A Better Picture of the Brain
A new imager that performs simultaneous MRI and PET scans could, among other applications, speed up the study of Alzheimer’s disease.
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.
Mathis says that one of the advantages of the combined imaging system is simply logistical. For patients who have Alzheimer’s disease, depression, or bipolar disorder, “getting [them] in is half the battle,” he says. Elderly and sick volunteers and their caretakers often have to go to the clinic on two different days for a PET and an MRI; each scan takes about an hour and can cause discomfort in feeble patients, who must lie still. Subjecting patients to one simultaneous scan, instead of two, would cut in half the time spent in the radiology department.
For studying the effects of pharmaceuticals on the brain, the element of time is critical, says Mathis, and performing PET and MRI scans separately means that researchers are making a lot of assumptions. Mathis suspects that, using the combined system, it will take “fewer experiments to nail down the source of pharmaceutical effects.”
Darrow says that Siemens has sent a few PET/MRI machines to researchers and is about 18 months from releasing the first commercial device. The company is also working on developing a whole-body simultaneous PET/MRI imaging system. The primary application for the whole-body system will be to search for cancerous tumors that have spread beyond their initial site.
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