Euromedim 2006 :1st European Conference on Molecular Imaging Technology

ANALYSIS OF DIFFERENT DETECTOR AND ELECTRONICS DEFECTS ON F18-FDG IMAGES

11 May 2006, 14:00

1h

Palais du Pharo, Marseille

poster S5-S6 medecine Poster session : Imaging systems, Molecular Imaging

Speaker

Dr Felicia Zito (Nuclear Medicine Department, Fondazione Ospedale Maggiore Policlinico, Mangiagalli e Regina Elena, Milan, Italy)

Description

Objectives Aim of the present work is to describe and quantitatively assess on reconstructed FDG-PET images, the effects of different levels of detector failures for the HR+, a full ring block designed scanner. Methods All the studies were performed in 2D mode (span 15) with the ECAT EXACT HR+ PET scanner (CTI-Siemens, Knoxville-USA). By using data acquired with PET scanner fully calibrated and perfectly functioning different detector defects were simulated: 1) one block detector with a 40% (B1 40%) and 100% (B1 100%) loss of efficiency; 2) one analog board involving 2 contiguous axial blocks, with 40% (AB 40%) and 100% (AB100%) loss of efficiency; 3) two opposite blocks in the same plane with 40% and 100% loss of efficiency (B1 40%B2 100%) and (B1 100%B2 100%). Different radioactivity distributions were also considered: -the 20 cm diameter uniform phantom containing a solid solution of 68Ge; scanned at high counting statistics and reconstructed with FBP ramp filter, -the anthropomorphic Alderson phantom filled with different 18F radioactive concentration and containing 4 spherical lesions acquired at clinical counting statistics and reconstructed with OSEM 2 iterations and 8 subsets (2i/8s) as routinely used in whole-body PET FDG studies and (4i/16s). The analysis of the faulty effects was performed taking as reference the original images without defects. Images were evaluated qualitatively, by visual inspection of expert observers, and quantitatively by drawing ROIs to assess % difference of SUV (ΔSUV%.) Results Visually and quantitatively, in spite of evident defect on sinograms (cold diagonal bands), reconstructed images are affected by slight artefacts when 1 block or 2 axial contiguous blocks even with 100% efficiency loss is considered. Artefacts appear as cold stripes following the Fan angle covered by the defective detector. More important variations are observed when two opposite blocks in the same plane, both with 100% inefficiency, are considered. In this case, depending on the geometrical position of the two blocks, a well structured artefact along the direct coincidence line is visible and a peak-valley count distribution affects the image. To this case |ΔSUV% | maximum values of 5.4% and 15.1% are respectively measured with 2i/8s and 4i/16s OSEM. Conclusions This study showed that for HR+ scanner, even a 100% loss of efficiency of a single block or an analog board (2 axial blocks) particular evident on sinograms does not substantially affect qualitatively and quantitatively FDG whole-body reconstructed images (SUV % variations less than 4%). When two opposite blocks in the same plane have 100% loss of efficiency larger inaccuracy should be expected and in this case stopping clinical activity is claimed. However, all defective block considerations referred as to be not relevant for scanner clinical use must be utilised to avoid practical problems in temporary patient management and not to change maintenance scheduler or lower the quality standards. Effects of malfunctioning blocks and electronics on 3D acquisition images is under evaluation.