Domenick Papa

HEAVY‐METAL CHELATES AND CESIUM SALTS FOR CONTRAST RADIOGRAPHY

The search for the ideal radiographic contrast medium continues unabated. To be sure, there have been many valuable contributions to contrast radiography of special organs: for example, air and other gases for the study of the central nervous system and various organic iodine derivatives for such techniques as cholecystography and pyelography. Nevertheless, the ideal medium for general radiographic use has successfully eluded discovery thus far. There is n ore or less general agreement about the basic characteristics desirable in such a medium: (1) satisfactory radiopacity (primarily a function of the atomic number of the element in the medium, but also related to concentration), (2) stability, (3) pharmacological inertness, and (4) minimal toxicity and sensitizing properties. In 1952, while studying chelation in relation to the detoxification of heavy metals, we became interested in the application of this principle to contrast radiography. On theoretical grounds, at least, it seemed that a heavy metal chelate might satisfy all of the criteria of the so-called ideal radiographic medium. Consequently, a long-term investigation of these compounds was initiated. The term chelation, derived from the Greek XqXq (claw), signifies the tenacious grip that certain organic compounds have on metals, particularly heavy metals and the alkaline-earth group. When a metallic ion combines with one of these organic compounds, t.he resulting substance is a metal complex. If the substance combining with the metal has two or more donor groups, so that a ring compound is formed, the latter is termed a chelate. Simple diagrammatic illustrations of complex formation and chelation may be schematized as follows: