Elsevier

Magnetic Resonance Imaging

Volume 44, December 2017, Pages 65-71
Magnetic Resonance Imaging

Technical note
High resolution simultaneous imaging of intracranial and extracranial arterial wall with improved cerebrospinal fluid suppression

https://doi.org/10.1016/j.mri.2017.08.004Get rights and content

Highlights

  • Simultaneous imaging of intracranial and extracranial arterial walls of 0.6 mm iso can be performed in about 7.5 min.

  • In volunteers, iSPACE uniformly suppresses CSF without compromising blood suppression at carotid arteries, nor image SNR.

  • In patients, the proposed technique improves visualization of plaques from the carotid to the intracranial arteries.

Abstract

Purpose

To develop a technique for three dimensional (3D) high resolution joint imaging of intracranial and extracranial arterial walls with improved cerebrospinal fluid (CSF) suppression and good blood suppression based on T1 weighted sampling perfection with application optimized contrast using different angle evolutions (T1w-SPACE) and to compare this technique (hereafter, iSPACE) with alternating with nutation for tailored excitation (DANTE) prepared SPACE sequence (DANTE-SPACE) for their CSF suppression performance around the mid cerebral arteries (MCA) and blood suppression at carotid arteries.

Materials and methods

Eight volunteers and twelve patients were prospectively recruited in this institutional review board approved study. A custom designed 32-channel coil set covering the intracranial and extracranial arteries was used for signal reception. Imaging was performed in each subject using DANTE-SPACE and iSPACE. Signal-to-noise ratios (SNR) of the vessel walls at the MCA and carotid arteries, and contrast-to-noise ratios (CNR) between vessel wall and CSF at the MCA and between vessel wall and lumen at carotid arteries from the two sequences were compared.

Results

In volunteers, contrast between CSF and white matter (surrogate for vessel wall signal) at the M2 segments in iSPACE was 67.9% higher than in DANTE-SPACE. At the carotid region, the SNR of vessel wall in iSPACE was 11.6% higher than DANTE-SPACE while the CNR in iSPACE was 13% higher than DANTE-SPACE. In patients, images with 0.6 mm isotropic resolution were obtained in 7.5 min. iSPACE showed 70.9% improvement in CNR between plaque and CSF at the M2 segments compared to DANTE-SPACE.

Conclusion

Simultaneous extracranial and intracranial arterial wall imaging using iSPACE improved CSF suppression significantly at the M2 segment of MCA while blood suppression was comparable to DANTE-SPACE. The technique achieved 3D images with 0.6 mm isotropic spatial resolution and took 7.5 min using a custom made coil set. Using this technique, intracranial plaque visualization was improved with no observable image SNR degradation.

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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