Grids Flashcards
What is a Grid?
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.
Who invented the Grid?
Dr. Gustave Bucky in 1913
What is the primary purpose of the Grid?
INCREASE radiographic contrast in the image by absorbing scattered radiation before it reaches the image receptor.
What two processes are responsible for creating the image?
- Transmission of Radiation
- Absorption of Radiation
What is scatter responsible for in radiographic imaging?
Fog and reduced contrast.
What factors increase scatter?
- kVp increases
- Field size increases
- Thickness of part increases
- Atomic Number (Z) decreases
What are the indications for Grid use?
- Part thickness > 10 cm
- kVp > 60
What is the basic construction of a Grid?
Radiopaque lead strips separated by radiolucent interspace material, typically made out of aluminum.
What is a disadvantage of using Grids?
Increases radiation exposure to the patient and grid lines can appear on the image.
What is the Grid Ratio?
Grid Ratio = height of lead strips (h) / distance between strips (D)
What is a typical range for Grid Ratios?
5:1 to 16:1
What does high lead content in a Grid indicate?
Increased removal of scatter and therefore increased contrast.
What are the different Grid Patterns?
- Linear
- Cross-hatched
- Focused
What is Grid Cut Off?
Loss of primary radiation that occurs when the images of the lead strips are projected wider than they would be with ordinary magnification.
What are the two types of linear grids?
- Focused
- Parallel
What is the purpose of the Focused Linear Grid?
Lead strips are angled to match the divergence of the beam.
What is the Focal Distance in relation to a Focused Linear Grid?
The distance at which the primary beam is parallel to the lead strips of the grid.
What is the difference between Stationary and Moving Grids?
- Stationary grids: attached to a cassette
- Moving grids: blur out the images of the lead strips during exposure
What happens to exposure factors when using a Grid?
Exposure to the IR will decrease, therefore exposure factors must be increased.
What does the Grid Conversion Factor (GCF) represent?
Required increase in technique when using a grid.
What is the Bucky Factor?
The ratio of the incident radiation falling on the grid to the transmitted radiation passing through the grid.
What are the criteria for evaluating grid performance?
- Selectivity
- Contrast Improvement Ability
What is the ‘K’ factor in relation to Grids?
Ratio of contrast obtained with the use of the grid to the contrast without the use of the grid.
What are common errors that produce Grid Cut Off?
- Off-level
- Lateral Decentering
- Distance Decentering
What is the Moire Effect?
Occurs in Computed Radiography when grid lines are parallel to scan lines.
What is an alternative to Grid use?
Air-Gap Technique using a long SID and long OID.
