The Radiological Society of North America Standardization Committee, Technical Bulletin No. 1

Abstract

Chapter I.—The Meaning of Dose, Exposure, and other Physical Terms used AS the art of radiology has developed, the significance of many of its technical terms has been altered, modified, or broadened to suit new methods or to conform with new knowledge. The meanings of the physical terms have now become fairly well established; nevertheless, the earlier changes have resulted in a vagueness in the use of these words that is responsible for many of the difficulties encountered in measuring x-rays. The terms that are used in this bulletin are discussed below with attention to some of the misunderstandings that frequently lead to difficulties. The exposure or dose2 is a measure of a property of the x-rays at a particular place. This place may be situated in air, in tissue or other material, or even in vacuum. The exposure is expressed as a number of roentgens. The exposure is a property of the radiation only. The number of roentgens is a partial description of the x-rays at the place where it is measured or calculated. It tells nothing about what happens to the radiation or about the effects which it produces. It does not indicate whether the radiation is absorbed or passes through the region without absorption. It tells nothing about the material which it penetrates. It applies only to the radiation itself. The average exposure rate, dosage rate, or intensity is obtained by dividing the exposure by the time required to deliver it. Hence the exposure is the product of the average exposure rate and the length of time it continues. The exposure rate is usually expressed in roentgens per minute. The exposure may be measured with an instrument called a dosemeter, exposure meter, or roentgen meter. The dosemeter is intended to measure the exposure only. The reading obtained with the instrument should not be considered as a measure of the absorption of the radiation or of the ionization produced by the radiation. The absorption and ionization necessary to operate the instrument are not the effects that the dosemeter has been calibrated to measure. Exposure, absorption, and ionization are three very different physical effects and should be distinguished clearly. Furthermore, the difference in exposure at two places, one of which is farther from the x-ray tube, is not a measure of the energy absorbed between the two places. The difference is due to three causes: divergence of the radiation, true absorption, and the presence of scattered radiation. The divergence of the radiation refers to the increase in cross-section of the x-ray beam as the distance from its source increases. This increase in size of the x-ray beam implies that the radiation is more widely distributed and, therefore, less intense.