Grid errors occur when the primary X-ray photons are absorbed by the lead strips within the grid instead of passing through to create the image. In digital radiography, this still results in an underexposed image. However, due to rescaling, the brightness may appear normal, making the error harder to detect.
In the film-screen era, grid errors produced bright, washed-out images that were easy to spot. Today, the main giveaway is low image contrast—despite using a grid, which is specifically designed to increase contrast.
A telltale sign of a grid error is what technologists call the “trifecta”:
An image taken with a grid to increase contrast
Technical factors that appear to be within normal range
A negative deviation index (DI) tending toward underexposure, even though your technique should not have produced that
For example, a portable chest X-ray at 100 kVp, 10 mAs, 40″ SID with a grid may show an acceptable technique on paper, but a DI of -2.7 suggests underexposure and possible grid error.
Out of focal range – The source-to-image distance (SID) is not within the grid’s specified focal range.
Angled against the lead strips – You can angle with the strips, but never against them.
Off-centering – The X-ray tube is not aligned with the center of the grid.
Upside-down grid – Rare, but can occur if the grid is positioned incorrectly.
By recognizing these signs, technologists can troubleshoot image quality issues before they impact diagnostic accuracy.
