Chapter 18 Notes Flashcards
a grid is used to
improve image contrast
grids are made of
lead strips alternating with an interspace material (usually aluminum)
what grids are more efficient at removing scatter
grids with higher ratios and lead content
what technical factor should be increased when using a grid
mAs
alternative to use of a grid
air-gap technique or grid replacement software applications
what is the primary source of scatter radiation
the patient
scatter radiation has no _____
diagnostic value
transmission on a radiograph
dark areas of image
absorption on a radiograph
light areas of image
what does scatter do to contrast
lowers contrast
scatter increases as
kVp increases; field size increases; thickness of part increases; and atomic number decreases
grids are recommended in patients with
part thickness greater than 10 cm
grids are recommended with kVp setting
greater than 60
who created grids
Dr. Gustav Bucky
when were grids created
1913
what was the design of early grids
wide lead strips (2” apart) with a crosshatched design
who made improvements to the early grid
Dr. Hollis Potter
how did Dr. Hollis Potter improve the grid
realigned lead strips to run in one direction; thinner strips
grid ratio =
height of lead strips / thickness of interspace material
(h/D)
typical grid ratio range
5:1 to 17:1
grid frequency
number of lead strips per inch or cm
grid frequency range
60-200 lines/in (lpi)
most common grid frequency
85-103 lpi
lead content measured in
mass per unit area
gcm(squared)
as lead content increases, removal of scatter _____; therefore contrast _____
increases; increases
linear grid pattern
grid lines running in one direction only
cross-hatched grids
two parallel grids; one on top of the other
the cross-hatched grid is not widely used due to
positioning limitations and dose
a linear grid allows
primary beam to be angled along direction that lines are running (typically along long axis of table)
short axis grids
grid lines run across short axis of grid
a short axis grid is useful for
portable chest procedures when IR is placed crosswise
focused linear grids
lead strips angled to match divergence of beam
focused linear grids must be used within
range of SIDs
what grid is least expensive but unpopular due to positioning limitations
parallel linear grids
stationary grids
grids that can be attached to IR cassette
Potter-Bucky diaphragm
moves grid during exposure
reciprocating grid movement
small motor drives grid back and forth during exposure
oscillating grid movement
electromagnet pulls grid to one side, releasing it during exposure
GCF stands for
grid conversion factor
GCF =
mAs with grid / mAs without grid
GCF is _____ dependent
kVp
grid conversion
mAs1/mAs2 = GCF1/GCF2
contrast improvement ability
“K” factor
K factor typically ranges between
1.5 and 3.5
higher “K” factor grids offer
higher contrast
Moire effect
grid lines parallel to scan lines
how can Moire effect be prevented
by using high-frequency grids designed for computed radiography
what air gap has similar clean-up of 15:1 grid
10”
grid replacement software is designed to
produce a grid-like image with no grid use
grid suppression software
removes grid lines from final image for review
