Focal spot

Radiopaedia • CC-by-nc-sa 3.0 • de

Focal spot is the area of the anode surface which receives the beam of electrons from the cathode. It is the apparent source of x-rays.

Basic concept

Size and shape of the focal spot is determined by the size and shape of the electron beam when it strikes the anode .

Size and shape of the electron beam is determined by:

  • dimensions of the filament tungsten coil
  • construction of the focusing cup
  • position of the filament in the focusing cup
  • the electric field created between the cathode and anode: focal spot enlarges as current increases due to the repulsion of adjacent electrons (blooming effect)

To produce sharp images, focal spots need to be small but able to withstand heat loading without melting the anode target. A small focal spot is used when spatial resolution is important, while a large focal spot is employed when a short exposure time is the priority.

The focal spot sizes commonly employed are:

  • 0.3 mm and 0.6 mm, usually for mammography
  • 1.0 mm and 1.2 mm, usually for general radiography

The effective focal length of a focal spot can be calculated using:

Effective focal length = Actual focal length x sin θ

where θ is the anode angle