Overview and Critical Appraisal of Arterial Spin Labelling Technique in Brain Perfusion Imaging

Figure 7

A drawing showing the mechanisms of echo-planar imaging and signal targeting with alternating radiofrequency (EPISTAR). The labelling image is acquired using a large inversion slab (a) during the application of the proximal tagging; (b) a saturation slab is applied to the imaging plane in order to remove tagging contamination; (c) the tagged image is obtained after a delay time (TI) during which the tagged blood water leaves the labelled plane and starts to disperse from the vascular system into tissues at the image plane (arterial arrival time). A control image is acquired using a distal large inversion slab (a) during the application of tagging; (b) then a saturation slab is applied (c) to obtain the control image after the same delay time where the tagged venous spins enter the control image. The ASL difference image (control−label) includes the tagged venous spins, which are negative and appear as focal dark spots. Static tissue spins are shown as green arrows and tagged blood water is shown as blue arrows.