Principles of CT Operation: How does a microfocus X-ray tube work?
In an evacuated tube electrons are emitted from a heated filament and are accelerated towards the anode by the potential difference UACC. Electrons enter through a hole in the anode into a magnetic lens which focuses the electron beam to a small spot of a few microns in diameter on the massive tungsten target (directional tube).
In the tungsten the electronis are abruptly decelerated whereby X-rays are generated. The focal spot represents a very small X-ray source which enables sharpest imaging with micrometer resolution. Latest nanofocus tubes (transmission tube) achieve a detail detectability down to 200 nanometers (0.2 microns) by using multiple electron lenses. The electron beam current is controlled by the bias voltage UG via the Wehnelt electrode („grid“).
