Exploratorium Magazine Online
picturing the body

Volume 23, Number 3

 

 page 3 of 5



HOW MAGNETIC RESONANCE IMAGING WORKS

Functional MRI is a new technique involving MRI technology. You've probably seen MRI chambers on TV. The machine surrounds the patient, who lies still on a pallet inside a narrow cavern. A powerful electromagnetic field, considered to be harmless, is generated around the person. The electromagnetic field causes the nuclei of the body's hydrogen atoms (each a single, positively-charged proton) to stop their random spinning and align like compass needles. Precise radio waves are then slammed into the flipped nuclei, making the nuclei snap back to their original configurations. As they do this, they release energy in the form of radio waves that, echo-like, can be picked up by a detector and sorted out by a computer. Regions dense with hydrogen atoms will emit more radio waves, allowing the computer to generate a high-resolution, three-dimensional density map of the body.

Functional MRI adds another dimension to this. When neurons (nerve cells) are active, their metabolism increases significantly, requiring increased blood flow to supply oxygen and carry away metabolic waste. Blood that's carrying oxygen to the neurons has different magnetic properties than deoxygenated blood; as oxygen is rushed to active neurons, it causes a temporary increase in MRI signal that a computer can detect and amplify, giving a four-dimensional map in time and space of brain activity.

This is in contrast to previous brain-imaging techniques such as X rays and CT scans, basically three-dimensional X-ray images, good for looking at soft tissue structures like the brain, but yielding little functional information.

Electroencephalograms (EEGs), recordings of the brain's electrical signals made by attaching electrodes to the scalp, have helped researchers identify large, generalized regions of the brain, such as the temporal lobe and visual cortex. But the sensitivity of an EEG is limited to synchronous firing in large groups of neurons, and it has poor depth resolution—since the electrodes can only be stuck to the scalp, it's difficult to tell how far down an activity is located.

Positron emission tomography (PET) scans can perform a function similar to fMRI by monitoring increased sugar metabolism. Active cells use more of the simple sugar glucose than resting cells do. If glucose is radioactively "tagged," and then injected into the subject, its path through the body can be observed. But subjects are generally reluctant to be injected with radioactive material, even at very low levels. In contrast, fMRI is completely noninvasive.

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