Northwestern University chemist Thomas J. Meade means no disrespect to his medical colleagues, but when he looks at the state of the art in diagnostics, he suggests that, for some procedures, physicians might as well use "stone knives." Take, for example, mammography. "You know going in that there's a one in five chance of a false positive or a false negative. You have an x-ray that's not even smart enough to differentiate a shadow cast by a calcium spot from a tumor. After reading the film and seeing a shadow, they do the prudent thing and stick a 16-gauge needle in you for a biopsy. Then you have to spend the next five days freaking out that you've got breast cancer until you get the results," he says.

Some of Meade's annoyance stems from the fact that his wife endured just such a false alarm-as will an estimated quarter to half of all women who undergo annual mammogram screening over the course of 10 years. But Meade's criticisms go well beyond the specific failings of mammography and breast biopsies; to him and a growing number of other medical researchers, today's diagnostic tools are too uncertain and invasive-just too primitive. Working at select academic centers and industrial labs around the world, these researchers are developing a suite of new tools that will enable doctors to spot disease instantly and accurately, without ever taking a scalpel or biopsy needle to their patients' skin.

The new discipline is called "molecular imaging," and it is fundamentally altering physicians' ability to view the body and its processes. Most conventional imaging tools, from x-ray to magnetic-resonance imaging, provide anatomical or structural information: is there a lump in the breast or a shadow in the lung? Molecular imaging goes beyond anatomical information to reveal functional data-the cellular activities that characterize tumor growth or inflammation, for example.

This is important because cancer and other diseases often begin with subtle cellular changes, well before a structural abnormality, such as a tumor, is detectable. What's more, the new advanced imaging methods can help distinguish between diseases that look similar but actually involve different molecular malfunctions-and thus require different treatments. "Disease is being redefined in terms of its molecular signature," explains Daniel Sullivan, associate director of the National Cancer Institute and head of the institute's biomedical-imaging program. "In the future, people will talk about cancer by the molecular abnormality, not by the organ of origin."