What is the Moiré effect, and how can it be prevented in grid-based radiography?

What is the Moiré effect, and how can it be prevented in grid-based radiography?

• A. Moiré is a random noise pattern caused by detector saturation.
• B. Moiré is a result of patient motion during exposure.
• C. Moiré is an aliasing artifact due to sampling frequency unrelated to grids.
• D. Moiré is a repetitive interference pattern from grid lines interacting with detector sampling; prevent with high-frequency grids or grid rotation.

Answer: (D)

The Moiré effect is an aliasing phenomenon that happens when two regular patterns overlap and are sampled, creating a new interference pattern that wasn’t in the original scene. In grid-based radiography, the grid has evenly spaced lead strips and the digital detector samples the image with its own fixed pixel grid. If the grid’s line spacing (grid frequency) is too close to the detector’s sampling frequency or the grid lines line up with the detector’s pixel grid, the interaction can produce visible, repetitive bands across the image instead of a uniform field. To prevent this, use a grid with a higher line density (higher grid frequency) so the grid pattern is well above the detector’s Nyquist limit, reducing the chance of aliasing. Alternatively, rotate or tilt the grid relative to the detector so the grid lines no longer align with the pixel grid, breaking the steady interference pattern.

The Moiré effect is an aliasing phenomenon that happens when two regular patterns overlap and are sampled, creating a new interference pattern that wasn’t in the original scene. In grid-based radiography, the grid has evenly spaced lead strips and the digital detector samples the image with its own fixed pixel grid. If the grid’s line spacing (grid frequency) is too close to the detector’s sampling frequency or the grid lines line up with the detector’s pixel grid, the interaction can produce visible, repetitive bands across the image instead of a uniform field. To prevent this, use a grid with a higher line density (higher grid frequency) so the grid pattern is well above the detector’s Nyquist limit, reducing the chance of aliasing. Alternatively, rotate or tilt the grid relative to the detector so the grid lines no longer align with the pixel grid, breaking the steady interference pattern.