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A dual modality SPECT/CT computed mammotomography (CmT) system for dedicated functional/structural breast
imaging is under development. In simultaneous, dual-modality imaging, contamination of the transmission (x-ray) image
by emission photons from the uncompressed, pendant breast and torso is an important consideration in the design of
hybrid imaging hardware. The lack of a collimator on the transmission image detector implies increased geometric
efficiency of primary and scattered emission photons from the breast and neighboring torso region that potentially
increase transmission image noise. This study investigates the nature and extent of this cross contamination. Projection
and tomographic x-ray images are obtained with and without emission activity in a realistic anthropomorphic torso and
various breast phantoms, and also with and without lead shielding on the torso for a variety of x-ray exposure times.
Results for emission-source contamination of transmission images are quantified in terms of a mean and standard
deviation of regions of interest. There was an observed trend of increased contamination with increasing emission
radioactivity in the projection images when the x-ray detector was located immediately beneath the torso phantom, but
no discernible effect when the detector was lateral to (and beneath) the torso. Torso shielding mitigated this
contamination somewhat. Indeed, in reconstructed CmT data, there was both a decrease in SNR and concomitant
decrease in mean attenuation coefficient with increasing emission radioactivity contamination. These results are
consistent with the expected increased noise due to a uniform emission irradiation of the detector and hence the resulting
apparent increase in detected x-ray transmission events (which yield a lower reconstructed attenuation coefficient value).
Despite the emission contamination in both projection and reconstructed images, the contamination is uncorrelated, and
indeed no reconstruction artifacts were observed under the various measured conditions. This indicates that a simple
contamination correction may be possible to the projection data prior to reconstruction.
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