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. [Illustration: Showing:] 1 ²⁴Na intravenously injected To lungs, To body, From arm, From right lung, From left lung 2 Geiger counter detects radiations from ²⁴Na 3 Ink writing recorder shows route of ²⁴Na Technetium-99^{m} Because of its short half-life of six hours, technetium-99^{m}[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. [Illustration: _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-99^{m}. This new, mild isotope can be mixed with other elements and these become the day’s supply of radioisotopes for other scans. Technetium-99^{m} decays in 6 hours. Thus greater amounts, with less possibility of injury, can be administered and a better picture results._] Thulium-170 and Gamma Radiography For years it has been recognized that there would be many uses for a truly portable device for taking X-ray pictures—one that could be carried by the doctor to the bedside or to the scene of an accident. Conventional X-ray equipment has been in use by doctors for many years, and highly efficient apparatus has become indispensable, especially in treating bone conditions. There is, however, a need for a means of examining patients who cannot be moved to a hospital X-ray room, and are located where electric current sources are not available. A few years ago, a unit was devised that weighed only a few pounds, and could take “X-ray pictures” (actually gamma radiographs) using the gamma rays from the radioisotope thulium-170. The thulium source is kept inside a lead shield, but a photographic shutter-release cable can be pressed to move it momentarily over an open port in the shielding. The picture is taken with an exposure of a few seconds. A somewhat similar device uses strontium-90 as the source of beta radiation that in turn stimulates the emission of gamma rays from a target within the instrument.