Lectures 3 + 4

Question 1

Breifly explain how the attenuation of X-rays can occur:

Answer 1

• Attenuation can occur by both attenuation of the beam as it passes through the patient and scatter

Question 2

What are the four main factors that influence the amount of attenuation in a tissue?

Answer 2

1. The tissue thickness (cm)
2. The density of the tissue
3. The beam energy (kV)
4. The atomic number of the tissue

Question 3

Example:

We have a good exposure for a 9cm thick piece of tissue at 70kV and we want to radiograph the same tissue at a 16 cm thickness. How is the new exposure calculated?

Answer 3

• <80kV = + 2kV/cm of increased thickness
• 80-100kV = +3kV/cm of increased thickness
• > 100 kV = +4kV/cm of increased thickness

Question 4

How would you adjust exposure settings going from soft tissue to bone?

Answer 4

• Bone = 1.5 x mAs of soft tissue

Question 5

For the photoelectric effect:

1. Breifly explain how it occurs
2. Explain the significance of it (3)
3. What are the factors (2) that influence the photo-electric effect? - breifly explain why

Answer 5

How it occurs:

1. Occurs at the inner shell electrons - X-ray energy transfered to inner shell ⇒ removed from its shell
2. Ejected electron has insufficient energy to have any further effect
3. Absorption of X-ray photon occurs
4. Original photon does not reach film ⇒ reduced radiographic density on the film

Significance on PE:

1. Increased patient dose
2. Reduced beam penetration
3. Greater image contrast

Main factors affecting the photo-electric effect:

1. Beam energy (kV) - PE more likely when kV is > but close to binding energy of inner shell electrons of tissue atoms - most likely with low energy beam (low kV)
2. Tissue atomic number - high Z tissues (bone - binding energy closer to inner sheel electrons)

Question 6

What is the mathmatical relationship between tissue atomic number and the photoelectric effect? - using this explain why bone has 8x more attenuation relative to soft tissue

Answer 6

• PE (photoelectric effect) = Z³
• Bone = 14³ = 2744
• Soft tissue = 7³ = 343
• 2744/343 = 8 x (bone is 8 x greater attenuation than soft tissue)

Question 7

For comptom scatter:

1. Explain what it is
2. The significance of it (3)
3. The factors that influence compton scatter (3)

Answer 7

Comptom scatter:

1. Outer shell interactions of X-ray - causes a change in direction resulting in energy loss
2. Scattered X-rays - reach the film ⇒ inappropriate density and fog OR miss the film in all directions ⇒ creates a concern for radiation safety

Significance of compton scatter:

1. Increased scatter production
2. Decreased film quality
3. Increased worker dose

Factors that influence compton scatter:

1. Tissue thickness - increased tissue thickness - > depth of atoms ⇒ more likely interaction occuring with outer shells
2. Tissue density
3. High beam energy (kV) - increased beam energy reduces interactions with inner shell electrons - means a decrease in absorption of the beam (PE)

Question 9

At what point does scatter normally become a problem when imaging tissues?

Answer 9

• Scatter is normally a problem above 10cm

Question 10

What are the two ways that scatter production can be decreased?

Answer 10

1. Decrease the kV (Compton)
2. Decrease the volume of tissue irradiated - most effective way of reducing the scatter production (Collimate, Collimate, Collimate !!!!!!!)

Question 11

What is the most effective method of reducing the amount of scatter that reaches the film?

Answer 11

• Grids - most effective method in this group

Question 12

When using a grid ______(1)______ exposure factors are required to compensate.

Answer 12

1. Increased

Question 13

What is the grid factor?

Answer 13

1. The increase in exposure required when using a grid