This article reports on PET-mode transaxial spatial resolution for a new hybrid PET/SPECT system. This system is based on a rotating dual-head tomograph using LSO/NaI scintillators and is capable of depth-of-interaction (DOI) measurement. For this LSO PET/SPECT, each detector head contains two 84/spl times/120 planar arrays of crystals-one array NaI, the other LSO. LSO and NaI crystals are each 0.44/spl times/0.44/spl times/1 cm. In PET mode, each head is radially offset 36 cm from FOV center. Relative orientation of the two heads is 158 degrees. Dual-level DOI discriminates LSO versus NaI by scintillation pulse decay time. While significant for good image resolution, dual-level DOI isolates gamma detection only to the (1 cm) long axis depth of each individual crystal. Critical to delivery of excellent spatial resolution are additional methods to precisely characterize the depth of the centroid of the probability of gamma interaction along the crystal long axis. This depth is measured to /spl plusmn/0.01 cm for each of 4 planar arrays and applied for more accurate LOR positioning in super-fast real-time rebinning hardware. With DOI and measured centroid depth applied. FWHM transaxial spatial resolution is 0.4 cm at FOV center and 0.6 cm at 10 cm off center. Without DOI, resolution degrades as much as 33%.