Lecture 9

Question 1

From what thickness does scatter begin to become a problem?

Answer 1

Tissues greater than 10 cm in thickness

Question 2

What are the main methods of scatter control?

Answer 2

1. Reduce the scatter production 2. Reduce scatter from reaching the film

Question 3

How can we reduce the scatter production of an X-ray?

Answer 3

1. Decrease the kV (Compton) - tissue thickness and density are limits on this method 2. Collimate

Question 4

What is the most effective method of reducing scatter production?

Answer 4

Collimation = the most effective method of reducing scatter production

Question 5

What are the four methods of reducing scatter reaching the film?

Answer 5

1. Air gap technique (involves increasing the distance beyond the film) 2. Preventing backscatter through the use of lead backed cassettes 3. Reduce the kV (reducing the energy of the scatter) 4. Grids

Question 6

What is the best technique for reducing the amount of scatter that reaches the film?

Answer 6

Grids = most effective method in this group

Question 7

What is increased exposure required when using a grid?

Answer 7

Absorbs scatter photons before they reach the film but also absorbs some of the transmitted beam and hence greater exposure factors are required to compensate

Question 8

What does a higher grid ratio indicate?

Answer 8

More strips of lead in a given area

Question 9

How does a grid work?

Answer 9

The scattered photons that are at a different angle to the beam are absorbed by the lead strips in-between

Question 10

What is a grid factor?

Answer 10

The grid factor = the exposure that must be increased when using a grid

Question 11

Name the following grids: (1-4)

Answer 11

1. Parallel grid
2. Focused grid
3. Cross hatched grid
4. Potter-Bucky grid

Question 12

What is the pattern you will see if you put a focused grid in upside down?

Answer 12

Alot of black in the middle of the film. Only a very small area in the middle of the film will be observed.

Question 13

What is the effect of an incorrect film focal distace?

Answer 13

Cutt-off at the edges of the film will be observed

Question 14

What is the effect of a not-centered or perpendicular to beam grid?

Answer 14

You will end up getting strips on the film that are not exposed to light

Question 15

What is the problem with this film?

Answer 15

Not centered and incorrect film focal distance

Question 16

What type of grid should you use for horses?

Answer 16

cross hatch grid

Question 17

What is the functions of placing aliminium between the beam and the patient, what is the recommendation?

Answer 17

1. Functions to increase the average beam energy
2. Reduces the patient dose- 3 mm of aliminium reduces the patient dose by 80%
3. The recommendation is 2.5 mm Aliminium for kV of greater than 70