Visualization of an internal organ by radioisotope scanning is primarily dependent on the selective deposition of a γ-emitting radioisotope in the specific organ under study. Just as in cholecystography and renal pyelography, physiological concentrating mechanisms are utilized to concentrate the material. With radioisotope scanning, however, it is not necessary to achieve the high concentration necessary to produce opacity to x-rays. The relative concentration of the radioisotope with respect to its surroundings is the important criterion rather than the absolute concentration.
To visualize the thyroid, one utilizes the ability of the gland to concentrate iodide. In liver scanning, 2 types of concentrating mechanisms are used.1 The dye, rose bengal, is concentrated by hepatic parenchymal cells prior to its excretion via the biliary tract into the intestinal tract. The second mechanism depends on the high concentration of particulate matter, such as colloidal gold particles, in hepatic reticuloendothelial cells after intravenous injection.