New technique allows scans of babies' brains with infrared light
7 September 2007
An improved technique that uses infrared light to scan
the brain using a head cap will enable infants to be scanned for studying
brain development or for assessing brain injury.
In a paper published by
the Proceedings of the National Academy of Sciences in July (1),
Dr Joseph Culver, assistant professor of radiology at Washington University
School of Medicine in St. Louis, and 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.
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.
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
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 fibre 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 fibre optic cables in the new scanning unit. With
54 fibre optic cables, high-density DOT has four times the density of
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
Culver is currently collaborating with paediatricians to
adapt the technology for use in neonatal and paediatric 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."
1. 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.
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