#### Imaging Chapter 18 Grids Flashcards

What does a grid improve?

image contrast

Scatter will reduce what?

contrast

If there are more scatter photons, there are more what?

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