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Localization and detection of isolated "hot" spots in single-photon emission computed tomography (SPECT) may be aided by the addition of anatomical information from X-ray computed tomography (CT). A method for registering SPECT and CT volumes was developed, using two planar transmission views of the patient acquired immediately before or after a SPECT scan by placing a sheet source on one head of a dual-head camera system. Corresponding reprojected CT (rCT) images were simultaneously registered to the transmission images, using a region of interest (ROI) containing the lung boundaries. Six parameters (translations in three dimensions and rotations through three Euler angles) were calculated by iteratively transforming and reprojecting until a correlation between the rCT and transmission data within the ROI was maximized. Final values of the transformation parameters were used to register the entire CT volume to the SPECT volume. An experiment was performed using a torso phantom and spherical lesions containing Ga-67, representing the distribution in a lymphoma patient. Accuracy was assessed by calculating the differences between coordinates of centroids of five tumors in the CT and SPECT images. The magnitudes of the differences in x, y and z averaged over the five tumors were 1.55 mm +/-1.30, 036 mm +/-2.23 and 0.13 mm +/-1.47. Precision was assessed by perturbing the orientation of the CT volume over a range of translations and rotations. Effects on the perturbed parameter and on the other parameters that were held constant were examined by calculating the standard deviation about the mean of each parameter's final value following registration. The values varied from 0.06 to 0.62 mm and from 0.05 to 1.68 deg for translation and rotation, respectively. Acceptable accuracy and precision for registration of CT to SPECT volumes were achieved with the use of the paired transmission and rCT images.