatial pre-saturation band exploit the movement of blood spin [17]; double inversion recovery (DIR) sequences use both the T1 properties of blood and the blood flow. Disadvantages of DIR sequences are that they are prone to flow artifacts and have long acquisition time. With 3D sequences the main mechanism used to saturate the signal of blood is intravoxel dephasing: the most common 3D BB sequences used in VW-MRI are turbo spin echo (TSE) sequences with variable flip angle refocusing pulse [18]. The names of these sequences vary according to the vendor: SPACE (sampling perfection with application-optimized contrasts using different flip angle evolution, Siemens), CUBE (General Electrics), VISTA (volume isotropic turbo spin-echo acquisition, Philips Healthcare).
In VW-MRI the CSF suppression techniques are useful for a proper outer wall boundary evaluation and are particularly important when studying the peripheral branches of intracranial arteries [19]. Delayed alternating with nutation for tailored excitation (DANTE) preparatory pulse and anti-driven-equilibrium (ADE; Philips Health) or restore (Siemens) sequences suppress CSF signals [20][21][20,21].
3. Current Insights
VW-MRI has become a valuable aid in investigating patients with acute cerebrovascular disease and shows a wide range of clinical applications. Recent advances in MRI technology and the diffusion of high magnetic field devices made VW-MRI widely available. Its value in the differential diagnosis of intracranial vasculopathy is already recognized, and VW-MRI may be included in the diagnostic workup of several patients, as it provides adjunctive information not obtainable with traditional luminal-based imaging techniques [3][22][3,125]. Further applications, like risk stratification in patients with intracranial atherosclerotic disease and intracranial aneurysms, offering the potential to tailor the best therapeutic approach in fields is of great interest, but it requires larger prospective studies [23][129]. Furthermore, the introduction of 7T MRI devices in clinical practice may further increase the possibility of VW-MRI, allowing for better signal-to-noise ratios and a better definition of intracranial lesions [24][25][26][130,131,132].
