Do PET scanners only detect the abnormal part of a persons e.g. brain?

The FDG will go to all areas of the brain/body, but more will go to the tumor (or another area with higher metabolic activity). The final image from a PET scan is artificially colored - the scan will return an intensity value for each voxel of somewhere between 0 and whatever the max of the scanner is, and a color scale is simply mapped to this range. The scale is designed/adjusted so that anything significant will stick out, while the background will be unobtrusive or left out completely if it is under a certain threshold, such that the resulting image will be meaningful and as easy to read as possible for the person interpreting the result.