The first method of imaging the interior of the human body came with the discovery of X-rays in 1895. A major advance in image resolution came with computerized axial tomography (CAT), a method of imaging which combines X-ray images from numerous axial viewpoints.
A new method is positron emission tomography (PET). PET allows us to obtain very precise tomographic information of a specialized nature. For instance, we can obtain information about cells which are metabolizing a large amount of glucose. In this method, an analog of glucose, 2-fluoro-2-deoxy-D-glucose, or FDG, is introduced into a person's bloodstream. The analog is incorporated into the cells along with normal glucose by the same carriers. Hexokinase converts FDG into FDG-6-phosphate, but its metabolism stops at that point, and the fluorine may be detected.
The key to this procedure is using the isotope 18F in FDG, since 18F is unstable to positron decay (half-life 110 minutes). In the production of this isotope, a cyclotron accelerates deuterons (2H) toward a target of 20Ne and H2.A deuteron and a neon nucleus collide to form 18F, which chemically reacts with the hydrogen gas to form hydrogen fluoride (H 18F). This is used in the synthesis of FDG.
The cells which are metabolizing a large amount of glucose will have a high concentration of FDG tagged with 18F. When a 18F nucleus emits a positron, the positron travels several millimeters before slowing to a stop. It then reacts with an electron to form two photons, which are emitted 180˚ apart. These photons are detected by a ring of detectors circling the person's body. Thus the original site of the decay can be reconstructed.
Another use of PET involves the incorporation of the isotope 15O (half-life 122 seconds) into water. This can be accomplished by bombarding a target of 14N with protons from a cyclotron. The water (with 15O) is introduced into the bloodstream. Detailed information can be obtained as to what capillaries are open or closed and therefore what areas of the brain are active.
If a sample of 15O is to have an activity of 10 mCi at a time 8 minutes after it is created and injected into the body, what must its activity be at the time it is synthesized?