Can functional MRI help diagnose Alzheimer's disease?

One of the frustrating things about suspecting a loved one has Alzheimer's disease is that no definitive test of any kind can make a firm diagnosis. Only an autopsy can confirm that a person had the disease. That's why scientists are working to use new types of brain imaging in hopes that such a test may soon be possible.

Two leading possibilities are functional Magnetic Resonance Imaging, or fMRI, and positron emission tomography, or PET.

When a person uses a particular part of the brain, blood flow increases to that specific anatomical area, and it uses more oxygen. That difference can be seen by an fMRI. For example, if a person moves his left hand back and forth while getting an fMRI scan, the part of the brain that controls the left hand gets more blood and oxygen. During the scan, a neuroradiologist can see that part of the brain "light up" with the movement.

It's not certain yet, but early investigative work by Duke University researchers suggests that there may be a site on the middle surface of the brain's parietal lobe which might reliably light up in Alzheimer's patients. If complicated (and tedious) memory tests are administered to an Alzheimer's patient during MRI scanning, areas of the brain normally responsible for memory function, such as the hippocampus, may be seen to activate much less intensely than in a normal patient. This would indicate that the patient cannot use this portion of the brain as well as normal patients can.

In other studies, the brains of patients with early Alzheimer's, or with the minimal cognitive impairment that can precede the disease, have been observed to activate more intensely and in many more areas associated with memory than normal individuals' might under similar circumstances. In these patients, regions of the brain seemingly unrelated to the task may light up. This may seem to contradict the Duke studies, but what it appears to show is that, in early-onset patients, the brain is trying to work harder than normal to perform memory tasks.

PET scanning may hold even greater promise than fMRI, providing a better physiological picture of the patient's brain. PET scans use radioactive molecules as biomarkers to detect hallmarks of Alzheimer's pathology in the brain. A promising study of a new radioactive tracer, AV-45, reported at the recent Alzheimer's Association International Conference, showed a high correlation between the presence of the Alzheimer's-associated protein amyloid on the PET scans of people at the end of their lives and the amyloid in their autopsied brains after they died.

For now, these fMRI and PET studies are of limited usefulness and availability. Because fMRI depends upon the cooperation of patients themselves, it is often not practical. PET scanning is very expensive and the radioactive materials used have extremely short half-lives, meaning that they must be injected into the patient within minutes of their production in a cyclotron, a kind of particle accelerator. However, although there is more work to do before they can be routinely used, both fMRI and PET hold great potential for early diagnosis of dementia.

Until then, well-performed, high-quality conventional MRI scans of the brain continue to be the mainstay of the imaging portion of dementia diagnoses. The main reason to perform this anatomic MRI is to rule out anything that can be addressed surgically or medically, such as brain tumors in certain locations, and to help differentiate between Alzheimer's disease and other medical conditions that can cause dementia.