Almost all medical x-ray tubes have two focal spot sizes. For some machines, the operator is able to select ‘Small’ or ‘Large’ focal spot sizes. On other machines, the different mA selections will have an ‘S’ or an ‘L’ after them (e.g., 100S or 300L) that corresponds to the same settings. But what does this mean, and when should you choose small or large?

 

X-rays are produced by heating up a filament (like a filament in an old incandescent light bulb) so hot that it ‘boils’ electrons off the filament (cathode). Then the big potential voltage (at the kilovolt level) in the x-ray tube grabs these electrons and slams them into the target (anode), which produces x-rays. The small and large focal spot sizes come from the size of the filament, so it is important to understand which one is best for which purpose.