X-ray Interaction with Matter (c) Flashcards
What is the source of x-rays?
an x-ray machine is the source of x-rays
X-ray photons pass from the tube, and some through the patient, to reach what?
x-ray photons pass from the tube, and some through the patient, to reach the image receptor
Interaction with different X alters the number of photons exiting the patient
X - tissues
The variation in numbers of X reaching the image receptor produces the radiographic appearance of different tissues
X - photons
What are the 4 different ways in which x-ray photons may traverse tissue?
x-ray photons may traverse tissue by:
- passing through unaltered
- changing direction with no energy loss
- scatter
- changing diection and losing energy
- scatter and absorption
- being stopped, depositing all energy within tissue
- absorption
What is attentuation?
attenuation is a reduction in number of photons (x-rays) within the beam
What is a reduction in number of photons (x-rays) within the beam known as?
a reduction in the number of photons (x-rays) within the beam is known as attenuation
What does attenuation occur as a result of?
attenuation occurs as a result of absorption and scatter
What occurs as a result of absorption and scatter?
attenuation occurs as a result of absorption and scatter
When all photons reach the image receptor, what will the image appear like?
when all photons reach the image receptor, the image appears black
When there is partial attenuation, what does the image appear like?
when there is partial attenuation, the image appears grey
When there is complete attenuation, what will the image appear like?
when there is complete attenuation, the image will appear white
What is the charge of the nucleus?
the nucleus has a positive charge
What is the atomic number equal to?
the atomci number is equal to the number of protons
What is the mass number equal to?
the mass number is equal to the number of protons + neutrons
Electrons in what shell have the highest binding energy?
electrons in the K shell have the highest binding energy
Outer shells have X binding energies
X - lower
The maximum number of electrons in the orbit is X in outer orbits
X - greater
What are the principle interactions of diagnostic x-rays in tissue?
principle interactions of diagnostic x-rays in tissue:
- photoelectric effect - absorption
- Compton effect - scatter and absorption
In the photoelectric effect, an x-ray photon interacts with an X shell electron
X - inner
In the photoelectric effect, the incoming photon has energy just X than the binding energy of the electron
X - higher
In the photoelectric effect, does the photon reach the image receptor?
no, the photon doesn’t reach the image receptor in the photoelectric effect
In the photoelectric effect, there is complete X
X - absorption
X effect:
- x-ray photon interacts with inner shell electron (usually K shell)
- photon has energy just higher than the binding energy of the electron
- x-ray photon disappears
- most of the photon energy is used to overcome the binding energy of the electron, remainder gives electron kinetic energy
- electron is ejected (photoelectron)
- atom has “hole” in electron shell and is positively charged
- unstable
- electron drops from outer shell, filling the void
- difference in energy between the 2 levels is emitted as light/heat (characteristic radiation)
- outer voids are filled by “free” electrons
- results in complete absorption of photon energy, the photon doesn’t reach the image receptor
X - photoelectric
The X effect results in complete absorption of the photon, preventing any interaction with the active component of the image receptor - the image appears white if all photons are involved, grey if some photons are not involved
X - photoelectric
What is the occurence of the photoelectric effect proportional to?
the occurence of the photoelectric effect is proportional to:
- (atomic number)3
- 1/(photon energy)3
- density of material
X effect:
- x-ray photon interacts with loosely bound outer shell electron
- photon energy considerably greater than the electron biinding energy
- electron is ejected, taking some of the photon energy as kinetic energy: recoil electron
- atom is then positively charged
- following collision, photon has lower energy (longer wavelength), called a scatter photon
- undergoes a change of direction
X - compton
In the compton effect, following scatter events - how is atomic stability regained?
with the compton effect, following scatter events, atomic satbility is regained by capture of free electrons
In what direction can scatter photons travel?
scatter photons can travel in any direction
What is the direction of scatter affected by?
the direction of scatter is affected by the energy of the scatter photon
If a scatter photon has a high energy, what direction will it travel in?
if a scatter photon has high energy, it will travel in a forward direction
If a scatter photon has a low energy, what direction will it travel in?
if a scatter photon has a low energy, it willl travel in a backwards direction
What is the probability of the Compton effect occurring proportional to?
the probability of the Compton effect occurring is proportional to the density of the material (electron density)
Is the probability of the Compton effect occurring dependent on atomic number?
the probability of the Compton effect occurring is independent of atomic number
Is the probability of the Compton effect occurring related to photon energy?
no, the probability of the Compton effect occurring is not related to photon energy
Scattered photons produced before the image receptor is reached (and scattered backwards) X reach the image receptor and X contribute to the image
X - don’t
Scattered photons produced beyond the image receptor and scattered back towards it may reach the image receptor and produce a X
X - darkening
As the path of Compton scattered photons is randomly altered, they do not contribute X information to the image
X - useful
Scattered photons produced beyond the image recepotr and scattered back towards it result in X of the image, reducing contrast and image quality
X - fogging
Scattered photons produced beyond the image receptor (and scattered back towards it) results in fogging of the image, reducing X and image quality
X - contrast
How can you reduce scatter?
Collimination and use of lead foil to prevent back scattered photons
Reduction of area irradiated (collimation), and therefore volume irradiated, will reduce the number of scattered photons produced as well as reducing what?
reduction of area irradiated (collimation), and therefore volume irradiated, will reduce the number of scattered photons as well as reducing patient dose
X within film packets prevents the back scatterd photons from oral tissues reaching the film (in addition to absorbing some of the energy in the primary beam). These are not used with digital receptors
X - lead foil
Under what circumstances is absorption of photons more likely?
absorption of photons is more likely if:
- object traversed has a high atomic number
- object traversed is thicker
- photon energy is lower
X of photons is more likely if:
- object traversed has a high atomic number
- object traversed is thicker
- photon energy is lower
X - absorption
What is radiographic contrast?
radiographic contrast is the difference in density of light and dark areas of radiographs
What is the difference in density of light and dark areas of radiographs known as?
the difference in density of light and dark areas of radiographs is known as radiographic contrast
An image showing both light and dark areas with clear borders has a X contrast
X - high
X is greatest when the difference in absorption by adjacent tissues is greatest
X - contrast
When is contrast at its greatest?
contrast is at its greatest when the difference in absorption by adjacent tissues is greatest
Photoelectric absorption results in deposition of all photon energy within tissue, giving an X patient dose but this is necessary for image quality
X - increased
X absorption results in deposition of all photon energy within tissue, giving an increased patient dose but this is necessary for image quality
X - photoelectric
X scatter results in deposition of some photon energy within tissue:
- adding to patient dose but does not give useful information
- may increase the dose to operators
X - compton
Compton scatter results in deposition of some photon energy within tissue:
- X to patient dose but not giving useful information
- may increase dose to operators
X - adding
Who may compton scatter increase the dose to?
Compton scatter may increase the dose to operators
A low tube potential difference (kVp) produces X energy photons
X - lower
When there is a low kVp, photoelectric interactions are X
X - increased
When there is a low kVp, contrast between different tissues X
X - increases
When there is a low kVp, the dose absorbed by the patient is X
X - increased
A high tube kVp produces X energy photons
X - higher
When there is a high kVp, photoelectric interactions are X
X - reduced
When there is a high kVp, contrast is X
X - reduced
When there is a high kVp, the dose absorbed by the patient is X
X - reduced
What is the chosen kVp a compromise between?
the chosen kVp is a compromise between diagnostic quality of image and dose
