Interest in the patient dose incurred in diagnostic fluoroscopy and radiography, together with a tendency toward certain changes in technic and equipment, led to the belief that some investigation into the effect of these changes on the patient dose was justified. The changes in mind are the tendency toward reduced tube current and higher voltages in fluoroscopy and the use of tube-supporting structures requiring longer shock-proof cables between the x-ray tube and the high-voltage generator. The assumption is often made that a reduction in tube current will bring about a directly proportional reduction in patient dose. This assumption may not be valid at the low currents used in fluoroscopic procedures, since the electrical capacity of shock-proof cables acts as a filter, having a tendency to alter the waveform to the x-ray tube in the direction of constant potential, an effect which is related to the length of the cable, the tube current, and the kilovoltage used. Even before the use of shock-proof cables, Taylor, in a paper published in 1933, pointed out that the capacitance of the then current aerial systems had a bearing on waveform and dose rate. The purpose of this discussion is to report the results of a study made in an effort to relate patient dose to these factors. Equipment The equipment used for the study was a full-wave rectifying unit rated at a maximum voltage of 130 kvp and a maximum current of 500 ma. A high-voltage rotating anode tube of similar ratings was used. From regular catalogue listings, four sets of cables were selected, with single cable lengths of 15, 24, 30, and 40 feet. It should be borne in mind that the total cable length involved in an installation is twice the length listed here. The nominal capacitance of the cable used was 65 micromicrofarads per foot. Since some of the data presented here were dependent upon tube current, the milliammeter in the control was carefully checked against a milliammeter placed in the high-voltage circuit near the x-ray tube. The two systems were found to agree within 1 per cent at currents greater than 1 ma, the minimum tube current used. Kilovoltage Calibration In a study such as this, one of the questions that always arises is the accuracy of the voltage calibration of the x-ray machine under the actual conditions of installation and use. Calibrations adequate for routine fluoroscopy and radiography may not be sufficiently exact for laboratory studies. Modern equipment design makes it difficult to check kilovoltage by means of the usual sphere-gap calibrations. Even when sphere gaps can be connected into the circuits, there is the difficulty in getting accurate readings at high tube currents where short times must be used. The system proposed by Newell and Renny was considered, but it was not adopted because it requires a sphere-gap calibration to establish the base line.
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Trout et al. (1960) studied this question.