High Field ASL

High-field imaging machines may represent an ideal application for arterial spin labelling, due to the intrinsic SNR, the use of proton-density-weighted images, and the longer T1 of blood, which improve sensitivity to labelled spins and decrease sensitivity to uncertainties in arterial arrival time [4, 5]. For example, at 3.0 T, the SNR is about twice that at 1.5 T. At 3.0 T, blood T1, which determines the half-life of the label, is 1.6 -1.8 s (vs 1.2-1.4 sat 1.5 T).

This results in a better SNR because the label decays more slowly, and, more importantly, in less sensitivity to arterial arrival time differences, because longer post-labelling delays are enabled. Such increased SNR sensitivity can be harnessed to decrease scanning time, to acquire higher resolution images, or to apply longer post-label delay times (Fig. 9.10).

Several recent papers have reported favourably on high-field ASL perfusion imaging [88 - 90]. Using CASL at 3.0 T with a 16-element phased-array receive coil and separate neck coil for labelling, Talagala et al. found a better SNR and obtained 3-mm isotropic resolution images in only 1 - 2 min [91]. Wang et al. showed a 33 % higher SNR at 3.0 T using a single coil and amplitude-modulated control pulse to reduce the effects of magnetization transfer [92].

Other recent applications of ASL are devoted to the study of regional perfusion territory imaging in order to visualize the vascular territory supplied by a specific labelled vessel.

Selective labelling may be useful in patients with high-grade or complete vascular occlusion to guide the choice between angioplasty and stenting, to evaluate cerebrovascular bypass grafts, or to estimate the likelihood of an embolic or atherothrombotic source in a patient with multiple bright regions on diffusion-weighted imaging.

Zaharchuk [93] first reported on this method by using continuous selective labelling at the level of the human carotid bifurcation with a separate labelling coil.

Medical Asl

Several investigations of regional perfusion territories using continuous [94] or pulsed [95-98] ASL methods have followed this first study.

Hendrikse et al., selecting a rectangular volume for labelling based on a previous MR angiogram, showed excellent visualization at 3.0 T of perfusion territories of isolated right carotid, left carotid and posterior circulation, demonstrating individual differences in vascular territories relating to different variants in the circle of Willis [96-98].

It is important to note that the only way to obtain such information used to be invasive catheter angiog-raphy. These data maybe helpful to gain abetter understanding of vascular dynamics in patients at high risk of stroke or being considered for corrective surgical procedures [99].

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