Techniques for Image Optimisation

Question 1

what is scatter radiation a result of

Answer 1

compton scattering
- incoming x-ray photon loses energy and changes direction

Question 2

what factors influence amount of scatter

Answer 2

increased patient thickness
increased x-ray field size
kVp (if we up kVp, we get more scatter)

Question 3

increase collimation

Answer 3

making field size smaller

Question 4

what does increasing collimation do

Answer 4

decrease patient dose
decrease scatter produced in patient
reduces scatter the IR is exposed to
increases radiographic contrast

Question 5

what happens to compton and photoelectric effect at high kVp

Answer 5

both compton and photoelectric effect become weaker
but compton interaction becomes more dominant over PE

Question 6

which is preferable high kVp or high mAs

Answer 6

higher kVp preferable for correct exposure because it has lower patient dose (if mAs simultaneously adjusted - 15%)

Question 7

what do large field sizes generate

Answer 7

greater amounts of exposed tissue generate scatter radiation

Question 8

part thickness

Answer 8

Imaging thick parts of the body results in more scatter than imaging thin body parts

Question 9

compression

Answer 9

Compression of anatomy improves contrast and lower the patient dose
- Mammography

Question 10

what happens as field size increases

Answer 10

relative quantity of scatter radiation increases

Question 11

what happens as collimation increases

Answer 11

field size decreases
patient dose decreases
scatter radiation decreases
radiographic contrast increase
digital: quantum noise increases

Question 12

what happens as collimation decreases

Answer 12

field size increases
patient dose increases
scatter radiation increases
radiographic contrast decreases
digital: quantum noise decreases

Question 13

where is collimator located

Answer 13

immediately below the tube windows

Question 14

what is collimator comprised of

Answer 14

a set of lead shutters

Question 15

automatic collimators

Answer 15

automatically limits size and shape of primary beam to size and shape of IR in Bucky

Question 16

what two pieces of technology improve image quality and reduce dose

Answer 16

1. automatic exposure control
2. grid

Question 17

AEC

Answer 17

automatic exposure control

Question 18

where does AEC sit

Answer 18

just in front of IR

Question 19

what does AEC measure

Answer 19

quantity of radiation that reaches IR

Question 20

what does AEC do

Answer 20

stops exposure at a point in time when the IR received required amount of radiation

Question 21

what do AEC control
what does radiographer control

Answer 21

AEC = exposure time and mAs
radiographer = mA, kVp

Question 22

how does DAP and AEC differ

Answer 22

DAP meter measures what comes out of x-ray tube
AEC measures what makes it through the patient

Question 23

what do AEC’s consist of

Answer 23

ionisation chamber
capacitor
exposure termination switch

Question 24

ionisation chamber in AEC

Answer 24

has two electrodes (neg and pos)
when x-rays come in, ionise some of air molecules and produce electrons
electrons move towards positive electrode

Question 25

what does capacitor do

Answer 25

store a specific quantity of electrical charge

Question 26

what is quantity of stored electrical charge in capacitor proportional to

Answer 26

radiation that the ionisation chamber has been exposed to when a certain pre-determined level is reached, a signal is sent to the termination switch

Question 27

what is ionisation chamber made of

Answer 27

radiolucent materials - they transmit x-rays

Question 28

backup time

Answer 28

max length of time the x-ray exposure will continue when using an AEC system

Question 29

why is there backup time when using AEC

Answer 29

safety mechanism when AEC system fails prevents overexposure protects x-ray tube and patient

Question 30

factors that affect AEC usage

Answer 30

patient positioning patient thickness chamber selection collimation minimum response time back up timer

Question 31

how can patient positioning affect AEC usage

Answer 31

need to position anatomy relative to chamber positions if incorrect positioning can lead to under or overexposure

Question 32

how does AEC deal with patient thickness

Answer 32

if AOI is thicker, the exposure time will lengthen to reach the pre set exposure to the detectors

Question 33

how does collimation cause overexposure with AEC

Answer 33

x-ray field collimated to closely detector doesn't receive sufficient exposure and may prolong exposure time

Question 34

how does collimation cause underexposure with AEC

Answer 34

x-ray field not collimated enough detector measures scattered and transmitted radiation exiting patient time turned off too soon when scatter is excessive, causes underexposure of AOI

Question 35

advantages of AEC

Answer 35

maintain consistency and quality of images reduce number of repeats --> reduces patient dose increase efficiency

Question 36

disadvantages of AEC

Answer 36

chambers can't differentiate between transmitted and scattered radiation dependent on radiographer positioning patient correctly

Question 37

what happens when primary x-rays leave the tube

Answer 37

come from point source are in straight lines they diverge

Question 38

what does grid do

Answer 38

filter out scattered radiation

Question 39

where is grid positioned

Answer 39

between patient and IR - in the bucky

Question 40

what does grid contain

Answer 40

rows of lead slats with radiolucent spaces at fixed intervals

Question 41

what feature is important on grids why

Answer 41

'moving' grid so that it does not appear on image as an artefact

Question 42

what happens to scatter with grids what happens to transmitted photon

Answer 42

scatter is absorbed by lead slats in grid transmitted photon passes straight through

Question 43

moving grid

Answer 43

grid vibrates horizontally at high speed to prevent artefact

Question 44

non-focused grid

Answer 44

lead lines run parallel

Question 45

focused grid

Answer 45

lead lines are angled to match angle of divergence of primary beam

Question 46

what is difference between focused and non-focused grid

Answer 46

focused grid allows more transmitted photons to reach IR

Question 47

what is important with focused grids why

Answer 47

to use the right SID so that primary x-rays can pass between slats

Question 48

grid factor equation

Answer 48

grid factor = mAs with grid / mAs without grid

Question 49

advantages of grids

Answer 49

reduces scatter from patient improves radiographic contrast between adjacent anatomical structures improves diagnostic quality for techniques that include many tissue types

Question 50

disadvantages of grids

Answer 50

increased exposure factors required increased dose to patient if used incorrectly, image quality can reduce - SID crucial to allow alignment of angled slats

Question 51

how do grids improve contrast

Answer 51

by absorbing scattered radiation and preventing them from intercepting the IR