Grids Flashcards
What is a grid?
A flat plate that is placed between the patient and the x-ray film to improve image contrast and reduce the scattered radiation.
What is the structure of a grid?
Alternating strips - material able to absorb radiation.
A radiolucent interspace - Aluminium or carbon plastic fibres.
Scatter will be absorbed by the lead strips.
What are the different grid types/patterns?
Parallel grid
Potter Bucky grid
Linear grid
Crossed grid
Focussed grid
Pseudo focussed grid
What is a radiographic grid cut off?
When an x-ray grid absorbs primary x-ray beams, preventing them from reaching the image receptor.
This occurs when the grid is used incorrectly such as positioning error or used upside down.
What is a parallel grid?
Very common.
Stationary grid.
Produces good results - grid cut off.
Strips are 90*C to the primary beam.
What is a Potter-Bucky grid?
Parallel grid is integrated into the table.
Moving grid - moves rapidly from side to side.
Elimination of fine lines usually visible with a stationary grid.
What is a focused grid?
Stationary.
Prevents grid cut off.
Central strips are vertical, outwardly begin sloping.
Has to be used the correct way up and centred correctly, at right angles.
What is a Pseudo-Focused Grid?
Stationary grid.
Strips are vertical.
Strips get progressively shorter towards the ends.
Reduces primary beam absorption by lead strips.
Have to be used at the correct film focal distance and used the correct way up.
What is a crossed grid?
Stationary grid.
Stripes running in two directions.
Remove more scattered radiation.
What is included in a grid parameter?
Grid factor.
Lines per centimetre.
Grid ratio.
What is the grid ratio?
The ratio of the height of the lead strips to the distance between them.
Used to calculate the grid factor.
What is the grid factor?
Must increase the mAs if using a grid.
Can lead to longer exposure times (s).
Increased movement blur.
mAs x Grid Factor = New mAs with grid.
What is the Grid Focus?
Distance from the tube head to the grid.
Image appears lighter with distinct white lines over the unexposed areas of the film.
