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X-ray Scatter

Anthony Greaves, Peter Cadusch

X-ray phase contrast is a relatively new modality of imaging with x-rays. Unlike the traditional absorption type x-ray radiograph, phase contrast imaging is ideally suited to imaging materials such as biological tissue which are composed of low atomic number elements with very small absorbance of hard x-rays. As a bonus, differences in material are highlighted by a diffraction fringe which can be likened to in-line holographic techniques.

The technique requires a source with good spatial coherence which scales inversely to the spot size (typically < 10 micron) and sufficient distance between object and image plane to allow propagation effects to show the near field diffraction as phase contrast. The images formed contain information on the complex refractive index of the material, showing both the real component (x-ray phase shift) and the imaginary part (x-ray absorption). Large changes or gradients in the phase component or discontinuities such as interfaces in the object cause interference in the image plane and hence intensity modulation. Sources of spatially coherent x-rays are synchrotrons, microfocus tubes and recently developed laser induced x-ray sources [1].

Our research aims to identify those aspects of the imaging chain that reduce or maintain good beam coherence throughout the process of phase contrast imaging to determine optimum image quality. This is done via a combination of experimental work done at CSIRO's Feinfocus microsource facility and computer simulation methods based on the Fresnel/Kirchhoff integral as an aid to testing various hypotheses.

One bonus besides better image quality is the potential of reduced patient dose since the beam energy may be increased which reduces photoelectric interaction and hence dose as the phase component is less sensitive to beam energy than the absorption component. Thus small differences in tissue composition such as between healthy and malignant tissue have the potential to be visually differentiated allowing better diagnostic ability.

[1] R. Toth, J. C. Kieffer, S. Fourmaux, and T. Ozaki: In-line phase-contrast imaging with a laser-based hard x-ray source. Review of scientific instruments 76, 083701 (2005)

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