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

Abstract

We report here on methods developed to reliably perform attenuation correction by post-injection transmissicln 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' 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 vanes 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 recored 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 Standardised Uptake Values for analyzing tumor activity.

Attenuation effects in gamma-camera coincidence imaging

“…This can be achieved by transmission scans containing significantly, but not orders of magnitude, more counts than in typical emission data. Such transmission data should be attainable in a reasonable amount of time, particularly in singles mode, perhaps using a source that emits at an energy sufficiently above 51 1 keV so that post injection transmission scans are possible [7,8]. Additionally, artificial removal of noise via image segmentation may be a useful approach [9].…”