THE DESIGN AND PHYSICAL CHARACTERISTICS OF A SMALL ANIMAL POSITRON EMISSION TOMOGRAPH

A small diameter positron emission tomograph, designed specifically fo r small animal studies, was constructed from existing, commercially av ailable, bismuth germanate (EGO) detectors and electronics. The scanne r consists of 16 EGO detector blocks arranged to give a tomograph with a diameter of 115 mm and an axial field of view (FOV) of 50 mm. Each block is cut to produce eight (axial) by seven (radial) individual det ector elements. The absence of interplane septa enables the acquisitio n of 3D data sets consisting of 64 sinograms. A 2D, data set of 15 sin ograms, consisting of eight direct and seven adjacent cross planes, ca n be extracted from the 3D data set. Images are reconstructed from the 20 sinograms using a conventional filtered backprojection algorithm. Two methods of normalization were investigated, based on either a rota ting Ge-68 rod source, or a uniform Ge-68 plane source, with a uniform cylindrical F-18 phantom. Attenuation of the emitted photons was esti mated using a rotating Ge-68 rod source. The transaxial resolution of the tomograph was measured as 2.3 mm full width at half maximum (FWHM) and 5.6 mm full width at tenth maximum (FWTM) at the centre of the FO V, degrading to 6.6 mm (radial) and 4.4 mm (tangential) FWHM and 10.4 mm (radial) and 14.4 mm (tangential) FWTM at 40.0 mm from the centre o f the Fov. The axial slice width was 4.3 mm FWHM, 10.3 mm FWTM at the centre of the transaxial field of view and 4.4 mm FWHM, 10.6 mm FWTM a t 20.0 mm from the centre of the FOV. A scatter fraction of 31.0% was measured at 250-850 keV, for an F-18 line source centred in a 60 mm di ameter, water-filled phantom, reducing to 20.4% and 13.8% as the lower energy discrimination was increased to 380 keV and 450 keV, respectiv ely. The count rate performance was measured using a noise equivalent count rate method, and the linearity of the dead time correction was c onfirmed over the count rates encountered during routine scanning. In 20 mode, the absolute sensitivity of the tomograph was measured as 994 8 counts s(-1) MBq(-1) at 250-850 keV, 8284 counts s(-1) MBq(-1) at 38 0-850 keV and 6280 counts s(-1) MBq(-1) at 450-850 keV.