Post-injection transmission scans in a PET camera operating without septa with simultaneous measurement of emission activity contamination

The authors report here on methods developed to reliably perform attenuation correction by post-injection transmission in a volume imaging PET scanner. The method directly measures the emission contamination during the transmission study, using a virtual transmission source position offset by 20 degree from the actual 68Ge transmission source. Events are recorded only if they meet a co-linearity requirement with either the real or virtual source position. The simultaneous measurements of the emission contamination and transmission data remove the need for complex corrections to the emission data and are not subject to activity redistributions between emission and transmission scans performed sequentially. Correction is necessary, however, for the extra deadtime that varies with the amount of emission activity in the FOV. The extra deadtime is determined from a lookup table of deadtime as a function of detector countrates, which are recorded during the study. The lookup table is based upon phantom measurements. Using patient and phantom data, with both pre- and post-injection transmission measurements, this method is shown to be reliable for attenuation correction in the body. In addition, it enables the calculation of Standardized Uptake Values for analyzing tumor activity