A geometrical mismatch between emission and transmission occurs in cardiac PET/CT because of respiratory and cardiac motions. Proposed solutions to this problem include fast CT during free breathing, with or without image alignment, and slow CT. To study this problem, including the variability of human breathing patterns and compliance with instructions, we performed real-time FISP MRI measurements of two freebreathing volunteer subjects. We have developed a method for simulating PET and CT coronal images from these image sequences, and for locating the left cardiac free wall semiautomatically. The PET geometry represents an average over the whole MR examination. The simulated CT geometry represented 28.8 mm axial extent and speeds of 83 mm/sec (fast) and 12 mm/sec (slow), representing a Senation-64 CT scanner running at fast and slow settings. We considered 400 start times for each CT geometry. The free wall was located in each CT image and was compared with the location expected in PET. The error in a given CT scan depends on the geometry, i.e. fast or slow scan, and also on the state of breathing at the time of the scan. A more accurate result can be realized by selecting the best-aligned of two or three fast CT scans. The average errors in fast, slow, best of 2, and best of 3 fast CT scans are respectively 5, 4, 4, and 3 mm. The errors in these scans have 98% likelihood of being less than, respectively, 14, 9, 9, and 7 mm.