Quantitative image reconstruction in single photon emission CT requires an accurate attenuation map of a cross section of an object. Several data acquisition geometries have been proposed to obtain the true attenuation map by means of gamma-ray transmission CT (TCT). In the transmission data scattered photons are sometimes measured and they reduce the accuracy of reconstructed TCT images. To investigate the effects of scattered photons in gamma-ray transmission CT, we performed Monte Carlo simulations for several types of data acquisition systems. Examined geometries were (a) an uncollimated flood source and a parallel hole collimator, (b) a collimated flood source and a parallel hole collimator, (c) an uncollimated line source and a symmetric fan beam collimator, and (d) a collimated line source and a symmetric fan beam collimator. The results showed that a fan beam collimator and a line source rejected most of the scattered photons generated inside an object, and that if we collimated emitted photons at the source side, almost all the scattered photons could be rejected at the collimator on the detector side.
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