FIGURE 1.

A simple cartoon depicting the differences between spatially selective and velocity selective labeling. The cartoon represents blood in the carotid and vertebral arteries flowing into the circle of Willis, which in turn distributes blood to the rest of the brain. In the left panel, inflowing spins are labeled as they flow through a labeling plane (such as in pseudo‐continuous arterial spin labeling [PCASL]), defined by the pulse. The labeling process is depicted by a color change from blue to red. The cartoon depicts the label at the beginning of the labeling pulse. However, the pulse is applied for an extended period such that the labeled spins fill the vascular space. The time between the spins being labeled at the neck and their arrival to the tissue is referred to as the arterial transit time and a significant amount of the label is lost because of T₁ relaxation. In contrast, velocity selective pulses label spins depending on their velocity, and not their location, as depicted in the right panel. As a result, the vascular space is immediately filled with labeled spins and the arrival time of the leading edge of the label to the tissue is dramatically reduced