The microPET Primate 4-ring system (P4) is an animal PET tomograph with a 7.8 cm axial extent, a 19 cm diameter transaxial field of view (FOV) and a 22 cm animal port. The system is composed of 168 detector modules, each with an 8 x 8 array of 2.2 x 2.2 x 10 mm3 lutetium oxyorthosilicate crystals, arranged as 32 crystal rings 26 cm in diameter. The detector crystals are coupled to a Hamamatsu R5900-C8 PS-PMT via a 10 cm long optical fibre bundle. The detectors have a timing resolution of 3.2 ns, an average energy resolution of 26%, and an average intrinsic spatial resolution of 1.75 mm. The system operates in 3D mode without inter-plane septa, acquiring data in list mode. The reconstructed image spatial resolution ranges from 1.8 mm at the centre to 3 mm at 4 cm radial offset. The tomograph has a peak system sensitivity of 2.25% at the centre of the FOV with a 250-750 keV energy window. The noise equivalent count rate peaks at 100-290 kcps for representative object sizes. Images from two phantoms and three different types of laboratory animal demonstrate the advantage of the P4 system over the original prototype microPET. including its threefold improvement in sensitivity and a large axial FOV sufficient to image an entire mouse in a single bed position.

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The 'EXACT3D' positron tomograph, which is now in routine clinical research use, was developed with the aim of achieving unprecedented sensitivity, high spatial and temporal resolution and simplicity of design using proven detector technology. It consists of six rings of standard detector blocks (CTI/Siemens EXACT HR+) with 4.39 mm x 4.05 mm x 30 mm elements, giving an axial field of view (FOV) of 23.4 cm. This extended FOV and the absence of interplane septa and retractable transmission rod sources has allowed greatly simplified gantry and detector cassette design. Operation in exclusive 3D mode requires an alternative to the conventional coincidence method for transmission scanning, and a single photon approach using a hydraulically driven 137Cs point source has been implemented. The tomograph has no other moving parts. A single time frame of data without any compression is very large (> 300 Mbyte) and two approaches are employed to overcome this difficulty: (a) adjacent sinograms can be summed automatically into different combinations and (b) listmode (event-by-event) acquisition has been instituted, which is both storage efficient (particularly for acquisition of sparse data sets) and maximizes temporal resolution. The high-speed I/O and computing hardware can maintain a sustained acquisition rate of about 4 million coincidence events per second. A disadvantage of the large axial FOV in 3D is the increased sensitivity to activity outside the coincidence FOV. However, this can be minimized by additional side shielding. The mean spatial resolution is 4.8 +/- 0.2 mm FWHM (transaxial, 1 cm off-axis) and 5.6 +/- 0.5 mm (axial, on-axis). Its absolute efficiency is 5.8% for a line source in air (just spanning the axial FOV) and 10% for a central point source (with thresholds of 350-650 keV). For a uniform 20 cm diameter cylinder, the efficiency is 69 kcps kBq(-1) ml(-1) (after subtraction of a scatter fraction of 42%). Sensitivity relative to the EXACT HR+ (with four rings of blocks) is 2.5 (3D) and 12 (2D) times respectively. The rate of random events in blood flow studies in the brain and body, using 15O-labelled water, can be controlled by limiting the administered dose and inserting additional side shielding.

A small diameter positron emission tomography, designed specifically for small animal studies, was constructed from existing, commercially available, bismuth germanate (BGO) detectors and electronics. The scanner consists of 16 BGO 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 detector elements. The absence of interplane septa enables the acquisition of 3D data sets consisting of 64 sinograms. A 2D data set of 15 sinograms, consisting of eight direct and seven adjacent cross planes, can be extracted from the 3D data set. Images are reconstructed from the 2D sinograms using a conventional filtered backprojection algorithm. Two methods of normalization were investigated, based on either a rotating 68Ge rod source, or a uniform 68Ge plane source, with a uniform cylindrical 18F phantom. Attenuation of the emitted photons was estimated using a rotating 68Ge 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 FOV, 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 of 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 at 20.0 mm from the centre of the FOV. A scatter fraction of 31.0% was measured at 250-850 keV, for an 18F line source centred in a 60 mm diameter, water-filled phantom, reducing to 20.4% and 13.8% as the lower energy discrimination was increased to 380 keV and 450 keV, respectively. The count rate performance was measured using a noise equivalent count rate method, and the linearity of the dead time correction was confirmed over the count rates encountered during routine scanning. In 2D mode, the absolute sensitivity of the tomograph was measured as 9948 counts s-1 MBq-1 at 250-850 keV, 8284 counts s-1 MBq-1 at 380-850 keV and 6280 counts s-1 MBq-1 at 450-850 keV.