B - Interaction, Attenuation, Scatter and Filtration Flashcards
What happens to X-rays we have produced?
Leakage
Spills out of the tube housing, not part of the primary beam
Primary radiation
Exits through a port in the housing
Secondary radiation
Originates within the patient, travels in all directions
Remnant radiation
Emerges from patient towards film cassette and produces the image. Is a mixture of scattered radiation and un-scattered radiation (carrying the image)
What is attenuation?
Attenuation is the loss of energy of a beam of radiant energy due to absorption, scattering, beam divergence, and other causes as the beam propagates through a medium
What are the processes of attenuation?
Coherent Scattering
Pair Production
Photo-disintegration
Photoelectric Effect/Absorption
Compton Scattering
What is coherent scatter?
If the energy of a photon is considerably less than binding energies of orbiting electrons of an atom, the photon may be deflected from its path with no loss in energy
No absorption, no energy has been permanently transferred to material
<5% of interactions in diagnostic energy range
Only interaction that does not cause ionisation
Not useful for diagnostic imaging as energy range of these photons is TOO LOW
What are other names for coherent scatter?
Classical
Rayleigh
Thompson
Unmodified
What is pair production?
Energy range of these photons is TOO HIGH for diagnostic radiography
Photon completely attenuated
Incident photon energy must be at least 1.022MeV (this is required to make matter from energy)
Photon interacts with strong electric field around nucleus
Two particles created from photon:
One electron
One positron
The electron and positron then interact with patient
Electron is quickly absorbed
Positron comes to rest and combines with a neighboring electron and the two particles neutralize each other, this is ANNHILATION RADIATION
During AR the two particles form two photons of electromagnetic radiation, travelling at 180 degrees to each other (this is a concept used in PET)
What is Photodisintegration?
X-ray photons possessing a minimum of 10 MeV of energy can interact directly with the nucleus of the atom
This causes a state of excitement within the nucleus, followed by the emission of a nuclear fragment
This process is referred to as photodisintegration
It does not occur in diagnostic radiography as photon energy TOO HIGH, but does occur in the nuclear industry
What is Photoelectric Effect?
X-ray photon involved in collision with orbiting electron
Photon gives up all its energy therefore disappears (absorbed)
Electron is ejected from atom
Absorption only if photon energy equal to or greater than electron binding energy
Photon is absorbed by an e- in an atom in the matter (patient) and, in effect, combines with it
This “photoelectron” from atom is displaced (ion pair created)
Other e- fall toward nucleus to fill the shell
Characteristic radiation is emitted, but does not often reach the image receptor
What is Compton Effect (Scatter)?
Compton Effect (modified/compton scattering) occurs when an incident x-ray photon strikes a target atom and uses a portion of its energy to eject an outer shell electron
The remainder of the photon’s energy proceeds in a direction different from that of the incoming photon
This process results in a recoil electron ejected from the outer shell, which travels until it combines with matter
Compton effect is extremely important because it is responsible for a majority of occupational worker exposure to radiation.
What is secondary radiation?
Occurs when x-rays interact with matter
Is emitted in all directions
Increases with higher tube voltage (kVp) and atomic number of material /tissue irradiated
Travels at speed of light
ends as soon as x-rays stop being emitted from tube
(x-rays do not make a room (or person!) radioactive)
Causes fog (scatter fog) if it hits the image receptor
What are the characteristics of radiation?
Quantity = number of photons in the beam (mAs)
Quality = energies of the photons (harder beam = higher quality) (kVp)
Intensity = (no. of photons) x (energy of the photons)
Attenuation = ↓ in intensity of beam as it travels through matter
Mostly from PE or Compton interactions
What 4 factors does Attenuation depend on?
- Thickness of tissue
- Energy (E) of the beam
↑ E → ↓ attenuation
3.Density
↑ density → ↑ attenuation
4.Atomic number (Z) of matter (tissue)
↑ Z → ↑ attenuation
What are the 5 radiographic densities?
Air, fat, water, bone, metal
Each has a different combination of interactions, so each attenuates x-rays to a different degree
What is the energy of the beam, kVp?
50 kV 79% is photoelectric, 21% Compton, < 1% no interaction
80 kVp 46% is photoelectric, 52% Compton 2% no interaction
110 kVp 23% photoelectric, 70% Compton, 7% no interaction
What is the effective atomic number of Fat?
6.3
