XR interaction 1 Flashcards
photoelectric absorption and compton scattering
What three things can happen to an X-ray photon incident on a object?
Case 1: X-ray photon passes straight through the matter without interacting
Case 2: X-ray photon is completely absorbed by the matter
Case 3: X-ray photon is scattered from its original path because of interacting with the matter
XR photon interactions
-X-ray photon interactions are random statistical processes
some interactions have a higher probability which is dependent on a number of factors
-For example
-it is more likely that a thicker object will absorb more X-ray photons than a thinner object made of the same material
-it is more likely that a dense object will absorb more X-ray photons than a less dense object
what are the 5 processes whereby x ray photons can interact with matter
-Photoelectric absorption
-Compton scattering
-Coherent (Rayleigh) scattering
-Pair production
-Photodisintegration
what is the definition of absorption
The transfer of energy from the X-ray beam to the medium
outline photoelectric absorption
- Photon Interaction: An X-ray photon with energy equal to or slightly greater than the binding energy of an inner-shell electron (usually K or L shell) interacts with the atom.
- Electron Ejection: The photon transfers all its energy to the electron, ejecting it from its shell and ionizing the atom.
- Energy Transfer: The energy of the ejected photoelectron is equal to the energy of the incident photon minus the binding energy of the electron.
4.Secondary Emissions: The vacancy in the inner shell is filled by an electron from a higher energy level, leading to the emission of characteristic X-rays or Auger electrons.
what does the probbaltiy of photoelectric effect depend on?
E - energy of the x-ray photon
Z - atomic number of the material
p - mass density of the material
energy of the XR photon regarding photoelectric absorption
Probability of photoelectric interactions is highest when the X-ray photons energy is slightly above the electron binding energy
If the X-ray photon energy is too low in cannot free the bound electron
If the energy is too high the probability significantly decreases due to the inverse relationship with the cube of the energy.
photoelectric absorption and image quality
Photoelectric effect produces excellent images because it enhances natural tissue contrast and does not produce scatter
photoelectric absorption and contrast
X-ray image contrast depends on some tissues absorbing more X-ray photons than other tissues
Contrast is greatest when the difference in absorption between adjacent tissues is greatest
Because the number of photoelectric reactions depends on the third power of the atomic number, the photoelectric effect magnifies the difference in tissues composed of different elements such as bone and soft tissue
Photoelectric Absorption & Patient Dose
The photoelectric effect results in patients receiving more radiation dose than from any other type of interaction
All the energy of an incident photon is absorbed by the patient
Effect of photoelectric interaction can be reduced using high-energy (kVp) techniques
photoelectric absorption and contrast media
-K-edge absorption
-Iodine & barium contrast media
define k edge absorption and contrast media
K-edge absorption refers to a sudden increase in the absorption of X-rays by an atom when the photon energy is just above the binding energy of the K-shell electrons.
Contrast media (or contrast agents) are substances used in medical imaging to enhance the contrast of structures or fluids within the body, making them more visible on radiographs or other imaging modalities.
K-edge Absorption & Contrast Media
-as photon energy increases, there is a sudden jump in attenuation.
-as the energy increases a larger no, of electrons become available for photoelectric absorption.
-the probability of the X-ray photon being attenuated by a photoelectric interaction significantly increases
- this is useful for materials such as iodine and barium- as it produces good contrast.
K-edge Absorption & Contrast Media- iodine and barium
Iodine contrast agent is often injected into blood vessels to make their structures visible on X-ray images—they would be otherwise invisible.
Barium contrast agent is often administered orally to make gastrointestinal structures visible of X-ray images—they would otherwise be invisible
what is compton scattering
A change in the direction of an X-ray photon due to interaction
An X-ray photon interacts with a loosely bound outer shell electron of the object atom
The bound electron absorbs some of the incident X-ray photon’s energy and is deflected
The X-ray photon having lose some of it’s energy is deflected (scattered)
what is compton scattering also called?
Compton scattering is also called incoherent scattering as the change of photon energy is not orderly and consistent
The change in energy of the X-ray photon depends on the angle through which it is scattering and not on the scattering medium
The larger the energy transfer from the incident X-ray photon to the loosely bound outer shell electron the….
-lower the energy of the scattered X-ray photon
-larger the angle of the deflected X-ray photon
-higher the kinetic energy of the deflected electron
the probability of compton scattering depends on:
The probability of Compton scattering depends on:
-Total number of electrons in absorber
-Density
-Number electrons per gram
-All elements contain approximately the same number of electrons per gram, regardless of atomic number
Therefore, probability of Compton scattering is almost independent of the atomic number Z but does depend on:
the density of the absorber—the higher the density the greater the Compton scattering
the energy of the incident X-ray photon—inversely proportional to photon energy
when does the probability of compton interaction decrease?
as incident photon energy increases (but I fairly constant across the diagnostic energy range).
what effect does compton scattering have on image quality
Degrades image quality
Arrives on the image detector from all directions leading to an increase in overall image density with reduced image contrast
At worst the image may be non-diagnostic
what radiation dose issues can compton scattering lead to
-Compton scatter that escapes from the patient or X-ray tube can travel in all directions
-Compton scatter can be absorbed by staff working in proximity to the patient
-Compton scatter can be absorbed by patient anatomy outside the primary X-ray bea
what are the key points about compton scattering
-Dominates at high photon energy
-Degrades radiographic contrast if incident on X-ray detector
-Can lead to staff and patient dose issues
