In this work we studied the techniques for determining the geometrical parameters for small animal SPECT imaging on an existing MicroPET scanner. A slit-slat collimator was inserted into the PET scanner's detector ring and the imaging subject was rotated to acquire SPECT tomographic data. The parameters to be calibrated included the center of rotation (COR) of the imaging subject, the slit-aperture center (SAC) positions and the tilt and twist angles of the slats. Based on acquiring multiple projections of point sources, we proved that, if the collimator has only one slit, two sets of projection data with each from a different point source location, and known distance between, is enough for uniquely determining COR and SAC. When the collimator consists of two or more slits, only one set of projection is enough. A least-square fitting procedure was used for determining the transaxial geometrical parameters including COR and SAC. A method to determine tilt and twist angles of the slats was proposed by using the axial information of the projection of point sources. Monte Carlo simulation and experimental studies were performed to evaluate the validity and accuracy of the calibration techniques. The reconstructed images with calibrated parameters showed satisfactory image quality.