Chapter 9 Production of X-rays
Diagnostic X-rays are produced in the target of the anode when high-energy projectile electrons are rapidly decelerated. Diagnostic X-ray imaging equipment provides the means for practitioners to control the quality and quantity of the X-ray beam. Consequently, it is important to understand the process of X-ray production and the factors that influence the characteristics of the beam. Practitioners familiar with the concepts and factors that influence quality and quantity are better able to control exposure factors to produce optimal radiographic images while minimizing patient dose.
In the X-ray tube, the purpose of the filament is to provide the free electrons necessary for X-ray production. As the rotor is activated the current passing through the filament heats to the point where electrons boil off. This process is referred to as thermionic emission. At this point, a space charge (cloud of electrons) forms around the filament. The focussing cup temporarily concentrates the free electrons and helps form them into a beam.
When the exposure begins, the primary circuit closes and a high voltage is applied across the anode (positively charged) and cathode (negatively charged). This causes electrons to stream towards the anode at a high rate of speed. The potential energy of each electron is one kiloelectron volt (keV) of energy for each kilovolt (kV) of voltage set for the exposure. Electrons (sometimes called projectile electrons) that travel from the cathode to anode make up the tube current.
When the high-speed projectile electrons collide with the X-ray tube target they interact with the orbital electrons or the nuclear field of the target atoms. Kinetic energy transferred from the projectile electrons to the target atoms converts into heat or X-rays. When projectile electrons strike outer target shell electrons it puts them in an excited state and as a result, infrared (heat) radiation is emitted. Approximately 99% of the energy of projectile electrons converts into heat. Only about 1% of the energy converts into X-ray photons. Two types of interaction produce X-ray photons: bremsstrahlung interactions and characteristic interactions.
Bremsstrahlung in German means ‘to brake radiation’ or braking radiation. Bremsstrahlung target interaction occurs when projectile electrons pass by outer shell electrons of target atoms and interact with the force field of the nucleus of the atom. Because atomic nuclei are positively charged and electrons are negatively charged, there is a mutual attraction between them. The nuclear force field causes the entering electron to slow down (or brake) and change direction. The loss of kinetic energy that occurs when a projectile electron slows down is emitted as an X-ray photon. These X-ray photons are known as bremsstrahlung photons or brems radiation (Fig. 9.1). In the diagnostic range, approximately 85% of X-ray emissions are the result of bremsstrahlung interactions.