Diffusive Optical Imaging in Neuroscience
Diffusive Optical Imaging (DOI) also known as Near Infrared Optical tomography (NIROT) or Diffuse Optical Tomography (DOT) is a technique that gives neuroscientists the ability to simultaneously obtain information about the source of neural activity as well as its time course. In other words, it allows them to "see" neural activity and study the functioning of the brain.
In this method, a near-infrared laser is positioned on the scalp. Detectors composed of optical fiber bundles are located a few centimeters away from the light source. These detectors sense how the path of light is altered, either through absorption or scattering, as it traverses brain tissue.
This method can provide two types of information. First, it can be used to measure the absorption of light, which is related to concentration of chemicals in the brain. Second, it can measure the scattering of light, which is related to physiological characteristics such as the swelling of glia and neurons that are associated with neuronal firing.
Typical applications include rapid 2D optical topographic imaging of the event-related optical signal (EROS) or Near infrared spectroscopy (NIRS) signal following brain activity and tomographic reconstruction of an entire 3D volume of tissue to diagnose breast cancer or neonatal brain haemorrhage. The spatial resolution of Diffuse Optical Tomography (DOT) techniques is several millimeters, comparable to the lower end of functional magnetic resonance imaging (fMRI). The temporal resolution of EROS is very good, comparable to electroencephalography, and magnetoencephalography (~milliseconds), while that of NIRS, which measures hemodynamic changes rather than neuronal activity, is comparable to fMRI (~seconds). DOT instruments are relatively low cost ($150,000), portable and immune to electrical interference. The signal-to-noise ratio of NIRS is quite good, enabling detection of responses to single events in many cases. EROS signals are much weaker, typically requiring averaging of many responses.
Important chemicals that this method can detect include hemoglobin and cytochromes.
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