Phase imaging with high-energy x-rays

17 Jun 2002

Conventional x-ray imaging methods detect how many x rays are absorbed as they pass through a sample. As objects get thicker it becomes necessary to use high-energy x-rays in larger amounts in order to penetrate and build contrast, causing considerable radiation damage to the sample. An alternative approach, requiring much smaller amounts of x-rays, is to image objects by measuring changes in phase. The phase shift can provide information about both the composition of the material and its thickness. Interferometric methods are difficult to carry out because the beam of x rays must be coherent, and currently lasers are only available at lower (softer) x-ray energies. However, Keith Nugent of the University of Melbourne in Australia and his colleagues have demonstrated a phase imaging technique that does not require the use of interferometry. Instead they convert measurements of how the x rays are re-directed through a sample into information about the phase shifts at different points in the sample. This "phase map," in turn, can be transformed into a direct physical image.

So far the researchers have used this method to image a carbon grid with lines 330 microns apart. Ultimately, the researchers expect potential resolutions of about 1 micron with current detector technology and are exploring the goal of making phase-based CAT scans of the interiors of objects, according to an upcoming paper in Physical Review Letters.