X-ray production 1 Flashcards
What is an x-ray tube?
X-rays are produced within an X-ray tube.
The X-ray tube is provided with its high voltage electrical supply.
An X-ray tube is a vacuum device within which electrons are accelerated into a metal target.
Some of the interactions between the electrons and the metal target result in X-ray emission.
Basic elements of an x-ray tube
source of electrons
anode/cathode
electrons
vacuum target
Electrons are attracted to the anode
- Negatively charged electrons attract to positively charged anode which creates a force of attraction called Coulomb force.
- Electrons interact with the anode material.
- Convert potential energy to kinetic energy , but when they interact they give up that energy in the form of radiation (x-ray photons)
Why is a vacuum needed?
So that there is nothing else that the electrons are going to interact with between the cathode and the anode.
What is the Electron Volt?
The electron volt is a convenient unit of energy for X-ray science
It is defined as the amount of energy gained by a single electron when moved across a potential difference of one volt
Radiographic factors
Tube voltage, kV or kVp, is the potential difference between anode and cathode.
Tube current, mA is the current flowing between anode and cathode.
Pulse duration, ms, is the time during which current flows in the tube.
mAs is the product of the tube current in milliamps and the time in seconds
X-rays are generated by two processes:
General Radiation (bremsstrahlung)
Involves the electrons interacting with the nucleus of the target atoms
(breaking radiation, breaking the electrons then producing radiation)
Characteristic Radiation
Involves the electrons interacting with the electrons in the shell of the target material
General Radiation (Bremsstrahlung)
Tungsten atom in our anode which has a lot of electrons.
Accelerated electrons are attracted to the nucleus because of the Coulomb force.
As the nucleus gets close to the its diverted from its path and gives up some of its kinetic energy.
This produces either heat or gives up some of its photon energy.
The closer it gets to the nucleus the more energy gets released because there is a higher breaking force.
Produces a range of energies.
General Radiation (Bremsstrahlung) General points
Electrons give up their energy with a number of such interactions
Not all X-ray photons are produced at the surface of the target
Head-on collisions with the nucleus do occur, but are rare – all the energy of the electron is given up in a single photon of equal energy
Bremsstrahlung produces a wide range of photon energy
Mostly heat
Energy depends on proximity of electron to the nucleus and the charge associated with the nucleus
The minimum energy depends on the filtration of the X-ray beam
The maximum energy emitted from the X-ray tube depends on the applied voltage
What is photon energy?
Photon energy is a measure of the ability of the X-ray photon to penetrate matter
Generally, the higher the energy of the X-ray photon the more likely it is to penetrate matter to some depth in matter compared to a lower energy photon
X-ray beam intensity
-An X-ray radiographic exposure is made up of a burst of X-ray photons (~one million per mm2 – but this varies widely depending on the application)
-These X-ray photons are randomly distributed in time and space
-Practical X-ray beam exposures are composed of many photons of different energies
-The intensity of the X-ray beam is defined by the number of photons per unit area per unit time
- XR beam distributed randomly, so the x-ray beam intensity wont be the same across the whole square. There will be some variation.
- Variation- quantum noise and if noise is too big It can obscure our images.
Reduce the noise by increasing the number of XR photons per mm squared
X-ray beam intensity & photon energy distribution
- High energy x-ray proton more capable in penetrating energy/ matter than the low energy electrons.
There is a distribution of energy.
X-ray Spectrum
- At lower energy levels, XR photons have been filtered by the XR tube.
- At higher energies they are more likely to penetrate matter and get out of the XR tube and avoid being absorbed.
As the energy goes down , the probability of them being absorbed goes up. Therefore they get attenuated more.
Characteristic Radiation
takes place when the incoming electrons collide with the electrons within the heavy metal and knock-out the electrons from the electron shell.
An incident electron with enough energy to overcome the binding energy of the K-shell electron in the target of the X-ray anode target material may dislodge the bound electron creating a vacancy in the K-shell
This vacancy can be filled by an electron from one of the other shells (e.g. L-shell or M-shell)
When this happens an X-ray photon is emitted that is equal to difference in the binding energies of the two shells.
x- Accelerated electrons interact with the k shell electrons
- If the electrons has sufficient kinetic energy to overcome the binding energy of thus k shell electron, it can eject it out of its orbit.
- That will leave a vacancy which is then filled from the higher shells above it.
- Electron drops down from L shell to K shell in doing so it gives of characteristic radiation.
The energy given off is equal to the difference in the binding energies between the two shells.
Characteristic Radiation General Points
Incident electron must have energy equal to, or greater than, the binding energy of the bound electron for it to be freed
Any excess energy is shared between the electrons
The ionized atom is viewed as having an excess of energy. The atom, in returning to its normal (or neutral) state, can get rid of this excess in two ways
Emit an Auger electron
Emit an X-ray photon (the latter is more likely in Tungsten at diagnostic X-ray energies)
Emitted photon has an energy equal to the difference of the binding energy between the two shells
X-ray Beam Spectrum
Needs enough energy to overcome the binding energy
- Max kinetic energy depends on the voltage. If voltage isn't high enough, we don’t see characteristic x-ray peaks.
Contribution of Characteristic X-rays for W target
Below 70 kVp: none
80 kVp - 150 kVp: 10% - 28%
X-ray Beam Quality
Beam Quality
is a term used to indicate the ability of the X-ray beam to penetrate an object or indicate the energy of the beam.
is generally a descriptive term rather than a formal definition
the beam quality is referred to as being hard or soft, meaning it is more or less penetrative.
X-ray Beam Spectrum: Effect of Altering Exposure Factors
Increasing the Tube Potential (kV)
Increases
Intensity
Average energy
Maximum energy
If the kV is high enough, the characteristic X-rays will also be produced
- Intensity increases, more XR photons per mm squared. - More higher energy XR photons, The average energy of that spectrum has increased, and that’s a consequence of increasing the KV. - Increases the XR tube potential, increases intensity, average energy and maximum energy. - The position of the peaks does not change, even if you increase the KV. This is because the binding energy of tungsten dictates the The peaks are characteristic of the anode material. - Difference between the low current and high current spectrum. - When increasing the current, we increase the intensity of the XR beam. - Don’t get any difference In the distribution of the energies. The average beam energy stays the same. Energy distribution doesn’t change because the number - Glass, envelope, light beam diaphragm are examples of matter that can get in the way of XR beam leaving the tube. - More filtration leads to more photons being absorbed. So you get a spectrum that is reduced. - Low energy XR photons can be easily absorbed by the patient. So we need to get rid of these as these are dangerous and can produce skin burns. To do this we add some extra filtration eg. 2mm of aluminium.
Summary: X-ray Production Processes characteristic radiation and bremsstrahlung
Characteristic Radiation
-Only accounts for small percentage of X-ray photons produced
-Bombarding electron interacts with inner shell electron
-Radiation released due to a bound electron dropping down into lower energy state
-Radiation released is of a specific energy
-X-ray photon energy depends on element of target atoms not tube voltage
Bremsstrahlung
-Accounts for ~80% of photons in X-ray beam (if characteristic radiation is produced)
-Bombarding electron interacts with nucleus of target atom
-Radiation released due to diversion of bombarding electron as a result of the Coulomb force
-Radiation released has a wide range of energies
-X-ray photon energy depends on tube voltage
Dose Measurement
Air kerma is a quantity that is used to express the radiation concentration delivered to a point, such as the entrance surface of a patient’s body
Air kerma is a measure of the amount of radiation energy, in the unit of joules (J), actually deposited in or absorbed in a unit mass (kg) of air
It is sometimes referred to as Entrance Surface Dose or Entrance Skin Dose and has units of Gray.
Air kerma is linearly proportional to the X-ray beam intensity
