A novel x-ray source, providing dichromatic beams for the application of dual-energy radiography, has been assembled and studied. The system works via Bragg diffraction, by monochromatizing the beam produced by a conventional W-anode x-ray tube with a mosaic crystal monochromator. The source generates a laminar beam (10 cm-high and 0.8 cm-wide), composed by two spatially superimposed quasi-monochromatic beams. The characteristics of the radiation field in terms of energy resolution and fluence have been reported, for three pairs of energies. A study of the spectra attenuated by several phantoms of breast equivalent tissue of different thicknesses shows that the optimal energy of the dichromatic beam for dual energy mammography application ranges between 18/36 keV and 18.6/37.2 keV and may be set as a function of the thickness or density of investigated tissue.
In angiography practice an iodate contrast medium is injected in patient vessels with catheters. The absorption of x-rays raises immediately after the iodine K-edge energy. In digital subtraction angiography, two images are used, acquired before and after the injection of the contrast medium, respectively. The vessels morphology result from the difference of images so obtained. This technique involves a non-negligible risk of morbidity or mortality, due to high concentration of injected contrast agent. We are investigating a new source which produces two thin parallel quasi-monochromatic beams - having peak energies centered before and after the iodine K-edge energy, respectively - by using a conventional x-ray tube and a highly oriented pyrolytic graphite mosaic crystal. The polychromatic x-rays incident on the crystal are monochromatized by Bragg diffraction and split in two thin parallel beams, by means of a collimating system. These two beams impinge on the phantom simulating patient vessels and are detected with solid-state array detectors. The image results as difference between the remaining intensities of two beams. We report a preliminary study of the new technique performed both with theoretical stimulations and experimental measurements. Results of computer simulation give information about characteristics as size and quality of the quasi- monochromatic beams, that should be considered in detail to design a system dedicated to the clinical practice. Experimental measurements have been performed on a small- field detector in order to shows the enhancement of image contrast obtained with the application of the new technique.
Highly Oriented Pyrolytic Graphite is a very efficient and well-known x-ray and neutron monochromator. The crystal macroscopic properties are determined by its microscopic structure. Our aim is to study the crystal internal structure and correlate it with the crystal optical behavior. We studied the texture of the crystal, in particular its spatial homogeneity, for different samples using x-ray diffraction topography. The experiment was performed at the ESRF beamline BM5 using a laminar 18 keV monochromatic beam. Several samples supplied by different manufacturers have been studied. Images of (002) reflected beam have been acquired at the Bragg angle for each sample, using a phosphor coated CCD digital detector. Contrast profiles have been obtained, and exponential fits has been performed allowing to deduce the secondary extinction coefficient. It has been found that some samples are quite perfect and the results agree with ideally imperfect crystals model. Other samples present well defined granular macrostructures (with dimensions of tens of microns) superposed to the well-known Gaussian-like crystallite distribution. The different behavior between different samples should be explained in terms of sample internal structure, which is also related to the different graphitization process used by manufacturers.
The existence of an optimal energy range for mammography has been demonstrated by several authors. Improvement in image contrast and reduction of patient dose can be achieved using narrow energy band X ray beams in the 16 - 24 keV range. Quasi-monochromatic X rays in the mammographic energy range have been produced via Bragg diffraction by making use of a conventional W-anode, Be-window X ray tube and a monochromator optical system based on a set of mosaic crystals. The mosaic crystals are high oriented pyrolytic graphite (002) which provide an interesting choice for monochromators because of their high integrated reflectivity compared to perfect crystals. The monochromator optical system consists of an array of ten crystals (2.8 X 6.0 cm2 of size) which are assembled so as to produce in the image plane an irradiation field obtained with adjacent reflected beams. A scanning technique of the optical system has been applied in order to remove the spatial non- uniformities of the entire irradiation field. The source has been characterized in terms of beam size and monochromaticity, photon flux and exposure rate, field uniformity, capability in low contrast detection, dose reduction, and spatial resolution properties. The system provides a large field (10.5 X 12.0 cm2) of quasi- monochromatic X rays ((Delta) E/E equals 12%) at the energy of 18 keV. The spatial resolution capabilities of the sources are affected by the introduction of an active optical element such as a mosaic crystal monochromator. They may be optimized by choosing the proper irradiation geometry. The mean glandular dose delivered to the standard breast by the quasi-monochromatic source is about a half of those delivered by the conventional mammography units.
X-ray instruments with mosaic crystals are proposed and used in many applications in synchrotron radiation, medical physics and astrophysics. These crystals present a parafocusing of the x-ray beam in the diffraction plane, which is thoroughly analyzed in this paper. We studied the evolution of the cross- section of the diffracted beam, using several samples of Highly Oriented Pyrolythic Graphite crystals coming from different suppliers. The experiment has been performed at the European Synchrotron Radiation Facility (beamline BM5). The results clearly show a parafocusing effect in the 1:1 magnification ratio along the diffraction plane and a defocusing effect along the perpendicular plane. The secondary extinction coefficient is also measured.
Quasi-monochromatic X-rays with energy in the 16 - 24 keV range have been produced in our laboratory by making use of a conventional W-anode X-ray tube and a monochromator optical system via Bragg diffraction. The optical system is based on an array of mosaic crystals which produces an irradiation field obtained with adjacent reflected beams. The field size is about 10.5 X 12.0 cm2 in the image plane. The introduction of an active optical component such as a mosaic crystal array may modify the imaging performances of the system. In order to investigate how the resolution properties of the quasi-monochromatic source are affected by the use of adjacent beams and the Bragg diffraction phenomenon, images of a hole pattern and a slit camera have been obtained with different magnification factors. The images of the hole pattern show duplication effects due to the partial superposition of adjacent beams. Slit camera images show an unexpected increase of the focal spot blurring along the diffraction plane. This behavior could be explained by considering each crystal monochromator of the array as a secondary source and consequently as a focal spot of the beam coming from it. Along the orthogonal direction the image blurring depends on the focal spot size of the X-ray tube.
It has been demonstrated both theoretically and experimentally that mammographic x-ray imaging with monochromatic beams could help to improve subject contrast and to reduce the dose delivered to the patient. To this aim, quasi-monochromatic x-rays have been produced in the mammographic energy range by making use of a conventional W- anode, Be-window x-ray tube and a monochromator optical system based on an array of mosaic crystals. The mosaic crystals are highly oriented pyrolytic graphite which provide a gain in flux as compared to perfect crystals because of their higher integrated reflectivity. The monochromator optical system consists of an array of three crystals which has been assembled so as to produce in the image plane an irradiation field obtained with adjacent reflected beams. The field size reflected by each crystal on the image plane is limited by the desired energy resolution along the horizontal direction and by the crystal size along the perpendicular one. The energy spread of the reflected beams is about 10 percent. The characteristics of the system in terms of energy resolution and fluence rate are reported. Radiographs of test phantoms imaged with quasi-monochromatic beams in the energy range of 18-21 keV have been obtained with a conventional screen/film combination. To remove the spatial non-uniformities of the entire irradiation field a correction procedure has been applied.Large field quasi- monochromatic x-ray beams with the same flux of a standard Mo-anode tube with an anode current of about 600 mA.