Which component of the x-ray tube contains the focal spot?

• A. Anode target
• B. Cathode filament
• C. Vacuum envelope
• D. Rotor bearing

Answer: (A)

The focal spot is the small area on the anode target surface where the high-energy electrons collide and X-rays are generated. When the tube is energized, electrons emitted from the cathode are accelerated toward the anode and strike this target; the interaction at that spot produces the photons that become X-rays. The anode target is the actual X-ray source, and its surface location defines where the focal spot lies. The other components don’t serve as the X-ray source: the cathode filament emits the electrons but does not emit X-rays itself; the vacuum envelope simply houses the tube’s internals; and the rotor bearing is a mechanical part that supports and cools the rotating anode, not the source of X-rays.