X-ray beam attenuation and scatter control

Question 1

What is the reduction in x-ray intensity that results from absorption and scattering?

Answer 1

Attenuation

Question 2

What factors effect the amount of attenuation that occurs?

Answer 2

1. The tissue thickness (cm)
2. The density of the tissue and bone for same thickness
3. The beam energy (kV) – PE (photoelectric effect)
4. The atomic number of the tissue - PE

Question 3

What will increasing tissue thickness affect?

Answer 3

↑ x-ray beam attenuation (absorption and scatter)

↓ # x-rays reaching the film

Question 4

How do you compensate for an increase in tissue thickness?

Answer 4

↑ the exposure (kV with mAs) = increased energy and penetration

Question 5

When do you measure the tissue thickness?

Answer 5

When in position

Question 6

What happens if you measure an animal’s tissue thickness whilst standing or sitting?

Answer 6

Over-exposed radiograph - measured thicker

Question 7

What is the attenuated density of bone, soft tissue and gas?

Answer 7

Bone = 1.5 (small animal) - 2 (large animal)
Soft tissue = 1
Gas = negligible

Question 8

This occurs when an incident x-ray is totally absorbed during the ionization of an inner-shell electron

Answer 8

Photoelectric effect (PE)

Question 9

Significance of PE

Answer 9

↑ patient dose
↓ beam penetration - more absorbed in patient
↑ image contrast

Question 10

Factors that influence PE

Answer 10

Beam energy (kV)
Tissue atomic number

Question 11

In Compton scattering, the incident x-ray interacts with an _______ electron and ejects it from the atom

Answer 11

Outer-shell

Question 12

X-rays throughout the diagnostic range can undergo an interaction with outer-shell electrons that not only scatters the x-ray but reduces its energy and ionizes the atom as well. This interaction is called_______

Answer 12

Compton scattering

Question 13

Compton scattering reduces _____ _____

Answer 13

Image contrast - creates a fog over image

Question 14

What factors increase Compton scatter?

Answer 14

↑ tissue thickness
↑ tissue density
↑ beam energy

Question 15

What causes scatter?

Answer 15

When an x-ray photon collides with a stationary outer-shell electron, transferring some of the energy and momentum to the electron
• Electron will be excited and ejected from its valence shell atom
• X-ray photon has less energy moving in a scattered direction randomly

Question 16

How can you reduce scatter reaching the film?

Answer 16

Grids

Question 17

How can you reduce scatter production?

Answer 17

Collimation

Question 18

What is beam collimation?

Answer 18

Utilise movable lead leaves and a light source to define the area of the beam to reduce scatter production

Question 19

When should you generally use a grid to reduce scatter reaching a film?

Answer 19

At 10cm tissue thickness

9cm bone, 15cm thorax

Question 20

What is the draw-back of using grids?

Answer 20

Absorb some of transmitted x-ray beam (primary beam as well as scatter)

Question 21

Name the 4 different types of grids

Answer 21

Parallel
Focus
Cross-hatch
Potter-Bucky

Question 22

What is the main difference in Compton radiation and the Photoelectric effect?

Answer 22

Compton’s incident x-ray interacts with an outer-shell electron, Photoelectric’s incident x-ray interacts with an inner-shell electron

Question 23

Bone has an atomic number of 13.8; soft tissue has an atomic number of 7.4. Which will absorb more x-ray?

Answer 23

Bone will absorb more x-ray because of its higher atomic number

Question 24

What is scatter radiation called when x-rays are bounced back towards to the incident beam?

Answer 24

Backscatter radiation

Question 25

X-rays that pass thru the body without interacting do what to the image?

Answer 25

Create black areas

Question 26

An increase in patient thickness results in a/an ____________ in attenuation

Answer 26

Increase

Question 27

The primary purpose of the glass envelope of an x-ray tube is to

Answer 27

Provide a vacuum