Chapter 14 - Grids Flashcards
What is the main reason to use a grid?
to clean up scatter radiation
what is the best equipment for reducing the amount of scatter that reaches the film
grids
when was the first grid made?
1913
Which type of grids are more effective, higher ratio or lower ratio?
higher ratio
What is the formula for grid change?
old grid factor
What is the formula for grid ratio?
GR = h/D
h- height of grid strips
D- thickness of interspace
What effect does higher ratio grids have on patient dose?
higher ratio grids = higher patient dose
What is the number of grid lines per inch or centimeter?
grid frequency
What is the purpose of interspace material?
to maintain the separation between lead strips
What are the lines imaged when primary x-rays are absorbed in the grid strips?
grid lines
What are the 4 grid types?
parallel
crossed
focused
moving
What is the simplest type of grid?
parallel grids
What is the original type of grids? This grid also has lead strips running parallel to both the long and short axis of the grid
crossed grid
Which type of grid is designed to minimize grid cut-off and has grid strips that are angled?
focused grid
Which grid moves while the x-ray exposure is being taken and helps prevent grid lines?
moving grid
What are the 2 basic types of moving grids?
reciprocating & oscillating
What is the most frequent error with grids?
improper positioning
What is the undesired absorption of the primary beam?
grid cut-off
4 types of grid cut-off
off level
off center
off focus
upside down
Off level grid occurs when:
tube or grid is angled
off center grid is caused by:
lateral shifting of the beam
Off focus grid occurs when:
focused grid is not used at proper SID
Your grid selection depends on what 3 factors?
kVp
patient dose
patient size
as grids are added or grid ratio increases, contrast _______
increases
as grids are added or grid ration increases, density _______
decreases
as grids are added or grid ratio increases, patient dose _______
increases
