Near infrared light
Absorption of light by blood
Cerebral Cortex
PET ( positron emission tomography ) is a nuclear medical tool for the measurement of human brain activities. Examinee is administered the compound labeled by some radioactive isotopes, which decay emitting a positron having a shorter half-life than the other isotopes. An emitted positron travels a little in the human body and it collides with a normal electron and then changes a pair of photons. The energy of emitted photons is 511 keV, equal to the electron energy. A pair of photons proceed just in the opposite direction, so it is possible to detect them by the counters surrounding by the brain, and analyze the position of the distribution of the tracer in the brain, using the similar technique of X-ray CT reconstruction tomography technology. For example, using FDG ( fluorodeoxyglucose ), labeled by F18 (half-life 110 min.), it is possible to monitor the glucose metabolism of the brain, and also using water substituted by O15 (half-life 2 min.) to monitor the blood volume. In other cases, C11 (half-life 20 min.) and N13 (half-life 10 min.) are also used. A cyclotron and a labeled tracer-synthesizer are used for the production of these artificial isotopes and the radio-pharmaceutical compound as a tracer. Thus, the PET system is large in size and expensive. PET also has radiation exposure both to the examinee and the examiner.