Two X-ray phase-contrast imaging techniques are compared in a quantitative way for future mammographic applications: Diffraction Enhanced Imaging (DEI) and Propagation. The first uses an analyzer crystal after the sample acting as an angular filter for X-rays refracted by the sample. The latter simply uses the propagation (Fresnel diffraction) of the monochromatic and partially coherent X-ray beam over large distances.
Experiments to compare both modalities have been performed at the Topography Beamline of the European Synchrotron Radiation Facility. The respective set-ups and experimental parameters are described in detail.
Depending on the object properties, the two techniques present a difference in area contrast and edge visibility. DEI shows an enhancement of area contrast for positions of the crystal corresponding to the tails of its rocking curve (RC) and a similar increase but inverted is also visible at the peak of its RC. At the tails, the contrast is mainly produced by ultra small angle scattering, at the peak, it is due to absorption and scatter rejection by the analyzer. At the flanks, it may disappear when attenuation and scattering effects compensate each other. However, an enhancement of the object edges is clearly noticeable, which mainly corresponds to the refracted part.
Propagation reveals an improvement of the edge visibility with the distance and shows negligible area contrast for non-absorbing, large structures.
Diffraction Enhanced Imaging (DEI) can significantly improve the expressiveness of mammography radiographs. Whereas the contrast in conventional radiographs is based on small X-ray absorption differences of tissues, the contrast mechanism of DEI is, in addition, partially related to the differences in X-ray refraction properties. DEI has been successfully applied to in-vitro mammography studies where little absorption tissue differentiation is present. In this paper we will present work on high-energy DEI mammography, which has been carried out by utilizing a tunable monochromatic X-ray beam. Since the refraction characteristics of soft tissues are much less energy dependent than absorption, the use of high energy X-rays is favoured. They can be employed in mammographic imaging without reducing the image contrast, while getting the benefit of reduced dose since the X-ray absorption falls off considerably. In-vitro images of an American College of Radiology (ACR) mammographic phantom using monochromatic X-rays through 50 keV have been obtained with a digital detector. High-energy mammography has been successfully performed at a significantly lower dose than that usually applied in clinical mammography without important contrast loss.
The display of low-contrast structures and fine microcalcifications is essential for the early diagnosis of breast cancer. In order to achieve a high image quality level with a minimum amount of radiation delivered to the patient, the use of different spectra (Mo or Rh anode and filters) was introduced. The European Synchrotron Radiation Facility is able to produce a monochromatic beam with a high photon flux. It is thus a powerful tool to study the effect of beam energy on image quality and dose in mammography. Our image quality assessment is based on the calculation of the size of the smallest microcalcification detectable on a radiograph, derived from the statistical decision theory. The mean glandular dose is simultaneously measured. Compared with conventional mammography units, the monochromaticity of synchrotron beams improves contrast and the use of a slit instead of an anti-scatter grid leads to a higher primary beam transmission. The relative contribution of these two effects on image quality and dose is discussed.
The European Synchrotron Radiation Facility Medical Research Beamline is now fully operational. One of the primary programs is the development of dual-energy transvenous coronary angiography for in vivo human research protocols. Previous work at this and other synchrotrons has been entirely devoted to the use of the dual-energy digital subtraction technique at the iodine k-absorption edge at 33.17 keV. The images are recorded in a line scan mode following venous injection of the contrast agent. Considerations of the patient dose, the dilution of the contrast agent in the pulmonary system and the arteries overlying the filled ventricles have limited the image quality. The ESRF facility was designed to allow dual- energy imaging at higher energies, for example at the gadolinium k-absorption edge at 50.24 keV. The advantages have been theoretically known for many years, with the higher energy promising higher image quality with less radiation dose. During the commissioning phase of the ESRF angiography program, the opportunity presented itself to image adult pigs in vivo with Gd contrast agent. This paper presents some initial results of the image quality in the Gd studies in comparison with iodine contrast agent studies, also carried out in adult pigs at the ESRF.