The secondary radiation grid; its effect on fluoroscopic dose-area product during barium enema examinations

The secondary radiation grid is placed between the patient and the image intensifying screen, during fluoroscopy, to attenuate the incident forward scattered radiation. This reduction in scatter improves the radiographic contrast and hence image quality. However, this improved quality is achieved at the expense of greater radiation exposure to the patient, as this higher exposure is required to produce satisfactory image density. Patients scheduled to have a barium enema were randomly allocated into two groups. One group had the procedure performed with the grid in situ throughout the procedure. The second group was examined with the grid removed for fluoroscopy but returned for image acquisition. The fluoroscopic dose-area product (DAP) was recorded for both groups using a dose-area product meter. The DAP was not recorded for spot film acquisition. The results suggest that there is approximately a 50% reduction in DAP to the second group. We can be 95% certain that, on average, the ratio of doses in patients without the grid in situ to those screened with the grid is between 0.40 and 0.58, with the best estimate being 0.48. Multiple regression methods to take account of the effects of weight, sex and time, revised this estimate as 0.57, with 95% confidence interval from 0.52 to 0.63. Sex, weight and exposure time accounted for 61% of variation in dose-area products, with time alone accounting for 41%. Grid status accounted for another 22% of variation, which represents over a half of the remaining variation once sex, weight and exposure time have been allowed for. We therefore recommend that the routine use of an antiscatter grid during fluoroscopy should be abandoned. The use of a grid should be restricted to those patients (usually obese) when poor visualization is obtained in its absence.