X-ray Interaction with Matter

Question 1

How can photons in a diagnostic X-ray beam interact with matter?

Answer 1

Transmission - passes through matter unaltered
Absorption - stopped by the matter
Scatter - changes direction

Question 2

How can X-ray photons be both scattered and absorbed?

Answer 2

Photon deflected by tissue
Partial deposition of energy into tissue
Photon continues in new direction - to be transmitted, absorbed or scattered again

Question 3

What is X-ray beam intensity?

Answer 3

The quantity of photon energy passing through a cross-sectional area of the beam per unit time

Question 4

How does attenuation lead to radiographic image?

Answer 4

Different tissues and materials have varying degrees of attenuation which determine how many photons reach the receptor and what colour is shown

Question 5

What is the photoelectric effect?

Answer 5

Photon in X-ray beam interacts with inner shell electron in subject, resulting in absorption of the photon and creation of a photoelectron

Question 6

What does the photoelectric effect cause?

Answer 6

Absorption by the photoelectric effect prevents X-ray photons reaching the receptor leading to a lighter area on the radiographic image

Question 7

What is the Compton effect?

Answer 7

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

Question 8

When does the photoelectric effect occur?

Answer 8

When the energy of the incoming photon is equal to or just greater than the binding energy of the inner shell electron

Question 9

When does the Compton effect occur?

Answer 9

When the energy of the incoming photon is much greater than the binding energy of the electron

Question 10

What influences the direction of scattered photons?

Answer 10

Energy of the incoming photon:
- higher energy photons are deflected more forward
- lower energy photon are deflected more backward

Question 11

Which type of scatter is most common?

Answer 11

Forward scatter

Question 12

Why is scatter important?

Answer 12

It is the reason why the controlled area needs to completely surround the patient

Question 13

Does backwards scatter affect radiographic image and why?

Answer 13

Photons will not reach the receptor so do not affect the image

Question 14

Does forward scatter affect the radiographic image and why?

Answer 14

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

Question 15

What is used to reduce scatter?

Answer 15

Collimation

Question 16

How does the photoelectric effect impact radiation dose?

Answer 16

Deposits all X-ray photon energy into tissue to increases patient dose but is necessary for image formation

Question 17

How does the Compton effect radiation dose?

Answer 17

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

Question 18

How does lowering the kV affect the X-ray?

Answer 18

Overall lower energy photons produced
Increased photoelectric effect interactions
Increased contrast between tissues with different Z (good)
Increased dose absorbed by patient (bad)

Question 19

How does raising the kV affect the X-ray unit?

Answer 19

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)

Question 20

What kV range is used for dental X-rays?

Answer 20

Option of either 60-70kV
GDH uses 70kV