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Physics > CT > Flashcards

CT Flashcards

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1
Q

Doses

A
  • CT Head - 2mSv
  • CT Chest - 7mSv
  • CT Abdo - 8mSv
  • CTPA - 10mSv
  • CT Abdo/Pelvis - 15mSv
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2
Q

Partial Volume

A

Makes high contrast objects appear larger

Makes low contrast objecto appear smaller

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3
Q

Beam Hardening

A

Causes a reduction in houndsfield units towards the centre of the image

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4
Q

Anode-cathode axis

A

Parallel with centre of scanner

In Z axis

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5
Q

Fan Beam

A

Is perpendicular to the anode-cathode axis

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6
Q

What is resolution in CT?

A

Measured in line pair/cm

7lp/cm in the transaxial plane

When directly compared with radiography:

1lp/mm vs 3lp/mm for radiography

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7
Q

Is contrast resolution better in CT than plain film?

A

Yes contrast resolution better in CT

(Spatial resolution better in plain film)

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8
Q

Does a narrow window increase or decrease noise?

A

Increases noise

Smaller FOV = less signal and more noise

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9
Q

What is anode cathode axis in CT?

A

Parallel to the axis of the scanner to minimise heel effect

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10
Q

What filtration is used in CT?

A

6mm of aluminium

or

0.5mm Copper which is equilivent

In newer scanners:

  • 3mm aluminium
  • 0.1mm copper

Total filtration 6mm equivilent

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11
Q

What collimation distance is used in CT?

A

50cm usually sufficient for most patients

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12
Q

Are detectors separated in CT?

A

Yes, this is to reduce light crossover

Reduces detection efficiency however (to 80%)

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13
Q

What is cone beam effect due to?

A

Due to divergence of beam in Z axis

Usually problematic when there are more than 4 slices

Increased number of slices = worse cone beam effect

Affects outer detectors rows more

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14
Q

For same mA and kV, does slice thickness affect dose?

A

No

Dose is independent of slice thickness

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15
Q

In an obese patient, is kV or mA increased? or both?

A

Just mA is increased

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16
Q

Which part of patient receives higher dose?

A

Periphery

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17
Q

Which scanner produces more scatter? 1st or 3rd gen

A

Third generation

18
Q

Are detector crystal embedded in a matrix in CT?

A

NO

19
Q

What is pixel size formula?

A

Pixel size = FOV /matrix size

20
Q

In single slice scanners, does pitch affect noise?

A

NO

21
Q

What is CTDIw

A

CTDIw = 1/3CTDI centre + 2/3CTDI periphery

Units = mGy

22
Q

What is CTDI vol?

A

CTDIw / Pitch

Units = mGy

23
Q

What is DLP?

A

CTDIvol x Total Scan length

Units mGy/cm

24
Q

What is effective dose?

A

DLP x fiddle factor (dependent on body part)

Units are mSv

25
Q

How to calculate noise in CT?

A

1/ square root of number of photon

Inversely proportional to the square root of number of photons

26
Q

What effect will doubling of mA have on the noise?

A

Will decrease noise by a factor of:

square root of 2

Halving rotation time also decreases noise by the square root of 2

27
Q

What is filter interpolation and 2 point interpolation used in?

A

Filter interpolation = Multislice scanners

Two point interpolation = single slice scanners

28
Q

What is normal CT matrix size?

A

512 x 512

For high resolution = 1024 x 1024

Fluoro CT uses 256 x 256 for reconstruction

29
Q

How to calculate CT number?

A
30
Q

What measures can be taken to improve cupping artefact?

A

Cupping artefact is otherwise known as beam hardening

Made better by:

  • Bow tie filter
  • Algorithms
31
Q

Which has more afterglow?

Sodium iodide

vs

Bismuth germinate

A

Sodium iodidie has more afterglow

Bismuth germinante is more preferable

32
Q

Which CTDI most closely corresponds with absorbed CT dose?

A

CTDIvol

CT dose = mA/pitch

33
Q

Is effective dose measured on every CT scan?

A

No

34
Q

What is pixel size in CT?

A

0.5mm usually

35
Q

In CT does magnification increase spatial resolution?

A

Yes

36
Q

Can CT gantry tilt in the vertical axis?

A

Yes

It can tilt up to 30 degrees in the vertical axis

37
Q

What is heat capacity in CT?

A

4MJ

38
Q

What is CT fluoroscopy used for?

A

Needle guidance in IR procedures

  • 180 degree rotation
  • low dose technique
  • higher skin dose than normal CT
  • lower effective dose
39
Q

When is post patient collimation used?

A

Used in single slice scanners

Used to reduce scatter

Results in increased dose

40
Q

Do 4th gen scanners result in a higher dose?

A

Yes

The patient to detector distance is greater

They have 6x the amount of detectors than other scanners

Calibration can be adjusted through each scanning cycle

Physics (10 decks)

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