3 - X-ray interaction with matter

Question 1

What are the different types of interactions photons can have with matter?

Answer 1

• transmission
• absorption
• scatter

Question 2

Describe a transmission photon interaction.

Answer 2

Photon passes through matter unaltered

Question 3

Describe an absorption photon interaction.

Answer 3

• photon is stopped by the matter
• energy is fulled absorbed into matter
• photon ceases to exist

Question 4

Describe a scatter photon interaction.

Answer 4

• photon changes direction
• partial absorption of energy into matter
• can then be transmitted, absorbed or scattered again

Question 5

Define attenuation.

Answer 5

Reduction in intensity of x-ray beam

Question 6

Define x-ray beam intensity.

Answer 6

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

Question 7

How is intensity related to energy and number of photons?

Answer 7

• proportional
• increases in current (number of photons) and increases in potential difference (photon energy) increase the intensity of beam

Question 8

How does minimal attenuation present on a radiographic image?

Answer 8

Black

Question 9

How does partial attenuation present on a radiographic image?

Answer 9

Grey

Question 10

How does total attenuation present on a radiographic image?

Answer 10

White

Question 11

What are the different types of attenuation interactions?

Answer 11

• photoelectric effect
• Compton effect

Question 12

Describe the photoelectric effect.

Answer 12

• complete absorption
• photon in x-ray beam interacts with inner shell electron in patient, resulting in absorption of photon
• photoelectron is created
• electrons from outer shells move down, emitting energy in the form of light and heat

Question 13

When does the photoelectric effect typically happen in human tissues?

Answer 13

• when energy of incoming photon is equal to, or just greater than the binding energy
• with lower energy photons
• human tissues have lower binding energies so can be displaced relatively easily

Question 14

How does the photoelectric effect affect the radiographic image?

Answer 14

Lighter area on the image, as the photons don’t reach the receptor

Question 15

How is contrast produced in radiographic images using the photoelectric effect?

Answer 15

• photoelectric effect is proportional to the atomic number cubed (Z^3)
• small steps in atomic number result is large changes in absorption = contrast

Question 16

Describe the Compton effect.

Answer 16

• partial absorption and scatter
• photons in x-ray interact with outer shell electron in subject
• creates a recoil electron and the photon is scattered

Question 17

When does the Compton effect happen in human tissues?

Answer 17

• when the incoming photon has much greater energy than the binding energy of the electron
• higher energy photons

Question 18

Describe the recoil electron.

Answer 18

• some photon energy is transferred to the electron to overcome binding energy and provide kinetic energy
• electron is ejected and can ionise other tissues

Question 19

How does energy affect the direction of photon scatter?

Answer 19

Higher = forward scatter
Lower = back scatter

Question 20

How does back scatter affect the radiographic image?

Answer 20

No effect

Question 21

How does forward scatter affect the radiographic image?

Answer 21

• photons scattered obliquely forward may interact with the wrong area of the receptor
• creates fogging of image or reduced contrast

Question 22

What affects the probability of the Compton effect happening?

Answer 22

Proportional to the density of the material being irradiated

Question 23

Describe the effect of collimation on scatter.

Answer 23

• reduced surface area reduces the volume of irradiated tissue
• less scattered photons are produced
• less scatter interacting with receptor which results in better contrast on the image

Question 24

How does the photoelectric effect affect the dose to the patient?

Answer 24

All energy deposited into tissue but necessary for image formation

Question 25

How does the Compton effect affect the dose to the patient?

Answer 25

Some energy deposited into tissue but scattered photons do not contribute to the image quality

Question 26

What is the dose of a periapical radiograph?

Answer 26

4μSv

Question 27

What is the dose of a bitewing radiograph?

Answer 27

4μSv

Question 28

What is the dose of a maxillary occlusal radiograph?

Answer 28

8μSv

Question 29

What is the dose of a OPT radiograph?

Answer 29

20μSv

Question 30

What is the dose of a lateral cephalogram radiograph?

Answer 30

3μSv

Question 31

What is the dose of a dento-alveolar CBCT radiograph?

Answer 31

60μSv