(5) Compton Scatter Distribution Function in Non-Uniform Attenuation in SPECT(2. Tomographic Imaging)

抄録

(Introduction) Major problems of single photon emission computed tomography (SPECT) which degrade images both qualitatively and quantitatively include attenuation, camera uniformity geometric collimator response, and scattered photons. Compton scattered photons degrade spatial resolution and reduce the accuracy and precison of quantiation in SPECT images. The majority of correction methods introduced to reduce influence of compton scatter component consider the scatter distribution function to be a symmetrical, monoexponential function of distance from source position. Here a more realistic approach has been taken to derive scatter distribution functions for usiform and nonuniform scattering geometries. (Method) Monte Carlo simulations of Tc-99m point and line sources were used to obtain detailed information on the spatial and energy distribution of scattered and non-scattered events in SPECT images. EGS4 simulation code was adapted for use with planar and SPECT imaging geometries and detection devices. Simulations were carried out in two steps : 1) photons were followed in attenuation media until they were completely absorbed or emerged. Photon energy, input and output coordinates, direction and scatter order were recorded for photons that emerged within the detector field of view. 2) The detection process was simulated in a second step which gave greater flexibility in changing detection parameters (collimator geometry, intrinsic spatial resolution, energy resolution, energy window settings etc.). Scatter in the collimator and septal penetration were not included. Energy resolution of the system was simulated by sampling from an energydependent Gaussian function. The data shown were obtained from a simulation of SPECT system with a LEHR collimator, radius of rotation of 13cm, an energy resolution of 13%, intrinsic spatial resolution of 3.6mm, an energy window of 20% centered at 140KeV and a pixel size of 1mm. Seperate images were obtained for nonscattered events and events of scatter order n=1-3, a single image was obtained for n>4. (Results) The scatter distribution function in non-uniform density media indicates that scatter correction techniqes based on single exponential scatter distribution function do not provide a valid correction for non-uniform scattering geometries. Further investigations of SDF in more complex non-uniform geometries are needed to develop accurate correction method for Compton scatter reduction in SPECT images. My results suggest that 3-D scatter correction function and reconstruction methods need to be applied.