Acceleration of ASL‐based time‐resolved MR angiography by acquisition of control and labeled images in the same shot (ACTRESS)

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Evaluation of cerebrovascular hemodynamics provides crucial information for accurate diagnosis, treatment selection, and follow‐up of diseases such as arteriovenous malformation, arteriovenous fistula, and steno‐occlusive disease. Recently, there has been an increasing number of clinical reports using arterial spin labeling (ASL) for noncontrast‐enhanced (CE) magnetic resonance angiography (MRA) 1, mainly due to the possibility of time‐resolved (4D) MRA as well as vessel specificity by restricting the labeling to a single vessel 7.
4D‐MRA can be achieved by acquiring images with different postlabeling delays (PLDs), for example, by using a Look‐Locker readout 9. The benefits of 4D‐MRA using ASL is that injection of the contrast agent is not required and that both high temporal and spatial resolution can be achieved. Unlike CE‐MRA, which must capture the quick first passage of the contrast agent and therefore suffers from a compromise between temporal and spatial resolution, labeling of arterial blood can be repeated until sufficient data are acquired to achieve both high temporal and spatial resolution, thus enabling detailed visualization of arterial flow hemodynamics by ASL. To achieve this advantage, ASL‐based 4D‐MRA usually requires a longer acquisition time than CE‐MRA. A second reason for the longer acquisition time of ASL‐based 4D‐MRA is that ASL techniques require the acquisition of two images: one in which the arterial blood is inverted (labeled image) and in the other in which the arterial blood is not inverted (control image). By subtraction of these two images, the background static tissue signal is cancelled out and the inflow of the labeled arterial blood is visualized. The mean acquisition time of previously reported ASL‐based 4D‐MRA is approximately 7 minutes (5 min–8.5 min) 1, which is not always fast enough to enable its use in clinical protocols.
In this paper, we present a novel, ASL‐based 4D‐MRA technique, named ACTRESS (ACquisition of conTRol and labEled image in the Same Shot), which nearly halves the acquisition time by restricting image acquisition to only the labeled condition. Subtraction can be performed by shifting the labeling pulse to the second phase of the Look‐Locker readout and using the first phase as the control image (Fig. 1). However, this approach potentially could induce several artefacts because the subtraction is performed between images acquired in different phases of the Look‐Locker readout. Moreover, the control image acquired with this approach might not perfectly cancel out the magnetization transfer (MT) effects, unlike conventional ASL sequences designed to keep label and control images as identical as possible with a similar level of MT effects.
The purpose of this paper is to optimize ACTRESS MRA so that it enables generation of 4D‐MRA images of a similar quality to the conventional ASL sequence in nearly half the scan time while avoiding possible artefacts caused by the absence of the separate acquisition of control images. This approach can significantly improve the clinical usability of 4D‐ASL due to shorter scan times, which also reduces the occurrence of motion artefacts associated with longer scan duration.

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