MR image to show how locus coeruleus (LC) signal intensity (SI) is typically calculated as a contrast ratio using the pons tegmentum as a reference are (blue circle). Regions of Interest are placed around the LC (green circles). The average SI of the mean right LC and mean left LC (LCSI) minus the mean signal of the pons tegmentum (SIPT) divided by the signal intensity of the pons tegmentum: (LCSI –SIPT)/SIPT provides the contrast ratio.