Radiation doses from diagnostic x-ray studies.

Abstract

The average person experiences no significant hazard from roentgen examinations, but in workers handling radioactive materials, such x-ray exposure may produce an excessive cumulative effect. Because the maximum permissible exposure at Brookhaven National Laboratory is 0.3 r per week total body radiation, it seemed desirable to determine the exposure for different types of diagnostic x-ray procedure. A search of the literature revealed insufficient data upon which to base expected radiation exposures (1 – 5). The x-ray apparatus2 used by the Medical Department had, therefore, to be calibrated. This report deals with the methods of measurement and the results. A Victoreen condenser r meter, number 2557, and 10- and 0.25-r chambers were used for calibrating. The 0.25-r chamber had been tested previously with the use of a standard radium gamma source and was found accurate to within 1 per cent of full scale. This chamber was used for all measurements except those made on the photofluoroscopic unit and the fluoroscopic unit, for which the 10-r chamber was used. Two corrections were made for each measurement. One, the barometric and temperature correction, was made in accordance with the tables supplied with the instrument. The second correction, concerned with the response of the r meter at soft x-ray frequencies, was made according to F. Day (6) and ranged between 1 and 10 per cent. Since the response of the r meter at low-kilovoltage potentials drops off, the correction factor was estimated finally after further personal correspondence with Day. A paraffin phantom was used in these measurements and the target-chamber distance in each case was equivalent to the target-skin distance. The phantom was a rectangular box having a base 25 × 30 cm. and a height of 19 cm., which approximates the thickness of a patient. The data were plotted as constant milliampere curves, with the kilovoltage and skin doserate in roentgens per second as the variables. Figure 1 shows the results of the measurements from 30 to 100 kv. Table II shows the exposure received from some common diagnostic examinations. The dose received from the photofluoroscopic unit used varies with the size of the patient, because the time of exposure is controlled by a phototimer. This accounts for the wide range in the dose delivered by this unit. In the technic used for the 14 × l7-inch chest films on employees exposed to ionizing radiation, the exposures were made at 6 feet, the instrument set at 200 ma., with an exposure time of one-tenth second, and with a filter of 0.5 mm. of aluminum in the beam. Thus the only variable between patients is the kilovoltage. Figure 2 shows the dependence of the skin dose on this factor. The broken curve of Figure 2 shows the dose measured without the phantom.