X-Ray Interaction with Matter

Question 1

Define transmitted.

Answer 1

Photon passes directly though tissue unchanged.

Question 2

Define absorbed.

Answer 2

Energy fully deposited in tissue and photon ceases to exit.

Question 3

Define scattered.

Answer 3

Photon deflected by tissue.
Partial deposition of energy into tissue.
Photon continues in new direction.

Question 4

Define attenuation.

Answer 4

Reduction in intensity of x-ray beam.

Question 5

What effect describes complete absorption of photon ?

Answer 5

Photoelectric effect.

Question 6

What effect describes partial absorption or scatter of photon ?

Answer 6

Compton effect.

Question 7

Define photoelectric effect.

Answer 7

Photon in x-ray beam interacts with inner shell electron in subject resulting in absorption of the photon and creation of photoelectron.

Question 8

What are the two important points of the photoelectric effect ?

Answer 8

Predominates with lower energy photons since human tissues have relatively low binding energies.
Can ionise and potentially damage adjacent tissues.

Question 9

Define Compton effect.

Answer 9

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 10

What are the 2 important points of the Compton effect ?

Answer 10

Predominates with higher energy photos and outer shell electrons which are loosely bound.
Recoil electron can ionise and potentially damage adjacent tissues.

Question 11

What influences the direction of scatter ?

Answer 11

Energy of incoming photon.

Question 12

What direction are higher energy photons deflected in ?

Answer 12

Forward - forward scatter.
So majority of scatter from 70kV X-ray tube are forward scatter.

Question 13

What direction are lower energy photons deflected in ?

Answer 13

Backwards - backwards scatter.

Question 14

What is the effect of backwards scatter on radiographic image ?

Answer 14

Does not affect image as photon will not reach receptor.

Question 15

What is the effect of forwards scatter on radiographic image ?

Answer 15

Causes darkening of image in wrong places.
Causes ‘fogging’ - reduces image contrast and quality.
As photons will reach receptor but will interact with wrong area.

Question 16

How can scatter be reduced ?

Answer 16

Collimation.

Question 17

What is the 5 functions of collimation ?

Answer 17

Reduced surface area irradiated.
Reduced volume of irradiated tissue.
Reduced number of scattered photons produced in tissue.
Reduced scattered photons interacting with receptor.
Reduced loss of contrast on radiographic image.
Reduced patient dose.

Question 18

What is the effects of lowering the kV on x-ray unit ?

Answer 18

Lower energy photons produced due to low PD.
Increases photoelectric effect interactions.
Increase in contrast between tissues.
Increased dose absorbed by patient.

Question 19

What is the effects of increasing the kV on x-ray unit ?

Answer 19

Higher energy photons produced due to high PD.
Decreased photoelectric effect interactions.
Increased forward scatter.
Reduced dose absorbed by patient.
Poorer contrast between tissues.

Question 20

What is the most suitable kV on X-ray tube - recommended by UK guidance ?

Answer 20

60-70kV.
GDH uses 70kV.