Radiography Interactions And Grids

Question 1

What does the radiograph or X-ray picture depend on?

Answer 1

Different amounts of radiation reaching the receptor and causing blackening

Question 2

What are the three factors that affect the absorption of X-rays by tissue?

Answer 2

• Atomic number (Z) of the tissue
• Density of the tissue
• Thickness of the tissue

Question 3

How does the atomic number affect X-ray absorption?

Answer 3

Higher atomic number tissues absorb more X-rays

Question 4

Which tissue has a higher atomic number, bone or soft tissue?

Answer 4

Bone

Question 5

What is the relationship between tissue density and X-ray absorption?

Answer 5

Higher density tissues absorb more X-rays; lower density tissues absorb fewer

Question 6

How does the thickness of tissue affect X-ray absorption?

Answer 6

Thicker areas will absorb more X-rays than thinner ones

Question 7

What appearance does bone have on a radiograph?

Answer 7

Nearly white due to high absorption of X-rays

Question 8

Fill in the blank: Gas appears ______ on the radiograph.

Answer 8

very dark

Question 9

What happens to the intensity of the X-ray beam as it passes through matter?

Answer 9

It decreases due to various effects.

Question 10

What is the photoelectric effect?

Answer 10

The incident X-ray photon collides with an electron and knocks it out of its shell around the nucleus of the atom.

Question 11

What is a photoelectron?

Answer 11

The electron displaced during the photoelectric effect.

Question 12

What happens to the energy of the photoelectron?

Answer 12

It eventually loses its energy as heat.

Question 13

What occurs after a photoelectron is displaced?

Answer 13

An electron from an outer shell fills the space, as it is going from a high energy position to a low energy position, this extra energy is lost as radiation.

Question 14

What is characteristic radiation?

Answer 14

Radiation emitted when an outer shell electron moves to fill a vacancy, characteristic to that atom.

Question 15

At what energy levels does the photoelectric effect occur?

Answer 15

It occurs at relatively low kVs, ie relatively low energy X-ray photons.

Question 16

What is the Compton effect?

Answer 16

An X-ray photon interacts with a loosely bound or free electron at higher kVs.

Question 17

What happens to the displaced electron in the Compton effect?

Answer 17

It is called a recoil or Compton electron and eventually loses its energy as heat.

Question 18

What is the energy of scattered radiation in the Compton effect?

Answer 18

It has lower energy than the incoming photon.

Question 19

In what directions can scattered radiation be sent?

Answer 19

It can be sent in any direction.

Question 20

What factor does the Compton effect vary with?

Answer 20

It varies with the density of the tissue.

Question 21

Fill in the blank: The photoelectric effect occurs at relatively low _______.

Answer 21

[kVs]

Question 22

Fill in the blank: The electron displaced in the Compton effect is called a _______ electron.

Answer 22

[recoil or Compton]

Question 23

What is the purpose of collimating the primary beam?

Answer 23

To restrict the size of the primary beam to the correct size for the area being radiographed.

This is done using the light beam diaphragm.

Question 24

How can scatter produced be reduced?

Answer 24

By collimating the primary beam and keeping the kV low.

Keeping kV low can be compensated for by increasing the mAs, but results may lead to under penetration.

Question 25

What are grids used for in radiography?

Answer 25

To absorb scattered radiation that reaches the receptor. ## Footnote Grids are usually used for chests over 15cm thick and abdomens greater than 10cm.

Question 26

What materials are grids typically made of?

Answer 26

Lead strips separated by a radiolucent substance such as plastic, wood, or aluminium. ## Footnote This configuration allows the primary beam to pass through while absorbing obliquely moving scattered radiation.

Question 27

True or False: Most primary beam x-ray photons pass through the grid.

Answer 27

True ## Footnote The grid is designed to allow the primary beam to pass while blocking scattered radiation.

Question 28

Fill in the blank: Grids consist of lead strips separated by a _______.

Answer 28

[radiolucent substance] ## Footnote Examples include plastic, wood, or aluminium.

Question 29

What is the effect of a grid on obliquely moving scattered radiation?

Answer 29

It is absorbed by the strips of lead. ## Footnote This helps improve the quality of the radiograph by reducing scatter.

Question 30

What is a parallel grid?

Answer 30

The cheapest type of grid, but some useful primary beam is absorbed at the edge, known as grid cut-off. Grid lines are also visible on the radiograph at the edge. ## Footnote Parallel grids are widely used due to their cost-effectiveness but have limitations in image quality.

Question 31

What is grid cut-off?

Answer 31

The absorption of useful primary beam at the edge of a grid. ## Footnote This phenomenon occurs in parallel grids and can affect image clarity.

Question 32

What is a focused grid?

Answer 32

A grid with angled lead strips designed to compensate for grid cut-off, requiring correct focal-film distance and orientation. ## Footnote Focused grids are more expensive and must be used properly to avoid image distortion.

Question 33

What must be considered when using a focused grid?

Answer 33

* Correct focal-film distance * The right way up ## Footnote Incorrect usage can lead to suboptimal imaging results.

Question 34

What is a pseudofocused grid?

Answer 34

A compromise between parallel and focused grids, with lead strips varying in depth to simulate slanting. ## Footnote Like focused grids, pseudofocused grids require correct focal-film distance and orientation.

Question 35

What is a criss-cross grid?

Answer 35

A grid with lead strips running in both directions, which absorbs more scatter but is more expensive. Also known as cross-hatch grids. ## Footnote Criss-cross grids provide enhanced image quality by reducing scatter radiation.

Question 36

What is a Potter-Bucky grid?

Answer 36

A grid that is moved quickly from side to side in a holder under the table to smooth out grid lines ## Footnote The table must be radiolucent for this.

Question 37

What are the advantages of moving grids?

Answer 37

Smoothes out grid lines ## Footnote Can be focused or parallel.

Question 38

What are the disadvantages of using moving grids?

Answer 38

* Noisy and table vibrates * Requires a longer exposure time * Can jam

Question 39

What does 'lines per inch' refer to in grids?

Answer 39

The number of lead lines per inch of grid, usually between 60-100 lines ## Footnote Fewer lines are cheaper but can break up the image more.

Question 40

What is grid ratio?

Answer 40

The ratio between the height of the slats compared with the space between them, ranging from 5:1 to 16:1

Question 41

What is the effect of higher grid ratios?

Answer 41

More scatter is removed, but more X-rays (higher mAs) are needed to ensure enough X-rays reach the film

Question 42

What is a common grid ratio used?

Answer 42

8:1

Question 43

What is grid factor?

Answer 43

The factor by which mAs has to be increased to allow for absorption, usually 2 - 4x

Question 44

What increases the grid factor?

Answer 44

* Increase in grid ratio * Increase in lines per inch * Increase in thickness of the lines

Question 45

What is a disadvantage of using a grid related to X-ray exposure?

Answer 45

Higher actual number of X-rays needed increases risk to patient and operator

Question 46

How does increasing exposure time affect X-ray imaging?

Answer 46

Increases the risk of movement blur