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By
Michael Purdy
Aug.
20, 2007 -- Researchers hoping to better understand
the development of the infant brain have long been stymied
by a formidable obstacle: babies just don't want to
sit still for brain scans.
"There
have been some studies that obtained brain scans of
infants while they were napping or sedated, but
what
we'd really like to do is to scan their brains when
they're sitting on a parent's lap, seeing new things,
hearing new words and interacting with the environment,"
says Joseph Culver, Ph.D., assistant professor of radiology
at Washington University School of Medicine in St. Louis.
In
a paper published by the Proceedings of the National
Academy of Sciences in July, Culver and his colleagues
report that they've improved a recently developed brain
imaging technique to the point where it will allow such
scans. In addition to aiding basic research, the technology,
known as high-density diffuse optical tomography (DOT),
should help clinicians treating infant brain injury
by making it possible to monitor brain function at infants'
incubators.
Using
scans to determine what parts of the brain become active
during a mental task, an approach known as functional
brain imaging, has been the source of many of neuroscience's
most important recent insights into how the human brain
works. But until now, it's been very difficult to apply
this approach to infants. One such brain imaging technique,
functional magnetic resonance imaging (fMRI), inserts
volunteers into a tightly confined passage through a
huge, noisy magnet, an environment that even adults
find unnerving and difficult to sit still in. Similarly,
computed tomography (CT) scans involve large, loud equipment,
and also expose patients and volunteers to radiation
exposure levels generally considered unacceptable for
research studies of infants.
The
DOT scanner, in contrast, uses harmless light from the
near-infrared region of the spectrum and is a much smaller
and quieter unit. "It's about the size of a small
refrigerator, and it doesn't make any noise," Culver
says. Diffuse optical brain imaging was originally developed
in the 1990s by research groups in the United States,
Europe and Japan. To scan a patient or volunteer with
high-density DOT, scientists attach a flexible cap that
covers the exterior of the head above the brain region
of interest. Inside the cap are fiber optic cables,
some of which shine light on the surface of the head,
and some of which detect that light as it diffuses through
tissue.
"The
fact that light will diffuse through tissue may seem
surprising at first, but almost everyone has held a
flashlight up to his or her hand and watched the light
shine through the other side," Culver notes. "The
flashlight's white light becomes visibly reddened, because
there's a window in the near infrared region of the
spectrum where human tissue absorbs relatively little
of the light."
Unlike
X-rays or ultrasound, near-infrared light passes through
bone with relatively little attenuation. Scientists
can use the diffusing light to determine blood flow
and oxygenation in blood vessels of the brain. When
these characteristics increase, researchers assume that
the area of the brain they are scanning is contributing
to a mental task.
Most
previous studies have not used diffuse optical imaging
in conjunction with tomography, a computerized approach
to data analysis that allows depth sectioning and is
more commonly applied to X-ray and positron emission
scans. Adding tomography became possible because of
the greater density of fiber optic cables in the new
scanning unit. With 54 fiber optic cables, high-density
DOT has four times the density of previous scanners.
To
prove that they achieved sufficient resolution for functional
brain imaging, scientists used high-density DOT on human
volunteers to link stimulation of parts of the visual
field to activation of corresponding areas in the brain's
visual cortex. "This is called retinotopic
mapping of the visual cortex, and it's a classic functional
brain imaging task that was used to establish the validity
of earlier neuroimaging techniques like fMRI and PET,"
Culver says. "Before the development of our high-density
DOT system, detailed retinotopic maps like this weren't
possible with non-invasive optical imaging."
In addition to enabling infant brain scans, high-density
DOT should make it possible for neuroscientists to scan
adults engaging in complex tasks that are difficult
in the tight confines of an fMRI scanner, such as playing
a game or engaging in conversation.
Culver
is currently collaborating with pediatricians to adapt
the technology for use in neonatal and pediatric intensive
care units. Scientists hope to use the technology to
assess the effectiveness of therapies for brain injury
in infants.
They
are also working to shrink the size of the unit further,
hoping to develop clinical systems "with a footprint
similar to a microwave."
Zeff
BW, White BR, Dehghani H, Schlaggar BL, Culver JP. Retinotopic
mapping of adult human visual cortex with high-density
diffuse optical tomography. Proceedings of the National
of Sciences, July 15, 2007.
Funding
from the National Institutes of Health, Washington University
in St. Louis and the Mallinckrodt Institute of Radiology
supported this research.
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