What does a grid improve?
image contrast
Scatter will reduce what?
contrast
If there are more scatter photons, there are more what?
shades of grey
An increase in atomic number will what?
increase scatter
An increase in kVp will what?
increase scatter
An increase in patient size will what?
increase scatter
An increase in field size will what?
increase scatter
When do we use a grid?
if a body part exceeds 10cm, anything done in bucky, and increased kVp
What does the lead on grids do?
absorb the scatter
The radiolucent interspaces on a grid does what?
allows primary radiation to pass through
What is a grid ratio?
ratio of the height of lead strips to the distance between them
Grid ratio = h/D
If the height of the strips is 2.4mm and the width of the
interspacing is .30, what is the grid ratio?
2.4/.30 = 8:1
An increase in grid ratio results when the distance between the lead
strips does what?
decreases
True/False: grid ratio increases when density of image decreases due
to more photon absorption/scatter
TRUE
True/False: the higher the grid ratio, the fewer number of scattered
photons reaching the film
TRUE
When is an 8:1 ratio grid used?
>90kVp
When is an 10:1 or higher ratio grid used?
<90kVp
What is a chest x-ray at the bucky grid ratio?
12:01
If you increase grid ratio, you do what to the technique?
increase
Is a high grid or low grid better for clean up of scatter and more efficient?
high grid
What are the formulas for changing from non grid to grid?
5:1 (mAs X 2)
6:1 (mAs X 3)
8:1 (mAs X 4)
12:1 (mAs X 5)
16:1 (mAs X 6)
If you have no grid and using 10mAs, then you switch to a 5:1 grid,
you'll use how much mAs?
5:1 (mAs X 2)
So you would do, 10 X 2 = 20mAs
True/False: higher frequency grids have thinner lead strips, which
decrease the visibility of grid lines on the film?
TRUE
What are the types of grid patterns?
linear and criss/cross grids
What are linear grids?
strips run in one direction, good for when using angle and want lines
to go in same direction
What happens if tube angle is against the grid lines?
grid cut-off
What is a criss/cross grid?
two linear grids places on top of each other, very uncommon, can't
use with angle
What happens if you use an angle with criss/cross grids?
grid cut-off
What is a focused grid?
central grid strips are parallel, as strips move away from center
they become more inclined
Why do strips become more inclined for a focused grid?
to match beam divergence
What is a stationary grid?
used for portable or surgery exams
What is a disadvantage of stationary grids?
grid lines can occur
How do you convert from one grid to another?
mAs1/mAs2 = GCF1/GCF2
What is the new mAs?
Original mAs=40
Original grid ratio=8:1
New ratio grid=16:1
40/X = 4(8:1)/6(16:1)
=4X=240
=60mAs
What are off level grid erros?
central ray directed across long axis of the grid
What causes off level grid errors?
pg. 266 (creates lightness on image)
putting grid under patient which causes angle, common during portables
What are off center grids?
pg. 267 (creates lightness of image)
tube must be centered along the central axis of the focused grid
What is EXTREMELY off centered grids?
1/2 of the image will be normal, other 1/2 of image will look bad
What are off focus grids?
must be used at specific distances, GRID CUT-OFF
When will you see grid cut off?
when not using right grid at different distances
What is an upside down grid?
pg. 269 18-16 (lateral margins light, center looks fine)
have lead strips that are angled, cut-off will occur if grid is
upside down
What is air gap technique?
when you don't have a grid and need one so you used air between
patient and IR
What is good about air gap technique?
decrease scatter and have better contrast
What is bad about air gap technique?
magnification of body part, reduction in detail, and sharpness