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FRCR Part 1 Physics > MRI > Flashcards

MRI Flashcards

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1
Q

The typical range for T1 is… (in ms)

A

200-2000ms

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2
Q

T1 (increases/decreases) with increasing magnetic field strength

A

T1 increases with increasing magnetic field strength

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3
Q

True/False:
For any tissue T1 is always longer than T2

A

True. T2 < T1.

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4
Q

The T2 of any tissue is the time taken (in msec) for the MR signal to decay to (?%) of its full value

A

The T2 of any tissue is the time taken (in msec) for the MR signal to decay to 37% of its full value

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5
Q

True/False:
The T1 relaxation time influences the repetition rate of a spin echo pulse sequence

A

False.

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6
Q

Spin Echo: Protons lose phase coherence after an RF pulse due to…?

A

magnetic field inhomogeneities

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7
Q

Spin Echo: how are the proton spins rephased?

A

Using the 180 degree pulse

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8
Q

Can Spin Echo generate both T1 and T2 weighted images?

A

Yes

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9
Q

How do we create different weighted images with Spin Echo?

A

By varying the TR and TE values

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10
Q

In Fast spin echo sequences, we get faster imaging at the expense of what…?

A

Contrast

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11
Q

What flip angle does Gradient echo use?

A

Usually 10-35 degrees

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12
Q

What 2 differences does gradient echo have compared to spin echo?

A
  1. Small flip angles
  2. Gradient field used instead of 180 degree RF pulse
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13
Q

Can Gradient echo sequences generate T2 images?

A

No, only T2*, or T1

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14
Q

What type of MR pulse sequence is often used in MR Angiography?

A

Gradient echo sequences

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15
Q

Can Gradient echo sequences compensate for any magnetic field inhomogeneities?

A

Yes, only the extrinsic field inhomogeneities (not intrinsic)

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16
Q

What is the resonant frequency of Hydrogen nuclei known as?

A

Larmor frequency

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17
Q

What is the Larmor equation?

A

Larmor frequency = Gyromagnetic ratio x Magnetic field strength (Bo)

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18
Q

In slice selection: How do we define the slice to be imaged?

A

Using a magnetic field gradient

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19
Q

How does Gadolinium contrast affect the T1 and T2 relaxation times of tissues?

A

Gadolinium contrast decreases both T1 and T2 relaxation times

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20
Q

Do hydrogen nuclei in fat and water precess at the same frequency? Why/why not?

A

No
There are differences in the magnetic field of their orbital electrons.

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21
Q

Spatial resolution of MR images is improved by (increasing/ decreasing) slice thickness

A

Decreasing slice thickness
This reduces voxel size for finer spatial sampling

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22
Q

Spatial resolution of MR images is improved by: (increasing/ decreasing) matrix size

A

Increasing matrix size
A larger matrix size (eg 128x128 to 256x256) -> more pixels covering the same field -> smaller pixel size -> spatial resolution will improve

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23
Q

Spatial resolution of MR images is improved by: (increasing/ decreasing) field of view

A

Decreasing field of view
A smaller field view means the same number of pixels are used to sample a smaller region of the patient. Which means smaller pixels -> better spatial resolution

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24
Q

Is Spatial resolution of MR images
improved by changing main magnetic field strength?

A

No effect.
Increasing Bo will increase the SNR, but will not have a direct effect on spatial resolution

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25
Q

Spatial resolution of MR Images is improved by changing which 4 factors?

A

Decreasing Slice thickness
Increasing matrix size
Decreasing Field of view
Increasing phase encoding steps

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26
Q

MRI Signal: Noise ratio is improved by changing which 2 factors?

A

Increasing Bo magnetic field strength
Decreasing Echo Time

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27
Q

The STIR sequence minimises signal from what tissue?

A

Fat. It’s a fat suppressed sequence.

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28
Q

How is Chemical shift artefact affected by a higher field strength?

A

More artefact produced
Higher field heighten the artefact, commonly in fat vs water.

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29
Q

Why does the chemical shirt artefact occur between fat and water protons?

A

The artefact occurs because of differences in the intrinsic magnetic properties of protons in water and fat molecules

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30
Q

MRI Signal: Noise ratio (increases/decreases) when increasing slice thickness?

A

Increases, as thicker slices contain more proton nuclei giving more signal

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31
Q

MRI Signal: Noise ratio (increases/decreases) when increasing echo time (TE)?

A

Decreases, because as TE increases there will be greater decay of the signal prior to readout

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32
Q

MRI Signal: Noise ratio (increases/decreases) when increasing receiver bandwidth?

A

Decreases, because this will open the receiver coil to more bandwidths of noise. Signal has a much narrower spectrum than noise.

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33
Q

MRI Signal: Noise ratio (increases/decreases) when increasing field of view?

A

Increases, because pixel size will also increase (the same number of pixels are sampling a larger area). Therefore, voxels get bigger and generate larger signals.

34
Q

True/False: MRI produces permanent biological effects > 3T

A

False.
there is no evidence of permanent biological effects < 8 T

35
Q

Are all metal objects attracted to the scanner magnetic field?

A

No, only ferromagnetic objects (ie those containing iron, nickel or cobalt) will be strongly attracted towards the scanner

36
Q

Biological effects: Which MR Technique can cause peripheral nerve stimulation?

A

Rapid gradient switching can cause nerve stimulation

37
Q

Biological effects: What is specific absorption rate?

A

a measure of tissue heating due to RF pulses

38
Q

What 5 materials are bright on T1 images?

A

Blood, Fat, Melanin and proteinaceous fluid. Also Contrast agents.

39
Q

What materials are bright on T2 sequences?

A

Fat and fluid

40
Q

MRI Safety: what strength of gauss line must be contained within the inner controlled area?

A

3mT Gauss line

41
Q

True/False: Specific Absorption Rate depends on patient weight

A

True

42
Q

MRI: a smaller field of view gives increased or decreased?:
1. Resolution
2. Signal strength
3. Gradient strengths needed?

A
  1. Higher resolution
  2. Lower signal strength
  3. Higher gradient strengths needed
43
Q

What is aliasing?

A

Wrap around artefact from when body parts being images are bigger than the field of view

44
Q

What is Nyquist limit?

A

Signal being sampled must be over twice the wavelength of the highest frequency

45
Q
A
46
Q

MRI Rectangular field of view: does this have a higher or lower signal to noise ratio?

A

Lower SNR

47
Q

What are the 2 problems with rectangular field of view (in MRI)

A
  1. Reduced SNR
  2. Aliasing can occur
48
Q

Gadolinium contrast (for MRI) is diamagnetic, parramagnetic or ferromagnetic?

A

Paramagnetic

49
Q

Mri sequence acquisition times equation?

A

Acquisition time = No. of excitations x No. of phase encoding steps x TR

50
Q

The centre of K space gives what info?

A

Low frequencies, gross contrast

51
Q

The outer K space gives what info?

A

High spatial frequencies, fine detail

52
Q

Fast spin echo description?

A

90 degree pulse followed by a train of 180 degree pulses

53
Q

Fast spin echo advantage?

A

Saves time, depending on echo train length

54
Q

Fast spin echo disadvantage?

A

Contrast is worse

55
Q

Why is an inversion recovery sequence useful?

A

The signal is flipped and if the 90 degree pulse applied at the right time, you can nullify the signal from a chosen tissue (fat - STIR, fluid - FLAIR)

56
Q

Inversion recovery: what is the formula for Inversion time?

A

TI= 0.693 x T1

57
Q

Gradient echo disadvantages? (3)

A

Lower Signal noise ratio
Can’t do T2
Susceptibility artefacts

58
Q

Gradient echo advantages? (3)

A

Fast acquisition
Sensitive to blood flow (angiography)
Low power/less heating side effecfs

59
Q

Magnetic susceptibility artefact: explain

A

Boundary between two tissues of different magnetic properties leads to signal voids

60
Q

Specific absorption rate in MR is proportional to flip angle: true or false?

A

True

61
Q

2 relative contraindications to Gadolinium contrast for MR?

A

Low eGFR
Pregnancy
Breastfeeding is SAFE

62
Q

With fast spin echo, the longer the echo train the more T1 weighted or T2 weighted?

A

More T2 weighted

63
Q

Movement artefact is much worse in T1 or T2 weighted sequences?

A

T2 weighted because of the longer time scale

64
Q

How do we minimise chemical shift artefact?

A

Increase receiver bandwidth

65
Q

Motion artefact occurs in which type of encoding?

A

Phase encoding only

66
Q

Fat: T1 and T2 times?

A

260, 80ms

67
Q

CSF: T1 and T2 times?

A

2400, 160ms

68
Q

Kidney: T1 and T2 times?

A

760, 30ms

69
Q

Diffusion weighted MRI:
- T1 or T2?
- restricted water appears bright/dark?
- restricted = long/short T2? ADC? b-900 image?

A

T2
Bright
Long T2, low ADC, bright b-900

70
Q

A short Tr is less than…

A

500ms

71
Q

A long Tr is more than…

A

1500ms

72
Q

A short Te is…

A

Less than 30ms

73
Q

A long TE is more than

A

90ms

74
Q

MRI image contrast can be improved with (3)

A

Using contrast agents
Increasing SNR
Fat suppression

75
Q

Hydrogen has a gyro magnetic ratio of…

A

42MHz per Tesla

76
Q

Proton density images had long/short TE and TR?

A

Long TR
Short TE

77
Q

Spectroscopy: choline is pathological if high/low

A

High

78
Q

Spectroscopy: NAA is pathological if high/low

A

Low

79
Q

Spectroscopy: peaks correspond to

A

Shifts of the resonant frequencies

80
Q

Rephrasing during a Gradient echo is achieved by…

A

Frequency encoding Gradient

81
Q

When these two things go down, SNR goes up. What 2 thugs.

A

Bandwidth
TE

FRCR Part 1 Physics (20 decks)

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