Brain PET Studies with a High Sensitivity Fully Three-Dimensional Tomograph

This chapter reports on a fully three-dimensional (3D) large axial field of view bismuth germanate positron tomograph. The aim was to develop a high sensitivity tomograph for both brain and whole body studies. The system operates exclusively in 3D mode and incorporates a number of innovative features to optimize and enhance performance for brain studies that were not necessary with previous 2D or combined 2D/3D systems. The absolute sensitivity of this tomograph is almost 6% for coincidences arising from within the field of view. Resolution is 4.5 mm full-width-at-half-maximum (FWHM) isotropic. Transmission scanning is accomplished with a 150-MBq 137 Cs point source. The axial field of view for single photons measures approximately 80 cm beyond the ends on the tomograph. To shield against these unwanted single photons in brain studies, detachable lead shields have been employed, which reduce the subject aperture from 65 to 35 cm. When using the extra shields, in the early phase of 11 C-labeled neuroreceptor ligand studies, a 70% reduction in randoms rates and a corresponding increase in true event rates due to lower detector dead time have been observed. The sensitivity of this tomograph has reduced the optimal administered dose to 200 MBq for individual [ 15 O]H 2 O rCBF “activation” studies. For 18 F-labeled tracers, the benefit is improved image quality, whereas for 11 C-labeled radiotracers, useful scanning periods have been extended up to 120 min and profit from the higher sensitivity for radiotracers with low cerebral uptake.