- A current is passed through the tungsten filament and heats it up.
- As it is heated up the increased energy enables electrons to be released from the filament through thermionic emission.
- The electrons are attracted towards the positively charged anode and hit the tungsten target with a maximum energy determined by the tube potential (voltage).
- As the electrons bombard the target they interact via Bremsstrahlung and characteristic interactions which result in the conversion of energy into heat (99%) and x-ray photons (1%).
- The x-ray photons are released in a beam with a range of energies (x-ray spectrum) out of the window of the tube and form the basis for x-ray image formation.
- Made of thin (0.2 mm) tungsten wire because tungsten:
- has a high atomic number (A 184, Z 74)
- is a good thermionic emitter (good at emitting electrons)
- can be manufactured into a thin wire
- has a very high melting temperature (3422°c)
- The size of the filament relates to the size of the focal spot. Some cathodes have two filaments for broad and fine focusing
- Made of molybdenum as:
- high melting point
- poor thermionic emitter so electrons aren’t released to interfere with electron beam from filament
- Negatively charged to focus the electrons towards the anode and stop spatial spreading
- Target made of tungsten for same reasons as for filament
- Rhenium added to tungsten to prevent cracking of anode at high temperatures and usage
- Set into an anode disk of molybdenum with stem
- Positively charged to attract electrons
- Set at angle to direct x-ray photon beam down towards patient. Usual angle is 5º – 15º
- Target, focus, focal point, focal spot: where electrons hit the anode
- Actual focal spot: physical area of the focal track that is impacted
- Focal track: portion of the anode the electrons bombard. On a rotating anode this is a circular path
- Effective focal spot: the area of the focal spot that is projected out of a tube
Stationary anode: these are generally limited to dental radiology and radiotherapy systems. Consists of an anode fixed in position with the electron beam constantly streaming onto one small area.
Rotating anode: used in most radiography, including mobile sets and fluoroscopy. Consists of a disc with a thin bevelled rim of tungsten around the circumference that rotates at 50 Hz. Because it rotates it overcomes heating by having different areas exposed to the electron stream over time. It consists of:
- Molybdenum disk with thin tungsten target around the circumference
- Molybdenum stem, which is a poor conductor of heat to prevent heat transmission to the metal bearings
- Silver lubricated bearings between the stem and rotor that have no effect on heat transfer but allow very fast rotation at low resistances
- Blackened rotor to ease heat transfer
Heating of the anode
This is the major limitation of x-ray production.
Heat (J) = kVe x mAs
Heat (J) = w x kVp x mAs
kVe = effective kV
w = waveform of the voltage through the x-ray tube. The more uniform the waveform the lower the heat production
kVp = peak kV
mAs = current exposure time product
Heat is normally removed from the anode by radiation through the vacuum and into the conducting oil outside the glass envelope. The molybdenum stem conducts very little heat to prevent damage to the metal bearings.
A higher heat capacity means the temperature of the material rises only a small amount with a large increase in heat input.
Temperature rise = energy applied / heat capacity
Each machine has a different capacity for dissipating heat before damage is caused. The capacity for each focal spot on a machine is given in tube rating graphs provided by the manufacturer. These display the maximum power (kV and mA) that can be used for a given exposure time before the system overloads. The maximum allowable power decreases with:
- Lengthening exposure time
- Decreasing effective focal spot size (heat is spread over a smaller area)
- Larger target angles for a given effective focal spot size (for a given effective focal spot size the actual focal spot track is smaller with larger anode angles. This means the heat is spread over a smaller area and the rate of heat dissipation is reduced)
- Decreasing disk diameter (heat spread over smaller circumference and area)
- Decreasing speed of disk rotation
Other factors to take into consideration are:
- By using a higher mA the maximum kV is reduced and vice versa.
- A very short examination may require a higher power to produce an adequate image. This must be taken into consideration as the tube may not be able to cope with that amount of heat production over such a short period of time.
Anode cooling chart
As well as withstanding high temperatures an anode must be able to release the heat quickly too. This ability is represented in the anode cooling chart. It shows how long it takes for the anode to cool down from its maximum level of heat and is used to prevent damage to the anode by giving sufficient time to cool between exposures.
Anode heel effect
An x-ray beam gets attenuated on the way out by the target material itself causing a decrease in intensity gradually from the cathode to anode direction as there is more of the target material to travel through. Therefore, the cathode side should be placed over the area of greatest density as this is the side with the most penetrating beam. Decreasing the anode angle gives a smaller effective focal spot size, which is useful in imaging, but a larger anode heel effect. This results in a less uniform and more attenuated beam.
** smaller angle = smaller focal spot size but larger anode heel effect **
Window: made of beryllium with aluminium or copper to filter out the soft x-rays. Softer (lower energy) x-ray photons contribute to patient dose but not to the image production as they do not have enough energy to pass through the patient to the detector. To reduce this redundant radiation dose to the patient these x-ray photons are removed.
Glass envelope: contains vacuum so that electrons do not collide with anything other than target.
Insulating oil: carries heat produced by the anode away via conduction.
Filter: Total filtration must be >2.5 mm aluminium equivalent (meaning that the material provides the same amount of filtration as a >2.5 mm thickness of aluminium) for a >110 kV generator
Total filtration = inherent filtration + additional filtration (removable filter)
Producing an x-ray beam
1. Electrons produced: thermionic emission
A current is applied through the cathode filament, which heats up and releases electrons via thermionic emission. The electrons are accelerated towards the positive anode by a tube voltage applied across the tube. At the anode, 99% of energy from the electrons is converted into heat and only 1% is converted into x-ray photons.
The accelerating potential is the voltage applied across the tube to create the negative to positive gradient across the tube and accelerate the electrons across the anode. It is normally 50-150 kV for radiography, 25-40 kV for mammography and 40-110 kV for fluoroscopy. UK mains supply is 230 V and 50 Hz of alternating current. When the charge is negative the accelerating potential is reversed (the cathode becomes positive and the anode becomes negative). This means that the electrons are not accelerated towards the anode to produce an x-ray beam. The ideal waveform for imaging is a positive constant square wave so that the electron flow is continuously towards the anode. We can convert the standard sinusoidal wave into a square wave by rectification.
Full wave rectification: the use of a rectification circuit to convert negative into positive voltage. However, there are still points at which the voltage is zero and most of the time it is less than the maximum kV (kVp). This would lead to a lot of lower energy photons. There are two rectification mechanisms that prevent too many lower energy photons:
- Three phase supply: three electrical supplies are used, each applied at a different time. The “ripple” (difference between maximum and minimum current) is about 15% of the kVp.
- High frequency generator: this can supply an almost constant potential. The supply is switched on and off rapidly (14kHz) which can then be rectified. They are much more compact than three phase supply and more commonly used.
Effect of rectification on spectrum
- Increased mean photon energy – fewer photons of lower energy
- Increased x-ray output – stays closer to the maximum for longer
- Shorter exposure – as output higher, can run exposure for shorter time to get same output
- Lower patient dose – increased mean energy means fewer low energy photons that contribute to patient dose but do not contribute to the final image
The current (usually 10 A) heats up the filament to impart enough energy to the electrons to be released i.e. it affects the number of electrons released.
This is the flow of electrons to the anode and is usually 0.5 – 1000 mA.
- Filament current is applied across the tungsten cathode filament (10 A) and affects the number of electrons released.
- Tube current is applied across the x-ray tube from cathode to anode and affects the energy and number of electrons released.
2. X-ray production at the anode
The electrons hit the anode with a maximum kinetic energy of the kVp and interact with the anode by losing energy via:
- Elastic interaction: rare, only happens if kVp < 10 eV. Electrons interact but conserve all their energy
- Ineleastic interaction: causes excitation / ionisation in atoms and releases energy via electromagnetic (EM) radiation and thermal energy
At the anode, electrons can interact with the atoms of the anode in several ways to produce x-ray photons.
- Outer shell interaction: low energy EM released and quickly converted into heat energy
- Inner shell interaction: produces characteristic radiation
- Nucleus field interaction: aka Bremsstahlung
1. Characteristic radiation
- A bombarding electron knocks a k-shell or l-shell electron out.
- A higher shell electron moves into the empty space.
- This movement to a lower energy state releases energy in the form of an x-ray photon.
- The bombarding electron continues on its path but is diverted.
It is called “characteristic” as energy of emitted electrons is dependent upon the anode material, not on the tube voltage. Energy is released in characteristic values corresponding to the binding energies of different shells.
Ek – El (aka Kα) = 59.3 keV
Ek – Em (aka Kβ) = 67.6 keV
- Bombarding electron approaches the nucleus.
- Electron is diverted by the electric field of the nucleus.
- The energy loss from this diversion is released as a photon (Bremsstrahlung radiation).
Bremsstrahlung causes a spectrum of photon energies to be released. 80% of x-rays are emitted via Bremsstrahlung. Rarely, the electron is stopped completely and gives up all its energy as a photon. More commonly, a series of interactions happen in which the electron loses energy through several steps.
|Only accounts for small percentage of x-ray photons produced||Accounts for 80% of photons in x-ray beam|
|Bombarding electron interacts with inner shell electron||Bombarding electron interacts with whole atom|
|Radiation released due to electron dropping down into lower energy state||Radiation released due to diversion of bombarding electron as a result of the atomic pull|
|Radiation released is of a specific energy||Radiation released is of a large range of energies|
|X-ray photon energy depends on element of target atoms not tube voltage||X-ray photon energy depends on tube voltage|
Summary of steps
- Filament current applied through tungsten filament at cathode.
- Heats up filament to produce enough energy to overcome binding energy of electrons (thermionic emission).
- Electrons released from filament.
- Tube voltage is applied across the x-ray tube.
- Electrons, therefore, are accelerated towards positively charged anode, which gives them a certain energy.
- The electrons strike the anode and the energy released via interaction with the anode atoms produces x-ray photons.
- These x-ray photons leave the x-ray tube through the window in an x-ray beam towards the patient.
- They pass through the patient to the detector to produce the x-ray image (this section is covered in the next chapter “Interaction with matter”).
The resulting spectrum of x-ray photon energies released is shown in the graph. At a specific photoenergy there are peaks where more x-rays are released. These are at the characteristic radiation energies and are different for different materials. The rest of the graph is mainly Bremsstrahlung, in which photons with a range of energies are produced. Bremsstrahlung accounts for the majority of x-ray photon production.
Beam quality: the ability of the beam to penetrate an object or the energy of the beam.
Beam quantity: the number of x-ray photons in the beam
Altering the x-ray spectrum
Increasing the Tube Potential (kV)
- Quantity of x-ray photons
- Average energy
- Maximum energy
If kV great enough, characteristic energy produced
Increasing the Tube Current (mA)
Increased quantity of x-ray photons
No change in:
- Characteristic energy
- Average energy
- Minimum energy
- Maximum energy
Fewer lower energy photons
- Average energy of photons
- Total number of photons
Waveform of Current
Having a more uniform current (rectified) results in increased:
- Average energy
- Quantity of x-ray photons
- Same maximum keV
Increasing Atomic Number of Target
- Quantity of x-ray photons
- Characteristic energy