Regions of interest in a PET image. (A) A CT slice on which the carotid artery, outlined in red, was delineated. (B) The arterial voxels and external voxels on the corresponding PET image recorded at 2 min. The blood-sampled activity at this time was 12.3 kBq. The arterial voxels containing the carotid artery and surrounding tissue are delineated by the solid black line, and the carotid artery is outlined on the PET voxels in (C). The external tissue, comprising the next layer of voxels out, is delineated by the dashed line.