Implementation of Sensitivity and Resolution Modeling for SPECT with Cone-Beam Collimator

We implemented a fully-3D ordered subsets expectation maximization (OSEM) algorithm with compensation of attenuation, distance-dependent blurring (DDB) and sensitivity modeling for SPECT performed with cone-beam collimator (CBC). The experimentally obtained detector response to point sources across FOV was fitted to a twodimensional Gauss function with its width (FWHM) varied linearly with the source-to-detector distance and with very weak sensitivity dependence on the emission angle. We obtained CBC SPECT scans of a physical point source phantom, Defrise phantom and a female patient, and investigated performance of our algorithm. Our studies indicated that to correctly simulate DDB and sensitivity a blurring kernel with a radius of up to 10 elements had to be used for 128x128 acquisition matrix and volumetric ray tracing rather than line element based ray tracing has to be implemented. In the point source phantom reconstruction we evaluated the uniformity of FWHM for the radial, tangential and longitudinal directions, and sensitivity vs. distance. An isotropic and stationary resolution was obtained at any location by OSEM with DDB and sensitivity modeling only when volumetric ray tracing was utilized. We analyzed axial and transaxial profiles obtained for Defrise phantom and evaluated the reconstructed breast SPECT patient images. The proposed fully-3D OSEM reconstruction algorithm with DBB and sensitivity modeling, and attenuation compensation with volumetric rays tracing is efficient and effective with significant resolution and sensitivity recovery.