The progress of nuclear medicine and x-ray computer imaging encourage the researchers to develop new dual modality positron emission/X-rays tomography (PET/CT) systems, to acquire both the anatomical and the functional information during the same measurement, relying on the same detection system and electronics at the same time. The aim of this work was to study a detector consisting of four LSO pixel crystals coupled to a Hamamatsu S8550 avalanche photodiode (APD) array. The measurements covered tests of the detectors in the PET and in the CT modes using counting and current modes, respectively. The measurements included a determination of the light output, energy resolution, non-proportionality of the light yield and time resolution for 511 keV annihilation quanta, characterizing the PET detector, and the dependence of a counting rate versus a mean current of APD in the X-ray detection.