Objectives: One approach to motion detection in SPECT is to observe the patient using optical cameras. Patient motion is estimated from changes in the images and is used to modify the reconstruction algorithm. An important subproblem is calibrating the optical camera and the gamma camera. That is, it is necessary to determine the transformation from the gamma camera coordinate system to the optical camera coordinate system such that given a gamma camera point, one may compute the corresponding optical camera point. Conversely, given an optical camera point, one may compute the corresponding patient ray. Methods: We have devised a calibration phantom that can be imaged using both optical and gamma cameras. The phantom comprises a set of lucite disks; each disk supports 2 low-intensity light bulbs and a 0.8mm diameter hole centered between the bulbs to hold a ^99mTc point source. The radioactive source location for each disk in image coordinates is taken to be the midpoint of the bulbs. The radioactive source location in gamma camera coordinates is found by segmenting the reconstructed source distribution and computing the centroid of the activity of each source. At least 6 such point pairs are needed, although 7 are used in practice to provide increased accuracy. Using procedure PROJ_MAT_CALIB of Trucco & Verri Introductory Techniques for Computer Vision, we compute the 11 parameters of the coordinate transformation and the residual error. Because we do not know in advance which optical camera points match which gamma camera points, an exhaustive search is used to find lowest-error matches. Results: We have been able to match optical and gamma camera points and determine the transformation. Tomographic reconstruction and segmentation take up most of the processing time; point matching and parameter calculation take less than 1 second of processor time on a Digital Alpha 433au workstation. Conclusions: