In the 1970s, researchers began using
computers to process information from
x-rays passed through the brain into a
single whole-brain image. This
technology provided the first direct
pictures of normal brain anatomy in
living humans. A variety of techniques
are helping researchers understand the
relationship between brain structure,
function, and human behavior. They also
are revolutionizing diagnosis and
treatment of many brain disorders.
For centuries, knowledge of normal
human brain structure and function was
rudimentary. Safe, practical ways to
study the normal living brain did not
exist. But this is changing radically as
scientists develop new techniques to
visualize the living brain.
The new techniques are helping
researchers:
Understand the relationship between
brain structure and functions such as
speech and memory.
Identify what goes awry in brain
disorders such as schizophrenia,
stroke, and depression.
Locate and treat epilepsy, brain
tumors, and other disorders with
precision.
Early animal studies showed that
dense tissue, such as bone, absorbs more
x-ray energy than softer tissue like
muscle. In the 1970s, scientists began
using computing technology to combine
brain x-rays taken from many different
angles into a single picture. This x-ray
computed tomography (CT) allowed
scientists a way to "see" into a subject's
brain without causing discomfort and
gave them their first glimpses of normal
brain anatomy in living humans. It also
showed where brains tumors and other
structural abnormalities were located,
greatly improving diagnosis of brain
disorders and the success of surgery.
CT's usefulness spurred interest in
other imaging strategies using
computers. Scientists soon tried injecting
small amounts of radioactive substances,
or isotopes, into the blood. The isotopes
release particles known as positrons,
which produce other particles called
photons that can be detected by a special
camera. When radioactive water, labeled
with the isotope oxygen 15, is injected
into the blood, it is taken into the brain
in proportion to increased blood flow and
acts as a measure of nerve cell, or
neuron, activity in different brain areas.
Using computing strategies similar to
those for CT, scientists could for the first
time make images of brain function. This
technique, called positron emission
tomography (PET), using other isotopes
also can image other body processes,
including glucose breakdown (the process
by which energy is produced), oxygen
consumption, and the effect of drugs on
the brain.
Using PET and oxygen 15 labeled
water, scientists can locate the regions
that become active while a person
speaks, listens to music, or performs
other activities. By comparing these
snapshots to those taken before or after
a task, they are gaining many new
insights about brain organization. Studies
show, for example, that the brain areas
used in a new task are often different
than those used in the same task after it
is learned. These findings are helping
researchers understand how humans
process information and which brain
areas must be preserved during surgery.
PET also helps reveal how drugs and
certain disorders, such as depression and
Parkinson's disease affect the brain.
Another imaging method, magnetic
resonance imaging (MRI), was developed
in the 1980s. MRI uses magnets to detect
signals from protons, particles with a
positive electronic charge that act like
compass needles in the magnetic field.
Protons abound naturally in the body, so
MRI does not require injections as does
PET. MRI images provide greater detail
than CT images.
In the early 1990s, scientists found
ways to adapt MRI to measure functional
changes in brain activity. Because the
amount of oxygen found in blood affects
its magnetic properties, MRI detects
regions with changes in levels of blood
oxygenation due to activity-related
changes in blood flow. MRI can provide
both anatomical and functional
information for each subject, helping
researchers accurately determine which
brain regions are active in each task.
A variety of other imaging techniques
are now available. One of the most
popular is single photon emission
computed tomography (SPECT), which is
similar to PET but detects a different type
of photon. SPECT provides lower
resolution but is much less expensive
than PET. Another method, called
magnetoencephalography (MEG),
measures millisecond-long changes in
magnetic fields created by the brain's
electrical currents. Scientists also are
experimenting with computer programs
than can alter or rearrange anatomical
brain images from MRI and PET to match
a standardized brain map, making it
easier to compare the anatomy and
function of different brains and to
measure specific brain structures
objectively. They also are using
combinations of imaging techniques to
obtain a comprehensive picture of the
brain in action.
Brain imaging with positron emission tomography
(PET) reveals the different regions of the human
brain active during various verbal tasks.