X-ray production Flashcards
What is Bremsstrahlung x-ray production?
X-rays created from the conversion of the electrons kinetic energy into electromagnetic radiation when they are decelerated by interaction with a target material
What is Bremsstrahlung radiation?
Electrons undergo inelastic collisions; their path is deflected by the +ve charged nucleus and there is kinetic energy loss. The radiation acompanying the KE loss is Bremsstrahlung (braking) radiation
What is Bremsstrahlung emission per atom proportional to?
Z^2 / m^2
Z = atomic number of absorber / m = mass of incident particle
Why is the xray spectrum continuous?
Bremsstrahlung x-ray photon energies can be any value up to and including the entire KE of the deflected electron. Many electrons undergoing Bremsstrahlung interactions = continous spectrum of x-ray energies
What voltages are used in DR?
20-150kV
Mammo 25-40kV; diagnostic imaging 40-150kV
What does the x-ray tube contain?
- electron source (cathode)
- target (anode)
- rotor/stator
- glass or metal envelope
- tube port (output window)
- cable sockets (out to the housing, then out of the housing)
Housing = lead for shielding and cooling; tube is evacuated
How does x-ray generation work?
- Appy potential difference between 2 electrodes (anode held at +ve voltage WRT cathode)
- Electrons travel from cathode to anode, accelerating due to voltage, gaining KE
- Electrons hit anode target
- KE is converted into other forms of energy
- Heat mostly, also Bremsstrahlung and characteristic xrays
What type of interactions occur at the anode?
- Mostly collisional; energy exchange with the target gives rise to heat
- Small fraction interact with nucleus +ve field, get deflected (Bremsstrahlung)
- Some electrons interact with inner shell (K) of bound electrons, ejecting one if their KE > binding energy (characteristic)
Which force is at play during Bremsstrahlung radiation?
Coulombic (electrical) force
electrons come within proximity of +ve nucleus, get influenced by its field.
Force attracts the electron, decelerating it, changing its direction, causing a loss of KE.
KE is emitted as an xray photon of equal energy
How does Coulomb force depend on distance?
1/(distance)^2
The amount of energy through loss of KE (causing Bremsstrahlung radiation) depends on what?
The distance between the incident electron and the target nucleus
If an electron interacts very near to the nucleus, what energy will the Bremsstrahlung radiation be?
close to nucleus = force acting on electron increases = greater deceleration = more KE lost = higher energy radiation emitted
What happens if an electron nearly directly impacts on a target nucleus?
Nearly all the KE will be lost, so the highest possible energy Bremsstrahlung xray radiation will be produced
this is rare!
How does the probability of an interaction resulting in a Bremsstrahlung xray radiation change with interaction distance r?
interaction distance = distance between nucleus and electron
circumference = 2.pi.r
Probability is proportional to r
Increasing distance from nucleus r = increased circumference = probability of interaction increases (but xray energy decreases)
If the probability of interaction increases, what happens to the emitted xray energy?
Energy decreases [as interaction probability increases]
How does the number of xrays produced scale with the energy they will have?
No. xrays produced decreases linearly with energy
(up to the maximum xray energy = incident electron energy)
A Bramsstrahlung spectrum shows….
the probability distribution of xray photons as a function of photon energy (keV)
For an unfiltered Bremsstrahlung spectrum, how does the number of xrays produced scale with their energy?
inverse linear relationship, i.e. lots produced but mostly low energy
For a filters Bremsstrahlung spectrum: when is the maximum number of xrays produced?
When E = 1/3 max
the number produced will decrease to zero as the energy increases to the
Filtered spectrum has no xrays < 10keV
What determines the highest energy an xray can have?
The peak voltage applied across the tube (kV)
What is xray filtration?
Removal of xrays by attenuation in materials that are inherent in the tube (e.g. glass window of insert) and materials placed in the beam to remove lower energy xrays (aluminium, copper)
Why would you want to filter an xray beam?
To remove lower energy xrays and adjust the spectrum for optimal low-dose imaging. If the energy is too low, the xrays would contribute to the patient dose but not to image formation process
What affects the efficiency of xray production?
- atomic number of the target material
- KE of the incident electrons
energy conversted to heat or low energy e-m radiation is not useful!
Name 3 common target materials
- tungsten W (Z=74)
- molybdenum Mo (Z=42)
- rhodium Rh (Z=45)
Mo and Rh used in mammography
