Phosphorus-32
The phosphate ion is a normal constituent of the blood. In many kinds of tumors, phosphates seem to be present in the cancerous tissue in a concentration several times that of the surrounding healthy tissue. This offers a way of using phosphorus-32 to distinguish between cancer cells and their neighbors. Due to the fact that ³²P gives off beta rays but no gammas, the counter must be placed very close to the suspected tissue, since beta particles have very little penetrating power. This fact limits the use of the test to skin cancers or to cancers exposed by surgery.
Some kinds of brain tumors, for instance, are difficult to distinguish visually from the healthy brain tissue. In such cases, the patient may be given ³²P labeled phosphate intravenously some hours before surgery. A tiny beta-sensitive probe counter then can be moved about within the operative site to indicate to the surgeon the limits of the cancerous area.
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.