1. A method for assessing the activity of an organ in a subject with the aid of a digital computer comprising: a) accessing by one or more computers a quantitative three-dimensional image of the organ that is represented as voxels, wherein each voxel contains information about the activity of a portion of the organ; b) calculating by the one or more computers a mean of the activity represented by the voxels, wherein voxels representing values at upper and lower extremes are excluded from calculation of the mean; c) calculating by the one or more computers a standard deviation of the mean obtained in step b), wherein voxels representing activity above a certain standard deviation of the mean indicate areas of the organ having increased activity and wherein voxels representing activity below a certain standard deviation of the mean indicate areas of the organ having reduced activity; and d) outputting by the one or more computers to an output device a representation of the organ showing areas of the organ having increased activity and/or reduced activity.