A Photographic Method of Measuring Fluoroscopic Dose to the Patient

Abstract

Investigators attempting to assess radiation exposure of the population from the medical use of x-ray equipment all agree that an important segment of the dose is contributed by fluoroscopes. In any estimate of patient dose there is an interest in several possible methods of expression: dose to the gonads, dose to the blood-forming or other organs of interest, and total integral absorbed dose. Various attempts have been made to measure and estimate the average dose for specific examinations. Because of the continuously changing location and size of the field, combined with the practice of switching the beam on and off and occasionally adjusting the brightness (dose rate), an accurate measurement of the dose per examination has been virtually impossible by the usual means. It is generally recognized that all of the following factors must be measured: entry dose rate, exposure time, size, shape and location of the field. As a fluoroscope is generally used, all of the above factors may vary, and most often do. The need to make measurements during actual examinations appears to be obvious. Thimble chambers and parallel-plate chambers, as used to monitor therapy beams, could be used to measure dose, but the recording of the important exposure pattern is difficult. Through the co-operation of the Eastman Kodak Company, specially packaged insensitive “Commercial” films in “Ready Pack” envelopes3 have been used to cover the panel of a fluoroscope prior to the examination of a patient. A number of exposures have been made by taping the films to the fluoroscope panel and indicating the location of the patient's shoulders and hips on the film (Fig. 1). After examination, the large films have been measured by means of a special densitometer designed to handle large film sizes. Since it was more convenient to supply us with standard 14 × 17-inch film, we are using two such films side by side, with a means for indexing them relative to each other. A single film would probably be easier to use, but the processing of oversize film presents problems. Preliminary investigation with films confirms our opinion that technics vary considerably even for the same type of examination. In most cases it seems feasible to make a single densitometer measurement for any particular rectangular exposure and evaluate the dose at any point where rectangles overlap by merely adding the various components. The second phase of the dosimetry problem has just been initiated and significant results are lacking. Phantom measurements are being made in which the various entry dose patterns are being reconstructed. For such measurements a technic previously described by one of the authors (E. R. Epp) is being used. It appears that the use of the original exposed films may be helpful in setting up the fluoroscope to expose the phantom. The only serious problem encountered so far has been in the case of certain examinations done by some radiologists, in which the patient is rotated during examination.