Positron Emission Tomography
Before about 1970, physicians had few techniques with which to examine the internal structure of the living human body. X rays, discovered by Wilhelm Conrad Röntgen in 1895, had been utilized for decades, but could only show bones (which absorb x rays) and some soft tissue around them. Viewing other organs was difficult.
In the early 1970s the field of medical informatics was developed by Robert Ledley and others, in which computers and other information technologies aid physicians in diagnosis. South African physicist Allan M. Cormack and British engineer Sir Godfrey Hounsfield independently developed the field of x-ray computed tomography, referred to as simply CT, or CAT. Computed tomography uses the fact that different tissues absorb varying amounts of x-ray energy--the denser the tissue, the more energy absorbed.
An x-ray beam sent through tissue will therefore exit with a reduced energy, depending on what tissue and organs it went through. The information obtained from many x rays passed through the body at many different angles can provide a picture of the body section. The CAT scan was superior to x rays, but still suffered from some soft tissue x-ray absorption. However, its development did provide scientists with the mathematical framework and computer algorithms necessary to reconstruct images from scattered beams.
Scientists next developed scanning methods using biological tracers. A tracer is a compound made radioactive by replacing one of its atoms with a radioactive atom. When the tracer is placed in the body through a blood vein, it will accumulate in an organ (such as the brain), and when it gives off a gamma ray (a high energy photon), that particle can be detected outside the body. Much like the CAT scan, multiple photon scans are performed and an image is reconstructed. This single photon emission computerized tomography (SPECT) has several drawbacks, as many of the photons either emitted are missed by the detectors or located only within a cone roughly centered around the location of their creation. Also, there is a limit to the amount of radioactive tracer that can be safely injected into the body.
Positron emission tomography (PET) combines the principles of CT and SPECT together. PET uses tracers too, but these tracers emit positrons, a particle with all the properties of the electron except for its positive charge. Common positron-emitting isotopes used as PET tracers such as fluorine-18, carbon-11 and a form of water, hydrogen combined with oxygen-15. When these tracers are injected into the body, the emitted positron comes to rest, and, attracted to a charge of the opposite sign, annihilates with an electron. This event produces two photons which fly away in opposite directions (so as to conserve momentum), and the PET detectors detect both, which arrive at nearly the same time but from opposite directions. CT methods are then used to make an image. This detection along a line makes PET about 10 to 50 times more sensitive than SPECT.
Positron emission tomography has proved especially useful for studying brain and heart functions. PET has been used to show which regions of the brain are active at any one time, and has provided real-time images of the physical changes associated with thought processes--a mapping of the neural systems responsible for thought. A typical PET can locate changes in activity to within just a few millimeters.
Since about 1997 scientists have been using yet another imaging stragegy, functional magnetic resonance imaging (fMRI) to map brain functions, mostly replacing PET for this particular application.
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