September 8, 2009

Study illuminates fMRI interpretation

Finnish and Canadian scientists say they've found brain cells called astrocytes play a major role in generating functional magnetic resonance imaging signals.

Professor Kai Kaila of the University of Helsinki and the Academy of Finland's Neuroscience Research Program, along with Professor Brian MacVicar of the University of British Columbia, found functional MRI imaging does not directly measure nerve cell activity. Instead, it actually measures changes in cerebrovascular circulation.

Helping to regulate that circulation are specialized support cells called astrocytes, which are far more numerous than nerve cells. The researchers discovered the signals generated by astrocytes change as a result of changes in nerve cell activity and the metabolic state of the brain. Signals from astrocytes help to regulate the diameter of the brain's blood vessels.

Kaila said the findings have helped define the fundamental mechanisms that generate fMRI signals, and show the interpretation of the fMRI results is more complex than had been assumed.

For example, it is generally believed that changes in fMRI signals associated with different brain diseases reflect changes in the function of nerve cells and neural networks, even though the explanation might lie in a pathological change in the characteristics and function of astrocytes, Kaila said. Likewise, the distinctive characteristics seen in fMRI signals measured from premature newborns is probably, in large part, based on the immaturity of astrocyte and blood vessel function.