What grid error causes normal exposure in the middle of an image but decreased exposure on the sides?

The correct answer is related to grid-focus decentering, which occurs when the grid is not properly aligned with the x-ray tube. In this situation, the primary beam is tilted in relation to the grid's focus lines. As a result, while the middle portion of the image may receive sufficient exposure due to alignment with the x-ray beam, the edges do not receive adequate exposure. This creates a gradient effect, where the central area appears properly exposed, but the sides are noticeably darker or underexposed.

This phenomenon is particularly relevant in radiographic imaging because grids are designed to enhance image quality by absorbing scatter radiation. When decentering occurs, it disrupts this function, leading to inconsistent exposure across the image. Grids have specific alignment requirements, and any deviation can lead to noticeable changes in the exposure levels captured.

Other options, while relevant to grid errors, do not lead to the specific pattern of exposure described in the question. An "off level" grid typically causes a uniform decrease in exposure across the entire image rather than a gradient; "cut-off" often refers to a sharp loss of exposure at the edges due to incorrect alignment, but it usually indicates a more severe loss rather than the gradation described; and "grid radius"