X-ray Interation With Matter Flashcards
How do photons travel?
travel in a straight line at the speed of light until they interact with something
How can photons in a diagnostic x-ray beam interact with matter?
in three ways
• Transmission (i.e. passes through matter unaltered)
• Absorption (i.e. stopped by the matter)
• Scatter (i.e. changes direction)
What happens to an absorbed photon?
• Energy fully deposited into tissue
• Photon ceases to exist
What happens to a scattered + absorbed photon?
• Photon deflected by tissue
• Partial deposition of energy into tissue
• Photon continues in new direction
• To be transmitted, absorbed or scattered again
In what scenarios does attenuation occur?
Scattered
Absorbed
What is the x-ray beam intensity?
Quantity of photon energy passing through a cross-sectional area of the beam per unit
time
What is the intensity proportional to?
• Energy of X-ray beam affected by potential difference across X-ray tube (kV)
• ↑ kV → ↑ average photon energy & ↑ maximum photon energy
• Number of photons in X-ray beam primarily affected by current in filament (mA)
• ↑ mA → ↑ number of photons
• Note: increasing potential difference will also increase number of photons since an electron with
more energy can undergo more interactions at the target
What does minimal attenuation look like on a radiograph?
Black
What does complete attenuation appear on a radiograph?
White
How can you predict x-ray photon interactions?
Unable to predict outcome of a single photon, but can estimate proportion of
interactions in an X-ray beam (which consists of millions of photons)
Predictions based on physical properties of matter being exposed
• e.g. thick lead → essentially all attenuation
• e.g. piece of paper → essentially all transmission
• e.g. enamel → mostly attenuation
• e.g. cheek → mostly transmission
What is the photoelectric effect?
Photon in X-ray beam interacts with inner shell electron in subject, resulting in absorption of the photon & creation of a photoelectron
How does the photoelectric effect occur?
Occurs when energy of incoming photon is equal to, or just greater than, binding energy of inner shell electron
• Therefore photoelectric effect predominates with lower energy photons (since
human tissues have relatively low binding energies)
What does the photoelectric effect result in?
Photon energy overcomes binding energy, resulting in inner shell electron
being ejected (now called a “photoelectron”)
• Any excess photon energy becomes kinetic energy of photoelectron
• Photoelectron can ionise (& potentially damage) adjacent tissues
Vacancy in inner shell is filled by cascade of outer shell electrons
• Produces light photons &/or heat
What does the photoelectric effect appear on a radiograph?
Absorption by the photoelectric effect prevents X-ray photons reaching the receptor → leads to lighter area on radiographic image
What is the probability of the photoelectric effect occurring?
• Proportional to atomic number cubed (Z3)
• Inversely proportional to photon energy cubed (1 / E3)
• Proportional to physical density of material (ρ)
p x Z3/ E3
What does increasing kV do to the beam?
• Increasing kV → less attenuation of the X-ray beam
What is the photoelectric effect proportional to?
Photoelectric effect proportional to Z3
What do small steps in z result in?
• Small steps in Z result in large jumps in absorption
• Results in good contrast between different tissues on radiographic image
What is the Compton effect?
• Photon in X-ray beam interacts with outer shell electron in subject, resulting in partial
absorption & scattering of the photon & creation of a recoil electron
When does the Compton effect occur?
Occurs when energy of incoming photon is much greater than binding energy
of electron
• Therefore Compton effect predominates with higher energy photons & outer shell
electrons (which are loosely bound)
What is a recoil electron and what can it do?
Some photon energy transferred to electron to overcome binding energy &
provide kinetic energy
• Electron ejected & called a “recoil electron”
• Recoil electron can ionise (& potentially damage) adjacent tissues
What happens to the photon in the Compton effect?
Photon loses energy & changes direction (i.e. is scattered)
• Can undergo Photoelectric effect & further Compton effect interactions
What is the direction of scattered photons?
Scattered photons can be deflected in any direction but are influenced by the
energy of the incoming photon
• Higher energy photons are deflected more forward → “forward scatter”
• Lower energy photons are deflected more backward → “back scatter”
What direction is the majority of scatter from an x-ray beam at 70kV?
Forward
Why does the controlled area need to completely surround the patient
Scattered
Why does the controlled area need to completely surround the patient
Scatter
What is the effect of photons scattered backwards, sideways or very obliquely forwards?
Will not reach the receptor and do not affect image
What is the effect of photons scattered slightly obliquely forwards?
Photons scattered slightly obliquely forwards may still reach the receptor but
will interact with the wrong area
• Causes darkening of the image in the wrong place
• Results in “fogging” of image → reduces image contrast/quality
What is the probability of Compton effect occurring dependent on
• Independent of Z
• Weakly proportional to photon energy
• Proportional to density of material
What will increasing the photon energy do to the Compton effect?
increasing photon energy has minimal effect on the likelihood of the Compton effect, but higher energy photons are more likely to scatter forwards,
reach the receptor, & degrade the radiographic image
How can scatter be reduced?
Collimation
What does collimation result in?
- ↓ surface area irradiated
- ↓ volume of irradiated tissue
- ↓ number of scattered photons produced in the tissue
- ↓ scattered photons interacting with receptor
- ↓ loss of contrast on radiographic image
What impact does the photoelectric effect have on radiation dose?
• Deposition of all X-ray photon energy into tissue
• ↑ patient dose but is necessary for image formation
What impact does the Compton effect have on radiation dose?
• Deposition of some X-ray photon energy in tissue
• ↑ patient dose but scattered photons do not contribute usefully to image
• May ↑ dose to operators (from back scatter)
What is the effect of lowering kV on x-ray unit?
Lower X-ray tube potential difference (kV) >
Overall lower energy photons produced >
↑ photoelectric effect interactions >
↑ contrast between tissues with different Z >
BUT
↑ dose absorbed by patient
What is the effect of raising kV on x-ray unit?
Higher X-ray tube potential difference (kV) >
Overall higher energy photons produced >
↓ photoelectric effect interactions (+ ↑ forward scatter) >
↓ dose absorbed by patient >
BUT
↓ contrast between tissues with different Z
What does deciding on kV compromise?
Decision is a compromise between image quality
& patient radiation dose
UK guidance advises a suitable range of 60-70kV
for intraoral X-ray units
• Units typically give the option of either 60kV or 70kV
• Glasgow Dental Hospital uses 70kV
Where do continuous/characteristic radiation interactions occur, what is the interaction between and what does it lead to?
• Occur in X-ray tube (at target)
• Electrons interacting with tungsten atoms
• Lead to production of X-ray photons
Where do photoelectric/ Compton effects occur, what is the interaction between and what does it lead to?
• Occur in patient/receptor/shielding
• X-ray photons interacting with atoms
• Lead to attenuation of X-ray beam
