Henri Becquerel was a French physicist who in 1896 discovered a new property of matter (which was later called radioactivity). This was accidentally observed when Becquerel found out that uranium salt was spontaneously emitting some kind of rays that went through different substances and were able to veil even a photographic film. Then Marie Curie and her husband Pierre Curie carried out more experiments to study, in a quantitative way, the radiation emitted by the salts Becquerel studied cualitatively, and other salts that contained uranium. During this process they found other radioactive substances, such as: thorium, polonium and radium.These radioactive isotopes have proven particularly effective as tracers in certain diagnostic procedures. As radioisotopes are identical chemically with stable isotopes of the same element, they can take the place of the latter in physiological processes. Moreover, because of their radioactivity, they can be readily traced even in minute quantities with such detection devices as gamma-ray spectrometers and proportional counters. Though many radioisotopes are used as tracers, iodine-131, phosphorus-32, and technetium-99m are among the most important. Physicians employ iodine-131 to determine cardiac output, plasma volume, and fat metabolism and particularly to measure the activity of the thyroid gland where this isotope accumulates. Phosphorus-32 is useful in the identification of malignant tumours because cancerous cells tend to accumulate phosphates more than normal cells do. Technetium-99m, used with radiographic scanning devices, is valuable for studying the anatomic structure of organs.
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