What causes the phenomenon of increased image brightness at the center compared to the edges?

The phenomenon of increased image brightness in the center compared to the edges is primarily due to vignetting. Vignetting occurs when the brightness of an image decreases towards the corners or edges, often caused by the optical design of the system, where light is less efficiently collected at these peripheries compared to the center.

In imaging systems, this results in pictures that appear darker at the edges, leading to a perception of increased brightness in the center area. Vignetting can be influenced by various factors, including aperture settings, lens design, and the way light focuses through the lens. Understanding this effect is crucial for interpreting images correctly in fluoroscopy and other imaging modalities, as it can affect diagnostic quality and the perception of anatomical details.

In contrast, lag refers to the persistence of an image after the radiation source has been removed, pincushion distortion relates to the geometric distortion of an image, where it appears pinched at the center, and quantum mottle is related to image noise due to insufficient radiation exposure. None of these phenomena specifically explain the brightness variation from center to edge as effectively as vignetting.