A solid state PET module for retrofitting to a standalone CT scanner generally includes a housing that is insertable into at least a portion of a bore opening of a gantry of the CT scanner. The housing includes a number of PET coincidence detectors, which are preferably solid state APD detectors. The module may include a base for supporting the module on a floor or platform of the CT scanner, or may be in the form of a disk securably fastened to the CT scanner gantry.