Sodium-24
Normal blood is about 1% sodium chloride or ordinary salt. This fact makes possible the use of ²⁴Na in some measurements of the blood and other fluids. The figure illustrates this technique. A sample of ²⁴NaCl solution is injected into a vein in an arm or leg. The time the radioisotope arrives at another part of the body is detected with a shielded radiation counter. The elapsed time is a good indication of the presence or absence of constrictions or obstructions in the circulatory system.
The passage of blood through the heart may also be measured with the aid of sodium-24. Since this isotope emits gamma rays, measurement is done using counters on the outside of the body, placed at appropriate locations above the different sections of the heart.
Technetium-99m
Because of its short half-life of six hours, technetium-99m[10] is coming into use for diagnosis using scanning devices, particularly for brain tumors. It lasts such a short time it obviously cannot be kept in stock, so it is prepared by the beta decay of molybdenum-99.[11] A stock of molybdenum is kept in a shielded container in which it undergoes radioactive decay yielding technetium. Every morning, as the technetium is needed, it is extracted from its parent by a brine solution. This general procedure of extracting a short-lived isotope from its parent is also used in other cases. We shall see later that radon gas is obtained by an analogous method from its parent, radium.
Using a “nuclear cow” to get technetium from its parent isotope. The “cow” is being fed saltwater through a tube. The saltwater drains through a high-radiation (hot) isotope. The resultant drip-off is a daughter such as technetium-99m. This new, mild isotope can be mixed with other elements and these become the day’s supply of radioisotopes for other scans. Technetium-99m decays in 6 hours. Thus greater amounts, with less possibility of injury, can be administered and a better picture results.