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Huda > Chapter 7 Radiography > Flashcards

Chapter 7 Radiography Flashcards

1
Q

electron source in x-ray tube

A

filament
negative cathode

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2
Q

positive anode in x-ray tube

A

target
usually tungsten

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3
Q

line focus principle

A

target/anode is angled at 15 degrees
-although a large area of the target is irradiated, this will appear small when viewed from the patient’s perspective

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4
Q

focal spot

A

area generating x-rays

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5
Q

evacuated envelope

A

offers electrical insulation and shielding

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6
Q

where do primary x-rays go through from xray tube?

A

x-ray tube window

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7
Q

leakage radiation from x-ray tubes

A

transmitted through x-ray tube housing
leakage Kair < 1 mGy/h at 1 m per regulation

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8
Q

what is secondary radiation in x-ray tube

A

sum of leakage and scattered radiation

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9
Q

where do energetic electrons strike?

A

the target
produce heat and x-rays
anode material stores heat energy

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10
Q

what is tube loading

A

heat energy deposited in focal spot

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11
Q

what does tube loading depend on

A

tube voltage
tube current
exposure time
total energy also depends on number of exposures

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12
Q

how do modern anodes spread heat loading over large area?

A

circular and rotate at high speed (10,000 rpm)

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13
Q

what are stationary anodes embedded in and where are they used

A

embedded in copper block
portable x-ray units

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14
Q

heat capacity of anode

A

several hundred thousand joules

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15
Q

why have the anode angle?

A

permits large area to be irradiated, helping to reduce heat problems, while maintaining a a small focal spot (less blur)

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16
Q

what is power rating

A

max kW that a focal spot can tolerate in a specified exposure time

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17
Q

power loading for large focal spot

A

100 kW

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18
Q

power loading for small focal spot

A

25 kW

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19
Q

how to achieve the required x-ray tube output if power loading is limited?

A

may have to increase exposure time

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20
Q

how much of the electrical energy supplied to x-ray tubes is converted to heat?

A

99%

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21
Q

how is heat transferred from focal spot to anode?

A

conduction
then anode radiates (via light) to tube housing

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22
Q

how are x-ray tube housings cooled

A

immersed in oil which aids heat dissipation by convection

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23
Q

what happens when anode heat capacity is reached?

A

anode must cool down before additional exposure is allowed

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24
Q

how long does it take a hot x-ray tube to cool?

A

several minutes

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25
Q

heel effect

A

-x-rays are produced within the tagert; thus they are attenuated as they travel through it
-attenuation is greater in the anode direction than in the cathode direction because of differences in path length within target
-thus get higher x-ray intensity at cathode end and lower at anode end
-i.e. anode end of image will be underexposed and cathode end overexposed
put cathode end at thicker side of patient to prevent this

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26
Q

how to reduce magnitude of heel effect

A

increase anode angle
increase source to image distance
decrease field size

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27
Q

where are low tube voltages used?

A

imaging extremities (bone)
thin body parts
infants
50-65 kV

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28
Q

intermediate voltages

A

70-90 kV

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29
Q

high voltages

A

120 kV
used in chest x-rays
-high kV reduce patient dose when Kair at image receptor is kept constant
-high kV reduces latitude- results in narrower ranged of detected signals that is easier to capture and process

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30
Q

what is tube output proprtional to at a given kV?

A

tube current
exposure time

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31
Q

tube currents in radiography

A

a few hundred mA

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32
Q

exposure time in radiography

A

very short < 10 ms
short < 100 ms

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33
Q

small focal spot size

A

0.6 mm
-sharp images, better spatial resolution

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34
Q

large focal spot size

A

1.2 mm
-tolerate high heat loading, therefore reducing exposure times

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35
Q

mA and exposure time of chest xray

A

200 mA
5 ms

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36
Q

mA and exposure time of abdominal x-ray

A

400 mA
50 ms

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37
Q

HVLs for 60, 80, 100, 120 kV in mmAl

A

2,3,4,5

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38
Q

tube output at 1 m (mGy/100 mAs) for 60, 80, 100, 120 kV

A

3.5, 6.5, 10, 14

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39
Q

what is source to image distance

A

distance from x-ray tube source to image receptor

40
Q

what is source to skin distance

A

distance from x-ray tube source to front surface of patient

41
Q

what is source to object distance

A

as it sounds

42
Q

geometric magnification

A

SID/SOD

43
Q

why does geometric magnification create distortion?

A

because it varies with patient depth

44
Q

what increases with longer SID?

A

More radiation is required = more exposure time and more motion blur

45
Q

most common SID in radiography

A

100 cm

46
Q

why are air gaps not commonly used in radiography?

A

air gap = magnification imaging
-increases focal spot blur

47
Q

scatter to primary ratio in radiography

A

> 1

48
Q

typical grid ratio for scatter removal grid

A

10:1

49
Q

grid lines are aligned along what?

A

anode cathode axis

50
Q

grid ratio vs voltage and body thickness

A

lower voltage- lower grid ratio (8:1)
thicker body part- higher grid ratio (12:1)

51
Q

when are grids optional?

A

body parts thinner than 12 cm

52
Q

what does grid do?

A

increase contrast at the cost of more dose

53
Q

digital x-ray material

A

scintillator- data is read out and transmitted electronically. More expensive
photostimulable phosphors- use a reader

54
Q

how does scatter/primary change with thickness and FOV?

A

-increases with FOV
-increases with body thickness

55
Q

explain automatic exposure control

A

-ionization detector is between patient being x-rayed and image receptor
-when signal from detector matches a pre-set treshold of Kair, the exposure is terminated
-yields consistent image quality despite variations in body shape
-prosthetic device- AEC may overexpose the image

56
Q

exposure index

A

-measure of Kair at image receptor
-1 uGy of Kair is EI of 100

57
Q

deviation index

A

-quantifies how closely Kair at receptor matches target value
- plus or minus 3 means the exposure is double or half the target Kair
(god forbid they made it + 2 and -2)

58
Q

SID for chest x-ray

A

180 cm
-large SID minimizes cardiac magnification, gives more reliable estimate of size of heart

59
Q

entrance Kair in PA chest x-ray

A

0.1 mGy
easy to transmit through the lungs

60
Q

chest imaging

A

easy to transmit through lungs
therefore 120 kV, to reduce image dynamic range and patient dose
-large focal spot size to reduce exposure to < 5 ms
-10: 1 grid ratio
-uses AEC
-Kair at receptor is 3 uGy, EI of 300- requires about 1 mAs

61
Q

SID in bedside radiography

A

100 cm

62
Q

bedside imaging

A

-manual techniques- technologist monitors EI
(too much EI = too much dose, low EI = suboptimal image)
-large focal spot to reduce exposure time <10 ms and minimize motion blur
-usually not practical to use gird- use 80 kV to reduce photon energy and reduce scatter
-processing of ICU chest x-rays using unsharp mask enhancement improves visibility of tubes, lines, catheters
-grids must be used for adult bedside abdominal imaging- grid ratio 6:1 is used as these are easier to align than standard grids

63
Q

abdomen/pelvis imaging SID

A

100 cm

64
Q

abdomen/pelvis imaging

A

-entrance Kair 3 mGy
-80 kV
-large focal spot reduces exposure time < 100 ms
-grid ratio 10:1
-AEC gives Kair at image receptor of 3 uGy- EI of 300
-requires 20 mAs

65
Q

extremity SID

A

100 cm

66
Q

extremity imaging

A

-50-65 kV
-small focal spot (0.6 mm) to improve sharpness (ex detect hairline fracture)
-reduce max power to 25 kW to reduce focal spot
-60 kV and 400 mA = 24 kW
-grids not necessary but can be used
-Kair at receptor 10 uGy, EI = 1000 (because photon energies are lower). At the same Kair, lower energy x-rays deposit less energy in detectors

67
Q

pediatric imaging

A

-large focal spot to minimize exposure time and blur (100 kW power)
-because children are small, magnification is low and focal spot blur is minimal
-optional grid for patients < 12 cm
-kV and use of grid increases as patient is bigger
-EI values ~ 300

68
Q

when is lesion contrast maximized?

A

-when voltage is set to just penetrate a patient
-lower voltage will not penetrate
-higher voltage reduce contrast

-penetration decreases with tissue thickness and increases with kV

69
Q

what does lesion CNR depend on?

A

-choice of kV and mAs
-when lesion Z is similar to background, reductions in lesion contrast with increasing kV will be modest. But when lesion Z is different from background, reducing kV will improve visibility of the lesion

-mAs impacts mottle. If mAs quadruples, image mottle is halved

70
Q

is noise fixed with AEC?

A

Yes
lesion CNR depends on kV
-use of high voltage reduces CNR and patient dose

71
Q

effect of time on resolution

A

reducing exposure time reduces motion blur and thus improves resolution

72
Q

impact of scintillator detector thickness on resolution

A

thicker = more light diffusion = less sharp

73
Q

impact of detector thickness for photostimulable phosphors

A

-thicker = readout laser light scatters more = less sharp

74
Q

pros and cons of Se-based photoconductors

A

-good resolution
-rarely used because they absorb little of incident x-rays

75
Q

limiting resolution of 35x43 cm detector (2kx 2.5 k matrix size)

A

-pixels are 175 um - 6 pixels/mm, limiting res is 3 lp/mm

76
Q

limiting resolution of 20x24 cm detector (2kx2.5 k matrix size)

A

-pixels are 100 um - 10 pixels/mm, limiting res is 5 lp/mm

77
Q

limiting resolution of digital detectors vs screen films

A

3 lp/mm vs 6 lp/mm (i.e. half)

78
Q

in digital radiography, what is usually more important, resolution or CNR?

A

CNR

79
Q

examples of artifacts

A

-false teeth, hairpins, jewellery
-spills of barium and iodine on bedclothes appear as “all white” artifacts

80
Q

when does grid cutoff occur

A

grid not aligned properly
-if grid is upside down, central region will have normal appearance but appear white towards edges
-grid cutoff can also occur when SID doesn’t match that of grid

81
Q

“ghost” image

A

when a region of an image has received an unexpected high or low exposure, this region can have a different sensitivity for a short period after the exposure
-can give ghost image when subsequent exposure is obtained
-ex: ghost of metallic implant can appear in images obtained after a radiograph on a patient with this implant
-minimize by waiting for digital detector to recover

82
Q

kerma area product for adult PA chest x-ray

A

-entrance Kair is 0.1 mGy, areas is 1000 cm^2, KAP is 0.1 Gy-cm^2
lateral projection is double (0.2 Gy-cm^2)

83
Q

kerma area product for skull x-ray

A

lateral = 0.5 Gycm^2, AP = 1 Gy cm^2

84
Q

kerma area product for AP abdominal radiograph

A

3 Gycm^2

85
Q

benchmark for KAP for adult body and head ( not chest)

A

1 Gy cm^2

lower for chest (0.1- 0.2 Gy cm^2) and for kids

86
Q

skin dose in radiography

A

<10 mGy
-no clinical importance

87
Q

embryo dose with abdominal radiography

A

1 mGy
very low risk

88
Q

extemety and chest patient effective doses

A

< 0.1 mSv

89
Q

skull, cervical spine, abdomen, and pelvis/hip effective dose

A

0.1- 1 mSv

90
Q

thoracic and lumbar spine effective dose

A

1 mSv

91
Q

anode tube dissipation rate

A

10 kW

92
Q

Are SIDs< 100 cm used?

A

Not in radiography
65 cm used in mammo

93
Q

limiting spatial res of human eye at 25 cm distance

A

5 lp/mm

94
Q

typical xray tube anode heat dissipation rate

A

10 kW

95
Q

average kerma-area product for complete xray examination

A

1 Gy-cm2

Huda (20 decks)

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