Singles transmission scans performed post-injection for quantitative whole body FDG-PET

Post-injection singles transmission scanning has been implemented in the septumless PENN PET 240H scanner (prototype of the GE QUEST). The method uses a 6 mCi point transmission source of ¹³⁷Cs at the axial center and 37 cm off transaxial center of the camera field of view. Singles transmission scans of 1.8 minutes per bed axial position provide similar scan count densities to 15 minute coincidence transmission scans with a 0.5 mCi ⁶⁸Ge rod transmission source. Scatter and emission contamination suppression are achieved by applying a narrow 662 keV transmission photopeak energy window. The residual 511 keV emission contamination constitutes a background of uniform spatial distribution. Accurate and low noise attenuation correction is achieved by segmenting the singles transmission images into lung and soft tissue volumes. Established 511 keV gamma ray attenuation coefficients are then applied and these images are forward projected for attenuation correction. Expectation maximisation or OS-EM reconstruction of the transmission and emission images is used to improve image quality. Both the segmentation and OS-EM reconstruction maintain quantitative accuracy in the fully corrected emission images compared to measured coincidence transmission correction. Thus, a clinical protocol involving 40 minutes of emission scans followed by 20 minutes of singles transmission scans allow the 60 cm of the human torso to be fully scanned within 60 minutes. These quantitative whole body FDG PET images may then be used for tumor grading and assessment of tumor response to treatment