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Investigation of emission contamination in the transmission image of a dual modality computed mammotomography system

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.