Light beam diaphragm collimation errors and their effects on radiation dose for pelvic radiography

Purpose: To investigate the range of collimation errors in x-ray rooms and to calculate their possible effects on the radiation dose for anteroposterior pelvic examinations. Methods: A collimator test tool was suspended at 3 heights (14, 21, and 28 cm) above the table Bucky in 9 x-ray rooms. Heights corresponded to the typical patient thickness (mean, ± 2 SD) of 67 patients undergoing anteroposterior pelvic radiography. The x-ray beam was visually collimated to the inner boundary of the test tool and exposed to radiation. Differences between the visualized field size and the resultant x-ray field size (corrected for magnification) indicated a collimation error. Next, using a pelvic phantom, minimum textbook collimation was set and then changed and verified to simulate a range of possible collimation errors. Phantom examinations used a standard anteroposterior technique with exposure termination using outer automatic exposure control chambers. Dose area product (DAP) was recorded. Results: All but 1 of the 9 x-ray machines had a smaller irradiated area than was visually set. Errors ranged from a 16% reduction in irradiated field size to a slight overirradiation by 0.4%. Assuming that these errors could be larger in other institutions, additional errors with a range of -27% to 18% were simulated. Increases in field size by 1 cm (superiorly/inferiorly) increased the DAP by 5%. Laterally, a 1-cm increase caused a 4% rise in DAP. Increases of 1 cm in both planes raised DAP by 4%. Discussion: Within a single clinical department, minimal collimation errors were demonstrated. Further evidence from multiple centers would be beneficial; however, such low incidences might reflect strict legislative requirements governing the use of ionizing radiation. Understanding the magnitude of any error is important, but it is also important to ascertain an error's influence on the effective radiation dose for any given examination. Conclusion: Overall, collimation errors were minimal and favored underirradiation. Small collimation errors can affect DAP and are more dose significant in the superior/inferior plane