Generation of X-rays

In this section, we discuss how X-rays are actually created in diagnostics.

The X-ray tube
X-rays are generated in an X-ray tube. An X-ray tube consists of the following components:

A filament (cathode)
A hit plate (anode)
A glass enclosure

A generator is also needed to provide the voltage and current, and there are additional components such as a focusing cup, filter and diaphragm.

Mnemonic

In radiation protection, a mnemonic is often used to remember the charge of the cathode and anode: KNAP, which stands for Cathode Negative Anode Positive

A current runs through the filament, the filament current, which causes the filament to heat up. Electrons in the filament become slightly free from the filament. The higher the glow current the more 'free' electrons. When a high voltage of, say, 60,000 volts (60kV) is switched across filament (cathode) and hit plate (anode), electrons will be pulled away from the filament by the highly positively charged anode. The number of electrons attracted by the anode is a measure of the tube current. The glass envelope creates a vacuum inside the X-ray tube. The electrons will collide with the positive anode at a very high speed. In the anode, which is made of a heavy metal, the electrons are suddenly slowed down. This generates a lot of heat and, to a small extent, X-rays.

The mAs number is the product of the tube current and time. The higher the tube current or the longer the time, the higher the mAs number, the more electrons have collided with the anode. If a short exposure time is desired (to avoid motion blur), a high tube current is needed.

A focusing ring or cup keeps the beam of electrons narrow so that the spot where the anode is hit is small. This is the focus. The greater the focus the more geometric blur in the image.

Figure 1 shows a schematic representation of a static anode. This is used in devices in dentistry. Furthermore, you mainly see dish anodes, as shown in figure 2. The anode rotates during a shot, optically the focus seems to stay in the same position, but the heat is distributed over a much larger area. This has made it possible to produce a very large amount of radiation in a short time without melting the anode material. This has enabled short scan times in CT, for example.

Leak radiation
X-rays arise in all directions, but are only wanted in one direction (the exposure field). Therefore, a lead jacket is fitted around the tube that blocks radiation in undesirable directions. Radiation is never stopped for 100%, radiation that does pass through that lead envelope is called leakage radiation.

The X-ray spectrum
X-rays arise largely from braking radiation production. The electron is negatively charged and in the vicinity of a positive core of the anode material, the electron is strongly slowed down, see Figure 4. The electron can convert much or little of its kinetic energy into braking radiation when decelerating, thus creating a spectrum of energies in the X-ray beam. From very low energies to the maximum photon energy. The maximum photon energy is equal in keV to the set tube voltage in kV. The lowest energies are so weak that they are captured by the glass envelope and filter. The spectrum looks like Figure 5. On top of the braking radiation spectrum, you can also see characteristic peaks. These are characteristic of the anode material and are determined by the different binding energies of the electrons in the shell of the anode material. This process is visualised in figure 5 of the previous lesson.

Sources:

Physics for imaging and radiotherapy; J. Scheurleer, 2017

Introduction to Radiation Hygiene; A.J.J. Bos et al; 2009