MicroPET II is a newly developed PET (positron emission tomography) scanner designed for high-resolution imaging of small animals. It consists of 17 640 LSO crystals each measuring 0.975 × 0.975 × 12.5 mm 3 , which are arranged in 42 contiguous rings, with 420 crystals per ring. The scanner has an axial field of view (FOV) of 4.9 cm and a transaxial FOV of 8.5 cm. The purpose of this study was to carefully evaluate the performance of the system and to optimize settings for in vivo mouse and rat imaging studies. The volumetric image resolution was found to depend strongly on the reconstruction algorithm employed and averaged 1.1 mm (1.4 µl) across the central 3 cm of the transaxial FOV when using a statistical reconstruction algorithm with accurate system modelling. The sensitivity, scatter fraction and noise-equivalent count (NEC) rate for mouse-and rat-sized phantoms were measured for different energy and timing windows. Mouse imaging was optimized with a wide open energy window (150-750 keV) and a 10 ns timing window, leading to a sensitivity of 3.3% at the centre of the FOV and a peak NEC rate of 235 000 cps for a total activity of 80 MBq (2.2 mCi) in the phantom. Rat imaging, due to the higher scatter fraction, and the activity that lies outside of the field of view, achieved a maximum NEC rate of 24 600 cps for a total activity of 80 MBq (2.2 mCi) in the phantom, with an energy window of 250-750 keV and a 6 ns timing window. The sensitivity at the centre of the FOV for these settings is 2.1%. This work demonstrates that different scanner settings are necessary to optimize the NEC count rate for different-sized animals and different injected doses. Finally, phantom and in vivo animal studies are presented to demonstrate the capabilities of microPET II for small-animal imaging studies.

The development of dedicated small animal PET (positron emission tomography) scanners has led to significantly higher spatial resolution and comparable sensitivity to clinical scanners. However, it is not clear whether we are approaching the fundamental limit of spatial resolution. This work aims to understand what is currently limiting spatial resolution during data formation and collection and how to apply that knowledge to obtain the best possible resolution for small animal PET without sacrificing sensitivity. Monte Carlo simulations were performed of the interactions of a 511 keV photon in a variety of detector materials to evaluate the modulation transfer function of the materials. Positron range, non-colinearity and pixel size were modelled to determine the contribution of additional components of data formation and collection on the complete modulation transfer function of a PET system. These simulations are shown to predict the intrinsic detector resolution of current high resolution systems very well. They also show that current detectors are not limited by inter-crystal scatter. An intrinsic resolution of 0.5 mm can be achieved, but would require a detector with a pixel size of around 250 µm that can be read out unambiguously. It is shown that a range of different detector materials, both scintillators and semiconductors, can be used in these high-resolution detectors. While this design relies on thin (∼3 mm) pieces of material, stacks of the material are shown to simultaneously provide spatial resolution near 0.5 mm and 60% efficiency. This work has shown that detectors with significantly better resolution and sensitivity can be developed for small animal PET applications.

Excellent spatial resolution is a requirement for preclinical PET imaging. In order to achieve spatial resolution of significantly better than one millimeter, an appealing possibility is to employ direct detector materials, such as cadmium telluride (CdTe). Prototype thin orthogonal strip detectors have been developed for testing. They have dimensions of 20 mm by 20 mm and are 0.5 mm thick, and have strips of 0.5 mm pitch on one side and 2.5 mm pitch on the other. Results are presented for the energy resolution (3% at 511 keV), intrinsic position resolution (equal to the 0.5 mm strip pitch), and timing resolution (3 ns FWHM in coincidence with an LSO detector, 8 ns FWHM for coincidence of two CdTe detectors) of the detectors. A PET scanner design is proposed using blocks made of the CdTe strip detectors, oriented in the blocks with their thin edges toward the center of the scanner. Simulation results suggest that this scanner, using a threshold of 250 keV, would have a sensitivity of 3.4% for a point source at its center.Index Terms-Cadmium telluride (CdTe), positron emission tomography (PET), small animal imaging.

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