Digital X-Ray Exposed: Focal Spots

Digital x-ray imaging is an essential diagnostic tool in modern healthcare. Let's explore how the size and angle of a focal spot impacts image quality to improve image detail.

The main characteristics of the focal spot angle that large and small focal spots are superimposed and that the anode angle helps direct x-rays downward. The focal spots are imbedded in the rotating anode disc and are angled in such a way that when the electrons strike the anode focal track, the angle of the focal spot makes it easier for the x-rays to be emitted in a downward direction because the anode face has an angle. While the projectile electrons do not “bounce” away from the target, the angle at which it is positioned allows more x-rays to be directed toward the image receptor or film.

The actual dimensions of a typical focal spot may be 1 mm X 2 mm along the focal track, however depending on the angle of the anode face, this will change the appearance of the focal spot as seen from below or where the x-ray image receptor is positioned. The anode angle and the resulting appearance of the focal spot as it would appear from the image receptor is known as the “Line Focus Principle'. As the target angle decreases, so to does the effective focal spot. The smaller the angle of the anode, the smaller the focal spot will appear smaller because you are looking up at it at an angle or obliquely. As the angle decreases, you actually see less of the focal spot dimensions and this will actually enhance the recorded details on the image. In general, the smaller the focal spot is, either real or as changed by the line focus principle, the better the detail will be on the finished image.