Needle imaged in animal-tissue haystack.

A figure silhouetted in bright sunlight casts a sharp shadow. In a thick fog, the figure and its shadow virtually disappear, smeared out by the way water droplets randomly scatter any light penetrating the fog.

Animal tissue also scatters light, making it difficult to use visible or infrared light as a probe to locate and characterize tumors within the body. However, researchers have now demonstrated that they can capture "shadowgraphs" of objects embedded in tissue by concentrating on the fraction of a light pulse that passes most rapidly through a tissue sample. With further development, this imaging scheme may offer an alternative to X-ray techniques for noninvasive screening of human breasts.

When a beam of light passes through a tissue sample or a milky fluid, a small fraction of the light travels in a nearly straight line. The remainder, scattered by the medium, follows a considerably more tortuous path before finally exiting the sample. Because this scattered light travels farther, it takes longer than the straight-line, or ballistic, light to pass through the material.

In other words, light travelling predominantly in the forward direction arrives at a detector first. Because only this early light produces a sharp shadowgraph of an embedded object, researchers need a way to isolate it from the rest of the light emerging from a sample.

"We're looking for the shadow," says physicist Robert R. Alfano of the City College of the City University of New York. At last week's Conference on Lasers and Electro-Optics, held in Baltimore, several research teams developing visible-light imaging systems presented their progress reports.

The idea is to synchronize the opening and closing of an electronic or optical "gate" in front of a detector with the entry of a short laser pulse into a sample. To get sufficient contrast and spatial resolution in the resulting image, researchers try to use the shortest possible laser pulses and the fastest, most sensitive gates available.

At the University of Michigan in Ann Arbor, Janis A. Valdmanis, Emmett N. Leith and their colleagues use a holographic technique for capturing 100-femtosecond slices of early light to produce a two-dimensional image. An electronic camera records the resulting holographic interference patterns, which are rapidly processed and averaged by a computer to generate an image on a video screen.

Using this system, the group has obtained clear images of two sewing needles, 0.5 millimeter in diameter, hidden behind 6 millimeters of raw chicken meat. Under continuous laser light and without the gate in operation, the chicken meat completely obscures the needles.

"We have also demonstrated the capability of imaging . . . objects buried in diffusing [scattering] material several centimeters thick," the Michigan researchers report.

Alfano and his co-workers use an optical gate known as a Kerr shutter to snap images of various patterns viewed through a milky suspension of tiny polystyrene spheres in water. Using 10-picosecond laser pulses, they can pinpoint the location of a spot 200 microns in diameter and resolve a rectangular bar 400 microns wide. The New York group has also imaged such patterns through samples of chicken and human breast tissue about 3.5 millimeters thick.

"Our plan is to increase how deep we can go [into tissue] and to use shorter pulses to see if we can get better resolution," Alfano says.

Although these visible-light imaging techniques show promise, they remain in the research stage. In the May 10 MORBIDITY AND MORTALITY WEEKLY REPORT, the Atlanta-based Centers for Disease Control warns that the efficacy of such "transillumination" techniques for detecting early stages of breast cancer has not yet been demonstrated, and cautions that any transillumination devices now being marketed do not provide meaningful clinical information.