30. P.E.T. image of brain. Courtesy of the Department of Radiology, UIHC.

Positron Emission Tomography or P.E.T. is unique among imaging techniques because it produces an image of organ or tissue function. Other imaging techniques such as X ray, CT, MR, and sonography depict organ or tissue anatomy but cannot discern physiological activity within them.

The physiology of an organ refers to the function and biochemistry of tissue and cellular processes. The information P.E.T. can provide about an organ's physiology enables physicians in many fields to increase diagnostic accuracy, improve assessment of treatment, and raise overall understanding of diseases.

To image a specific biochemical activity of an organ, a radioactive substance, called a radiotracer or radiopharmaceutical, is injected into the body or inhaled. The tracer is usually a radioactive equivalent of a substance that occurs naturally within the body such as water or sugar. The radioactive isotope is identical to the body's own nonradioactive isotope except that its atoms have a different number of neutrons.

P.E.T. scanning uses isotopes which are unstable (hence the term radio isotope) and decay very quickly by the emission of positrons. A positron is a particle which is identical to an electron except that it is positive (+) whereas the electron is negative (-). When a positron collides with an electron, they annihilate each other. Their original mass is converted into two photons (beams of light) called gamma rays. These gamma rays travel in opposite directions and strike radiation detectors positioned around the patient. Each time a gamma ray hits the detector, a computer records its location. From this information the computer produces cross-sectional images or pictures of the organ under investigation.

31. Use of fluoroscope during surgery. From United States Army X-ray Manual, Paul B. Hoeber, New York. 1919.