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A small animal single photon emission computed tomography (SPECT) system has been developed for acquiring in vivo images of the biodistribution of radiopharmaceuticals in unrestrained, awake small animal models. This system uses two dedicated gamma-ray cameras and an infrared based animal position tracking system mounted on a rotational gantry with an infrared transparent mouse burrow in the center. The original implementation of this system had two compact gamma cameras with an active area of 5 cm×5 cm. Two new gamma camera detector heads with active areas of 10 cm×20 cm have been built, tested and installed in the SPECT gantry to replace the 5 cm×5 cm detectors. The new gamma cameras are based on pixellated NaI(Tl) crystal scintillator arrays, coupled to arrays of compact position-sensitive photomultiplier tubes (PSPMT) to achieve high spatial resolution in a compact, low profile device. A novel cost-effective readout is utilized. The two detector heads are based on a 4×8 array of Hamamatsu R8520-C12 position sensitive photomultiplier tubes. Each PSPMT array is coupled to a pixellated 10 cm×20 cm NaI(Tl) scintillator crystal array with individual crystal elements of 2 mm×2 mm×15 mm in size and a septum of 0.25 mm between elements. Samples of phantom and animal studies are presented. The detectors are capable of imaging 125I and 99mTc labeled radiopharmaceuticals in live mice.