Basic Definition
An X-ray tube is a device for the production of X-rays, found in most lab microCT systems. Inside the X-ray tube, an electron gun emits and accelerates electrons from a cathode to an anode within a vacuum to form a beam. When the electron beam hits the anode, the electrons decelerate and emit electromagnetic radiation in the form of X-rays.
As opposed to conventional X-ray tubes, where the anode is also the target, in microfocus and nanofocus X-ray tubes, the electron beam is transmitted through a hole in the anode where it is then focused onto a small spot on the target to generate a tiny, yet bright X-ray source.
There are two different types of X-ray tube, open and sealed X-ray tubes, which are differentiated by their housing. Both types can be equipped with a directional or transmission target.
Microfocus X-ray tube
A microfocus X-ray tube has a focal spot smaller than 200 micrometers.
Nanofocus X-ray tube
A nanofocus X-ray tube has a focal spot smaller than 1 micrometer (1,000 nanometers).
Open X-ray tube
An open X-ray tube can be opened to maintain and replace parts such as the filament and target. This type of tube offers a virtually unlimited life, but requires regular maintenance. Because the critical parts can be replaced, open tubes can be operated close to their physical limits. In view of this, they offer smaller focal spots, making them suitable for the highest magnification requirements (e.g. X-ray microscopy).
Sealed X-ray tube
Unlike an open tube, a sealed tube has a permanent vacuum. This type of tube is maintenance free, but has only a limited life because it is not possible to replace the filament or target. This type of tube is generally used in systems with lower requirements in terms of magnification and detail detectability, for instance.
More Information
Figure 7.7 in Chapter 7, Section 2, “X-ray Generation”, from Medical Imaging Systems: An Introductory Guide [Internet] illustrates the structure of an x-ray tube and describes its function.
Citation: Berger M, Yang Q, Maier A. X-ray Imaging. 2018 Aug 3. In: Maier A, Steidl S, Christlein V, et al., editors. Medical Imaging Systems: An Introductory Guide [Internet]. Cham (CH): Springer; 2018. doi:10.1007/978-3-319-96520-8