University of Helsinki Report Series in Physics Hu-p-d113 Diffraction-enhanced X-ray Imaging of in Vitro Breast Tumours

The motivation behind this thesis is illustrated by the fact that many breast cancers are missed at X-ray mammography, insufficient contrast being the most important reason. The present thesis considers the diffraction-enhanced imaging (DEI) technique for imaging in vitro breast tumours. DEI is one of the novel methods in which the change in X-ray wavefront phase or propagation direction at the boundaries and interfaces of the object is used for contrast formation. The purpose here was to evaluate the potential of the DEI technique for mammographic imaging. Other issues discussed include X-ray interactions with matter, radiation dose and generic aspects of breast diagnostics. The formation of refraction contrast was studied quantitatively, using well-characterised test objects and incoherent radiation from an X-ray tube. A new type of device for fine rotation of the analyser crystal was designed, constructed and calibrated. Detailed comparison was made between the histopathological sections of 18 excised human breast tumour specimens and radiographic images. Both planar and transaxial images were acquired with the DEI technique using coherent synchrotron radiation, and the same specimens were imaged with screen-film mammography and computed tomography (CT) units at a hospital. Refraction contrast in X-ray imaging was as much as 20 times larger than the absorption contrast. The fine-rotation device proved to be a very stable tool for examining refraction effects with nanoradian angular resolution. The typical morphologic features of lobular carcinoma invasion were evident in the medium-resolution DEI-CTs, while the changes were vague or ambiguous in the CTs. The visibility of microcalcifications and fine structural details was greatly enhanced in the high-resolution DEI images compared with the screen-film mammograms. Regarding the clinical applications, it is essential that refraction contrast prevails with incoherent radiation from the X-ray tube and that small X-ray beam deviations can be revealed with compact instrumentation. Development of compact monochromatic X-ray sources having sufficient flux to allow short exposure times for clinical purposes will remain a challenging task, and the latest developments in this field are discussed. DEI provides improved visibility of early signs of breast abnormalities, which is crucial for detecting tumours at early stages and small sizes, both being the most important prognostic indicators. In addition, lower radiation doses than those currently employed in mammography make DEI a promising method even for breast CT with clinically acceptable doses. However, the factors affecting image formation need to be examined further to yield direct correspondence between the contrast and structure of the object.