Focal Spot Size

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.

The difference between the two focal spots is just a matter of geometry. The longer the filament is, the fuzzier or wider the shadow (also called penumbra) around the x-ray image can be. It works just like light and shadows. A shadow formed from a pinpoint-like light source will be really sharp and crisp, and a shadow formed from a large fluorescent tube will be much fuzzier.

Diagram of the penumbra effect

The point to having small and large focal spots in your x-ray tube is that it gives the machine the versatility to image something as small as a finger and something as large as the side of the lumbar spine. On a smaller object, a small focal spot is better, but larger objects require a larger focal spot to reduce the size of the shadow.

In practice, this means that the smaller focal spot size should be used for more detailed imaging procedures like looking for hairline fractures in the hand or foot, and the larger focal spot size should be used for when you are looking at larger anatomical features like the curvature of the spine. Most of the time, the machine will force you to use the correct focal spot size based on the selected mA setting, but it is still important to understand the difference between the two filament sizes.