Figure 1:

ASL planning. In A), the labeling plane (yellow) and the image volume (red) can be moved freely and independently. In this ideal case, the labeling plane should be placed perpendicular to the carotid arteries around C2/C3 and the image volume along the AC-PC line in concordance with other used transversal sequences. In B) and C) the labeling plane and the image volume are not individually moveable but follow each other. In B) the orientation of the image volume corresponds to the standard orientation (i.e. the AC-PC line) and can be more easily compared to other conventional MR sequences; however, the radiologist must accept a potential reduction in the labeling efficiency that may vary (for example) between the anterior and posterior circulation. In C) the orientation of the imaging volume deviates from the AC-PC line, but the labeling efficiency should not be compromised. Ideally, option (A) is used and will lead to both optimal labeling efficiency and a familiar imaging slice orientation. When A) is not attainable, the decision between B) and C) rests upon the neuroradiologist and should consider the interpretative objectives of the particular case, though more uniform labeling efficiency is typically desired to avoid regions of artifactual hypoperfusion.