GRID VERSUS AIR-GAP - A COMPARISON OF CEPHALOMETRIC TECHNIQUES

This study compares two cephalometric radiography techniques: one that uses a grid, the other an air gap, both of which increase image contrast by reduction of scatter radiation to the film. A 12.0 cm water phantom was used with a modified cephalometric radiography unit operating at 71 kVp. Air gaps (patient-to-film distances) of 0.0, 7.5, 12.5, 17.5, 25.5 cm were compared with a 10:1 ratio stationary focused grid at 0.0 cm. Comparisons were made from the measured patient surface exposure, the calculated contrast improvement factor, nominal magnification, and image resolution at each distance. Radiation exposure to the patient was 2.52 times greater with the grid than without. Using regression formulas, the contrast improvement factor of the grid was calculated to equate to an air gap of 9.5 cm. The 9.5 cm air gap would reduce patient exposure by 59.6% with similar image contrast and resolution as compared with the grid. Overall magnification was increased by 6.6% with the 9.5 cm air gap.