Needles from Haystacks

In a streamlined dentists' office just off Manhattan's Fifth Avenue a decided improvement in X-ray could be seen last week. It consisted of some amazing X-ray transparencies. Each one appeared fuzzy to the naked eye, like seeing double. But through polaroid eyeglasses each transparency was a clear, three-dimensional, stereoscopic view into a body. A pencil moving over a film of the chest seemed to move among the ribs, poke the heart. Three dentist brothers, Edward, Milton, Harold Klein, perfected the method.

The trouble with most previous stereoscopic X-rays, the Kleins explain,* was that the two exposures were made the same way stereoscopic photographs are made: i.e., by moving the X-ray tube two and a half inches (average distance between eye pupils) between shots. But an X-ray tube does not correspond to an eye, as a camera does. It corresponds to a light source which shoots rays through the subject to make shadows on the film beyond. Moving the tube two and a half inches produces too much disparity in the resulting films. Viewed through a stereoscope, such pictures will not fuse.

The Kleins figured that, to prevent seeing double, no silhouette should overlap more than three-eighths of an inch in the final transparency. To achieve this result, the distance a tube is moved must vary with the thickness of the object being Xrayed. Instead of mounting their transparencies side by side, as in a stereoscope, the Kleins stuck them on opposite sides of a sheet of transparent polaroid.

The farther a piece of anatomy is from the film during exposure, the more its two superimposed silhouettes overlap in the resulting composite transparency. The method of calculating a bullet's depth is to measure the overlap of its silhouette, and compare that with the overlap of some body landmark, preferably a bone. The basic principle of this depth-determination technique is as old as geometry. But the Kleins hope that Army & Navy surgeons can use their stereoscopic Xray.

*In the May issue of the American Journal of Roentgenology and Radium Therapy.

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