Grids

Question 1

What is a Grid?

Answer 1

A device that has a series of thin lead strips separated by radiolucent interspacing material.

Most efficient way of removing scatter radiation from large radiographic fields.

Question 2

Who invented the Grid?

Answer 2

Dr. Gustave Bucky in 1913

Question 3

What is the primary purpose of the Grid?

Answer 3

INCREASE radiographic contrast in the image by absorbing scattered radiation before it reaches the image receptor.

Question 4

What two processes are responsible for creating the image?

Answer 4

• Transmission of Radiation
• Absorption of Radiation

Question 5

What is scatter responsible for in radiographic imaging?

Answer 5

Fog and reduced contrast.

Question 6

What factors increase scatter?

Answer 6

• kVp increases
• Field size increases
• Thickness of part increases
• Atomic Number (Z) decreases

Question 7

What are the indications for Grid use?

Answer 7

• Part thickness > 10 cm
• kVp > 60

Question 8

What is the basic construction of a Grid?

Answer 8

Radiopaque lead strips separated by radiolucent interspace material, typically made out of aluminum.

Question 9

What is a disadvantage of using Grids?

Answer 9

Increases radiation exposure to the patient and grid lines can appear on the image.

Question 10

What is the Grid Ratio?

Answer 10

Grid Ratio = height of lead strips (h) / distance between strips (D)

Question 11

What is a typical range for Grid Ratios?

Answer 11

5:1 to 16:1

Question 12

What does high lead content in a Grid indicate?

Answer 12

Increased removal of scatter and therefore increased contrast.

Question 13

What are the different Grid Patterns?

Answer 13

• Linear
• Cross-hatched
• Focused

Question 14

What is Grid Cut Off?

Answer 14

Loss of primary radiation that occurs when the images of the lead strips are projected wider than they would be with ordinary magnification.

Question 15

What are the two types of linear grids?

Answer 15

• Focused
• Parallel

Question 16

What is the purpose of the Focused Linear Grid?

Answer 16

Lead strips are angled to match the divergence of the beam.

Question 17

What is the Focal Distance in relation to a Focused Linear Grid?

Answer 17

The distance at which the primary beam is parallel to the lead strips of the grid.

Question 18

What is the difference between Stationary and Moving Grids?

Answer 18

• Stationary grids: attached to a cassette
• Moving grids: blur out the images of the lead strips during exposure

Question 19

What happens to exposure factors when using a Grid?

Answer 19

Exposure to the IR will decrease, therefore exposure factors must be increased.

Question 20

What does the Grid Conversion Factor (GCF) represent?

Answer 20

Required increase in technique when using a grid.

Question 21

What is the Bucky Factor?

Answer 21

The ratio of the incident radiation falling on the grid to the transmitted radiation passing through the grid.

Question 22

What are the criteria for evaluating grid performance?

Answer 22

• Selectivity
• Contrast Improvement Ability

Question 23

What is the ‘K’ factor in relation to Grids?

Answer 23

Ratio of contrast obtained with the use of the grid to the contrast without the use of the grid.

Question 24

What are common errors that produce Grid Cut Off?

Answer 24

• Off-level
• Lateral Decentering
• Distance Decentering

Question 25

What is the Moire Effect?

Answer 25

Occurs in Computed Radiography when grid lines are parallel to scan lines.

Question 26

What is an alternative to Grid use?

Answer 26

Air-Gap Technique using a long SID and long OID.