Technical noteHigh resolution simultaneous imaging of intracranial and extracranial arterial wall with improved cerebrospinal fluid suppression
Introduction
Stroke is a common cause of death, and ischemic stroke accounts for about 80% of the cases [1]. Intracranial atherosclerotic disease (ICAD) and extracranial atherosclerotic disease (ECAD) are two important causes for ischemic stroke [2], [3]. In magnetic resonance imaging of stroke patients, carotid and intracranial arteries are usually imaged together in contrast enhanced angiography but this is not so in vessel wall imaging.
MR vessel wall imaging can identify non-stenotic atherosclerotic plaques undetected by angiography, and is a useful tool in the diagnosis of ischemic stroke [4]. To image extracranial and intracranial arterial walls simultaneously, the technique would need: (a) good blood suppression, especially at the carotid bifurcation; (b) uniform suppression of cerebrospinal fluid (CSF) surrounding the intracranial arteries to improve vessel wall delineation [5]; and (c) high spatial resolution. Recently, three dimensional (3D) vessel wall imaging of extracranial arteries (ECA) and intracranial arteries (ICA) in one setting using gradient echo techniques was proposed [6]. Flowing blood was well suppressed but there was no provision for CSF suppression. Meanwhile, T1 weighted three dimensional TSE (or T1w-SPACE) has been used for high resolution 3D intracranial arterial wall imaging [7] while the DANTE module (Delay Alternating with Nutation for Tailored Excitation) module [8] has been found useful in suppressing of both blood and CSF signals [9]. Hence, in another method, DANTE and T1w-SPACE were combined (referred as DANTE-SPACE) to jointly image extracranial and intracrainial arterial walls [10]. However, evaluation of CSF suppression in DANTE-SPACE was limited to the cervical spinal cord region. Moreover, the spatial resolutions of both methods [6], [10] were between 0.74 and 0.8 mm. The low spatial resolution makes depiction of distal MCA difficult. Performance of DANTE in suppressing CSF around that region remains unknown.
We evaluated the ability of DANTE to suppress CSF at the distal MCA regions through high resolution vessel wall imaging of ECA and ICA. The results prompted us to improve DANTE-SPACE for uniform CSF suppression around the MCA without affecting blood suppression at the carotid region. When combined with a custom designed coil set, the improved technique would achieve an isotropic spatial resolution of 0.6 mm, which is higher than currently available techniques [6], [10] and allows visualization of arteries beyond M1. In this study, simulations would first be used to show how DANTE-SPACE could be improved to achieve this purpose. Healthy volunteer studies would then be performed to validate the simulation results. Patient studies would follow to demonstrate the clinical relevance of the technique. Preliminary results were first reported in [11].
Section snippets
Imaging sequence
DANTE-SPACE was designed to suppress both blood and CSF signals in intracranial and extracranial vessel wall imaging [10], Yet, assessment of its performance on CSF suppression was only limited to the cervical region. The sequence in this study improved CSF suppression by modifying DANTE-SPACE in the following way: (a) the flip-down pulse was reinstated at the end of echo train [12]. It improved T1 contrast but reduced image signal-to-noise ratios (SNR); (b) the flip angles for the refocusing
Simulations
Fig. 2 showed that DANTE-SPACE and iSPACE differ in several aspects. Fig. 2a showed that the use of T1/T2 = 1000/150 ms gave higher flip angles than the use of T1/T2 = 940/100 ms in computing the refocusing flip angles. The signal change of vessel wall and CSF over the series of refocusing flip angles for the two sequences was shown in Fig. 2b. The CSF signal was significantly reduced by the flip-down pulse in iSPACE (Fig. 2b and c). The vessel wall signal was also reduced, as noted in an earlier
Discussion
This study showed that the proposed iSPACE suppresses CSF uniformly at the distal MCA regions without affecting the sequence's blood suppression performance at the carotid arteries. The sequence reduce image SNR but this side-effect is mitigated by increasing the flip angles of the refocusing pulse series slightly by using different T1/T2 values in the refocusing flip angle calculation process. The study also showed that, using iSPACE with the new coil set, simultaneous 3D imaging of ICA and
Acknowledgements
This work is funded by the National Key R&D Program (Grant No. 2016YFC0100100, No 2017YFC0112903), Basic Research Program of China (973 program: 2013CB733800/2013CB733803), Natural Science Foundation of China (No.81470077, No.81120108012), Science Foundation of Shenzhen (No.JCYJ20140417113430589, No. JSGG20141020103440414, No. GJHS20160331190804880), Key Laboratory for Magnetic Resonance and Multimodality Imaging of Guangdong Province (Grant No. 2014B030301013)
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