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.

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