Monday, May 21, 2007

Changing the Physics behind X-Ray Imaging

Researchers integrate refraction information to improve x-ray resolution.

By Katherine Bourzac

Refracted rays: A metal plate can filter radiation into coherent waves, making it possible to record refraction information for potentially higher-resolution x-ray imaging. Credit: Courtesy Roberto Accorsi and Richard Lanza

MIT researchers are developing a new kind of x-ray imager that uses information that traditional machines ignore. By looking at how tissue refracts the rays, not simply at how it absorbs them, the researchers hope to increase the resolution of mammography, enabling doctors to detect smaller tumors earlier.

The basic physics behind x-ray imaging hasn't changed in more than 100 years. Most hospitals have gotten rid of film and gone digital, but their images still record the same kind of information: how a part of the patient's body absorbs the rays.

When radiation hits a material, many other things happen to the waves besides absorption, say Richard Lanza and Antonio Damato, a research scientist and a graduate student, respectively, in nuclear engineering at MIT. The pair is developing a prototype x-ray mammogram system that will record information about absorption and refraction of the radiation as it passes through an object.

Refraction is a change in the direction of a wave as it passes from one medium to another; it's the same effect that makes a straw in a glass of water look broken.

"Cancers have very similar x-ray absorption to normal tissue," says Daniel Kopans, director of breast imaging at Massachusetts General Hospital, in Boston. But cancers may have different refractive properties. Kopans says that preliminary studies using a synchrotron accelerator have shown that cancers appear to refract x-rays differently than normal tissue does. If this is so, recording this information during an x-ray scan might indeed help doctors better delineate tumors.

Since the widespread adoption of mammogram screenings in the early 1990s, the breast-cancer death rate has gone down by 25 percent in the United States. But breast cancer is still the second leading cause of cancer death in women. These tumors remain particularly difficult to detect because breast tissue has a variable structure. Five millimeters is the limit of detection with conventional mammography, so doctors know that they are missing tumors, says Richard Moore, research director in the breast-imaging center at Mass General.

Moore says the hallmarks of breast cancer that doctors look for in mammograms reflect disease processes that have been under way for a long time: tiny calcifications that signal cancer-cell death, and healthy surrounding tissue that's pinched, like fabric. Although they don't know what they'll find, Moore says that Lanza's x-rays might provide new information about breast tumors that enable doctors to see disease sooner, before the calcifications and pinching effects visible in a conventional mammogram even develop.

It's practically impossible to measure how a sample refracts waves unless the waves all start out going in the same direction at the same frequency. Conventional x-ray tubes--such as those used in mammography--can't do this, and x-rays can't be focused with lenses and mirrors. One way to achieve the uniformity, or coherence, necessary for refraction measurements is to run the x-rays through a particle accelerator. But these are too large and expensive for hospital use.