Attenuation corrections for coincidence studies by low energy stationary line source

A Gd-153 line source inside a stationary lead cylinder with slit apertures is used for transmission attenuation corrections (TAC) of dual head gamma camera (DHGC) PET (coincidence) studies. The source holder is placed inside a collimator consisting of lead baffles oriented in transaxial direction. The source holder is positioned next to the DHGC crystal face at the edge of FOV to prevent geometrical truncation of the central part of an object. This assembly results in an off center fan beam geometry for multiple point sources. Source open/close positioning is achieved by source holder rotation with slit apertures directed towards opposite camera head, or collimator lead frame. TAC acquisition is performed in two energy windows: one for Gd-153 and one 25 keV higher for correction of downscattered 511 keV photons. A watershed segmentation algorithm is applied to OS-EM iterative reconstructed data to replace measured attenuation map by known for 511 keV attenuation factors for air, lungs, and soft tissue. This technique is under evaluation at several clinical sites. It is concluded that the Gd-153 TAC technique gives adequate special resolution, does not require mechanical scan, provides better contrast and requires less shielding than higher energy transmission sources. The obvious disadvantage of presence of downscattered 511 keV photons is overcome by scatter correction using the second energy window.