EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE 3D SPECT PHOTON DETECTION KERNEL FOR NONUNIFORM ATTENUATING MEDIA

Experimental tests for non-uniform attenuating media are performed to validate theoretical expressions for the photon detection kernel, obta ined from a recently proposed analytical theory of photon propagation and detection for SPECT. The theoretical multidimensional integral exp ressions for the photon detection kernel, which are computed numerical ly, describe the probability that a photon emitted from a given source voxel will trigger detection of a photon at a particular projection p ixel. The experiments were performed using a cylindrical water-filled phantom with large cylindrical air-filled inserts to simulate inhomoge neity of the medium. A point-like, a short thin cylindrical and a larg e cylindrical radiation source of Tc-99(m) were placed at various posi tions within the phantom. The values numerically calculated from the t heoretical kernel expressions are in very good agreement with the expe rimentally measured data. The significance of Compton-scattered photon s in planar image formation is discussed and highlighted by these resu lts. Using both experimental measurements and the calculated values ob tained from the theory, the kernel's size is investigated. This is don e by determining the square N x N pixel neighbourhood of the gamma cam era that must Ix:connected to a particular radiation source voxel to a ccount for a specific fraction of all counts recorded at all camera pi xels. It is shown that the kernel's size is primarily dependent upon t he source position and the properties of the attenuating medium throug h Compton scattering events, with 3D depth-dependent collimator resolu tion playing an important but secondary role, at least for imaging sit uations involving parallel hole collimation. By considering small poin t-like sources within a non-uniform elliptical phantom, approximating the human thorax, it is demonstrated that on average a 12 cm x 12 cm a rea of the camera plane is required to collect 85% of the total count recorded. This is a significantly larger connectivity than the 3 cm x 3 cm area required if scattering contributions are ignored and only th e 3D depth-dependent collimator resolution is considered.