FB-TCT truncation suppression by combining two shifted scans with extrapolation techniques

The convergent beam geometry of fan beam (FB) transmission computed tomography (TCT) often results in image truncation; producing ring artifacts, distorted body contours, and erroneously high pixel values in filtered-backprojection reconstructed images. A method is described which reduces the degree of truncation by performing two patient-shifted FB-TCT scans, and then uses an extrapolation technique to complete the projection set. By scanning the patient when shifted left and right of the axis of rotation, the effective field-of-view is substantially increased. If any truncation remains, the total amount of attenuating medium present is calculated from untruncated views. For truncated views, missing projections are filled until this total is reached. The method has been tested by scanning a large (38×26 cm) anthropomorphic thorax phantom on a three camera single photon emission computed tomography (SPECT) system. The two run method was successful in reducing the degree of truncation to only a slight amount for this phantom. Application of the extrapolation method helped suppress ring artifacts in this case. To further test the extrapolation method projection sets were truncated to a greater degree using software. Extrapolation helped improve SPECT attenuation compensation, but some artifact remained in the images