Exam 3 Ch 22- Scatter Production Flashcards
3 main factors that contribute to production of scatter-
kVp, field size, & patient thickness
The only x-ray photons that are incident on an IR are-
those that didn’t interact with the patient
3 main tools for controlling scatter-
beam restriction, grids, & compression (not talked ab a lot)
3 types of x-rays responsible for imaging on a radiograph-
transmitted & scattered (both show up black) & absorbed (white, not talked ab a lot)
image-forming x-rays-
incident on IR
As SID decreases, what increases?
magnification
As kVp increases, what also increases?
-relative compton interaction
-scatter production
As kVp increases, what decreases?
-photoelectrons rapidly
-likelihood of any kind of single photon during a compton interaction
What does compton add to a radiograph?
fog/scatter
compton is scatter, but photons can-
scatter more than once
Why can you not use a low kVp technique?
-increases pt. dose
-would have fewer transmitted photons (black)
collimation reduces ____ ____ & improves ______.
reduces pt. dose & improves contrast
How does collimation help reduce scatter?
by restricting the beam
Field size affects-
amount of scatter produced
As field size increases, what also increases?
scatter
As field size increases, what decreases?
contrast
As patient thickness increases, what also increases?
scatter
with increasing pt. thickness, more x-rays undergo-
multiple scattering
What does pt. thickness result in?
greater average angle of scatter
What can be used to control patient thickness?
sometimes, compression devices
contrast-
visible difference between light & dark areas of an image
contrast resolution-
ability to image & distinguish the difference
as scatter increases, what decreases?
contrast
3 beam restrictors-
aperture diaphragm, cones & cylinders, & variable aperture collimator
Use a grid when part thickness gets above __, ___, or __. or when kVp is ____.
-when part thickness gets above 10, 12, or 13
-kVp is greater than 60
4 types of grids-
parallel, crosses, focused, & moving
2 reasons to use beam restrictors-
-only the tissue being examined should be exposed
-large field size increases scatter & decreases contrast
aperture diaphragm-
simplest & most restrictive beam restricting device (any kind of pitch aperture distorts it)
Aperture diaphragm is designed on film to-
leave 1 cm around field to prove collimation
What kind of construct is the aperture diaphragm?
lead/lead lined metal construct
aperture diaphragm opening is designed to be-
1 cm smaller on all sides of film
cones & cylinders is a modification to-
modification to aperture diaphragm
cones & cylinders resulting exposure-
resulting exposure is circular
cones & cylinders position & distal end determines-
field size
What is 1 problem with cones & cylinders?
cone-cutting
cone-cutting displayed by-
grid cut-off
What is the collimator also known as?
variable aperture diaphragm
collimator 1st stage-
enhances shutters designed to control off-focus radiation
collimator 2nd stage-
shutters work in pairs for square/rectangle fields
collimator allows for
infinite # of field sizes
collimator is more ___ & the least ____.
more versatile & the least __
collimator is the most commonly used-
most commonly used beam restrictor
PBL was mandated by-
US FDA for all x-ray machines in 1974
PBL was removed in 1994 bec-
bucky tray has sensors that detect-
size & alignment of cassette
synchronous motors are actuated to-
adjust to appropriate field size
even with PBL, tighter collimation will-
reduce pt. exposure
the grid is used to-
remove scattered x-ray after the subject & before IR
the grid was firs demonstrated by-
gustaf bucky in 1913
the grid consists of-
series of alternating radiopaque & radiolucent materials
radiopaque materials are-
led
radiolucent materials are-
plastic/carbon
radiopaque strips are called-
grid strips
radiolucent strips are called-
interspace material
grid ratio formula-
height of led strips divided by width of interspace (H/D)
high-ratio grids are more effective at-
“cleaning up” scattered x-rays
grids range from-
5:1-16:1
5:1 ____ cleanup-
85%
16:1 _____ cleanup=
97%
grids take ____ away where it shouldn’t be & adds ____.
takes away black, adds white
scatter puts ____ where ____ should be-
puts black where white should be
grid frequency-
Number of grids per cm/inch
high frequency grids show-
less distinct grid lines than low frequency grid
frequency is increased by-
adding grid strips & thinning interspace material
purpose of interspace material-
maintain precise separation b/w delicate led strips
aluminum advantages-
-less visible grid lines
-nonhygroscopic
-easier to manufacture precise specifications
non hygroscopic-
doesn’t attract water molecules
interspace materials used are-
aluminum or plastic fiber (neither preferred, they do ab the same)
aluminum disadvantages-
-absorption of primary beam resulting in higher pt. dose as mAs is compensated
-at low kVp, pt. dose may be increased by up to 20%
grid casing is primarily made of-
aluminum or plastic
grid casing provides-
rigidity & helps seal out moisture
grid casing helps-
maintain cassette contact & protects grid
grid casing job-
hold precise manufactured grid strips together in its proper place & seals out moisture from getting in
contrast improving factor (k)-
ratio of contrast of a radiograph w: grid to the contrast of radiograph made w/o grid
k for most grids-
b/w 1.5 & 2.5
bucky factor (B) also called-
grid factor
bucky factor (B) increase in-
amt. of technique required to achieve the same receptor exposure on a radiograph
bucky factor (B) increases-
pt. dose
parallel grid properly used-
results in grid cut off on lateral margins
in a parallel grid, why doesn’t grid cut-off occur on top & bottom?
beam divergence doesn’t’t cross grid lines
optical density decreases towards-
edges
parallel grids more pronounced at-
short SID
parallel grids clean up scatter-
in only 1 direction
cross/cross hatch grids clean up scatter-
in all directions
2 serious disadvantages of cross/cross hatch grids-
-positioning in critical
-any tube angle against grid will produce grid cut-off
focused grids designed to-
minimize grid cut-off
focused grids when used properly-
grid cut-off not visible
focused grids are marked with-
intended focus distance & which side of grid faces tube
in focused grids, led strips are angled to-
match beam divergence
Dr.Hollis improved on Bucky’s grid by-
moving grid during exposure
moving grids are called-
potter-bucky diaphragm
2 types of moving grids-
-reciprocating (to & fro movement)
-oscillating (circular motion)
disadvantaged of moving grids-
increased OID, increased image blur, & bulkiness of mechanics
virtual grids are-
reconstructions of captured data
virtual grids algorithms can-
identify scatter & remove their effect from radiograph
virtual grids lead to-
lower pt. dose w: high quality images
most common problem of grids-
improper positioning
off-level grid-
most often caused by tube head mis-positioning
off-level & off-center grids are
the same thing
off center grid, tube is off center ___ & called ____ _____.
off center laterally, called lateral decentering
off-center grid causes grid cut-off along-
entire radiograph
off-focus grid is a focused grid using-
wrong SID
off-center grids are further from focus distance the more-
severe the grid cut-off
grid cut-off is more severe toward-
periphery (edges)
off center grids have to have-
what side is toward the tube & what the focal distance is
upside down grid-
if turn grid upside down, entire grid is an x-ray catcher
proper grid ratio selection depends on 3 things-
-kVp increases, grid ratio should increase
-degree of clean up increases as ratio increases
-pt. dose in general- as ratio increases, dose increases
pt. radiation dose increases w:-
increasing grid ratio
high-ratio grids are used with-
high kVp exams
pt. radiation dose at ____ ____ is less than at ____ ____.-
at high kVp is less than @ low kVp
air-gap technique-
method of removing radiation scatter w/o using grid
air-gap technique is an alternative to-
grids
air-gap technique increases-
OID to 10-15 cm
air-gap technique allows scattered x-rays to-
miss IR
disadvantage of air-gap technique-
increase magnification & loss of recorded detail
