X-ray Interaction With Matter Flashcards
What 3 ways can diagnostic photons interact with matter?
Transmission - pass through
Absorption - stopped by matter and energy deposited into tissue. Photon ceases to exist
Scatter - changes direction, partial energy deposition
Absorbed and scattered = attenuation (decrease in intensity of x-ray beam)
What is the X-ray beam intensity?
Energy of photons in a cross section of the beam per unit of time
Intensity is proportional to energy and number of photons
How is an image produced on the receptor?
Attenuation
Different materials have varying degrees of attenuation
Black on X-ray = minimal attenuation
White = complete attenuation
What two attenuation interactions can occur? Which is worse for patient dose?
Photoelectric (complete absorption)
Compton effect
Both increase dose but photoelectric is necessary for image formation, compton can also increase operator dose through back scatter
Describe the photoelectric effect
When does it happen?
Why is this important?
Photon interacts with inner shell electron of atom in the subject.
Results in absorption of photon and creation of a photoelectron.
When photon energy is equal or just greater than binding energy of inner electron.
A released photoelectron can ionise and potentially damage adjacent tissues.
Why are gold and lead good for shielding in terms of photoelectric effect?
PE proportional to atomic number ^3
Soft tissue = 7 - 343
Gold = 79 - 493,039
Lead = 82 - 551, 368
Hence lead and gold absorb lots of the x-ray beam
Describe the Compton effect
How does it occur?
Why important?
Photon interacts with outer shell electron, resulting in some
absorption and creation of a recoil Compton electron.
Occurs when incoming photon energy is much greater than binding energy of electron.
Recoil electron can ionise and potentially damage tissues
Which way do photons typically scatter? How is scatter eliminated?
Higher energy = forward
Lower = backwards
Typically at 70keV = mostly forward
Eliminated by a lead collimator
What are the benefits of collimation?
Reduced scatter and improved image
Changes beam to receptor shape
Decreased patient dose
Decreased area and volume of tissue exposed to radiation dose
What happens if kV is lowered on x-ray unit?
Lower PD across anode-cathode
Lower energy photons produced
More photoelectric effect
Increased contrast of tissues
Increased dose
What happens if kV is increased in unit?
Higher PD across anode cathode
Higher energy photons
Less photoelectric interactions
Decreased image quality
But
Lower patient dose
