Bit depth is defined as the:

• A. Total line pairs per millimeter (lp/mm) that can be resolved by the digital system
• B. Total number of brightness levels that can be produced by the digital system
• C. Smallest exposure difference that can be detected by the digital system
• D. Total number of x-ray photons striking the digital receptor

Answer: (A)

Bit depth is defined by how many brightness (grayscale) levels a pixel can display in the digital image. It doesn’t measure how finely spatial details are resolved in the scene—that’s spatial resolution. More bits per pixel mean more possible gray shades, which expands the dynamic range and improves the ability to distinguish subtle differences in tissue density. For example, 8-bit depth gives 256 brightness levels, while 12-bit depth provides 4096 levels. The other ideas described—how small a detail is spaced (spatial resolution), or how small a change in exposure can be detected (exposure sensitivity/noise dynamics), or how many x-ray photons strike the receptor—are about different aspects of image quality, not the grayscale depth per pixel.