Investigation of approaches to reduce truncation of attenuation maps with simultaneous transmission and emission SPECT imaging

The limited field of view (FOV) associated with fan beam collimators can lead to truncation of the reconstructed attenuation maps in transmission imaging. The authors investigated through simulations, 5 approaches for reducing the truncation of attenuation maps with simultaneous transmission and emission SPECT imaging. These include: 1) the use of longer focal length collimators (65 vs. 104 cm); 2) using a 4 cm lateral table offset to bring the heart closer to the center of the fully sampled region (FSR); 3) using body contouring orbits instead of circular orbits to bring the camera closer to the body; 4) using a larger camera to increase the FSR; and 5) use of an asymmetric fan beam collimator to enlarge the FSR. The three dimensional (3-D) mathematical cardiac torso (MCAT) phantom was used to simulate the activity and attenuation maps. Simulations were carried out for two MCAT configurations with the heart located within, as well as extending beyond, the edge of the FSR for a 65 cm fan beam collimator. As expected, results indicate that employing a larger camera effectively removes the truncation of attenuation maps seen in normal sized patients. With a standard camera size of 40 cm, the asymmetric fan beam collimator gives results comparable to that obtained using a 104 cm symmetric fan beam collimator. Body contour orbits generally provided better cardiac uniformity at the shorter focal lengths than circular orbits. If any portion of the heart walls extends beyond the FSR for a 65 cm focal length collimator, a judicious combination of using longer focal length collimators along with table offset greatly reduces the amount of truncation in the attenuation maps, improving cardiac uniformity.