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 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 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
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 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. To top
Save
this page on del.icio.us
|
