Evaluation of a scatter correction technique for single photon transmission measurements in PET by means of Monte Carlo simulations

Purpose: Single photon transmission (SPT) measurements offer a new approach for the determination of attenuation correction factors (ACF) in PET. A ma jor drawback of this method is the high fraction of scattered photons in th e transmission sinogram resulting in a marked underestimation of the ACFs. It was, therefore, the aim of the present work, to evaluate a scatter corre ction algorithm proposed by C. Watson by means of Monte Carlo simulations. Methods: SPT measurements with a Cs-137 point source were simulated for a w hole-body PET scanner (ECAT EXACT HR+) in both the 2D and 3D mode. To exami ne the scatter fraction (SF) in the transmission date, the detected photons were classified as unscattered or scattered. The simulated data were used to determine (i) the spatial distribution of the SFs, (ii) on ACF sinogram from all detected events (ACF(tat)) and (iii) from the unscattered events o nly (ACF(unscattered)), and (iv) on ACF(cor) = (ACF(tot))(]+K) sinogrom cor rected according to the Watson algorithm. In addition, density images were reconstructed in order to quantitatively evaluate linear attenuation coeffi cients. Results: A high correlation was found between the SF and the ACF(to t) sinogroms. For the cylinder and the EEC phantom, similar correction fact ors kappa were estimated. The determined values resulted in on accurate sca tter correction in both the 2D and 3D mode. Conclusions: The algorithm prop osed by Watson allows on accurate correction of scattered radiation in SPT measurements. The correction factor kappa can by determined experimentally using simple phantoms and then applied to more complex objects. SPT measure ments should be performed in the 3D mode, in order to increase the total nu mb of counts and/or to reduce the measurement time.