  This paper analyzes the information content of medical images, with 3-D MRI
images as an example, in terms of information entropy. The results of the
analysis justify the use of Pixel Difference Coding for preserving all
information contained in the original pictures, lossless coding in other words.
The experimental results also indicate that the compression ratio CR=2:1 can be
achieved under the lossless constraints. A pratical implementation of Pixel
Difference Coding which allows interactive retrieval of local ROI (Region of
Interest), while maintaining the near low bound information entropy, is
discussed.
