**6. K edge filters**

138 Imaging of the Breast – Technical Aspects and Clinical Implication

One method utilizes mosaic crystals to produce quasi-monochromatic X-ray beams. When an X-ray hits the crystalline structure of a material, constructive interference takes place in

where, is the exiting X-ray photon wavelength, d is the spacing between atomic planes of the crystal and is the diffraction angle and n is an integer. The d-spacings are substancespecific and like the have an inverse relationship with the energy of the output beam. At different , different X-ray energy photons are reflected back from the crystal. This is called Bragg reflection and this method was proposed by Baldelli et al. (Baldelli, Taibi et al. 2003) to generate monochromatic X-rays. However, this method has some drawbacks. First of all, because it is a Bragg reflection from a plain surface it provides only a monochromatic fan beam and not a cone beam; furthermore, the intensity of the diffracted beam is very low because only a very small portion of the total incoming X-ray spectrum is reflected back. Moreover, because of the variation of crystal angle in the mosaic crystal and the finite width of the slits the X-ray beam is only quasi monochromatic with an energy bandwidth of ~E.

A prototype of a quasi monochromatic diffraction system has also been developed for mammography (Baldelli and et al. 2005). Advantages of the system are that it is tunable in the range of 18-24keV, and according to the authors, the dose can be decreased by half. Disadvantages are that it uses little of the available X-ray flux and requires scanning, as it produces only a sheet beam, so the scanning time is a few seconds long (possible motion blur). The resolution of the system is also lower than the best digital mammo systems can

A similar system to that of Baldelli's (Baldelli and et al. 2005) was developed for a combined breast SPECT – CT system (Gambaccini, Fantini et al. 2001). It had the same advantages and

2d × sinθ = λ × n (3)

accordance with the Bragg equation:

Fig. 3. Quasi monochromatic X-ray beam by X-ray diffraction.

**5.2 Applications for quasi monochromatic beams** 

disadvantages as the mammography system.

produce.
