Investigation of large field-of-view transmission imaging for SPECT attenuation compensation with Gd-153, Tc-99m and Ce-139 sources

A recently developed, large field-of-view (FOV), dual head SPECT system was investigated for fast sequential fan-beam (FB) transmission computed tomography (TCT) with various transmission sources. For TCT, the system utilizes a fixed line source at the 77 cm focal line of asymmetric FB collimator. The TCT maps are used for non-uniform attenuation compensation (NUAC) of Tc-99m myocardial SPECT. The heads each have three degrees of freedom (transaxial tilt, radial linear and transaxial linear travel). When the heads are tilted and height adjusted to accommodate different size FOVs (34-53 cm diameter), the symmetric FB geometry only samples part of the FOV. Thus, with full 360 degree acquisitions, the pseudo-asymmetric FB TCT detector utilizes measured conjugate views to avoid truncation artifacts without sacrificing spatial resolution, and minimizes emission contamination compared with other TCT geometries. Transmission line source energies of 100 to 166 keV (from Gd-153, Tc-99m, and Ce-139 line sources) were utilized with rod and sphere, and anthropsmorphic phantoms. The TCT images were reconstructed with iterative ordered subsets estimation maximization (OSEM). Comparisons were made between the emission reconstructions utilizing filtered backprojection (without and with iterative and multiplicative Chang NUAC) and OSEM (without and with NUAC). Based on a derived noise equivalent count rate determination of the three sources, the highest energy yet weakest intensity TCT source outperforms the other sources in the primary myocardial region of interest by approximately a factor of two. The emission contamination for the highest energy transmission source was an order of magnitude smaller than for the other sources with this acquisition geometry. These line sources each demonstrate the potential for suitably compensated images and quantitative myocardial activity profiles with this geometry. Higher energy transmission sources in particular result in more physically accurate attenuation maps useful for SPECT quantitation.