Intracranial 3D and 4D MR Angiography Using Arterial Spin Labeling: Technical Considerations

Abstract

In the 1980's some of the earliest studies of arterial spin labeling (ASL) MRI have demonstrated its ability to generate MR angiography (MRA) images. Thanks to many technical improvements, ASL has been successfully moving its position from the realm of research into the clinical area, albeit more known as perfusion imaging than as MRA. For MRA imaging, other techniques such as time-of-flight, phase contrast MRA and contrast-enhanced (CE) MRA are more popular choices for clinical applications. In the last decade, however, ASL-MRA has been experiencing a remarkable revival, especially because of its non-invasive nature, i.e. the fact that it does not rely on the use of contrast agent. Very importantly, there are additional benefits of using ASL for MRA. For example, its higher flexibility to achieve both high spatial and temporal resolution than CE dynamic MRA, and the capability of vessel specific visualization, in which the vascular tree arising from a selected artery can be exclusively visualized. In this article, the implementation and recent developments of ASL-based MRA are discussed; not only focusing on the basic sequences based upon pulsed ASL or pseudo-continuous ASL, but also including more recent labeling approaches, such as vessel-selective labeling, velocity-selective ASL, vessel-encoded ASL and time-encoded ASL. Although these ASL techniques have been already utilized in perfusion imaging and their usefulness has been suggested by many studies, some additional considerations should be made when employing them for MRA, since there is something more than the difference of the spatial resolution of the readout sequence. Moreover, extensive discussion is included on what readout sequence to use, especially by highlighting how to achieve high spatial resolution while keeping scan-time reasonable such that the ASL-MRA sequence can easily be included into a clinical examination.

Recurrent transient ischemic attacks in a moyamoya syndrome patient with ultra-long imaging follow-up

“…The balance of hemodynamics and compensated perfusion of this patient relies on slow flow from the collaterals of the posterior circulation, which prolongs the mean transit times of arterial blood flow and creates ATA on 3D-pCASL images. 4D MRA is a 3D-pCASL-based dynamic MRA with long labeling duration, which can ensure visualization of the complete arterial trees from proximal to distal vessels with adequate image quality (29). The patient's latest 4D-MRA clearly shows the presence of collaterals mainly from the bilateral PCAs.…”

A novel sequence for simultaneous measurement of whole‐brain static and dynamic MRA, intracranial vessel wall image, and T1‐weighted structural brain MRI

“…By adding these 2 imaging contrasts, the time efficiency of iSNAP has been greatly improved over the previous ASL-based dMRA techniques, for which the clinical acceptability is hampered by the long acquisition time. 16 We also proposed another mechanism for generating MRA image with iSNAP based on the PSIR technique. 6 As discussed later, this new MRA mechanism can serve as a complement to the ASL subtraction-based MRA.…”

“…Using ASL approaches to obtain sMRA or dMRA has been extensively studied, as summarized by Suzuki et al 16 Radial acquisition with undersampling was shown to be highly suitable for such ASL‐based MRA imaging due to the high sparsity of the images and the flexibility in balancing spatial and temporal resolution when GA acquisition order is used 17‐20 . Previous studies mainly adopted a regular acquisition order for the 3D radial readout, 18,19 or 1D GA for 2D radial and 3D stack‐of‐star readout 17,20 .…”

“…The other key innovation in this study is that we further explored the feasibility of imaging intracranial vessel wall and generating a whole‐brain T1W brain MRI with iSNAP by leveraging the inversion recovery process in the sequence. By adding these 2 imaging contrasts, the time efficiency of iSNAP has been greatly improved over the previous ASL‐based dMRA techniques, for which the clinical acceptability is hampered by the long acquisition time 16 . We also proposed another mechanism for generating MRA image with iSNAP based on the PSIR technique 6 .…”

MR Imaging in the 21st Century: Technical Innovation over the First Two Decades

“…This technology further combined with arterial spin labeling (ASL) perfusion imaging can generate MR angiography and perfusion imaging in a single acquisition. 64 …”