**4. Monochromatic X-ray imaging**

Beam hardening has a significantly deleterious effect on the precision of CT reconstruction. It can be avoided by using monochromatic X-ray beams. Furthermore, for each imaging object there is an ideal energy at which enough X-ray photons make it through the object to allow for low noise imaging while still using a low enough X-ray energy to provide the best contrast ratio. Synchrotrons are ideal sources of monochromatic beams of high flux. Some experiments on breast imaging with synchrotrons were made and compared to normal mammographic images (Fiedler and et al. 2004). A further advantage of monochromatic imaging is that it allows for phase contrast imaging, which greatly improves the contrast even between different soft tissues. However, the size and price of a synchrotron make it impractical for general radiographic use. X-ray tubes with monochromators can provide monochromatic x-ray beams but the flux is too low for practical applications (Donath, Pfeiffer et al. 2008).
