Imaging Software
I. Imaging J (Image Processing and Analysis in Java) 
ImageJ is a public domain Java image processing program inspired by NIH Image for the Macintosh. It runs, either as an online applet or as a downloadable application, on any computer with a Java 1.4 or later virtual machine. Downloadable distributions are available for Windows, Mac OS, Mac OS X and Linux.
It can display, edit, analyze, process, save and print 8-bit, 16-bit and 32-bit images. It can read many image formats including TIFF, GIF, JPEG, BMP, DICOM, FITS and "raw". It supports "stacks", a series of images that share a single window. It is multithreaded, so time-consuming operations such as image file reading can be performed in parallel with other operations. MicroCT imaging data can be analyzed with Imaging J.
Reference link: http://rsbweb.nih.gov/ij/
II. AMIDE
AMIDE is a medical imaging data examiner. You will be able to analyze the image data for both of microCT and microPET. AMIDE is a competely free tool for viewing, analyzing, and registering volumetric medical imaging data sets. It's been written on top of GTK+ , and runs on any system that supports this toolkit (Linux, Windows, Mac OS X with fink, etc.).
Reference link: http://amide.sourceforge.net/