focalspot

The focal spot is the region on the anode target of an X-ray tube where electrons emitted by the cathode strike to produce X-rays. The physical area of this spot is called the actual focal spot, while the projected size seen at the image receptor is the effective focal spot. The effective focal spot is determined by the geometry of the tube, particularly the anode angle, and by the line-focus principle.

Line-focus principle describes how angling the anode reduces the apparent size of the focal spot at the image receptor. A smaller effective focal spot improves spatial resolution and sharpness, but concentrates heat on a smaller portion of the anode. To manage heat while maintaining a small effective spot, most tubes use rotating anodes or larger actual focal spots with carefully chosen angles. The choice of focal spot size reflects a trade-off between image sharpness and heat loading, as well as the intended clinical task.

Focal spot size also interacts with geometry to determine geometric unsharpness or blur. Unsharpness increases with larger focal spots and with greater object-to-image distance, and decreases with greater source-to-image distance. Typical radiography tubes offer focal spot sizes in the range of about 0.3 to 1.0 mm for general use, with smaller (microfocus) sizes around 0.1–0.3 mm for high-detail work and specialized applications. The actual versus effective focal spot and the anode design are important considerations in image quality, dose management, and equipment performance. See also anode heel effect and line-focus principle.