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FRCR Part 1 Physics > CT (Erin) > Flashcards

CT (Erin) Flashcards

(116 cards)

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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
Usual HU of water, CSF, urine etc
Around 0
26
Usual HU of grey matter
35-45
27
Usual HU of white matter
20-30
28
Usual HU of muscle
40
29
Usual HU of bone
2000
30
Usual HU of metal
>2000
31
What is the cause of structural noise?
Causes by reconstruction algorithms
32
What is the cause of electronic noise
Arises due to slight variations in the current within the circuitry
33
What is the effect of a higher pitch on resolution?
Lower resolution
34
What is the effect of a lower pitch on resolution
Higher resolution
35
What generation are most modern CT scanners
3rd generation
36
What are the typical focal spot sizes used in CT
0.6mm 1.2mm
37
What shape of filter is often used in CT
Bow tie
38
What are the typical tube voltages used adults studies
120kVp (mean 60keV)
39
What are the typical tube voltages used in paediatric studies
80kVp (mean 40keV)
40
What are typical filter materials used in CT
Copper Aluminium
41
What are the ceramic scintillaiton detectors, used in CT, typically made from
Gadolinium oxysulphide
42
What is overbeaming
The additional exposure to the penumbral radiation
43
When is the dose consequence from overbeaming the largest?
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
44
In a multislice CT scanner what is slice thickness determined by
Detector size in Z axis
45
Describe axial CT scanning (Step and Shoot)
1.X-ray tube rotates through 360o around the patient 2. Bed moves to new position & the next slice is acquired
46
For which studies is axial scanning often used
brain scanning and high resolution lung scanning
47
Which is faster spiral CT or axial CT
Spiral CT
48
What is the effect of lower pitch on patient dose
Increased patient dose
49
What is overranging in CT
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
Is overranging required in spiral or axial CT
Spiral CT Not needed in axial CT
51
In filtered back projection: What is the effect of using a smoother filter on noise and SR
Less noise But reduced SR
52
In filtered back projection: What is the effect of using a sharper filter on noise and SR
More noise But better SR
53
What is the main drawback of filtered back projection?
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
For which studies are sharper FBP filters usually used
Bone studies which require better SR
55
For which studies are smoother FBP filters usually used
Brain exams or liver tumour assessment To reduce image noise and enhance low contrast detectability
56
Describe the steps of iterative reconstruction
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
What are 2 advantages of FBP?
Robustness Speed of computation
58
What is a disadvantage of IR
slow
59
Typical CT number for air
-1000
60
What is the window width
The range of CT number that are displayed in the image
61
What is the window level
Midpoint of the range of the CT numbers displayed in Window width
62
What happens to the range of CT numbers displayed as WW increases
Wider range of CT numbers
63
What is the effect of image noise on constrast
Reduced contrast
64
How can the focal spot reduce geometrical unsharpness
Using a small focal spot
65
What is the effect of a larger patient on noise
More noise
66
What type of artefact can patient motion cause
Streak artefact
67
Describe beam hardening
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
What is the effect of beam hardening on HU?
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
In which CT study is beam hardening particularly common?
Posterior fossa CT head scan due to the dense petrous bones
70
Describe cupping artefact
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
What type of artefact does photon starvation produce
Streak
72
Describe photon starvation
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
What can cause motion artefact
Patient swallowing Patient breathing Pulsatility of heart and vessels Patient moving
74
What type of artefacts can metal cause?
Beam hardening Photon starvation
75
What causes ring artefact
Faulty detector (or set of detectors)
76
How is ring artefact often fixed
By recalibrating the scanner
77
What is the effect of increasing slice thickness on partial volume effect
Increased partial volume effect
78
what is the effect of increased pitch on partial volume effect
Increased Partial volume effect
79
What is the effect of decreased pitch on partial volume effect
Decreased partial volume effect
80
Is the calculated CT number dependent on mA
No
81
Is the calculated CT number dependent on kVp
Yes
82
How does reduced scan time improve SR
Reduced motion artefact
83
Definition of CTDI
Dose to the phantom from single gantry rotation
84
What units is CTDI measured in
mGy
85
What size is the head phantom (cm)
16cm
86
What size is the body phantom?
32cm
87
What is the definition and equation for weighted CTDI (CTDIw) ?
CTDI that is adjusted for spatial variation of dose CTDIw = 1/3 CTDIcentre + 2/3 CTDIperiphery
88
What units is CTDIw measured in?
mGy
89
How is volume CTDI (CTDI vol) calculated?
CTDI vol = CTDIw / pitch
90
Definition of CTDI vol
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
What unit it CTDI vol measured in?
mGy
92
How is DLP calculated for CT?
DLP = CTDIvol x distance scanned
93
How is CTDI measured in a scanner?
Measured using an ionisation chamber at 5 different positions in an acrylic phantom
94
How is effective dose measured in CT?
EDLP x DLP
95
Divergence in which axis causes cone beam artefact?
Z axis
96
Does reducing beam filtration increase or decrease beam hardening artefact?
Increases beam hardening
97
Air is used as a negative contrast agent in CT T or F?
T
98
What is the effect of increasing pitch on image noise
Increased noise
99
What is the effect of decreasing pitch on image noise?
Reduced noise
100
What is the effect of increasing pitch on spatial resolution
Reduced SR
101
What is the effect of decreasing pitch on spatial resolution
Improved SR
102
What is the effect of increasing pitch on patient dose
Reduced patient dose
103
What is the effect of decreasing pitch on patient dose
Increased patient dose
104
What is the effect of increasing matrix size on SR
Improved SR
105
What is the effect of increasing matrix size on noise?
Increased noise (reduced SNR)
106
Do CT detectors have a high or low dynamic range?
High
107
What gases were used in ionisation chamber detectors
Krypton/Xenon
108
What is the detection efficiency of solid state detectors in CT
90%
109
What is the effect of doubling mA on patient dose
double dose
110
What is the effect of doubling rotation time on patient dose
Double dose
111
What is the effect of doubling pitch on patient dose
Halving dose
112
What is the effect of increased kVp on patient dose
Increased patient dose
113
What is the cause of incomplete projection
Variation of partial volume artefact in which an object is present in the xray beam in some projections but not in others
114
What is the solution to correct for incomplete projection artefact
Thinner slices
115
What type of phantom is used to assess for cone beam artefact
Capthan test phantom
116
What type of artefact does metallic implants cause
Beam harening Photon starvation

FRCR Part 1 Physics (20 decks)

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