What is the Moire effect, and why is it a concern with digital detectors using certain grids?

• A. Moire occurs when grid lines produce random noise
• B. Moire occurs when grid lines are perfectly aligned with detector sampling
• C. Moire is not a concern with digital detectors
• D. Moire occurs when grid lines interfere with detector sampling, producing a false banding pattern

Answer: (D)

Moiré is an aliasing artifact that happens when a repeating structure, like grid lines, interacts with the digital detector’s sampling grid. The detector records the image in discrete pixels, so if the grid line frequency is close to the detector’s sampling frequency (or its harmonics), the patterns don’t map one-to-one and instead produce an artificial, false banding or striped pattern. This is a concern with digital detectors using certain grids because the interaction between the grid’s regular lines and the pixel sampling can reveal as visible Moiré artifacts rather than true anatomy. To minimize it, use grids with frequencies that don’t alias with the detector’s pixel pitch, or employ a moving grid during exposure (or other anti-Moire approaches) to blur the pattern.