MAGNETIC RESONANCE IMAGING WORKS
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.
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.
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.
(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 resolutionsince
the electrodes can only be stuck to the scalp, it's difficult
to tell how far down an activity is located.
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.