Exam 3 Ch 22- Scatter Production Flashcards by Adi Miller

3 main factors that contribute to production of scatter-

kVp, field size, & patient thickness

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The only x-ray photons that are incident on an IR are-

those that didn’t interact with the patient

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3 main tools for controlling scatter-

beam restriction, grids, & compression (not talked ab a lot)

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3 types of x-rays responsible for imaging on a radiograph-

transmitted & scattered (both show up black) & absorbed (white, not talked ab a lot)

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image-forming x-rays-

incident on IR

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As SID decreases, what increases?

magnification

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As kVp increases, what also increases?

-relative compton interaction
-scatter production

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As kVp increases, what decreases?

-photoelectrons rapidly
-likelihood of any kind of single photon during a compton interaction

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What does compton add to a radiograph?

fog/scatter

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compton is scatter, but photons can-

scatter more than once

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Why can you not use a low kVp technique?

-increases pt. dose
-would have fewer transmitted photons (black)

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collimation reduces ____ ____ & improves ______.

reduces pt. dose & improves contrast

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How does collimation help reduce scatter?

by restricting the beam

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Field size affects-

amount of scatter produced

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As field size increases, what also increases?

scatter

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As field size increases, what decreases?

contrast

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As patient thickness increases, what also increases?

scatter

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with increasing pt. thickness, more x-rays undergo-

multiple scattering

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What does pt. thickness result in?

greater average angle of scatter

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What can be used to control patient thickness?

sometimes, compression devices

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contrast-

visible difference between light & dark areas of an image

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contrast resolution-

ability to image & distinguish the difference

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as scatter increases, what decreases?

contrast

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3 beam restrictors-

aperture diaphragm, cones & cylinders, & variable aperture collimator

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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

Exam 3 Ch 22- Scatter Production Flashcards

(115 cards)