X-ray Interaction with Matter Flashcards
How can photons in a diagnostic X-ray beam interact with matter?
Transmission - passes through matter unaltered
Absorption - stopped by the matter
Scatter - changes direction
How can X-ray photons be both scattered and absorbed?
Photon deflected by tissue
Partial deposition of energy into tissue
Photon continues in new direction - to be transmitted, absorbed or scattered again
What is X-ray beam intensity?
The quantity of photon energy passing through a cross-sectional area of the beam per unit time
How does attenuation lead to radiographic image?
Different tissues and materials have varying degrees of attenuation which determine how many photons reach the receptor and what colour is shown
What is the photoelectric effect?
Photon in X-ray beam interacts with inner shell electron in subject, resulting in absorption of the photon and creation of a photoelectron
What does the photoelectric effect cause?
Absorption by the photoelectric effect prevents X-ray photons reaching the receptor leading to a lighter area on the radiographic image
What is the Compton effect?
Photon in X-ray beam interacts with outer shell electron in subject, resulting in partial absorption and scattering of the photon and creation of a recoil electron
When does the photoelectric effect occur?
When the energy of the incoming photon is equal to or just greater than the binding energy of the inner shell electron
When does the Compton effect occur?
When the energy of the incoming photon is much greater than the binding energy of the electron
What influences the direction of scattered photons?
Energy of the incoming photon:
- higher energy photons are deflected more forward
- lower energy photon are deflected more backward
Which type of scatter is most common?
Forward scatter
Why is scatter important?
It is the reason why the controlled area needs to completely surround the patient
Does backwards scatter affect radiographic image and why?
Photons will not reach the receptor so do not affect the image
Does forward scatter affect the radiographic image and why?
Photons still reach the receptor but will interact with the wrong area
Causes darkening of the image in the wrong place resulting in fogging of the image
What is used to reduce scatter?
Collimation
How does the photoelectric effect impact radiation dose?
Deposits all X-ray photon energy into tissue to increases patient dose but is necessary for image formation
How does the Compton effect radiation dose?
Deposits some of X-ray photon energy into tissue so increases dose but scattered photons don’t contribute usefully to image
May increase dose to operators from back scatter
How does lowering the kV affect the X-ray?
Overall lower energy photons produced
Increased photoelectric effect interactions
Increased contrast between tissues with different Z (good)
Increased dose absorbed by patient (bad)
How does raising the kV affect the X-ray unit?
Overall higher energy photons produced
Decreased photoelectric effect interactions and increased forward scatter
Decreases dose absorbed by patient (good)
Decreases contrast between tissues with different Z (bad)
What kV range is used for dental X-rays?
Option of either 60-70kV
GDH uses 70kV
