arterial spin labeling mri is used to provide a patient specific correction factor to correct a image provided by a non - asl imaging modality . more specifically , a first blood flow image is taken using the non - asl imaging modality , and a corresponding second blood flow image is taken with asl . some or all of the voxels in the first image are selected according to a predetermined selection method . a correction factor is computed to be the ratio of second image bf to first image bf averaged over the selected voxels . thus , cf is the average of asl / non - asl blood flow over the selected voxels . this correction factor is applied to all voxels of an image equally , but can differ from patient to patient . this correction can be applied to one or more non - asl blood flow images .