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In human emission tomography, combined PET/CT and SPECT/CT cameras provide accurate attenuation maps for sophisticated scatter and attenuation corrections. Having proven their potential, these scanners are being adapted for small animal imaging using similar correction approaches. However, attenuation and scatter effects in small animal imaging are substantially less than in human imaging. Hence, the value of sophisticated corrections is not obvious for small animal imaging considering the additional cost and complexity of these methods. In this study, using GATE Monte Carlo package, we simulated the Inveon small animal SPECT (single pinhole collimator) scanner to find the scatter fractions of various sizes of the NEMA-mouse (diameter: 2-5.5 cm , length: 7 cm), NEMA-rat (diameter: 3-5.5 cm, length: 15 cm) and MOBY (diameter: 2.1-5.5 cm, length: 3.5-9.1 cm) phantoms. The simulations were performed for three radionuclides commonly used in small animal SPECT studies:99mTc (140 keV), 111In (171 keV 90% and 245 keV 94%) and 125I (effective 27.5 keV). For the MOBY phantoms, the total Compton scatter fractions ranged (over the range of phantom sizes) from 4-10% for 99mTc (126-154 keV), 7-16% for 111In (154-188 keV), 3-7% for 111In (220-270 keV) and 17-30% for 125I (15-45 keV) including the scatter contributions from the tungsten collimator, lead shield and air (inside and outside the camera heads). For the NEMA-rat phantoms, the scatter fractions ranged from 10-15% (99mTc), 17-23% 111In: 154-188 keV), 8-12% (111In: 220-270 keV) and 32-40% (125I). Our results suggest that energy window methods based on solely emission data are sufficient for all mouse and most rat studies for 99mTc and 111In. However, more sophisticated methods may be needed for 125I.

In human emission tomography, an additional transmission scan (x-ray CT or external gamma-source) is often required to obtain accurate attenuation maps for attenuation correction (AC) and scatter correction (SC). These transmission-based correction methods have been translated to small animal imaging although the impact of photon interactions on the mouse/rat-reconstructed images is substantially less than that in human imaging. Considering the additional complexity in design and cost of these systems, the necessity of these correction methods is questionable for small animal emission tomography. In this study, we evaluate the requirement of these corrections for small animal positron emission tomography (PET) through Monte Carlo simulations of the Inveon PET scanner using various sizes of MOBY voxelized phantoms. The 3D sinogram data obtained from simulations were reconstructed in 6 different conditions: Accurate AC+SC, simple (water) AC+SC, accurate AC only, simple AC only, SC only and no correction (NC). Mean error% for 8 different ROIs and 6 different MOBY sizes were obtained against the accurate scatter and attenuation corrections (first on the list). In addition to simulations, real mouse data obtained from an Inveon PET scanner were analyzed using similar methods. Results from both simulation and real mouse data showed that attenuation correction based on solely emission data should be sufficient for imaging animals smaller than 4 cm diameter. For larger sizes, a scatter correction employing an additional transmission scan can also be included depending on the objective of the study.

It is desirable to use mean population attenuation coefficients to reduce the statistical fluctuations in measured attenuation maps. In addition, the use of mean attenuation coefficients is also expected to facilitate algorithmic approaches to attenuation correction. Because of the large variation in lung attenuation coefficients both within and among individuals, there is concern about the errors using mean attenuation might create. To test the magnitude of this problem, myocardial metabolic studies were simulated. Attenuated projections were generated from 20 realistic emission and transmission distributions spanning the heart volume. The projections were corrected with both the actual attenuation map and with a mean attenuation map. Quantitative comparisons of the reconstructed images demonstrate no significant differences in the myocardial distributions. However, errors in the lung field exceeded 300% in some regions.

Routine calibration of on-line arterial blood detectors used primarily for PET blood flow studies while necessary is time consuming and if performed frequently, exposes staff to unnecessary levels of radiation. A method has been developed and implemented to account for changes in detector sensitivity over time and for variations in the diameters of tubes used to withdraw blood. The method is most applicable to positron detectors but can also be used for gamma systems. Errors are reduced from +/-5% to less than 1%.

A prototype SPECT (single photon emission computed tomography) system was constructed to demonstrate the performances of a novel multislice design for high-resolution imaging. The system consists of a stationary cylindrical detector composed of multiple bars of NaI(Tl) scintillator and a rotating collimator. The collimator consists of an assembly of multiholed modules arranged with a four quadrant symmetry. The collimator modules are individually focused for discrete sampling in the transverse plane but parallel-collimated for continuous sampling in the axial direction. Fan beam sampling for each detector bar in the transverse plane is accomplished after a 360 degrees rotation of the collimator. An array of photomultiplier tubes coupled to the outside of the detector cylinder identifies the bar and the axial position along the bar for each detected event. The current performance of this prototype system was evaluated. The useful imaging volume has a circular cross section (20-cm diameter) with a 5-cm axial field-of-view. The average local energy resolution is 11.5% and the system sensitivity is 800 cps/ mu Ci/cm/sup 3//cm. The spatial resolution in the reconstructed plane is 8 mm FWHM (full width at half maximum). The tomographic images of resolution and anthropomorphic phantoms demonstrate high image quality. >