THE CONVERSION FACTOR FOR IMAGE INTENSIFIERS.

Abstract

Early roentgen electron optical image intensifiers were said to produce an increase in brightness of 500 to 1,000 times when compared with a standard fluoroscopic screen. The light levels were, therefore, of the order of 0.003 cd∕m2 to 3 cd∕m2. Different manufacturers now claim between 1,000 and 6,000 times intensification, but they give very vague information about the screen used for the comparison. Often a Patterson B-2 or CB-2 screen is said to have been employed, but no data are given concerning its age and other parameters. Sometimes special screens have been utilized which are not available on the market. Frequently no mention is made of the radiation quality employed for the measurements. In 1956 the International Commission on Radiological Units (ICRU) formed a Subcommittee to establish standards for measurements of image intensifiers. This Subcommittee finished its work in 1962 and the reports are published in the National Bureau of Standards Handbook 89 (August 1963) (1). It is important to summarize and comment on some of the recommendations and definitions made by this Subcommittee. Terminology A roentgen image intensifier is defined as a device which instantaneously converts a roentgen radiation pattern into a corresponding light image of higher energy density. This is partly a misconception as the energy density is not higher in the output light image than in the input radiation pattern. The light photon density at the output screen is considerably raised compared with the roentgen photon density at the input screen of the image intensifier. The high-energy density of the primary roentgen-ray pattern, however, is never reached. The conversion factor of a roentgen image intensifier is defined as the ratio of the luminance of the output screen to the input exposure rate of the applied roentgen radiation (Table I). Thus, it is a measure of the efficacy of the image intensifier. The symbol for conversion factor is Gx and the unit is cd × m−2 × (mR)−1 × sec [= (cd∕m2)∕(mR∕sec)]. The luminance of the output screen is to be expressed in candela per square meter (cd∕m2) which is the internationally accepted unit. The light units (2) are, for the time being, an impenetrable jungle of jargon for the uninitiated, and ample possibilities exist for misunderstanding. The recommended unit cd∕m2 is also called nit (Table II). This unit is easier to use but is presently less well known. In English and American literature the unit foot-lambert, which equals 3.426 nit, is often employed. On the European continent the unit apostilb (asb) is quite common, but is actually incorrectly used.