X-ray interactions Flashcards
What is the photoelectric effect?
When a photon knocks an innner-shell electron out of its shell
What is the name of the electron knocked out of its shell due to the photoelectric effect?
a photoelectron
What normally happens to an electron knocked out of its shell in the photoelectron effect?
Absorbed almost immediately by surround matter
What end products are produced by the photoelectric effect?
Characteristic radiation (as an electron drops down a shell and releases it’s excess energy in the form of a photon)
A negative ion (the photoelectron)
A positive ion (atom deficient one electron)
What occurs to the characteristic radiation x-rays produced by the photoelectric effect?
(when x-rays interact with the body)
They are usually absorbed almost immediately?
What increases the chance of the photoelectric effect occurring?
If the energy of the x-ray is close to the binding energy of a given electron (although it must have more energy than the binding energy)
The tighter an election is bound in its shell the more likely it is be involved in the photoelectric effect
Photo electric effect is proportional to:
atomic number cubed/ incident photon energy cubed
Why are characteristic radiation x-rays produced in the body more likely to be absorbed, than those produced in an x-ray machine?
They have a lot less energy.
Instead of high energy electron hitting inner shells of high atomic number atoms it’s x-rays hitting inner shells of average atomic number atoms.
What are the main types of interactions that x-rays have with tissues?
Photo-electric effect
Compton Scatter
Coherent (Rayleigh) scattering
Does photoelectric effect have more of on important role in x-rays of soft tissues (muscle, fat) or hard tissues (bone)?
soft tissues
helps to create contrast int these tissues
What is the disadvantage of relying on the photoelectric effect for x-ray images?
increased patient radiation, all the energy from the x-rays is absorbed
(not the case with Compton scatter)
What occurs in Compton scatter?
The x-ray interacts with an electron in the outer shell., displacing it.
A new x-ray is created, travelling in a slightly different direction.
What factors increase the amount of energy absorbed from a photon during Compton scatter
The more of an angle the photon is ‘deflected’ the more energy is lost.
Lower energy photons also lose a greater proportion of their energy for a given angle of scatter.
Is compton effect more or less likley with higher keV?
Less likley, although only slightly.
However, it becomes more relevant than the photoelectric effect, which has a vanishingly small chance of happening.
What affect the probability of Compton Scatter occurring?
Slightly less likley with increasing keV photons.
Electron density.
Although all elements contain roughly the same electron density so nothing really.
What metric is used to measure the amount of scattered radiation in an x-ray?
Scatter to primary ratio
What are methods to reduce scatter in an x-ray image?
limit the x-ray field (collimation)
limit the irradiated thickness (e.g. squish breasts for mammography)
anti-scatter grids
Luminescence
Materials absorbing energy and converting it to light
Flourescence
Materials absorbing energy and converting it to light within 10^-8 seconds
Phosphorescence
Materials absorbing energy and converting it to light in > 10^-8 seconds
What does CR stand for in x-ray technology?
Computed Radiography
What is a phosphor
a chemical compound that emits light when exposed to light of a different wavelength.
How is an x-ray CR cassette read?
laser light (red) stimulates emission of trapped energy.
Visible light (blue) is released from the plate
The visible light strikes a photo-multiplier tube and produces an electric signal.
What is thermoluminescence
where heating liberates energy, which is released as light
What uses are there for thermoluminescence in radiology?
personal dosimetry
patient dose measurement
measurement of scatter
What is Rayleigh scattering also called?
Coherent scattering
What is Coherent scattering also called?
Rayleigh scattering
What occurs on Rayleigh scattering?
incident photon excites a total atom.
a scattered photon is released with the same energy/ wavelength as the incident photon
Is there ionisation in Ryleigh scattering?
No
Is there ionisation in Compton scattering?
Yes
Is there ionisation in Photo-electric interactions?
Yes
What units are used to measure attenuation in x-rays?
Linear attenuation co-efficient (μ):
A quantitative measurement of attenuation per cm of the absorber
Mass Attenuation Coefficient
divide linear attenuation coefficient by material density
What factors influence the degree of attenuation of an x-ray beam?
The spectrum of x-rays
The density of the material
What is the unit of measurement for linear attenuation co-efficient?
μ
OR
cm^-1
What word is used to describe an x-ray beam that has a spectrum/ range of energies? (2)
polyenergetic
polychromatic
What are the main x-ray contrast materials used? (2)
Barium
Iodine
Through what mechanism do phosphors work usually?
The photo-electric effect from a high atomic number component.
Does the photo-electric effect produce scattered radiation?
no
What’s the fundamental difference between the photoelectric effect and Compton scattering
Photo electric effect knocks an inner shell electron out of orbit
Compton scatter knocks an outer shell electron out of orbit
Photoelectric effect more likely if electron energy much higher than the binding energy of an electron?
No, photoelectric effect is more likely if the energies are similar (although electron energy still needs to be a little higher than the binding energy)
Compton Scatter more likely if electron energy much higher than the binding energy of an electron?
Yes, more likely as electron energy increases
