Comparison of transaxial resolution in 180 degrees and 360 degrees SPECT with a rotating scintillation camera.

Using circular 180 degrees and 360 degrees SPECT acquisition modes the transaxial resolution of line sources in air and water were measured at different positions in the field of view. With the 180 degrees acquisition mode, all line sources in air located off the axis of camera rotation (AoR) showed an oval distortion. This distortion was systematically related to the starting point of the rotating detector. On axis line sources in air were undistorted, regardless of the 180 degrees acquisition starting angle. The 360 degrees acquisition images of the line sources in air showed a similar effect but in a very mild form. In water, transaxial reconstructions of line sources (off axis) showed an enhancement of the oval distortion for both the 180 degrees and 360 degrees acquisitions. Computer simulations of the line source measurements were performed and correlated well with the experimental data. The line source results are explainable by the inherent depth dependent response of the scintillation camera. In clinical SPECT studies, distortions of this nature will be most appreciable with 180 degrees imaging of small organs that are located off the AoR.