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FRCR Part 1 Physics > CT (Erin) > Flashcards

CT (Erin) Flashcards

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1
Q

What is the effect of increasing slice thickness on SNR?

A

Increases SNR

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2
Q

What is the effect of thinner slices on SNR?

A

Decreases SNR

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3
Q

What is the effect of increasing mA on SNR?

A

Increases SNR

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4
Q

What is the effect of decreasing mA on SNR?

A

Decreases SNR

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5
Q

What is the effect of increasing kVp on SNR

A

Increases SNR

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6
Q

What is the effect of decreasing kVp on SNR

A

Decreases SNR

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7
Q

How is pitch calculated?

A

how far the table moves per gantry rotation/slice thickness

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8
Q

What is the dominant interaction in CT?

A

Compton scatter

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9
Q

Does compton scatter or the photoelectric effect dominate in CT?

A

Compton scatter

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10
Q

What is the usual kV used in CT?

A

120

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11
Q

What is the effect of increasing/decreasing window width on spatial resolution in CT

A

No effect

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12
Q

What is the effect of increasing window width on image contrast in CT

A

Decreases contrast

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13
Q

What is the effect of decreasing window width on image contrast in CT

A

Increases contrast

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14
Q

What is a normal pitch number for CT

A

1.5

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15
Q

What is the effect of higher pitch on CT scanning time

A

Faster scan

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16
Q

What is the effect of lower pitch on CT scanning time

A

Longer scan

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17
Q

What is the effect of a higher pitch on SR

A

Worse SR

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18
Q

What is the effect of a lower pitch on SR

A

Better SR

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19
Q

What is meant by CTDI

A

When the patient is in the scanner and the gantry rotates once through 360 degrees while x-rays are being emitted, the amount of dose received by the patient during that one rotation is referred to as the CTDI.

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20
Q

What does CTDI stand for

A

CT Dose Index

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21
Q

What is CTDI measured in?

A

mGy

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22
Q

What is the effect of reduced scan time on motion artefact

A

Reduced motion artefact

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23
Q

Usual HU for lungs

A

-500 to -1000

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24
Q

Usual HU of fat

A

–100

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25
Q

Usual HU of water, CSF, urine etc

A

Around 0

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26
Q

Usual HU of grey matter

A

35-45

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27
Q

Usual HU of white matter

A

20-30

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28
Q

Usual HU of muscle

A

40

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29
Q

Usual HU of bone

A

2000

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30
Q

Usual HU of metal

A

> 2000

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31
Q

What is the cause of structural noise?

A

Causes by reconstruction algorithms

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32
Q

What is the cause of electronic noise

A

Arises due to slight variations in the current within the circuitry

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33
Q

What is the effect of a higher pitch on resolution?

A

Lower resolution

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34
Q

What is the effect of a lower pitch on resolution

A

Higher resolution

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35
Q

What generation are most modern CT scanners

A

3rd generation

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36
Q

What are the typical focal spot sizes used in CT

A

0.6mm
1.2mm

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37
Q

What shape of filter is often used in CT

A

Bow tie

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38
Q

What are the typical tube voltages used adults studies

A

120kVp
(mean 60keV)

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39
Q

What are the typical tube voltages used in paediatric studies

A

80kVp
(mean 40keV)

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40
Q

What are typical filter materials used in CT

A

Copper
Aluminium

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41
Q

What are the ceramic scintillaiton detectors, used in CT, typically made from

A

Gadolinium oxysulphide

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42
Q

What is overbeaming

A

The additional exposure to the penumbral radiation

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43
Q

When is the dose consequence from overbeaming the largest?

A

When the total beam width is small
For smaller beams the penumbra represents a larger portion of the total beam width and results in LOWER DOSE EFFICIENCY

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44
Q

In a multislice CT scanner what is slice thickness determined by

A

Detector size in Z axis

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45
Q

Describe axial CT scanning (Step and Shoot)

A

1.X-ray tube rotates through 360o around the patient

  1. Bed moves to new position & the next slice is acquired
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46
Q

For which studies is axial scanning often used

A

brain scanning and high resolution lung scanning

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47
Q

Which is faster spiral CT or axial CT

A

Spiral CT

48
Q

What is the effect of lower pitch on patient dose

A

Increased patient dose

49
Q

What is overranging in CT

A

Images at very beginning at very end of spire scan require date from z axis projections beyond the defined scan boundaries

Typically additional half rotation is needed at the beginning and end of the scan range resulting in an additional complete rotation

50
Q

Is overranging required in spiral or axial CT

A

Spiral CT
Not needed in axial CT

51
Q

In filtered back projection:
What is the effect of using a smoother filter on noise and SR

A

Less noise
But reduced SR

52
Q

In filtered back projection:
What is the effect of using a sharper filter on noise and SR

A

More noise
But better SR

53
Q

What is the main drawback of filtered back projection?

A

It cannot account for poisson noise in the projections which is amplified during reconstruction
Resulting in high image noise

(also susceptible to streak artefacts)

54
Q

For which studies are sharper FBP filters usually used

A

Bone studies which require better SR

55
Q

For which studies are smoother FBP filters usually used

A

Brain exams or liver tumour assessment
To reduce image noise and enhance low contrast detectability

56
Q

Describe the steps of iterative reconstruction

A

CT data acquisition produces measured projections
2. A first CT first image “estimate” is generated
3. A set of simulated projections are generated for this image via forward projection
4. Simulated and measured projections are then compared
5. Any differences between the projections are used to correct/update the first image
estimate based on the characteristics of the underlying algorithm.
6. This correction of image and projection data is repeated until a condition predefined by the algorithm is satisfied and the final image is generated

57
Q

What are 2 advantages of FBP?

A

Robustness
Speed of computation

58
Q

What is a disadvantage of IR

A

slow

59
Q

Typical CT number for air

A

-1000

60
Q

What is the window width

A

The range of CT number that are displayed in the image

61
Q

What is the window level

A

Midpoint of the range of the CT numbers displayed in Window width

62
Q

What happens to the range of CT numbers displayed as WW increases

A

Wider range of CT numbers

63
Q

What is the effect of image noise on constrast

A

Reduced contrast

64
Q

How can the focal spot reduce geometrical unsharpness

A

Using a small focal spot

65
Q

What is the effect of a larger patient on noise

A

More noise

66
Q

What type of artefact can patient motion cause

A

Streak artefact

67
Q

Describe beam hardening

A

As beam passes through dense area
Lower energy photons more likely to be absorbed and higher energy photons are more likely to remain
Results in higher mean beam energy

68
Q

What is the effect of beam hardening on HU?

A

Lower HU unit is assigned (represented as more black)

As focally increase mean beam energy is interpreted as being due to the beam passing through less attenuating Material relative to the surroundings

69
Q

In which CT study is beam hardening particularly common?

A

Posterior fossa CT head scan due to the dense petrous bones

70
Q

Describe cupping artefact

A

Caused by beam hardening
Centre of object is usually thickest and therefore the beam will become harder in the centre than at the periphery
Centre therefore assigned lower HU number

71
Q

What type of artefact does photon starvation produce

A

Streak

72
Q

Describe photon starvation

A

When projections have to travel through more material (e.g across shoulders)
More xray photons are absorbed/removed from the beam

Results in smaller proportion of signal reaching the detector and therefore a larger proportion of noise

73
Q

What can cause motion artefact

A

Patient swallowing
Patient breathing
Pulsatility of heart and vessels
Patient moving

74
Q

What type of artefacts can metal cause?

A

Beam hardening
Photon starvation

75
Q

What causes ring artefact

A

Faulty detector (or set of detectors)

76
Q

How is ring artefact often fixed

A

By recalibrating the scanner

77
Q

What is the effect of increasing slice thickness on partial volume effect

A

Increased partial volume effect

78
Q

what is the effect of increased pitch on partial volume effect

A

Increased Partial volume effect

79
Q

What is the effect of decreased pitch on partial volume effect

A

Decreased partial volume effect

80
Q

Is the calculated CT number dependent on mA

A

No

81
Q

Is the calculated CT number dependent on kVp

A

Yes

82
Q

How does reduced scan time improve SR

A

Reduced motion artefact

83
Q

Definition of CTDI

A

Dose to the phantom from single gantry rotation

84
Q

What units is CTDI measured in

A

mGy

85
Q

What size is the head phantom (cm)

A

16cm

86
Q

What size is the body phantom?

A

32cm

87
Q

What is the definition and equation for weighted CTDI (CTDIw) ?

A

CTDI that is adjusted for spatial variation of dose

CTDIw = 1/3 CTDIcentre + 2/3 CTDIperiphery

88
Q

What units is CTDIw measured in?

A

mGy

89
Q

How is volume CTDI (CTDI vol) calculated?

A

CTDI vol = CTDIw / pitch

90
Q

Definition of CTDI vol

A

We don’t scan single slices.

The concentration of the dose along a patient is determined by the pitch. The higher the pitch, the larger the gaps between slices and the lower the dose.

Taking into account the pitch gives us the volume CTDI

91
Q

What unit it CTDI vol measured in?

A

mGy

92
Q

How is DLP calculated for CT?

A

DLP = CTDIvol x distance scanned

93
Q

How is CTDI measured in a scanner?

A

Measured using an ionisation chamber at 5 different positions in an acrylic phantom

94
Q

How is effective dose measured in CT?

A

EDLP x DLP

95
Q

Divergence in which axis causes cone beam artefact?

A

Z axis

96
Q

Does reducing beam filtration increase or decrease beam hardening artefact?

A

Increases beam hardening

97
Q

Air is used as a negative contrast agent in CT T or F?

A

T

98
Q

What is the effect of increasing pitch on image noise

A

Increased noise

99
Q

What is the effect of decreasing pitch on image noise?

A

Reduced noise

100
Q

What is the effect of increasing pitch on spatial resolution

A

Reduced SR

101
Q

What is the effect of decreasing pitch on spatial resolution

A

Improved SR

102
Q

What is the effect of increasing pitch on patient dose

A

Reduced patient dose

103
Q

What is the effect of decreasing pitch on patient dose

A

Increased patient dose

104
Q

What is the effect of increasing matrix size on SR

A

Improved SR

105
Q

What is the effect of increasing matrix size on noise?

A

Increased noise
(reduced SNR)

106
Q

Do CT detectors have a high or low dynamic range?

A

High

107
Q

What gases were used in ionisation chamber detectors

A

Krypton/Xenon

108
Q

What is the detection efficiency of solid state detectors in CT

A

90%

109
Q

What is the effect of doubling mA on patient dose

A

double dose

110
Q

What is the effect of doubling rotation time on patient dose

A

Double dose

111
Q

What is the effect of doubling pitch on patient dose

A

Halving dose

112
Q

What is the effect of increased kVp on patient dose

A

Increased patient dose

113
Q

What is the cause of incomplete projection

A

Variation of partial volume artefact in which an object is present in the xray beam in some projections but not in others

114
Q

What is the solution to correct for incomplete projection artefact

A

Thinner slices

115
Q

What type of phantom is used to assess for cone beam artefact

A

Capthan test phantom

116
Q

What type of artefact does metallic implants cause

A

Beam harening
Photon starvation

FRCR Part 1 Physics (20 decks)

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