A P4 and a Focus 220 microPET small animal scanner (Siemens, Knoxville, TN) are provided by the laboratory. The scanners create images with measured spatial resolution of 1.72 mm (P4) and 1.44 mm (Focus 220) FWHM corresponding to 5 and 3 microliter volumetric resolution. The Focus 220 is used routinely for in vivo PET brain imaging of rhesus macaques at the Harlow Center for Biological Psychology. The P4 scanner, which is well suited for rat studies, is located at the Waisman Center. Head holders are provided along with electronic vitals monitoring and isoflurane anesthesia equipment for each study. Four rats can be scanned simultaneously, resulting in significant PET tracer cost savings.
In the P4 scanner, scintillator crystals are arranged in 32 rings of 336 elements each, which provides 7.8 cm axial and 19 cm transaxial field of view with 0.8% sensitivity (350-650 keV, 6 ns). The Focus 220 scanner has 48 rings of 504 elements each, providing 1.8% sensitivity with 7.6 cm axial and 19 cm transaxial field of view. Detector modules consisting of 8×8 arrays (P4) and 12×12 arrays (Focus 220) of 2.2 mm x 2.2 mm x 10 mm (P4) and 1.5 mm x 1.5 mm x 10 mm (Focus220) lutetium oxyorthosilicate (LSO) crystals are coupled to position sensitive photomultiplier tubes.
The LSO crystals have good light output and fast time response permitting 16.2% (P4) and 18.5% (Focus220) energy resolution and 3.2 ns timing resolution. Energy and timing windows are user selectable and typically set at 350 – 650 keV and 6 ns, respectively. Emission data are acquired in 3D list mode with the time and crystal pair position recorded for each coincidence event. A transmission scan is performed using a mechanism that carries a singles point source on a helical path. Lists are histogrammed into sinograms with post hoc dynamic framing. Sinograms are corrected for accidental coincidences and scaled to correct for radioactive decay and deadtime. Images are typically generated using filtered back projection (FBP) or ordered subset expectation maximization (OSEM) algorithms with 2D rebinning of the 3D sinograms. During reconstruction, histograms are normalized for detector sensitivity, and transmission-based attenuation and scatter corrections are applied.
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