X-ray / x-ray
The capitalization of the noun and the verb.
The noun takes a capital X and a hyphen: a chest X-ray. The same spelling holds when the noun modifies another noun: an X-ray machine. The verb takes a lowercase x: The technician will x-ray the wrist. Capitalizing the verb is common, but it is a variant, not the main spelling.
Examples
UseThe doctor ordered a chest X-ray. (The noun names the image, so the X is capitalized.)
UseThe technician will x-ray the wrist. (The verb names the act of imaging, so the x is lowercase.)
AvoidThe doctor ordered a chest xray. (The hyphen is missing, and the noun needs a capital X.)