COLLOQUIUM

The grating based phase-contrast X-ray imaging technique is a novel X-ray imaging modality that might revolutionize medical X-ray imaging. The method delivers - besides the conventional attenuation image - two images with different information about the object: the differential phase image and the dark-field image. The differential phase image is sensitive shows phase-shifts of the X-ray waves imposed by object boundaries between regions with differing electron densities. Therefore, it provides improved soft tissue contrast. The dark-field image shows averaged structural information about the object on a micrometer scale, which is smaller than the system resolution. The dark-field signal measures the roughness of the X-ray wave front passing the object.Grating based phase-contrast imaging might be applicable especially in mammography, because of the absence of strongly absorbing structures, low photon energies and the small detector pixels used in current systems. In our phase-contrast measurements on dissected breast tissue, we have found a significant dark-field signal in some mamma carcinoma at typical dose levels. Some tumors that are invisible in attenuation based imaging, appear clearly in the dark-field image. The signal is caused by distributions of very small grains (micrometer scale) of hydroxylapatite. The grains are too small to be resolved individually and the local mass density of these micro-calcifications is so small, that they do not lead to significant attenuation above background.In the first part of this talk, the principle of grating-based phase-contrast imaging is explained in detail. Phase-contrast and dark-field images of phantoms and biological objects are discussed. The second part of this talk focuses on the application of the method in mammography. The origin of the observed dark-field signals is explained and challenges for practical integration of the method are discussed.