Of the many imaging techniques in use today, each has its limits and its strengths. Such factors as cost, labor intensiveness, and fineness of detail in the image help to determine the best uses for each form of imaging, whether in research or in clinical practice. One technique that shows promise in both areas, but that has made a name for itself thus far primarily in research, is positron emission tomography, or PET.
A PET scan measures the distribution in the body of a radioactively “labeled” substance that the patient has received shortly before the scan. Adding a radioactive label to compounds such as glucose (the main source of energy for the brain) permits researchers to the monitor metabolism—roughly, the rate at which energy is being used, or the rate of activity going on—at particular sites. Brain cells take up the radioactively tagged glucose for use; the glucose is metabolized, and the transiently radioactive atoms remain inside the cells, giving off positively charged particles. These “positrons” quickly collide with nearby electrons, then give rise to gamma radiation, which can be detected outside the body and mapped by a computer.
The resulting image of the brain, which is often enhanced with color to make the different values easier to see, quite clearly distinguishes those parts of the brain that are using more glucose from those that are using less. Thus PET can show where nerve cells are more active and where they are less so, not only in cases of disease or disorder but during a particular task or thought or emotion. As an even more flexible measurement of function in the normal brain, PET can detect changes in local blood flow. Small blood vessels respond very rapidly to the needs of nerve cells, axons, and dendrites, so that a full scan's worth of information can be gathered in 40 seconds, as compared with the 45 minutes required to measure glucose metabolism.
Although the PET scan may be familar to most readers as a series of still pictures, the technique is in fact highly dynamic and well suited to giving an ongoing picture of moment-by-moment changes in the working brain. In this aspect it borrows from several earlier techniques that were developed to