Patient x-ray exposure during fluoroscopic examinations with an analysis of contributing parameters.

Abstract

Throughout a fluoroscopic examination a patient is exposed to an x-ray beam whose dimensions and position relative to the patient are undergoing change. By considering the total examination as a series of exposure time intervals short enough that the exposed area is nearly constant for each interval, the total surface exposure can be expressed by: where: SEI is the surface exposure integral in R-cm2. R is the table-top output rate of the machine in R∕sec. Ai is the beam area in cm2 during each time interval. T is the total number of time intervals in the examination. It is convenient to let each time interval be one second. Then T is the total exposure time in seconds. By introducing a beam-quality factor as suggested by Adams (see Miller, 1), the total energy (E) in ergs can be estimated by: where: HVL is the x-ray beam first half-value layer expressed in millimeters of aluminum. The total x-ray dose (D) received by a specific organ (ovaries, testes, etc.) can be expressed as: where: Xi is a function relating organ dose to the surface exposure. It involves an effective distance and direction between the organ and exposed surface area, beam quality, anatomical composition of the patient, and is itself related to the exposed area. These relationships are useful when considering the factors which contribute to the wide distributions of both surface exposure and organ dose observed during fluoroscopic examinations. A determination of SEI or imparted energy does not involve the function X and is, therefore, relatively simple to make. Several instruments have been developed for the purpose (1–3). The method used in this study was to record the exposure area for each one-second time interval on tape and use a digital computer to determine the SEI for each examination studied (4). Data on approximately 1,200 fluoroscopic examinations at Emory University Hospital have been collected and analyzed. The distributions of SEI for barium enema and gastrointestinal examinations are shown in Figures 1 and 2. From Equation 1 it is apparent that variations in SEI can be attributed to variations in the x-ray machine output rate, the average exposure area, and the total exposure time. By applying a multiple regression analysis to the data it is possible to show to what extent these three factors contributed to the variations observed in SEI. This is shown in Table I. The mean values of SEI, average area, and exposure time for both the staff radiologists and radiology residents in training are shown in Table II along with a comparison of mean SEI values reported by Miller (1).