Scientists at the Washington School of Medicine in St. Louis (Missouri) have developed an instrument that relies on diffuse optical tomography (DOT), a technique that until now had been limited to small regions of the brain.
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The new DOT instrument covers two-thirds of the head and involves shining LED lights on it; for the first time, it can image brain processes taking place in multiple regions and brain networks, such as those involved in language processing and self-reflection (daydreaming). What's more, it avoids the radiation exposure and bulky magnets that positron emission tomography (PET) and magnetic resonance imaging (MRI), respectively, require.
The new approach is ideally suited for children and for patients with electronic implants, such as pacemakers, cochlear implants, and deep brain stimulators (used to treat Parkinson’s disease). The magnetic fields in MRI often disrupt either the function or safety of implanted electrical devices, whereas there is no interference with the optical technique.
“When the neuronal activity of a region in the brain increases, highly oxygenated blood flows to the parts of the brain doing more work, and we can detect that,” says senior author Joseph Culver, Ph.D., associate professor of radiology. “It’s roughly akin to spotting the rush of blood to someone’s cheeks when they blush.”
The technique works by detecting light transmitted through the head and capturing the dynamic changes in the colors of the brain tissue. It has the potential to be helpful in many medical scenarios as a surrogate for functional MRI (fMRI), the most commonly used imaging method for mapping human brain function. fMRI also tracks activity in the brain via changes in blood flow. In addition to greatly adding to our understanding of the human brain, fMRI is used to diagnose and monitor brain disease and therapy.
Because DOT technology does not use radiation, multiple scans performed over time could be used to monitor the progress of patients treated for brain injuries, developmental disorders such as autism, neurodegenerative disorders such as Parkinson’s, and other diseases.
Better still, DOT technology is designed to be portable, so it could be used at a patient’s bedside or in the operating room.
The researchers have many ideas for applying DOT, including learning more about how deep brain stimulation helps Parkinson’s patients, imaging the brain during social interactions, and studying what happens to the brain during general anesthesia and when the heart is temporarily stopped during cardiac surgery.
For the current study, the researchers validated the performance of DOT by comparing its results to fMRI scans. Data was collected using the same subjects, and the DOT and fMRI images were aligned. They looked for Broca’s area, a key area of the frontal lobe used for language and speech production. The overlap between the brain region identified as Broca’s area by DOT data and by fMRI scans was about 75 percent.
In a second set of tests, researchers used DOT and fMRI to detect brain networks that are active when subjects are resting or daydreaming. Researchers’ interests in these networks have grown enormously over the past decade as the networks have been tied to many different aspects of brain health and sickness, such as schizophrenia, autism and Alzheimer’s disease. In these studies, the DOT data also showed remarkable similarity to fMRI—picking out the same cluster of three regions in both hemispheres.
While DOT doesn’t let scientists peer very deeply into the brain, researchers can get reliable data to a depth of about 1 cm of tissue. That centimeter contains some of the brain’s most important and interesting areas, with many higher brain functions—such as memory, language, and self-awareness—represented.
During DOT scans, the subject wears a cap composed of many light sources and sensors connected to cables. The full-scale DOT unit takes up an area slightly larger than a phone booth, but Culver and his colleagues have built versions of the scanner mounted on wheeled carts. They continue to work to make the technology more portable.
The results are now available online in Nature Photonics; for more information, please visit http://dx.doi.org/10.1038/nphoton.2014.107.
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