Performance characteristics of an integrated small animal SPECT/CT unit

Our laboratory is evaluating a combined micro-SPECT/CT unit (CTI-Concorde) equipped with dual pixelated SPECT detectors each coupled to a square 3×3 array of Hamamatsu H8500 position sensitive photomultiplier tubes (PSPMTs). The CT component consists of a CCD X-ray detector and a micro-focus X-ray source (40 μm focal spot). SPECT detector properties investigated using Tc-99m, include system sensitivity, detection efficiencies, detector uniformity, energy resolution, spatial resolution, detector dead time, and collimator count rate performance. Pinhole tomographic In-111 and Tc-99m SPECT data was generated using a 3D-OSEM algorithm with geometrical misalignment corrections. In-111-DOTA-8-AOC-BBN(7-14)NH₂ and Tc-99m-HDP uptake profiles were evaluated in mice. Sensitivities and detection efficiencies were measured to be 95 (0.009%), 1012 (0.028%) and 1984 (0.198%) cps/MBq for the 0.5, 2 and 3 mm pinhole apertures, respectively. Intrinsic uniformity for the central field of view was 1.42% differential and 2.99% integral. Energy resolution (FWHM) at 140 keV was 14.3% ±1.5%. Tomographic spatial resolution of 1.2 mm was achieved with a hot-rod Tc-99m SPECT phantom. The pulse resolving time was 35.33 μsec for 960 μCi of Co-57. Count rate linearity performance was achieved up to 1 and 1.2 mCl for the 2 mm parallel and 2 mm pinhole collimators, respectively. Osteoblastic bone lesions 0.6 mm in diameter measured by CT were also detectable with SPECT Tc-99m-HDP imaging. In-111 SPECT studies using a mouse xenograft tumor model revealed tumor uptake which correlated with pharmacokinetic results and co-registration with CT data provided valuable molecular/anatomical information. The performance characteristics of this unit demonstrate that this unit is capable of high resolution SPECT/CT imaging. System sensitivity improvements either by employing multipinhole collimation and/or larger crystal elements will significantly enhance the performance of this SPECT instrument in pre-clinical imaging.