Whole-brain vessel wall MRI: A parameter tune-up solution to improve the scan efficiency of three-dimensional variable flip-angle turbo spin-echo

Abstract

Purpose: To propose and evaluate a parameter tune-up solution to expedite a three-dimensional (3D) variable-flip-angle turbo spin-echo (TSE) sequence for whole-brain intracranial vessel wall (IVW) imaging.

Materials and methods: Elliptical k-space sampling and prolonged echo train length (ETL), were used to expedite a 3D variable-flip-angle TSE-based sequence. To compensate for the potential loss in vessel wall signal, optimal combination of prescribed T₂ and ETL was experimentally investigated on 22 healthy volunteers at 3 Tesla. The optimized protocol (7-8 min) was then compared with a previous protocol (reference protocol, 11-12 min) in terms of signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), vessel wall sharpness, and wall delineation quality on a 4-point scale (0:poor; 3:excellent) in 10 healthy volunteers. A pilot study of five patients was performed and lesion delineation score was used to demonstrate the diagnostic quality.

Results: A protocol with ETL = 52 and prescribed T₂ = 170 ms was deemed an optimized one, which, compared with the reference protocol, provided significantly improved wall SNR (12.0 ± 1.3 versus 10.0 ± 1.1; P = 0.002), wall-lumen CNR (9.7 ± 1.2 versus 8.0 ± 0.9; P = 0.002), wall-CSF CNR (2.8 ± 1.0 versus 1.7 ± 1.0; P = 0.026), similar vessel wall sharpness at both inner (1.59 ± 0.18 versus 1.58 ± 0.14, P = 0.87) and outer (1.71 ± 0.25 versus 1.83 ± 0.30; P = 0.18) boundaries, and comparable vessel wall delineation score for individual segments (1.95-3; P > 0.06). In all patients, atherosclerotic plaques (10) or wall dissection (5) were identified with a delineation score of 3 or 2.

Conclusion: A parameter tune-up solution can accelerate 3D variable-flip-angle TSE acquisitions, particularly allowed for expedited whole-brain IVW imaging with preserved wall delineation quality.

Level of evidence: 2. Technical Efficacy: Stage 1 J. MAGN. RESON. IMAGING 2017;46:751-757.

Keywords: 3D TSE; intracranial vessel wall; magnetic resonance imaging; vessel wall imaging; whole brain.

Figure 1

Effects of prescribed T₂ (a & b.) and echo train length (ETL) (c & d.) on wall signal-to-noise ratio (SNR), CSF SNR, wall-lumen contrast-to-noise ratio (CNR), wall-CSF CNR, and white-gray matter CNR in selected scenarios.

Figure 2

Comparison of three expedited whole-brain IR-SPACE protocols (echo train length [ETL] = 52) to the reference protocol (i.e. previously proposed long-duration protocol whereby ETL = 36). For SNR/CNR comparisons (a), the expedited protocol with a prescribed T₂ of 170 or 200 ms provided significant (marked with *) improvements. For wall sharpness comparisons (b), a general trend of decrease in vessel wall sharpness was observed as prescribed T₂ increased, especially at the outer wall boundary. However, deterioration was not statistically significant with any of the expedited protocols.

Figure 3

Comparison between the reference protocol (i.e. previously proposed protocol whereby echo train length [ETL] = 36, prescribed T₂ = 100 ms) and the optimized protocol (i.e. ETL = 52, prescribed T₂ = 170ms) in one healthy volunteer. Example images of the basilar (BA), internal carotid artery (ICA) C4, middle cerebral artery (MCA) at M1, M2, M3 segments, anterior cerebral artery (ACA) at A1, proximal A2 segments, and posterior cerebral artery (PCA) at P1, proximal P2 segments. Comparable image quality is observed between the two imaging protocols.

Figure 4

Example patient images with curved multi-planar reconstruction. a) In a 58-year-old male patient, intracranial vessel wall (IVW) imaging reveals a focal lesion at the left middle cerebral artery M1 segment. It is characterized by eccentric wall thickening (arrows), suggestive of a plaque, on pre-contrast images and strong enhancement (arrows) at the plaque-lumen interface instead of in the entire thickened wall (arrowheads in the zoomed-in cross-sectional images) on post-contrast images. b) In a 51-year-old male patient, dissection at the left internal carotid artery, IVW imaging reveals a lesion (arrows) at the left internal carotid artery with hyper-intense, crescent-shaped intramural hematoma and longitudinally diffuse involvement.

Figure 5

Comparison between the optimized protocol (a) and the reference protocol (b) in a 28-year-old female patient with dissection at the left vertebral artery. Longitudinally diffuse hyper-intense intramural hematoma was depicted comparably by both protocols.