CLINICAL UNIT BASED RESEARCH
Optical Oximeter Brain Activity Measurement
This is a procedure where low level of invisible laser light is applied to the forehead by fiber-optic connectors. A small amount of the light penetrates the skull and bounces back. It is detected by a sensitive receiver which uses information about the light waves to determine brain activity. This is often used in hospitals to monitor the brain activity of premature infants. An individual sits and wears a spandex cap which holds small quarter-sized connectors against the forehead for about 30 minutes. While wearing the cap individuals are sometimes asked to perform computer tasks or think of a specific memory. This technique works because biological tissue is relatively transparent to light in the near infrared range.
Lasers emitting four different near infrared wavelengths are coupled to the individual's head via a fiber-optic cable (optode). Since light is highly scattered after entering tissue, two receiving optodes (placed ~ 2 and 4 cm away from the emitter), collect light that has passed through the underlying tissue in a banana-like path. Oxy and deoxyHgb have different absorption spectra, and their concentrations are calculated using the modified Lambert-Beer law from the degree of absorption at these wavelengths. The near optode will primarily detect photons taking a shallow path through extracerebral tissue. The far optode will primarily detect photons following a deeper path through the underlying PFC. The Somanetics Invos Cerebral Oximeter system (used in our laboratory) permits simultaneous measures of left and right PFC (2 laser optodes, 4 receivers).
For more information see Villringer, A. and Chance, B., Non-invasive optical spectroscopy and imaging of human brain function., Trends Neurosci., 20 (1997) 435-442