Isotope of Silver Used in Diagnostic Imaging

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Silver is a white, lustrous, soft, metallic element with the chemical symbol Ag (from Latin argentum, derived from Proto-Indo-European h2erg ‘shiny, white’). It occurs naturally in its pure free element form, as an alloy with gold and other metals, and in minerals such as argentite and chlorargyrite. Silver is used for ornaments, jewelry, tableware, and coins. It is also a major component in photographic film and in compounds such as silver nitrate and silver iodide, and it is an essential ingredient in some medicines.

The isotope of silver most commonly used in medical applications is the radioisotope iridium-109 (Ir-109). Ir-109 decays with a half life of just over 24.6 seconds to its daughter isotope, silver-109m, which is a very effective radioisotope for use in diagnostic imaging.

Ir-109 is produced in a variety of ways, most often using iridium targets as an activator (e.g., iridium nitrate). The isotope is then collected on a high-purity glass fiber that is wrapped around a Geiger counter and placed in a small “house” of lead bricks to minimize background radiation. The counter then plots the number of beta particles detected over time on a monitor, producing a real-time histogram that looks exponential.

In a similar manner to how a photograph develops in a darkroom, the isotope of silver-109 becomes a radioisotope that can be easily and rapidly eluted from the iridium target using physiologically acceptable ionic and aqueous buffer solutions. The isotope may then be administered directly to a patient, if necessary, within a matter of seconds.