Siemens has developed a prototype brain-imaging machine that can perform magnetic resonance imaging (MRI) and positron emission tomography (PET) simultaneously. This will save patients in clinical trials time and allow researchers to make more-accurate correlations between activity at different regions of the brain and at the cellular level. The device is the first to combine MRI, which gives information about the structure of the brain and about blood flow to brain regions, with PET, which allows researchers to monitor metabolic activity at the cellular level. The combined imaging method may help research into the basis of Alzheimer's disease and provide a more accurate picture of drugs' effects on the brain.

Currently, researchers must perform MRI and PET scans sequentially. "Each device only looks at part of the picture," says Doug Darrow, director of operations for molecular imaging at Siemens. When combining the images from MRI and PET scans, researchers must make assumptions about what happened in a patient's brain during the time elapsed between scans, and then correct for it. For example, levels of a drug in the brain might fluctuate, so the brain-activity levels pictured in an MRI image might not correlate with the concentration of a drug pictured in a PET image. A machine that combines the imaging techniques, says Darrow, gives simultaneous information about the structure and metabolism of a patient's brain.

Darrow suggests that simultaneous PET/MRI imaging will eventually be used to help diagnose Alzheimer's in its early stages and help doctors predict how fast a patient's disease will progress. The system can also be used to image brain tumors.

Radiologists already use PET and MRI in clinical trials to study the changes in the brain characteristic of Alzheimer's disease. Using specially designed chemical probes, PET allows researchers to follow the buildup of amyloid plaques, the clumps of protein that accumulate in the brain with Alzheimer's disease. MRI allows researchers to follow the structural changes associated with the disease--such as accelerated shrinkage of the brain.

The two imaging techniques are also used to study how drugs like antidepressants operate in the human brain over time. Researchers can use functional MRI to monitor how a drug affects regional brain activity by monitoring blood flow. Using PET, they can monitor where in the brain the drug binds, and to what kind of receptors--dopamine or serotonin, for example.

Chester Mathis, professor of radiology and director of the PET facility at the University of Pittsburgh School of Medicine, cautions that the use of the combined imaging to accurately monitor the course of Alzheimer's is still far off. But he says that his department has applied to the National Institutes of Health for funding to purchase one of the PET/MRI machines, and he does expect it to speed the pace of the researchers' work.