Dedicated mammography: Imaging with monochromatic X-rays and a clinical mammography unit

The aim of the current studies is to implement monochromatic X-rays on a conventional diagnostic mammography unit for the improvement of image quality and/or the reduction of radiation dose. The experimental setup consists of a conventional mammography unit with monochromator module mounted at the exit of the X-ray tube and a digital linear array detector instead of the screen-film system for image acquisition. The heart of the monochromator module is a curved HOPG (highly oriented pyrolytic graphite) crystal and a slit collimator. The monochromator is adjusted to 17.5 keV (Kα-emission line of the Mo-anode). For image acquisition the object moves through the X-ray beam, which is fixed in space and directed on the detector (12 bit sensitivity, operated at 54 μm resolution). The final images were reconstructed from the measured images, a reference image and the detector background. Lead calibration grids, a contrast detail phantom and gadolinium (Gd) or iodine (I) containing contrast media were investigated. The exposure dose was measured. The results reveal that only the 17.5 keV Kα-emission line of the Mo-anode passes the monochromator while the Kβ-emission line and the bremsstrahlung spectrum are rejected. The photon flux is considerably lower for monochromatic than for polychromatic X-rays under identical geometric conditions but sufficient to investigate 5 cm thick objects. The contrast response functions (CRF) for polychromatic and monochromatic X-rays are very similar while contrast values are in favor of the monochromatic illumination. In comparison to polychromatic X-rays, the 17.5 monochromatic X-rays lead to higher contrast for Gd or I containing contrast media. Monochromatic imaging has a high potential in diagnostic mammography: image contrast is improved and radiation dose can be reduced in comparison to polychromatic imaging. For clinical installation, high-power X-ray tubes are mandatory and the imaging procedure should be adapted to the slot-scan technique