Life & Death of a Photon (part 1)
Imaging factors and technique are starting to come together for me. Topics such as mA, time, and kVp don’t seem so complicated in the beginning, but they do get a little more complicated as the study continues. Let’s take a look at how x-rays are produced and what happens to them from that point…
The Creation of an X-Ray
In the x-ray tube, the filament (small or large) is heated through the process of applying electrical current. These filaments are located on the negatively charged cathode side of the tube. These filaments are made of tungsten. As they are heated, electrons from the tungsten atoms are boiled off through a process called thermionic emission. These electrons that are boiled off form a cloud around the filament called a space charge. The focusing cup around the filaments has a negative charge and forces the negatively charged electrons in the space charge to remain together.
While thermionic emission is taking place, the rotor on the anode (positively charged) side of the tube spins the rotating anode target up to speed. After the anode target (also made of tungsten) is spinning at full speed, kVp (kilovoltage) is applied across the tube from the cathode to the anode. The application of this voltage creates a highly negatively charged cathode and a highly positively charged anode. The high positive charge of the anode strongly attracts the negatively charged electrons that have formed on the cathode side of the tube. When this potential difference occurs, the electrons that have been created in the space charge race from the cathode to the anode side of the tube. Tube current is the term used to describe the flow of electrons from cathode to anode. As electrons race from the cathode side of the tube, they crash into the rotating anode target. The interactions during this ‘collision’ are where x-rays are formed.
The interactions between the speeding electrons and the tungsten atoms that make up the anode target are the source of x-ray photons. There are two possibilities for the creation of an diagnostic radiation when the electrons interact with the tungsten atoms:
1. Bremsstrahlung (Brems) Radiaton
Brems radiation is created when the incoming electron passes close to the nucleus of the target tungsten atom. The positive charge of the nucleus attracts the incoming electron causing it to change course and lose some speed. When the electron slows down and changes course, it loses some of its energy. This energy is lost by the electron in the form of heat and an x-ray photon. The path that the x-ray photon takes is totally random. The amount of energy the x-ray photon has depends on how much interaction it had with the tungsten nucleus. The closer the electron passes to the nucleus, the more energy it will scrub off in the form of heat and the x-ray photon. Heterogenous or polyenergetic are terms used to describe the varying energies of Brems Radiation. Isotropic is the term used to describe the path photons take when they are created. They travel in all directions from the source.
2. Characteristic Radiation
Incoming electrons may enter the tungsten atom and collide with an inner-shell (K-Shell) electron, causing the inner-shell electron to be ejected from its orbit around the nucleus. This makes the tungsten atom unstable. Electrons from the outer shells cascade in to fill the holes. The energy difference created by the outer shell electron falling into the K-shell creates another x-ray photon. Electrons from outer shells of the tungsten atom may also be ejected by the incoming electron. These ejections also cause a cascade of electrons to fill the hole, but the energy of the x-ray photons created by those cascades are very low energy and not useful for diagnostic purposes. Characteristic radiation differs from Brems radiation because the photon emitted when the outer shell electron drops in to fill the empty spot in the K shell is a known and fixed amount of energy. The radiation created by this process is homogenous, but like Brems radiation, it is also isotropic.
The process of creating x-ray radiation is very inefficient. 99% of the energy harvested from the process is in the form of heat, while only 1% comes in the form of x-ray photons. Since the generation of x-ray photons is isotropic (spreading in all directions), a lot of the radiation is absorbed by the protective lead lining of the x-ray tube. The diagnostic portion of the radiation exits the tube through the tube window. The collimator in the tube is used to shape the beam leaving the tube to the desired dimensions.
… to be continued …
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