How many shades of gray can be displayed by an 8-bit digital radiography system?

An 8-bit digital radiography system can display 256 shades of gray. This is derived from the fundamental principle of binary numbers, where each bit can represent two states (0 or 1). Therefore, the total number of different values that can be represented with n bits is calculated as 2^n.

In the case of an 8-bit system, the calculation would be 2^8, which equals 256. This range of values allows for a more detailed and nuanced representation of images, enabling better differentiation of tissue densities in radiographic images. Therefore, 256 shades of gray can provide clinicians with improved diagnostic capabilities, as they can discern subtle differences in density that may be present in the tissues being examined.

Other choices represent incorrect calculations based on the understanding of bit depth. For example, 128 shades correspond to a 7-bit system (2^7), while 64 shades are from a 6-bit system (2^6). The 512 shades of gray would come from a 9-bit system (2^9), which is not applicable here. Thus, the correct answer stands firmly at 256 shades of gray for an 8-bit system.