New Math, High-Tech Imaging Solving Old Riddle: How Does The Brain Operate When We Think?

The work, done in animals, offers promise for studying the brain at work, by measuring not only how much extra blood gets sent to thinking areas, but also by calculating how much oxygen those areas are using. Using a kind of imaging called functional MRI (fMRI), scientists have long known that the amount of oxygen used by a particular area of the brain can change due to damage or activation of that area.

"The technique of fMRI has been available for eight years, but no one knew exactly how to interpret changes in the magnetic signals caused by changes in brain activity," says Peter C. van Zijl, Ph.D., professor of radiology.

The team's work, which appears in the February 1998 issue of Nature Medicine, was supported by the National Institutes of Health, the American Heart Association, the Finnish Academy of Sciences, the Magnus Ehrnrooth Foundation, the North Savo Fund of the Finnish Cultural Foundation, and the Saastamoinen Foundation. A second article now in press shows how the new technique can be used to study the early stages of stroke.

"Our new mathematical formulas let us figure out how much more work is involved when a particular area of the brain is being used for thinking," van Zijl adds. "This is a major advance in fMRI that will lead to applications in the study of stroke, cancer and cardiac disease, which are diseases involving changes in blood flow and oxygen use in different organs. fMRI now has the potential to become much more like a routine blood test that lets doctors diagnose diseases by comparing levels of substances in the blood with normal ranges."

The type of fMRI the team used shows changes in brain activity due to increases or decreases in oxygen concentration. Called blood-oxygen-level dependent (BOLD) fMRI, it is based on the fact that magnetic properties of hemoglobin, the oxygen-carrying molecule in blood, change in brain areas that become active. That's because when hemoglobin is not carrying oxygen, it increases the strength of the magnetic field set up by MRI in that area of the body. In turn, this alters MRI signals released by molecules in there.

The researchers demonstrated the accuracy of their math by comparing normal animal brain activity to abnormal activity caused by decreased oxygen in the blood. The team lowered the amount of oxygen in animals for about 45 minutes, then recorded changes in MRI signals due to the decrease in the number of hemoglobin molecules carrying oxygen. The oxygen levels were then returned to normal levels and the signals again measured.

Other authors of the study include Scott M. Eleff, John A. Ulatowski, Aziz M. Ulug and Richard J. Traystman (Johns Hopkins), Joni M.E. Oja (Johns Hopkins and University of Kuopio; Kuopio, Finland) and Risto A. Kauppinen (University of Kuopio; Kuopio, Finland).