Ch. 7 X-Ray Interactions with Matter Flashcards
classical interactions are also commonly known as what
coherent scattering or Thomson scattering
incident photon interacts with an orbital electron of a tissue atom and the atom absorbs the energy and then releases it in a new direction
classical interactions
what energy levels does classical interactions take place in
low energy x-ray photons usually less than10 keV
do classical interactions ionize the atom
No
what is the released energy during the classical interaction called after it is reemitted in a new direction
scatter photon
what happens to most classical scatter photons
get absorbed in the body, adding slightly to patient dose not the image
how much energy does the reemitted classical photon have
same energy when the incident photon energy, just in a new direction
what energy levels does Compton scattering involve
moderate energy x-ray photons usually between 20-40 keV
incident photon interacts with an orbital electron of a tissue atom and removes it from its shell, incident photon loses up to 1/3 original energy and gets deflected in new direction
Compton scattering
In Compton interaction the incident photon can interact with one of the middle orbital electrons, knocking it out and then causing the cascade effect. What are the cascade released photons called
secondary photons - they are low energy photons contributing to patient dose
what is the ejected electron in the Compton Scattering called
Compton electron or secondary electron - enough energy to go on to interact with other atoms
what is the incident photon called after it is deflected in a new direction
Compton scatter photon - enough energy to interact with another atom or contribute to image receptor
what is the most prevalent/common interactions between x-ray photons and the human body
Compton Interactions
What is responsible for most of the scatter/fog on the image
Compton Scatter
What does Compton scattering depend on
Photons energy - more energy the less likely to interact and more likely it is to penetrate through the body, but more energy more common the chance of Compton interactions related to photoelectric interactions (with k-shell electron)
what does a deflection of 0 degrees in the Compton scattering indicate
no energy transfer
what does a deflection of a photon180 degrees indicate in a Compton scatter interaction
max energy transfer and deflection
how much energy does the scattered photon in a Compton scatter interaction usually keep
2/3 it’s original energy, only loses 1/3 energy
what is a major source of occupational exposre
Compton scatter photon - the incident photon that interacts and gets ejected in a new direction
this interaction involves inner-shell orbital electrons of the tissue atom; incident photon removes one of these inner-shell electrons from orbit, but expends all of it’s energy in doing so and gets totally absorbed
photoelectric interaction
what is the resultant ejected electron called in the photoelectric interaction
photoelectron
how much energy does the photoelectron leave with
incident photons energy minus the binding energy energy
which photoelectron will have more energy, one coming from bone or from soft tissue
soft tissue because the binding energy of soft tissue is much less; both will still have enough energy to go on to interact on their own though
what contributes significantly to patient dose
photoelectric interaction due to total absorption of the incident photon
Photoelectric interactions also create a cascade event since the k-shell electron is being ejected; just like the Compton interaction, what does this cascade produce
low energy Secondary photons which contribute primarily to patient dose
what energy level does the photoelectric interaction usually take place in
throughout the diagnostic range, usually between 20-120 kVp
what does a photoelectric interaction depend on
energy of incident photon and the atomic number of the tissue atom it would interact with
the greatest number of photoelectric interactions occurs when
incident photon energy is equal or only slightly greater than the binding energy of the inner-shell electron, as the incident photons energy increase, it’s chances of penetrating increase and therefore interaction decreases
what is the relationship between photoelectric interaction and the incident x-rays energy
inverse cubic relationship - probability of photoelectric interaction is inverse to the 3rd power of the x-rays energy
what is the relationship between photoelectric interaction and atomic number of atom being interacted with
direct cubic relationship - chance of photoelectric interaction is direct to the 3rd power of the atomic number of the absorbing material
what is used to increase photoelectric effects in tissues with low atomic numbers
barium or iodine - have high atomic numbers to increase photoelectric events
Shielding is also used to increase photoelectric effects to reduce scatter
What are the two photon interactions not in the diagnostic realm
pair production and photodisintegration
a positively charged electron
positron
what is another name for electron
negatron
for each particle in a pair production interaction exist, how much energy must they have
0.51 MeV each
what is the energy of an electron
0.51 MeV
the difference between x-ray photons that are absorbed photoelectrically and those that penetrate the body
differential absorption
x-rays photons that pass through the body and reach the image receptor
transmission
photons that are attenuated by the body and do not reach the image receptor
absorption
body structures that readily absorb x-rays
radiopaque
less dense structures that have a lower probability of absorption are
radiolucent
where is radiation dose and damage more more apparent
macrolevel
large molecules made up of thousands of atoms
macromolecules
three most common effects of energy transfer in photon interactions
main-chain scission, cross-linking, point lesions
most sensitive of moleucles
DNA
x-ray photons interacting with water molecules within the body; ionizes the water molecule, creating an ion pair - positive water molecule and negative free electron and both unstable and undergo interactions to regain stability
radiolysis of water
radiolysis of water can create these which that can indirectly damage molecules and cells
harmful free radicals
positive contrast media is administered to increase what type of interactions
photoelectric
