FoundationalConceptsNeuroscience

person’s circulatory system, it will flow throughout the body and be
absorbed into cells just as if it is normal glucose. However, once inside
the cell, the fluorinated glucose is recognized as an imposter and can
no longer participate in the chemical reactions leading to energy
generation. It accumulates in the cell, and that cell becomes a hot spot
of radioactivity. By detecting radioactive hot spots in the brain of a
person injected with radioactively labeled glucose, locations of great-
est neural activity can be determined. This is precisely what PET does.
A PET scanner consists of an array of many detectors designed to
measure gamma rays. The person lies in the scanner, the head sur-
rounded by these detectors. The person is injected with radioactive
(fluorine-18) glucose, and the location of the radioactive glucose in
the brain is determined by measuring the sites of positron decay—
sweet.
Another PET scan method uses radioactive water (HzO) in which
the oxygen is O-15, the isotope of oxygen that decays by positron
emission. Radioactive water is injected into the subject’s blood. It
flows throughout the body. It so happens that the brain adjusts the
flow of blood within it so that regions generating more neural signals,
and thus requiring more energy, receive a more robust flow of blood.
The regions receiving a more robust flow of blood will also have a
larger quantity of radioactive water. The flow of radioactive water,
and thus the flow of blood, can be measured in the PET scanner by de-
tecting the gamma-ray photons resulting from the positron decay of
oxygen-15. Thus, both fluorine-18 glucose and oxygen-15 water can
be used to measure the amount of neural activity in different regions
of the brain.
The radioactive carbon-11 isotope is used in a different way.
Suppose one wants to measure the locations of dopamine receptors in
a person’s brain. One may choose a molecule that sticks to dopamine