X-ray Beam Production Flashcards
What happens during excitation?
(3)
Energy is transferred to an orbiting electron
The electron jumps from lower to higher energy levels
The atom becomes excited
What happens during ionisation?
(3)
Energy is transferred to an orbiting electron
The electron is removed from the electric field of the nucleus
The atom is ionised
What causes heating?
When an electric current runs through a wire.
More current=more heat=more electrons=more interactions
What is thermionic emission?
The release of electrons due to heat.
What are the 2 processes that produce x-rays?
Characteristic x-rays
Bremsstrahlung
What happens in characteristic x-rays to produce x-rays?
(6)
Electrons crash into the target material, hoping that they’ll hit some orbito (inner) electrons in the target material
When they collide, it causes a vacancy
The atom becomes unstable
The inner shell wants to fill the vacancy, as they only have 1 electron instead of 2
In order to replace the electron, characteristic x-rays are produced, which stabilises the atom
These x-rays use the outer electron to replace the inner electron. The change of energy causes photon x-rays
How do auger electrons come about?
After the inner shell vacancy is filled by characteristic x-rays, it’s possible for an outer shell electron to be ejected instead of a photon- this emitted electron is an Auger electron
How many times smaller is the nucleus compared to an atom?
The nucleus is 10 000x smaller than the atom
What charge does the nucleus have?
Positive charge (+)
What charge do electrons have?
Negative charge (-)
How does Bremsstrahlung radiation produce x-rays?
(4)
The electron interacts with the nucleus of the target material (rather than colliding with inner electrons like in characteristic x-rays)
The electron loses (kinetic) energy and slows down
The electron changes path
The release in energy causes the release of photon energy
What type of photon is produced during Bremsstrahlung when the electron loses lots of energy?
A high energy photon
What type of photon is produced during Bremsstrahlung when the electron loses a little bit of energy?
A low energy photon
When the electron collides directly with the nucleus during Bremsstrahlung, how much energy does it lose?
All its energy
What is kVp?
The peak voltage
What determines whether we get characteristic x-rays or not?
The changing energy in the x-ray tube, which changes the maximum energy of electrons
Why are filters needed?
To remove low energy x-rays because they get absorbed by the patient but don’t contribute to the image. This contributes to radiation dose
What does attenuated mean?
Absorbed
What type of energy radiation is more likely to be attenuated/absorbed?
Low energy radiation
When would we see a greater change in radiation?
When the radiation is passing through more material in the low energy part of the spectrum
What is beam hardening?
When the average energy of the beam increases
What does a change in quantity mean?
The distribution of energies stays the same, but the number of photons change
How can we change the quantity?
By changing the mAs of the tube, e.g. running more current in the filament to cause more interactions
What does a change in quality mean?
The distribution of energies changes as well as a change in the whole number of energies
How can we change the quality?
By changing the kV of the tube, e.g. providing more energy to the electrons causing the x-ray production
What does changing the quality result in?
A change to the shape and height of the spectra
What does the material in an x-ray tube have to be able to do?
Withstand heat
What does the filament in an x-ray tube have to be able to do?
Sustain/tolerate high amounts of thermionic emission
What does the target in an x-ray tube have to be able to do?
(2)
Dissipate heat
Produce x-rays in the desired energy range
What does the housing do in the x-ray tube?
It shields the undesired x-rays
In which direction are x-rays produced in?
All directions
What does the window in an x-ray tube do?
(2)
It allows the x-rays to go in the desired direction
It contributes to inherent filteration
What is the focal point?
An area of the target which leads to blurring but allows better heat dissipation
What do physical limitations of the focal point cause?
They cause blurring, but allows for better heat dissipation
What is the focal point size controlled by?
(2)
The focusing cup
Filament size
What does a small focal point do to the unsharpness?
It causes a smaller amount of unsharpness
What is unsharpness?
A shadow
What’s good about having a small focal point?
There’s a smaller amount of unsharpness (shadow), so there’s a higher resolution of the image.
When can a small focal point be used?
When using small amounts of energy
What does a larger focal point do the unsharpness?
There’s higher amounts of unsharpness
Why are larger focal pints worse?
They cause higher amounts of unsharpness, which causes a lower resolution of the image
When can a larger focal point be used?
When using big amounts of energy
How can we minimise unsharpness?
(2)
Put the detector close to the patient
Use the smallest focal spot size you can
What happens if the area of the target material being hit is wider?
(3)
More electrons hit more of the target material
More heat is dissipated over a larger area
This causes more unsharpness
What does a large focal spot size allow us to do to the anode?
It allows us to put more hat into the anode without damaging it
What does increasing the anode angle do?
(4)
It results in a less steep slope facing the filament
It increases focal spot size, which reduces the resolution and increases the unsharpness
It improves heat loading
It increases the field coverage
What does the anode heel mean?
It’s the idea that when x-rays are produced, electrons will be emitted at different angles. The electrons that pass through more of the target material will have less energy, and the electrons that pass through less of the target material will have more energy
Where dos attenuation occur?
In the target material
What does a longer path within the anode do to the attenuation?
It increases attenuation
Where is the anode heel effect useful in radiography?
In mammography
What is the anode heel impacted by?
The anode angle
What does the anode heel cause?
It causes different intensities of photons across the x-ray field
What minimises/eliminates the effect of the anode heel?
Digital corrections
Why is the anode heel useful in mammography?
Because in mammography, we expect a difference in anatomy thickness across the x-ray field
What does a larger anode angle do to the anode heel effect?
It reduces the anode heel’s effect
What does the x-ray spectra depend on?
(3)
Accelerating voltage
Target material
Window material
Where is the advanced tube design used?
In mammography
What does the advanced tube design say?
It says that the adjustment (change) of both the target material and the tube window can cause changes in very different x-ray spectra
