Ratio maps. Ratio images from the [11C]Pittsburgh Compound B ([11C]PIB) scans of the two subjects shown in Figure 3: top images, control subject; bottom images, Alzheimer's disease (AD) patient. Images generated by dividing the mean activity concentration of each voxel from 40 to 60 minutes post injection (centre) or from 60 to 90 minutes post injection (right) by the mean activity concentration of cerebellar grey matter (white arrow) of the same time interval. The T1-weighted structural magnetic resonance (MR) images in the left column are shown for reference only. The colour bars at the bottom of the columns indicate the image minimum and maximum ratios. A visual comparison with the 10 to 90 minute sum images shown in Figure 3 immediately reveals the much higher levels of image noise in these late images due to the 11C half-life of 20.4 minutes.