Standardization of Roentgen Dosage by Means of Methylene Blue

THE ionization produced by roentgen rays in air can be determined with considerable accuracy if the measurements are properly made and adequate precautions are taken. There are, however, several reasons why it would be of advantage to have available as well a simple and satisfactory chemical method for measurements of x-ray dosage. It seems probable that such a method can be worked out, for many organic substances undergo changes when they are irradiated while in solution. A dilute aqueous solution, containing substances built up from light atoms, has approximately the same absorption and scattering abilities as tissue. Containers filled with the solution of such a substance may be sealed and sent to different places for irradiation and returned to the central laboratory for examination. Such a method would make it easy to obtain reliable standards for the different x-ray machines without very much expense and in the absence of persons with enough training in physics to handle delicate ionization apparatus. In order that a chemical method for measurements and standardizations of roentgen dosage may become practical, the following requirements must be fulfilled: 1. Absorption and scattering of roentgen rays in the medium must approximate that of tissue. 2. The chemical change to be measured must have a simple relation to the dose. The same dose must produce the same change at all times. 3. The medium must be stable enough so that it may be left for at least some few days before measurements are made. 4. The method for determining the amount of change must be relatively simple and should permit determination of the dosage to an accuracy of at least 5 per cent. 5. The medium (solution) should be easy to obtain and reproduce. 6. The change needed for measurements must be produced by a reasonable dose. 7. A fairly small amount of solution should be sufficient for each exposure. Although a number of pitfalls have to be avoided, we believe these requirements can be fulfilled by a properly prepared solution of methylene blue. It is our intention to describe how we succeeded in overcoming some of the difficulties, how the solutions were prepared and irradiated, and how the measurements were carried out Some of the details will be omitted as they have already been described in an earlier publication (4). It is necessary to have in mind that the effect of radiation upon the dissolved substance is influenced by the solvent. In all our experiments distilled water was used. It was necessary to use very dilute solutions in order that the change produced by a reasonable amount of radiation could be determined with fair accuracy. After having tried a number of substances, we chose methylene blue as suitable because it was influenced considerably by irradiation and because the color change could be measured simply and quickly by means of a spectrophotometer.