Doctors currently perform any number of medical proceduresfrom needle biopsies to amniocentesesusing ultrasound images to guide them. The tactic is invaluable but tricky to master: the doctor must look away from the patient at a screen to see the images, which breaks ordinary hand-eye coordination. Now George Stetten, a biomedical engineer at the University of Pittsburgh and Carnegie Mellon University, has come up with a better alternative. He and his colleagues have created a so-called sonic flashlight that, by way of an optical trick, lets doctors practically look through a patient's flesh (see image).

The process, which Stetten presented under the name of tomographic reflection at a recent meeting of the IEEE and ACM, is not the first to attempt to place medical images within a surgeon's direct line of sight. But it is the first to succeed without requiring the user or the patient to wear any sort of tracking device. Instead, Stetten's team superimposes the ultrasound images perfectly on a patient's body by employing a half-silvered, translucent mirror.

The researchers position an ultrasound scanner and a flat-screen ultrasound monitor on opposite sides of the mirror such that the see-through glass bisects the angle between them. Doctors can then look through the mirror to see the patient's skin and scanner on the far side. In front, on the near side of the glass, they see the ultrasound image projected from the monitor. "We are actually merging the virtual image in 3D with the interior of the patient," Stetten explains. "The reflected image is optically indistinguishable from the corresponding space within the patient."

The deviceas well as a portable version Stetten has developed for use in doctor's officesneeds further refinement before clinical testing can begin. "It has some significant potential application," says Jules H. Sumkin, chief of radiology at Magee Women's Hospital, who has tried the sonic flashlight although not on patients. "But I want to see his next prototype."