  The fundamentals of near infrared spectroscopy (NIRS) are reviewed. This
technique allows to measure the oxygenation of the brain tissue. The particular
problems involved in detecting regional brain oxygenation (rSO2) are discussed.
The dominant chromophore (light absorber) in tissue is water. Only in the NIR
light region of 650-1000 nm, the overall absorption is sufficiently low, and
the NIR light can be detected across a thick layer of tissues, among them the
skin, the scull and the brain. In this region, there are many absorbing light
chromophores, but only three are important as far as the oxygenation is
concerned. They are the hemoglobin (HbO2), the deoxy-hemoglobin (Hb) and
cytochrome oxidase (CtOx). In the last 20 years there was an enormous growth in
the instrumentation and applications of NIRS. . The devices that were used in
our experiments were : Somanetics's INVOS Brain Oximeter (IBO) and Toomim's HEG
spectrophotometer. The performances of both devices were compared including
their merits and drawbacks. The IBO is based on extensive efforts of an R&D
group to develop a reliable device, which measures well the rSO2. It is now
used efficiently in operating rooms, saving human lives and expenses. Its use
for research however has two drawbacks: the sampling rate is too small and the
readings are limited to only two significant digits. The HEG device does not
have these drawbacks, but is not developed sufficiently at this time to measure
rSO2. We have measured the HEG readings and compared them with the rSO2
readings of the IBO. Our findings show that the HEG can be used to measure
relative changes of rSO2.
