Monochromatic x-rays in digital mammography.

Rationale and Background: The emission spectrum of an x-ray tube is determined by the anode and filter materials as well as by the high voltage being used. For mammography, typical anode materials are molybdenum (Mo), rhodium (Rh), and tungsten (W); molybdenum, rhodium, and aluminum are favored for filters. Mammography is a soft tissue imaging modality demanding a high spatial resolution as well as a high detector sensitivity. Low-energy photons are only absorbed in tissue and have no contribution to the image; nevertheless, they increase the dose. High-energy photons mostly penetrate soft tissue and generate a background noise as a result of strong scattering that deteriorates the image quality. For mammography, the optimal energy window is in a range from 17 and 25 keV. From a theoretical perspective, one would favor monoenergetic x-rays (eg, the Mo-emission line at 17.5 keV). This article presents the realization of imaging with monochromatic x-rays using a diagnostic mammography unit.
Methods: Basically, a monochromatic module was added to a conventional mammographic system. The monochromatic module can be mounted at the end of the x-ray tube and it consists of a curved HOPG (highly oriented pyrolytic graphite) crystal and a slit collimator. For image generation, the object is moved through the fan-shaped monochromatic radiation field. In addition to the conventional polychromatic 2-dimensional case, the polychromatic irradiation was also able to be performed under similar conditions. For image acquisition, image plates or a linear array detector were used. Exposure doses were measured for both poly- and monochromatic radiation. The initial evaluation of the system performance was carried out by imaging a contrast-detail phantom and biologic specimens.
Results: The monochromatic x-ray beam has a size of approximately 35 mm x 200 mm in the object plane. The photon flux of the monochromatic x-rays is considerably lower than the photon flux of the polychromatic x-rays but adequate for initial studies of phantoms, biologic tissue, or small animals. The comparison of the results obtained with the monochromatic and polychromatic imaging modalities reveal a conspicuous increase of image contrast in the monochromatic case.
Conclusion: The results suggest that the experimental setup for monochromatic excitation shows clear potentials for improvements of the image in comparison to the conventional polychromatic case.