Grids Flashcards
Grid ratio
height of lead strips : width of interspacing material
o relationship between the height of the lead strips and the width of the spaces between them
o determines how much variation in the direction of the incoming photon is allowed without the photon being absorbed by the grid
o ↑ grid ratio = ↓ variation permitted
o higher ratios require more precise alignment of the x-ray beam
o higher ratios require more exposure, have better scatter clean-up and have much more positioning accuracy
Grid frequency
number of lead strips per inch
Grid radius
distance at which primary beam is parallel to lead strips
Bucky
A moving grid
o moving the grid during the exposure blurs the image of the grid lines so that the grid image is not visible on the film
Stationary grids
do not move during the exposure
o should have many very fine lines per inch to avoid objectionable grid lines
o commonly used today in upright film holders
Compare high and low ratio grids for positioning, exposure, scatter cleanup.
· higher ratios require more exposure, have better scatter clean-up and have much more positioning accuracy
When is a grid used?
To decrease scatter
What is a focused grid?
· Grids for general purpose
· Lead strips are aligned to the direction of the diverging primary x-ray beam
· Lead strips of a focused grid are precisely aligned to the x-ray beam at a specific SID, which is called the grid radius
What are grid cut-off and its causes?
· Undesirable attenuation of the primary x-ray beam by the grid
· Due to misalignment between grid and x-ray beam
· Appears as radiographic density
· Prevented by ensuring the x-ray beam is properly aligned to the center of the grid at the appropriate distance
· USE A GRID WHEN THE THICKNESS OF THE BODY PART EXCEEDS 12 CM
What is the “air-gap” principle?
· Chest radiography with a space (at least 10 inches) between the subject and film. Instead of using a grid, this method uses the geometry and X-ray absorption by the air to remove scattered radiation.
