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FRCR Part 1 Physics > CT > Flashcards

CT Flashcards

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1
Q

What is the usual HU for water/urine/CSF?

A

Around 0

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2
Q

What is the usual HU for Bone?

A

500-1000

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3
Q

What is the usual HU for fat?

A

-20 to -100

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4
Q

What is the usual HU for metal?

A

> 2000

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5
Q

What is the typical tube voltage of CT in adults?

A

120 kVp, mean energy of 60 keV

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6
Q

What is the typical tube voltage of CT in paediatrics?

A

80 kVp, mean energy of 40 keV

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7
Q

What are the focal spot sizes in CT and X-ray? (Same)

A

0.6mm, 1.2mm

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8
Q

What is the predominant mode of interaction in CT?

A

Compton scatter

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9
Q

What determines the slice thickness in single-slice CT?

A

Collimator width

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10
Q

What determines the slice thickness in multi-slice CT?

A

Detector size

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11
Q

What level of filtration and of what material are used in CT?

A

Up to 6mm Al/ Cu

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12
Q

What is the least significant type of noise in CT?

A

Electronic noise

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13
Q

What happens to image contrast with increased window width?

A

Reduced contrast

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14
Q

CT: What happens to spatial resolution with increased window width?

A

Window width does not affect spatial resolution

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15
Q

How to reduce partial volume effect? (3)

A
  • decrease slice thickness
  • decrease field of view
  • decrease pixel size/increase matrix size
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16
Q

The CT number represents the average…

A

Average LAC of tissues in the voxel

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17
Q

What effect does thick slices have in noise?

A

Less noise

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18
Q

CT detector arrays use what type of scintillators in their detectors?

A

Ceramic scintillation detectors (gadolinium oxysulphide)

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19
Q

How do CT detectors work?

A

Scintillation layer detects attenuated X-rays and then emits light, converted to electricity by photodiodes

20
Q

How do we eliminate multi slice CT overbeaming?

A

We need to increase the width of the Xray beam to allow the penumbras to fall to the side of the active detectors

21
Q

CT pitch = what?

A

Table feed per rotation/width of acquired slices
It’s a Measure of overlaps

22
Q

CT pitch - what does it mean when it’s <1, 1 and >1?

A

Pitch < 1 = overlapping slices
Pitch 1 = complete sampling no overlap
Pitch > 1 = gaps in data acquired

23
Q

What is CT overranging ?

A

Getting additional data on either end of the scanned area to be able to reconstruct the image

24
Q

CT filtered back projection: choosing filters is a trade off between…?

A

Noise and resolution, which are proportional

25
Q

Drawbacks of filtered back projection?

A

Streak artefact
Susceptible to Poisson noise

26
Q

Quantum noise in CT is reduced by…? (3)

A

Use thicker slices
Increase kVs
Increase mAs

27
Q

How do we decrease structural noise? (in CT)

A

increase number of projections used to create image

28
Q

When does beam hardening artefact commonly occur?

A

At bone:soft tissue junctions

29
Q

CT Number is dependent on which 2 factors?

A

CT Number is the average LAC of the tissues.
It depends on kVP and filtration

30
Q

CT Spatial resolution depends on what…? (5)

  1. … focal spot
  2. … number of smaller detectors
  3. … scan time
  4. … pixels
  5. Lack of …
A
  1. Small focal spot
  2. Increased number of smaller detectors
  3. Increasing scan time
  4. Small pixels, for finer sampling
  5. No motion/artefacts
31
Q

CT Noise is generally increased by…? (2)

A
  1. less x-rays hitting detector
  2. increased matrix size
32
Q

CTDI measurements units?

A

mGy
CTdI= Area under axial dose profile/slice width

33
Q

What is CTDI?

A

Average dose delivered to a standard plastic phantom

34
Q

what is the relationship between CTDI and current (mA)?

A

Proportional to each other.

35
Q

What effect does pitch have on CTDI?

A

High pitch = faster scan = Low CTDI

36
Q

CT imaging dose is proportional to…? (1)

A
  1. Tube current
    Dose increased by anything that causes more x-rays to be detected
37
Q

Typical CT spatial resolution is…? Mins Pairs per cm?

A

10mins pairs per cm.
(1lp/mm therfor less SR than plain film)

38
Q

CT gantry speeds can travel up to…

A

5 revs per second

39
Q

When is CT overbeaming more of a problem?

A

When the X-ray beam is narrow, so the penumbras make up relatively more of the dose the patient is receiving

40
Q

Iterative reconstruction advantages?

A

Very accurate, fewer artefacts

41
Q

How does Hybrid iterative reconstruction work?

A

Initial image generated with FBP
Iterative methods are then used to optimise image characteristics eg noise

42
Q

Advantages of Hybrid Ir? (4)

A

Less patient dose
Less noise
Better than FBP alone
faster than IR alone

43
Q

What is DLP in terms of Ct
What is the equation?

A

Scan length x average CTDI volume
A measure of the total dose in a CT study

44
Q

Will thin Ct slices increase or decrease contrast?

A

Increase

45
Q

How to decrease Noise in CT? (5)

A
  1. Bigger pixels
  2. Thicker slices
  3. Thinner patients
  4. More X-rays (kvP, mA, filtration) but will affect dose
  5. Soft reconstruction filters
46
Q

How to reduce beam hardening artefact in Ct?

A

Use filtration or correction software

47
Q

CT: What does Cone beam artefact do

A

Can cause streaking in images, especially in outer detector rows

FRCR Part 1 Physics (20 decks)

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