We are currently developing a monochromatic x-ray source for small animal tomographic imaging. This source consists
of a conventional cone beam microfocus x-ray tube with a tungsten target coupled to a filter that uses Bragg diffraction
to transmit only x-rays within a narrow energy range (~3 keV FWHM). A tissue-equivalent mouse phantom was used to
a) evaluate how clearly CT imaging using the quasi-monoenergetic beam is able to differentiate tissue types compared to
conventional polyenergetic CT, and b) to test the ability of the source and Bragg filter combination to perform dual
energy, iodine contrast enhanced imaging. Single slice CT scans of the phantom were obtained both with polyenergetic
(1.8 mm Al filtration) and quasi-monoenergetic beams. Region of interest analysis showed that pixel value variance was
signifcantly reduced in the quasi-monochromatic case compared to the polyenergetic case, suggesting a reduction in the
variance of the linear attenuation coefficients of the tissue equivalent materials due to the narrower energy spectrum. To
test dual energy iodine K-edge imaging, vials containing solutions with a range of iodine contrasts were added to the
phantom. Single-slice CT scans were obtained using spectra with maximum values at 30 and 35 keV, respectively.
Analysis of the resulting difference images (35 keV image - 30 keV image) shows that the magnitude of the difference
signal produced by iodine exceeds that of bone for iodine concentrations above ~20 mg/ml, and that of muscle and fat
tissues for iodine concentrations above ~5 mg/ml.
Pinhole imaging is a promising approach for high spatial resolution single gamma emission
imaging in situations when the required field of view (FOV) is small, as is the case for small
animal imaging. However, all pinhole collimators exhibit steep decrease in sensitivity with
increasing angle of incidence from the pinhole axis. This in turn degrades the reconstruction
images, and requires higher dose of radiotracer. We developed a novel pinhole SPECT system
for small animal imaging which uses two opposing and offset small cone-angle square
pinholes, each looking at half of the FOV. This design allows the pinholes to be placed closer to
the object and greatly increases detection efficiency and spatial resolution, while not requiring
larger size detectors. Iterative image reconstruction algorithms for this system have been developed. Preliminary experimental data have demonstrated marked improvement in contrast and spatial resolution.