Detection of small differences in mass using a direct digital dental X-ray system

Objectives: To evaluate experimentally how in direct digital radiography heterogenous detector response may influence the depiction of small mass differences. Methods: A step wedge of a bone-simulating resin was imaged at 70 kVp using Radio-VisioGraphy ZHR 32000. Exposure time and step wedge position on the detector varied. The pixel values of the stored images of the step wedge were converted to luminance values of the displayed images. Results: The relationship between pixel and luminance values was non-linear. The difference in luminance between adjacent steps varied with position of step wedge. Regardless of position, there was an increase in difference of luminance between steps with increasing exposure. A greater relative change in luminance was consequently seen in the thinner parts of the step wedge. The failure to perceive individual steps of the wedge in its thicker parts was related to insufficient relative change in luminance between steps. Conclusions: The heterogeneous response of the detector resulted in a varying ability to depict small mass changes over the detector area.