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We report on an approach toward the development of a high-resolution single photon emission computed tomography (SPECT) system to image the biodistribution of radiolabeled tracers such as Tc-99m and I-125 in unrestrained/unanesthetized mice. An infrared (IR)-based position tracking apparatus has been developed and integrated into a SPECT gantry. The tracking system is designed to measure the spatial position of a mouse's head at a rate of 10-15 frames per second with submillimeter accuracy. The high-resolution, gamma imaging detectors are based on pixellated NaI(TI) crystal scintillator arrays, position-sensitive photomultiplier tubes, and novel readout circuitry requiring fewer analog-digital converter (ADC) channels while retaining high spatial resolution. Two SPECT gamma camera detector heads based upon position-sensitive photomultiplier tubes have been built and installed onto the gantry. The IR landmark-based pose measurement and tracking system is under development to provide animal position data during a SPECT scan. The animal position and orientation data acquired by the tracking system will be used for motion correction during the tomographic image reconstruction. Index Terms--Motion tracking, multimodality imaging, small animal single photon emission computed tomography (SPECT).

A new x-ray computed tomography (CT) system is being developed at Oak Ridge National Laboratory to image laboratory mice for the purpose of rapid phenotype screening and identification. One implementation of this CT system allows simultaneous capture of several sets of sinogram data, each having a unique x-ray energy distribution. The goals of this paper are to (1) identify issues associated with the reconstruction of this energy-dependent data and (2) suggest preliminary approaches to address these issues. Due to varying numbers of photon counts within each set, both traditional (filtered backprojection, or FBP) and statistical (maximum likelihood, or ML) tomographic image reconstruction techniques have been applied to the energy-dependent sinogram data. Results of reconstructed images using both algorithms on sinogram data (high- and low-count) are presented. Also, tissue contrast within the energy-dependent images is compared to known x-ray attenuation coefficients of soft tissue (e.g. muscle, bone, and fat).

Two versions of a new high-resolution x-ray computed tomography system are being developed to screen mutagenized mice in the Oak Ridge National Laboratory Mammalian Genetics Research Facility. The first prototype employs a single-pixel cadmium zinc telluride detector with a pinhole collimator operating in pulse counting mode. The second version employs a phosphor screen/CCD detector operating in current mode. The major system hardware includes a low-energy x-ray tube, two linear translation stages and a rotational stage. For the single-pixel detector, image resolution is determined by the step size of the detector stage; preliminary images have been acquired at 100 [[micro]meter] and 250 [[micro]meter] resolutions. The resolution of the phosphor screen detector is determined by the modulation transfer function of the phosphor screen; images with resolutions approaching 50 [[micro]meter] have been acquired. The system performance with the two detectors is described and recent images are presented.

A novel method for measuring dead time in nuclear pulse processing circuitry has been developed using the autocorrelation measurement capability of the Nuclear Weapons Inspection System (NWIS). Initially developed for active neutron interrogation of nuclear weapons and other fissile assemblies, NWIS employs a custom gallium arsenide application specific integrated circuit and a new signature analysis software package to simultaneously acquire and display the autocorrelation and cross-correlation spectra of up to five detector/electronics systems. The system operates at clock frequencies up to 1 GHz, permitting the collection of timing pulses in bins as narrow as 1 ns. In normal operation NWIS uses well characterized detectors and constant fraction discriminators, but it may also be configured to accept pulses from any circuit and to use the autocorrelation spectrum to accurately determine dead-time. Unlike traditional dead-time assessment techniques that typically require multiple sources and an assumed dead-time model, NWIS provides single-measurement assessment of circuit dead time and does not require an assumed dead-time model or a calibrated high count-rate source.

We present performance results for a prototype PET detector module consisting of 64 LSO scintillatot crystals (3x3x20 mm) coupled on one end to a single photomultiplier tube and on the opposite end to a 64 pixel array of 3 mm square silicon photodiodes (typical pixel parameters are 5 pF capacitance, 300 pA dark current, and 73% quantum efficiency at 415 nm). The photomultiplier tube (PMT) provides an accurate timing pulse and energy threshold for all crystals in the module, the silicon photodiodes (PD) identify the crystal of interaction, the sum (PD+PMT) provides a total energy signal, and the PD/(PD+PMT) ratio determines the depth of interaction. With 32 of the channels instrumented, the detector module correctly identifies the crystal of interaction (where 'correct' includes the adjacent 4 crystals) 79[+ or -]4% of the time with high detection efficiency. The timing resolution for a single LSO detector module is 750 ps fwhm, while its pulse height resolution at 511 keV is 24[+ or -]3% fwhm. The depth of interaction (DOI) measurement resolution is 8[+ or -]1 mm fwhm.

MicroPET is a high resolution positron emission tomography (PET) scanner designed for imaging small laboratory animals. It consists of a ring of 30 position-sensitive scintillation detectors, each with an 8 x 8 array of small lutetium oxyorthosilicate (LSO) crystals coupled via optical fibers to a multi-channel photomultiplier tube. The detectors have an intrinsic resolution averaging 1.68 mm, an energy resolution between 15 and 25% and 2.4 ns timing resolution at 511 keV. The detector ring diameter of microPET is 17.2 cm with an imaging field of view of 112 mm transaxially by 18 mm axially. The scanner has no septa and operates exclusively in 3D mode. Reconstructed image resolution 1 cm from the center of the scanner is 2.0 mm and virtually isotropic, yielding a volume resolution of 8 [mm.sup.3]. For comparison, the volume resolution of state-of-the-art clinical PET systems is in the range of 50 - 75 [mm.sup.3]. Initial images of phantoms have been acquired and are reported. A computer controlled bed is under construction and will incorporate a small wobble motion to improve spatial sampling. This is projected to further enhance spatial resolution. MicroPET is the first PET scanner to incorporate the new scintillator LSO and to our knowledge is the highest resolution multi-ring PET scanner currently in existence.