Postinjection attenuation correction using singles transmission on a positron tomograph without interplane septa

Singles transmission is ideally suited for attenuation correction on PET scanners without interplane septa, producing low dead time and no randoms. However, sinogram windowing can not be used in singles mode to reject scatter or the emission background activity which is present after isotope injection. The authors propose that a simple subtraction of the singles emission background from the singles transmission scan will produce an accurate attenuation correction. The emission background is measured by performing an additional singles mode scan, but without actually exposing the /sup 137/Cs point source. Phantom studies with uniform cylinders show that the measured attenuation coefficients are restored to the true values after the emission background is subtracted. The effect on noise equivalent count rate is analogous to randoms subtraction because the singles emission background is relatively uniform throughout the field of view. The time required to perform a postinjection transmission scan is increased over a preinjection scan because the singles emission background must be measured separately.