Another way to make coherent radiation is to filter the waves generated by a conventional x-ray tube through a tiny hole in a lead shield. The rays come at the shield every which way, and they exit the hole in uniform circular waves. But because only a small proportion of the waves generated by the tube can squeeze through the hole, using this kind of filter takes a lot of power and generates a very weak image. In such a system, a patient would have to hold still and be scanned for a very long time, says Lanza.
The system Lanza and Damato are developing combines a traditional x-ray tube with a filter that has hundreds of tiny holes for generating coherent beams of x-rays. First, these coherent waves pass through the sample. Next, the waves pass an interferometer, which collects information about how each of the hundreds of sets of waves was refracted by the sample. Then they pass to a traditional absorption detector, which collects information about how each of the hundreds of sets of waves was absorbed by the sample.
Hundreds of interference images are then combined with the hundreds of absorption images into a single picture that a radiologist can read. Berthold Horn, a member of MIT’s computer-science and artificial-intelligence laboratory, is developing algorithms to process the x-ray images. “The computer becomes an integral part of the imaging system,” he says.
Similar filters are used in x-ray astronomy, in which the objects of interest–the stars–are very, very far away. Horn and Lanza were the first to apply this technique, called coded-aperture imaging, to nearby objects. They have already employed it to improve the resolution of a gamma-ray imaging technique used on lab animals, and they believe they will see similar results with x-ray imaging.
If the MIT x-ray system does prove to have a higher resolution, as the gamma-ray system did, it could help doctors find smaller tumors. But it will need to go through randomized control trials to prove that it saves lives. “You want to make sure you don’t make a patient’s life any worse,” says Moore. “They’re coming in for screening before they have any symptoms.” He hopes that more-sensitive mammography will not highlight nonlethal tumors, as a recent controversy suggests that lung CT scans may, but will help eliminate false positives.
For every 1,000 women screened for breast cancer, 80 are called back. “All 80 think they have cancer, and they feel terrible,” says Moore. But only 20 will be called back for biopsies, and only 5 will have cancer. “The funnel is wide at the top, narrow at the bottom,” says Moore. He hopes that better x-rays will help doctors do better.
For now, says Horn, the researchers are working on their prototype and hoping they can “demonstrate that there’s information there that’s not in the absorption images.”
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