A Study of the Limiting Diaphragm Method of Collimation

Abstract

During recent studies on the distribution of radiation in a phantom it was observed that the doses in the peripheral region of the x-ray beam were 20 to 30 per cent lower than the doses in the central axis at the same level. The distribution of radiation was also asymmetrical, the variations being greater on one side of the field than the other. (While these facts have been observed by others, they are frequently overlooked.) The measurements were made in a pressed wood (masonite) phantom with a thimble type ionization chamber. X-rays generated at 200 kv.p. with a half-value layer of 1.15 mm. copper were collimated by means of a cone with a limiting diaphragm, 28.5 cm. from the target, designed for a field size of 10 × 15 cm. at 80 cm. target-surface distance. The term limiting diaphragm, as used in this article, refers to a diaphragm placed close to the target for the purpose of limiting the x-ray beam to a specified field at a specified target-surface distance. A roentgenogram (Fig. 1) of the field irra...