Final Review: Grids Flashcards by Melissa Unknown

-improves radiographic contrast in image
-absorbs scatter radiation before it reaches IR
-patient is primary source of scatter radiation
-scatter radiation has no diagnostic value

purpose of grid

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responsible for dark images

transmission

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responsible for light areas

absorption

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lowers contrast

scatter

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-recommended in patients with part thickness greater than 10 cm
-kvp greater than 60

Grids

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Order
grid, IR, patient

patient, grid, IR

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Why do we use a gride when decreasing atomic number

less absorption, more transmission

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produces nice image because it cleans it up and less scatter

Grids

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What material is in the grid

Lead

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Why do you use a grid when increasing field size

because of increasing scatter

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created by radiologist, Dr.Gustav Bucky

Grids

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Grid contruction

-radiopaque lead strips
-separated by radiolucent interspace material (aluminum and carbon fiber-low dose grids)
-encased in aluminum or plastic for protection

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this individual took Gustav Bucky’s design and made it better, made a grid that moves so it blurs out the grid lines, making them invisible

Potter Bucky

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Grid Ratio

H/D
Height of radiopaque strips over distance between strips

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Higher grid ration

more effective it is cleaning up scatter and higher the technique needed (increasing pt. dose)

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Higher grid ratio

more black and white

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lower grid ratio

more shades of gray

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primary disadvantages of grid use:

-grid lines on film
-some absorption of primary radiation if not used properly

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-allow primary radiation to reach image receptor
-absorbs most scattered radiation

Grids

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what grid ratio is more efficient in removing scatter and requires more exposure

Higher grid ratio

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typical grid range

5:1 to 17:1

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Practice grid ration examples

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number of lead strips per inch or cm, how often it is occuring

grid frequency

high lead count

high grid ratio and low frequency

as lead content increases, removal of scatter increases therefore what happens to contrast

contrast increases

is the undesirable interception of primary beam photons, by the grid strips

grid cut off

artifact for grids

grid cut off

most important factor in grids efficiency

lead content

grid lines running in one direction only

Linear

two parallel grids; one on top of the other

criss-cross or cross hatched

two types of grid patterns

parallel and focused

parallel

non focused, strips are just parallel to one another, not canting

what happens to receptor exposure when you increase grid ration

receptor exposure will decrease

as you increase grid ration, decrease receptor exposure, increase contrast, what happens to the patient dose

patient dose increases

has canting lines to go with the divergence of the beam

focused grid

which grid cant be angles

crosshatched

what happens if you are below or above a certain SID

you will get a grid cut off

allows primary beam to be angled along directions that lines are running

linear grid

-grid lines across short axis of grid -useful for portable chest procedures when ir is placed crosswise

short axis grids

grids that can be attached to image receptor cassette

stationary grids

Grid is incorporated into cassette structure

Grid cassettes

small motor drives grid back and forth during exposure

reciprocating

electromagnet pulls grid to one side releases it during exposure and spring activated

Oscillating

Practice grid conversion factor

K factor

comparing contrast

grid erros

-off level -central ray angulation across grid lines -off center -off focus -upside down -moire effect

grid lines parallel to scan lines same grid lines in the same direction of sampling frequency

moire effect

how to prevent moire effect

using high frequency grids designed for computer raduography

-alternative to grid use -scatter radiation lost in air space between patient and receptor -10 inch air gap has simialr clean up of 15:1 grid -SID needs to increase equal amount to maintain image resolution

AIR gap technique

off leveling

receptor exposure only on one side

off centering

not centering correctly, not lining up with divergence of the beam

off focus and off radius

not within the SID range that it needs to be

only has receptor exposure in the center

upside down grid

creating oid (6in of oid) cleaning up scatter, increasing contrast and increasing oid because scatter is not reaching ir

Air gap

Final nimage contrast principle determined by:

Look up table

recomended technique wirh digital receptors

high kvp and reduction in mAs

Grid conversion factor for No grid 5:1 6:1 8:1 12:1 16:1 33334443

none=1 16:1=6 5:1=2 6:1=3 8:1=4 12:1=5