An electronic position timer for the fluoroscope.

Abstract

As many as 30 Per cent of the spot-films taken during fluoroscopy may be improperly exposed if the radiologist merely estimates the exposure time.2 Be-fore phototimers were used, the spot-film exposure time was determined by meas­uring the thickness of the abdomen. It occurred to one of the authors (E. R. M.) that the fluoroscopic table top and the fluoroscopic screen mounting could be used as a calipers to measure patient thickness. From the measurement thus obtained, optimum spot-film exposure time could be determined. It is the purpose of this paper to describe a method for con­struction of a spot-film exposure time scale based upon abdominal thickness and to present an electronic timer which uses this scale to control spot-film exposure. The Time Scale An optimum spot-film exposure time, giving a film density and contrast most pleasing to the radiologist, can be de­termined from the thickness of the ab­domen for each patient. On this basis, it becomes possible to construct a scale of optimum exposure times in relation to abdominal thickness for any particular fluoroscope. The fluoroscope used by the authors had the following characteristics: 90 kv, 150 ma, 2 mm. Al filter, 18-inch target-table top distance. For a patient with an abdomen 19 cm. thick an ex­posure time of 0.1 second was required for an optimum gastrointestinal series or barium-enema spot-film. An abdomen 30 cm. thick required an exposure time of 2 seconds. Spot-films were made at several other abdominal thicknesses between these two points, and a curve was plotted of patient thickness vs. exposure time (Fig. 1). This curve is actually the sum of two curves: (a) an exponential curve and (b) an inverse-square-law curve. The quantity of x-ray energy which reaches the spot-film is a function of patient thickness and the distance of the film from the x-ray source. The exponential curve is con­cerned with patient thickness. The in­verse-square-law curve is related to the target-film distance. From the graph reproduced in Figure 1, a time scale was constructed. This scale was attached to the fluoroscopic tower and a scale indicator was fastened to the fluoroscopic screen mount. The proper exposure time required for each patient could be read directly from the scale. At the University of California Hospital such a scale was used successfully for a period of months prior to the installation of a phototimer. Before the scale was used, a large fraction of the fluoroscopic spot-films were unsatisfactory as a result of improper exposure. After installation of the time scale, no films had to be discarded for this reason. The Electronic Spot-Film Timer A simple electronic timer was constructed to adjust automatically the spot-film exposure time in accordance with the time scale just described. The circuit diagram is shown in Figure 2. This timer uses a variable resistor in a resistor-condenser timing circuit. The R-C circuit is attached to the grid of an 884 gas tube.