Patient exposure requirements for high contrast resolution in digital radiographic systems

The contrast resolution of modern digital radiographic equipment is primarily limited by quantum mottle. The derivation of the contrast resolution of a system as a function of radiation exposure is presented. Calculations based on this derivation show that achieving 0.8% contrast resolution (128 meaningful gray levels) with 1-mm spatial resolution may require a patient exposure of 1.7 rad (1.7 cGy) per image of a thick body part (20 cm tissue). Acceptable clinical studies often can be obtained with only 5% contrast resolution (20 meaningful shades of gray), but even this level of contrast may require a patient exposure of nearly 90 mrad (0.9 mGy) per image. A comparison of three commonly used methods of reducing patient radiation exposure (pulsed imaging, temporal averaging, and recursive filtering) shows that, in theory, a pulsed system achieves the highest contrast resolution for the same total exposure.