The two most vaunted medical-imaging technologies have led separate existences. Positron emission tomography (PET) provides snapshots of the human metabolism, while magnetic resonance imaging (MRI) captures high-resolution pictures of anatomical structures. MRI could pinpoint a tumor's location, for example, while PET revealed whether a drug was working to cut off its blood supply. Yet the two techniques have not been used simultaneously in humans; the metal inside the PET camera wreaks havoc with the MRI scanner's powerful magnet.

Researchers at King's College in London and the University of California, Davis, are working on a combination scanner that places a PET camera made of plastic and fiber optics inside an MRI magnet. They have built shoebox-size models that can image metabolic changes and anatomy simultaneously in the bodies of mice and rats (the image at left is of a rat's head) and hope to eventually build a prototype large enough for human testing.