Several years ago, colloidal preparations of thorium dioxide were introduced into medicine for use in radiography.1 Thorium is opaque to roentgen rays and throws a deep shadow in roentgenograms. It was found that thorium dioxide sol could be introduced into body cavities and even into the blood stream with little immediate deleterious effect. When injected into a vein, the particles are picked up by the reticulo-endothelial system, especially in the liver and spleen, where the material remains indefinitely; the compound thus serves to make these organs less penetrable to the x-rays. Other structures can be visualized by direct injection of thorium dioxide sol into their cavities; these include the cerebral ventricles, the urinary tract, the uterus and fallopian tubes and various accessible blood vessels.
The immediate utility of colloidal thorium dioxide preparations has tended to obscure an important characteristic of this substance: its radioactivity. Over four years ago the