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Development of a clinically practical whole-brain intracranial vessel wall MRI technique at 3 Tesla
Journal of Cardiovascular Magnetic Resonancevolume 18, Article number: P350 (2016)
T1-weighted variable-flip-angle 3D TSE has emerged as a promising intracranial vessel wall imaging technique. To increase spatial coverage and cerebrospinal fluid (CSF) attenuation, a whole-brain 0.5-mm-reoslution protocol based on an inversion-prepared 3D TSE sequence has recently been proposed at 3T. However, its 11-12-min scan time renders it clinically impractical. This work aimed to develop an expedited protocol and validate it on patients.
Elliptical data sampling and prolonged echo train length (ETL) can be exploited to expedite the 3D TSE acquisition. However, this would reduce SNR and compromise vessel wall delineation. On the other hand, SNR is intimately related to the refocusing flip angles that are calculated for a prescribed signal evolution of a tissue with specific T1 and T2 values (denoted here as simulation T1 and T2). We hypothesized that an appropriate choice in the combination of ETL and simulation T2 may help achieve an efficient protocol.
On a 3T system, the effects of simulation T2 and ETL on wall SNR, wall-CSF CNR, and white-gray matter CNR (indicative of T1 contrast weighting) were first explored on 9 healthy subjects. Simulation T2 varied (50, 80, 110, 140, 170, 200 ms) while ETL was held fixed at 36 (5 subjects) or 60 (3 subjects), and ETL varied (36, 44, 52, 60, 68) while simulation T2 was held fixed at 170 ms (1 subject). The range of potential protocols (i.e. combinations of ETL and simulation T2) was then narrowed. Specifically, ETL = 52 combined with a simulation T2 of 140, 170, and 200 ms were respectively tested on 7 healthy subjects. In addition, a combination of ETL = 36/T2 = 100 ms was used as a reference. An optimal imaging protocol was determined from the four scans and finally applied to a pilot study comprising 18 patients with various known arterial wall disease.
Increasing simulation T2 boosted SNR and CNR (Figure 1 a and b). As expected, SNR and CNR were reduced as ETL increased (Figure 1c). An ETL of 52 appeared to allow the scan time to reduce to 8 min while avoiding drastic SNR/CNR sacrifice. ETL = 52/T2 = 170 ms was shown to provide significantly increased wall SNR (p = 0.012), wall-CSF CNR (0.049), and white-gray matter CNR (0.019), compared with those obtained by the original 12-min protocol (Figure 1d). This combination was chosen as an optimal imaging protocol with which wall abnormalities (Figure 2) were correctly detected in all patients using their clinical diagnosis as the reference. The 8-min scan was well tolerated according to a survey. The abnormalities detected were atherosclerotic plaque in 10, vasculitis in 4, dissection in 2, aneurysm in 1, and Moyamoya disease in 1. The T1-mediated signal features within various wall pathologies facilitated definitive diagnosis.
Whole-brain intracranial vessel wall evaluation at 3T is feasible within a clinically acceptable scan time - 8 min. A large-scale trial on using the technique for diagnosis of stroke etiology is underway to establish its clinical usefulness.