3.1: The X-ray Tube
- A vacuum sealed x-ray tube is made up of a cathode (negative electrode) and anode (positive electrode). High voltage is applied between the electrodes, and the electrons will accelerate from the cathode to the anode to strike the target.
- The cathode is made up of a tungsten wire filament, a circuit that supplies the filament current, and a focusing cup.
- When appropriate heat is applied to the filament, electrons will emit by the process of thermionic emission.
- The negatively charged focusing cup semi-surrounds the filament to direct the electrons toward the anode to strike the focal spot.
- The anode is made up of a thick copper rod with a tungsten target at the end facing the cathode.
- Tungsten is used as the target material because it has a high atomic number and a high enough melting point to withstand the heat generated by electron bombardment.
- This heat is removed by the copper rod and radiated into the oil reservoir surrounding the tube that absorbs heat by conduction. The oil bath also serves to insulate the tube from the high voltage.
- Rotating anodes are used in diagnostic x-rays to reduce the time that any point on the focal spot surface is in contact with the heat.
- Another method to minimize overheating is to utilize the principle of line focus. Using smaller focal spots will produce sharper radiographic images, but doing so generate more heat per unit area of the target.
- When the surface of the target is steeply inclined, the surface area of the focal spot increases in dimension, but the apparent or projected focal spot size will be small.
- An inclined target will produce x-rays from varying depths, and consequently, the intensity of the beam will decrease on the side of the cathode.
- This variation across the x-ray beam is referred to as the heel effect and can be minimized by using a compensating filter.
- The cathode is made up of a tungsten wire filament, a circuit that supplies the filament current, and a focusing cup.
3.2-3.4: Basic X-ray Circuit, Voltage Rectification, & High-Output X-ray Generators
- The circuit of a x-ray tube is divided into a high voltage and a low voltage circuit.
- The high voltage circuit is the voltage that is applied between the electrodes to provide the accelerating potential.
- The low voltage is used to supply the heating current to the filament for electron emission. The filament current controls the tube current, i.e. the intensity, which is caused by the flow of electrons across the electrode.
- An x-ray tube that runs on AC voltage will have a positive voltage for just half of the voltage cycle.
- The voltage will reverse for the next half-cycle, but tubes that are self-rectified will prevent the tube current from flowing in the reverse direction. Consequently, x-ray production will be paused until the voltage returns to positive.
- The inverse voltage cycle is a clear disadvantage of the self-rectified circuit because the output of the machine will be low. Furthermore, if the target overheats and electrons are emitted from the target by thermionic emission, these electrons can travel towards the cathode due to the change in the voltage relationship between the cathode and the anode. The cathode will become bombarded with electrons and cause damage to the filament.
- Voltage rectifiers can be used to prevent the tube from conducting during the inverse voltage cycle, and can further be arranged to provide full-wave rectification so that the tube current flows during both half-cycles.
- A three-phase x-ray generator uses 3 voltage wave-forms that are moments out of phase/sync with one another resulting in the voltage across the tube remaining near the maximum. When full-wave rectification is used in combination, there will be less fluctuation in tube current or “ripple” compared with schemes mentioned above.
- To provide a near constant potential to the x-ray tube, a high frequency x-ray generator is used. The circuit involves a DC voltage that is converted into a high-frequency low voltage AC. By a step-up transformer, this low voltage AC becomes a high voltage AC that is then rectified and smoothed to provide a nearly constant high-voltage potential with minimal ripple.
3.5 – 3.8: Physics of X-ray Production, X-ray Spectra, & Operating Characteristics
- The characteristic x-ray is the result of a collisional interaction between an incident electron and an orbital electron.
- An incident electron collides with an orbital electron to eject it from the atom and leaves the atom ionized. A threshold energy is required for this ejection and is dependent upon the binding energy of the shell.
- The vacancy will be filled by an outer orbital electron, and a characteristic x-ray will be emitted at discrete energies (difference between binding energies) as the outer shell electron transitions down to fill the vacancy.
- The bremsstrahlung x-ray is the result of a radiative interaction between an incident electron and a nucleus.
- When an electron approaches near a nucleus, the electron deflects from its regular trajectory and bends around the nucleus due to the Coulombic forces of attraction.
- A bremsstrahlung x-ray is emitted during the interaction, and the resulting photon can have up to the initial energy of the incident electron. Thus, bremsstrahlung photons have a continuous energy spectrum or a continuous distribution of energies.
- As the kinetic energy of the electron increases, the x-ray will become increasingly in the forward direction.
- Thus, transmission type targets are used in megavoltage x-ray tubes where x-rays bombard the target from one side and an x-ray beam is produced on the other.
- The probability of bremsstrahlung production varies with Z^2 of the target material.
- Filtration is used to improve the penetrating power of the beam by removing lower energy photons in a spectrum.
- Filtration will raise the average energy of the beam commonly referred to as “beam hardening”, but the intensity of the beam will decrease.
- Thus, filtration must be carefully used to improve beam penetration while also maintaining an acceptable intensity.
- The average x-ray energy of a beam can be approximated by taking one-third of the maximum energy.
- The x-ray beam is heterogeneous in energy due to the following:
- Bremsstrahlung interactions: the energy of the emitted bremsstrahlung x-ray will vary based on the energy of the incident electron as the AC voltage varies the energy of the beam. Even if all of the incident electrons were monoenergetic, the extent to which the electrons interact with their respective nuclei will emit varying energies. Furthermore, these electrons can undergo multiple bremsstrahlung interactions.
- AC voltage applied to the tube: even the high-frequency generator has minor amounts of ripple which will result in variations in electron energy and intensity arising from the filament. Thus, the electrons incident on the target will vary in energy even prior to bombardment.
- Filtration: low energy bremsstrahlung x-ray are preferentially filtered out by a filter to improve the penetrating power of the beam. However, the filter is still a medium by which the incoming beam can interact with to spread its energy spectrum.
- The output of an x-ray machine can be expressed in terms of the quantity exposure which refers to the amount of ionization it produces in air.
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