Final Review: Grids Flashcards
-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
responsible for dark images
transmission
responsible for light areas
absorption
lowers contrast
scatter
-recommended in patients with part thickness greater than 10 cm
-kvp greater than 60
Grids
Order
grid, IR, patient
patient, grid, IR
Why do we use a gride when decreasing atomic number
less absorption, more transmission
produces nice image because it cleans it up and less scatter
Grids
What material is in the grid
Lead
Why do you use a grid when increasing field size
because of increasing scatter
created by radiologist, Dr.Gustav Bucky
Grids
Grid contruction
-radiopaque lead strips
-separated by radiolucent interspace material (aluminum and carbon fiber-low dose grids)
-encased in aluminum or plastic for protection
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
Grid Ratio
H/D
Height of radiopaque strips over distance between strips
Higher grid ration
more effective it is cleaning up scatter and higher the technique needed (increasing pt. dose)
Higher grid ratio
more black and white
lower grid ratio
more shades of gray
primary disadvantages of grid use:
-grid lines on film
-some absorption of primary radiation if not used properly
-allow primary radiation to reach image receptor
-absorbs most scattered radiation
Grids
what grid ratio is more efficient in removing scatter and requires more exposure
Higher grid ratio
typical grid range
5:1 to 17:1
Practice grid ration examples
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
