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Radiation Safety Evaluation of Fluoroscopes

1958; Radiological Society of North America; Volume: 71; Issue: 2 Linguagem: Inglês

10.1148/71.2.227

1527-1315

John Hale, David B. Kusner, Robert O. Gorson, James R. Bartsch,

Nuclear and radioactivity studies

The problems of estimating patient exposure and the scattered radiation exposure to the operator during fluoroscopy are made difficult by the nature of fluoroscopic procedures. Field size and position change widely and rapidly and, for a given examination, the extent of exposure will depend to a large extent on the technic of the fluoroscopist. In each case the exposure to both patient and operator is related to the exposure dose rate, the quality of the radiation produced by the machine, and the scatter from the machine. These factors may be called the inherent radiation characteristics of the machine, since they depend on the x-ray unit itself and not on the manner of its use. During the past ten years we frequently have had the opportunity to examine fluoroscopes with regard to inherent radiation safety factors. While it is not possible to determine the safety of a particular unit according to an absolute scale, it is possible, by comparison, to discover whether a given machine measures up to a good standard in reference to experience and the criteria set forth by the National Committee on Radiation Protection in National Bureau of Standards Handbook 60 (1). The purpose of this paper is to present the results of a review of our calibration and safety survey procedures from the standpoints of accuracy, completeness, and practicality. Method The basic information required in the calibration of fluoroscopes is the table-top exposure dose rate and the half-value layer under specified operating conditions. To evaluate the importance of the parameters that might affect determinations of these values, measurements were made on a typical, modern fluoroscope, a G.E. Model KX-8 500-ma diagnostic unit. The x-ray tube was a G.E. Type HRT-I-2. Its equivalent filtration was specified by the manufacturer to be 1.25 mm. of aluminum. The voltage calibration was checked by a sphere gap at 85 kvp and is thought to be accurate to about 2 kvp. Targettable top distance was 18inches. The tabletop filtration was found to be equivalent to 0.7 mm. of aluminum at 85 kvp. All measurements were made with a Victoreen 25-r condenser ionization chamber. 2 A second chamber was used to monitor the constancy of the x-ray beam whenever possible. With the use of the monitor, experimental data could be determined to a precision of about ±3 per cent. Exposure Dose Rate in Air at Table Top Handbook 60 specifies that the exposure dose rate in air from fluoroscopes shall be less than 10 r per minute at the table top. Our measurements of exposure dose rate have been made with a condenser ionization chamber in the center of a 10 × 10-cm. field in contact with the surface of the fluoroscopic table. For comparative purposes, calibrations are made at a standard operating potential of 85 kvp. The accuracy of exposure dose measurements on fluoroscopes, however, is compromised by the effect of scattered radiation that can be avoided in the calibration of most other types of machine.