Could imaging with monochromatic x-ray beams become a reality in all our hospitals?

Abstract

X-ray beams used every day for imaging in our hospitals contain a wide range of photon energies. This is inevitable because of the way in which they are produced. Metal targets (e.g. x-ray tube anodes made of tungsten) are irradiated with beams of electrons and, although the electrons are monoenergetic, interactions with the metal generate Bremsstrahlung x-ray photons with energies extending from that of the incident electrons downwards. The electrons lose energy when they interact with the metal atoms as the beam penetrates the anode and x-rays produced from deep within the anode are attenuated as they leave. The net result is that a wide range of Bremsstrahlung x-ray energies are produced from the thick metal anodes with an intensity distribution that decreases with increasing photon energy. Contrast between different tissues in medical x-ray images occurs primarily because of the photoelectric effect, which gives large differences in attenuation resulting from variations in elemental composition, since the photoelectric attenuation coefficient is proportional to the cube of atomic number. Compton scattering, the other interaction process, gives some differentiation in tissue attenuation, but this depends only on tissue density and the contribution to image contrast is more limited. The probability of photoelectric interactions declines as photon energy increases, so the relative contributions from the two interactions vary across the energy range. Although we might know what x-ray energy would give the best images when, for example, iodine contrast material has been injected into a blood vessel, we have to use our standard x-ray beams containing photons with these wide ranges in energy. But what if we could select the particular x-ray photon energies that result in optimal contrast? Studies using monochromatic x-rays generated from high intensity beams from synchrotrons have shown excellent contrast in mammographic imaging with significant reductions in dose (Burrattini et al 1995, Arfelli et al 1998). But synchrotrons are large and expensive. They are not suitable for use in hospitals, even if the National Health Service could afford them. Characteristic fluorescent x-rays in narrow energy bands are emitted when transitions occur between electron energy levels. If tightly bound electrons are dislodged from inner atomic energy levels and are subsequently replaced by electrons from outer levels of the atom, x-rays within narrow energy bands that are characteristic of the particular metal are emitted. Electrons removed from the inner K-shell have the highest binding energies and so are the ones relevant for imaging. In a conventional x-ray tube some bound electrons are ejected from the metal atoms in the anode, and so some characteristic x-rays are produced. But their intensities are low compared with those of the broadband Bremsstrahlung x-rays, so they play an insignificant role in most x-ray equipment. But these fluorescent x-rays have the potential to provide narrow energy x-ray sources, if only we could remove the Bremsstrahlung. A way to produce characteristic x-rays without Bremsstrahlung would be to excite bound electrons in ametal target with x-ray photons rather than electrons. However, this would require very intense x-ray beams. One of the limitations in x-ray tube technology arises from the need