An investigation of factors affecting detector and geometric correction in normalization of 3-D PET data

Normalization in three-dimensional (3-D) positron emission tomography (PET) comprises two aspects: correction for differential detector response and correction for geometric effects. Comparison of rotating rod source and uniform cylinder data suggests that the position of the source used to correct for sensitivity should be similar to that of the emission data. A plane source method has been devised that uses a moving line source that traverses the transaxial field-of-view, emulating a plane source, but without the problems associated with using a conventional plane source in 3-D (uniformity, scatter, cost, etc.). This device has been used to record high count density acquisitions for direct normalization of emission data and also to examine geometric effects with increasing azimuthal (φ) and polar (θ) angles in the 3 D data set. The data have confirmed observations of two distinct geometric patterns seen previously in two-dimensional PET: an overall transaxial sensitivity profile that decreases toward the center of the projection and a crystal interference profile that changes with position in the block. Correction for the first geometric component removes a low-sensitivity “hole” in 3-D PET reconstructions, and correction for the second component removes “ring” artifacts. The direct normalization approach produces an artifact along the central axis of the scanner. A quantitative index of nonuniformity for 1-pixel-thick annular regions of interest showed a reduction from 60% nonuniformity with no corrections to less than 15% when the plant source data were used to directly normalize the emission data. The moving line source provides high quality data and may be an appropriate normalization device for 3-D PET