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 ima ge intensifying screen, during fluoroscopy, to attenuate the incident forward scattered radiation. This reduction in scatter improves the ra diographic contrast and hence image quality. However, this improved qu ality is achieved at the expense of greater radiation exposure to the patient, as this higher exposure is required to produce satisfactory i mage 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 examin ed with the grid removed for fluoroscopy but returned for image acquis ition. The fluoroscopic dose-area product (DAP) was recorded for both groups using a dose-area product meter. The DAP was not recorded for s pot 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 bes t estimate being 0.48. Multiple regression methods to take account of the effects of weight, sex and time: revised this estimate as 0.57, wi th 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 var iation, which represents over a half of the remaining variation once s ex, weight and exposure time have been allowed for. We therefore recom mend that the routine use of an anti-scatter grid during fluoroscopy s hould be abandoned. The use of a grid should be restricted to those pa tients (usually obese) when poor visualization is obtained in its abse nce.