Imaging
X-ray imaging
Conventional imaging with x-rays has been in use for over 100 years. It is
based on the absorption of the radiation; the contrast produced in a conventional
x-ray image results from differing absorption of components in the object caused
by varying composition, thickness or density. Effectively, a shadowgraph is
obtained, and this has worked well when there are very large differences in
absorption between constituents, such as bone and soft tissue. However, conventional
x-ray imaging of soft tissues such as skin, cartilage, ligaments, tendons, lungs,
breast tissue and tumours produces information of relatively poor quality.
Pioneering work at The University of Melbourne, CSIRO, and the Synchrotron
Laboratory at Daresbury in the UK has shown, however, that conventional x-ray
imaging discards much of the valuable information that is generated by the refraction
of the x-ray beam by soft tissues. The different refractive indices of the various
types of soft tissue cause changes in the direction and phase of the illumination
and this can be detected when the highly collimated tuned radiation of a synchrotron
is used as a light source together with a very sensitive detection technique.
Diffraction enhanced x-ray imaging
Diffraction enhanced imaging, sometimes called phase contrast imaging, relies
on the very small range of angles over which a perfect crystal reflects x-rays.
For the most commonly used cut of silicon crystal, this angular range is approximately
3 microradian. If the crystal, placed just after the sample as an analyzer,
is rocked through this small angular range, it will act as a very narrow slit,
and can separate out the diffraction information from the absorption information
in the image. When a diffraction enhanced image is reconstructed from this information,
very fine detail can be obtained. The resolution is usually improved by a factor
of at least 1000.
T-ray imaging
The advent of high-brilliance synchrotron light has also opened up new possibilities
for imaging in the terahertz (THz) region of the spectrum. So-called T-ray imaging
uses pulsed far infrared light. It has great potential as a medical imaging
tool because there is no ionization hazard for biological tissue and Rayleigh
scattering is many orders of magnitude less for THz radiation than for neighbouring
infrared and optical regions of the spectrum.
The THz frequencies correspond to energy levels of molecular rotations and vibrations of
DNA and proteins, and these may provide characteristic fingerprints to differentiate biological tissues in
a region of the spectrum not previously explored for medical use. In addition, THz wavelengths are
particularly sensitive to water, which can indicate tissue condition.
The Australian Synchrotron, which is a high-energy third generation design,
is not well-suited to generating THz radiation, and so this capability will
not be included in the initial set of beamlines. However synchrotron science
continues to develop rapidly, and in the longer term it may be possible to incorporate
this capability at the new national facility.