Low-dose X rays can sharpen fine details

Inspecting a fuzzy X-ray picture for signs of a lung or breast tumor can challenge even the most skilled physician. Now, a high-resolution X-ray technology previously used only in special settings could make diagnosis easier, a new study shows.

 The technique, called phase-contrast X-ray imaging, reveals details that get lost in conventional X rays, especially subtle distinctions between tissues that absorb radiation weakly. Scientists at CSIRO's Division of Materials Science and Technology in Victoria, Australia, describe the method in the Nov. 28 Nature.

 In addition to discerning finer details than current methods, this technique could require a lower overall X-ray dose to produce an image. "We have done some simple model calculations and simulations which indicate that the reduction could be quite significant," says study coauthor Stephen W. Wilkins. "Reduction in absorbed dose by 50 percent or more in mammography seems plausible."

 A conventional X ray relies on differences in the amount of radiation that various tissues absorb. Dense tissues, such as bones, absorb more radiation and cast sharp shadows that show up on a piece of film, while soft tissues block the beam less effectively.

 A phase-contrast X ray, on the other hand, records information from the beams themselves after they have passed through different tissues. All tissues cause the X rays to slow down, resulting in what's known as a phase shift. The size of the shift depends on the type of tissue.

 For conventional X rays, "when you detect the intensity, the phase information is gone," says Werner Meyer-Ilse of the Lawrence Berkeley (Calif.) Laboratory, "unless you have an experimental setup which converts the phase shift into intensity shift so that it can be recorded."

 In the experimental setup that Wilkins' group used, the X-ray beam had a spot size of 20 micrometers or less, which makes the wave crests well matched, although not as completely correlated as in a laser. Also, the distances between object and image are larger than those used in X-ray imaging, Wilkins says.

 Scientists currently use phase-contrast X rays to examine industrial materials and biological samples, says Meyer-Ilse, whose own work includes X-ray microscopy of cells. Those experiments typically require X rays of a single wavelength, so samples must be taken to the nearest high-energy synchrotron. For medical diagnosis, "that's not practical."

 The CSIRO group's technique, on the other hand, makes use of typical clinical X-ray sources, which emit a range of wavelengths. A commercial phase-contrast system, Wilkins says, "may even be cheaper than existing systems, since it involves a lower-power--but more focused--X-ray source."