Scatter radiation in cephalometric radiography: the effects of grid and collimation

OBJECTIVES: To measure the scatter fraction in cephalometric radiography with and without a grid and to estimate the patient's dose reduction by using a specially fabricated collimator without a grid. METHODS: A 16 cm thick water-equivalent phantom was used to measure the scatter fraction. The projection geometry was the same as our conventional one, i.e. the distance from the X-ray source to the centre of the phantom was 200 cm. A photostimulable phosphor plate was used as an X-ray sensor and was 20 cm from the centre of the phantom. The measurements were performed in four combinations: with an antiscatter grid in place, with a specially fabricated collimator in place, with neither in place, and with both in place. The images of a human dried skull were obtained by using the above parameters and were subjectively assessed for a visibility of several reference points. RESULT: The scatter fraction in the regularly collimated geometry with the antiscatter grid in place was 0.35 for a relative exposure of 1. With the antiscatter grid removed, the scatter fraction increased to 0.68, but the relative exposure was reduced to 0.3. With a special collimator, the scatter fraction was reduced to some extent in all instances, but the exposure was reduced to 0.45. The visibility of the reference points was acceptable in all conditions. CONCLUSION: In cephalometric radiography, removal of the antiscatter grid yields a significant reduction in exposure with no significant loss of information.