Quantifying the effects of defective block detectors in a 3D whole body pet camera

A comparison study was conducted in order to assess the image quality of a clinical 3D whole body PET scanner (Siemens Biograph 16 HiRez) in the condition of failure of one or more block detectors. A data set was acquired using the NEMA image quality phantom when all detectors were functioning normally. The ratio of the activity in the four smallest spheres to the background region was 8.27:1. Defective blocks were then simulated by zeroing the appropriate lines of response in the sinograms. Eight different combinations of defects are considered ranging from the case of no defect up to a complete bucket failure (12 blocks). Images were reconstructed with both OSEM and FBP using the manufacturer´s software. The images were examined both qualitatively and according to the NEMA NU 2-2001 protocol for contrast, variability, and residual error. The results show that despite very visible artefacts appearing in the images the NEMA contrast analysis was very similar for all defect cases. The variability increased for all cases with simulated defective blocks. The contrast results demonstrate that a method of qualitatively evaluating the images is required in addition to the quantitative analysis. The preliminary data examined in this study suggest that data acquired when there are two defective blocks in the system might still produce clinically useable images.