11. Scatter Control and Radiographic Grids

Question 1

what 2 things are photoelectric radiation interaction responsible for

at what energies is it more likely to occur at

Answer 1

absorption of photons and tissue contrast

lower energies

Question 2

contrast is improved (high) with ___ kV

Answer 2

decreased

Question 3

what are compton radiation interaction responsible for

at what energies is it more likely to occur at

Answer 3

scattering of photons

higher energies

Question 4

scatter is more likely to degrade image quality at ___ kV

Answer 4

increased kV

Question 5

large IR has what effect on the scattered radiation detected

Answer 5

more scattered radiation detected

Question 6

ideally what do beam restrictors do

Answer 6

decrease the field size to anatomy of interest

Question 7

what 4 things happen when FS is decreased

Answer 7

decreases unnecessary tissue exposure

scatter decreases

scale of contrast shortens

visibility of detail increases

Question 8

are 3 types of beam restrictors

Answer 8

aperture diaphragms/cones/cylinders

collimators

ancillary devices

Question 9

what do aperture diaphragms cones used for

Answer 9

reduces unsharpness as compared to aperture diaphragm

gives specific shape of beam

closer collimation for specific shape

Question 10

what do aperture diaphragms cones introduce and what shape does it have

Answer 10

introduce area of unsharpness = penumbra

almost divergent shape

Question 11

what do aperture diaphragms cylinders used for

what is its effectiveness at reduction compared to the cones

Answer 11

further reduction in unsharpness as compared to cone

Question 12

what 2 components do collimators have

Answer 12

light localizing field light

2 sets of shutters that permits infinite number of field size

Question 13

what do the 2 sets of shutters do in a collimator

what does it permit

Answer 13

length and width of field independently controlled

permits rectilinear collimation

Question 14

which shutters of the collimator reduces the penumbra

why

Answer 14

bottom shutters

geometric unsharpness around periphery of image- edge unsharpness

Question 15

what does collimators do to the image edge

Answer 15

improves sharpness of recorded image edge

Question 16

what does off focus radiation occur from

Answer 16

areas of the x-ray tube other than focal spot areas

Question 17

upper shutters of the collimator reduces what in terms of radiation reaching the IR

Answer 17

reduces off focus radiation reaching IR

Question 18

off focus radiation produces images where

Answer 18

beyond exposed field of radiation

image shadows

Question 19

beam restriction does what

Answer 19

limit the x-ray beam field size

Question 20

what 2 things does limiting the x-ray beam field size do

Answer 20

limit patient dose

reducing the amount of scatter

Question 21

beam restriction needs to consider what

Answer 21

the full circular x-ray field

Question 22

in terms of an aperture diaphragm there is significant unsharpness due to what

Answer 22

owing to proximity to x-ray source

Question 23

what happens inside a collimator

Answer 23

light comes from lightbulb and mirror angle directs where x-ray beam is going to be

Question 24

why is there poor quality with wide collimation

Answer 24

lots of scatter

Question 25

the light reflected off the mirror is used to do what

Answer 25

project the coverage of x-ray beam

Question 26

the projection of the x-ray beam should be accurate to within +/- ___ % of the ___

Answer 26

+/-2% of the SID

Question 27

mirror adds what kind of filtration to the system

Answer 27

inherent filtration

Question 28

what are the 3 features of collimator housing

Answer 28

central ray must be marked alignment light helps center beam with IR some units project the AEC sensor locations in the light field

Question 29

what are Positive beam limitation devices/what do they do

Answer 29

automatically collimates beam to size of IR

Question 30

what are 2 types of ancillary devices

Answer 30

lead blockers/shields lead masks attached to the collimator

Question 31

what do lead blockers do

Answer 31

absorb scatter

Question 32

what do grids do

Answer 32

absorb and limit the amount of scattered radiation that reaches the IR improves the quality of radiograph

Question 33

less scatter on the image has what effect on contrast

Answer 33

increases contrast

Question 34

what do grids do in terms of the transmitted radiation

Answer 34

absorbs the transmitted radiation exiting the patient and therefore reduces the radiation reaching the IR

Question 35

what is the grid conversion formula

Answer 35

mAs1/mAs2 = grid ratio1/grid ratio2

Question 36

what are radiographic strips

Answer 36

device that has very thin lead strips with radiolucent interspaces, intended to absorb scatter radiation emitted from the patient

Question 37

what is the grid frequency

Answer 37

measure of the number of grid lines per unit distance

Question 38

what is the grid ratio equation in terms of h and D

Answer 38

ratio = h/D ``` h = height of lead strip D = interspace distance ```

Question 39

where are grids placed

Answer 39

between the patient and the IR

Question 40

what is the grid frequency equation

Answer 40

freq = 1/(t + D) t = thickness of lead strip

Question 41

increased grid ratio has what effect on scatter

Answer 41

more scatter removed = improved contrast

Question 42

what are the 2 grid types

Answer 42

linear and cross-hatched

Question 43

the focused grid does what to the x-ray beam and photons

Answer 43

matches the divergence of the x-ray beam and allwos more photons to reach the IR

Question 44

what happens if the grid isnt focused

Answer 44

if the grid isn't focused, may eliminate some of the radiation you were going to use

Question 45

the convergent line or point of a focused grid falls within ___ ____

Answer 45

a focal range

Question 46

what is the grid conversion factor

Answer 46

measure of the required mAs adjustment to keep exposure constant with and without the grid

Question 47

what is the GCF equation

Answer 47

GCF = mAs with grid/mAs without grid

Question 48

increase in GCF has what effect on the required exposure

Answer 48

required exposure to the IR increases

Question 49

what is the limitation of stationary grids

Answer 49

stationary grids produce visible grid lines on the radiography

Question 50

what are reciprocating grids

Answer 50

slightly moving the grid during the x-ray exposure will blur the grid lines which will therefore be less visible

Question 51

reciprocating grid motion is controlled how

Answer 51

electronically

Question 52

what is the grid cutoff

Answer 52

decrease in the number of transmitted photons that reach the IR because of grid misalignment

Question 53

what are the 4 causes of grid unalignment

Answer 53

upside down focused grid off level grid off centre grid off focus grid

Question 54

what is the moire effect

Answer 54

artifact demonstrated when a stationary grid is used during CR imaging and the grid frequency is similar to the laser scanning frequency

Question 55

higher ratio grids will increase what 3 things

Answer 55

scatter absorption patient exposure potential for grid cuttoff