1. A method for assessing the activity of an organ in a test subject without comparison to a control subject with the aid of a digital computer, the method comprising: a) accessing by one or more computers a quantitative three-dimensional image of the organ of the test subject that is represented as voxels, wherein each voxel contains information about the activity of a portion of the same organ; b) constructing by the one or more computers a histogram of values represented by all voxels, fitting a curve to the histogram, and selecting voxels for inclusion for determining a reference level of activity of the organ if their values fall within a portion of the histogram that is symmetrical about a mean of values represented by all voxels; c) calculating by the one or more computers a mean of the activity represented by the voxels selected for inclusion in step b) and a standard deviation (SD) of the mean, wherein the mean of the activity represented by the voxels selected for inclusion represents a reference level of activity of the organ of the test subject; d) rescaling the voxels and calculating an advance/decline ratio to eliminate any bias, which is stored for future reference; e) identifying areas of the organ of the test subject having increased activity above the reference level of activity of the organ of the test subject as represented by voxels having activity above a certain standard deviation of the mean calculated in step c), and/or identifying areas of the organ of the test subject having reduced activity below the reference level of activity of the organ of the test subject as represented by voxels having activity below a certain standard deviation of the mean calculated in step c); and f) outputting by the one or more computers to an output device a representation of the organ of the test subject showing areas of the organ having increased activity above the reference level of activity of the organ of the test subject and/or reduced activity below the reference level of activity of the organ of the test subject, thereby assessing the activity of an organ in the test subject without comparison to a control subject.