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FRCR Part 1 Physics > MRI (Erin) > Flashcards

MRI (Erin) Flashcards

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1
Q

How is the larmor frequency calculated

A

Magnetic field strength and the gyromagnetic radion

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2
Q

What is the gyromagnetic ratio of hydrogen in a 1T machine?

A

42.5 MHz

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3
Q

What is the gyromagnetic ration of hydrogen in a 1.5T machine

A

63 MHz

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4
Q

Which is longer T1 or T2

A

T1
Apart from in water where T1= T2

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5
Q

What is the T1 time constant

A

the time taken for Mz to recover to 63% of its maximal value

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6
Q

Fat T1 and T2 numbers?

A

260ms
80ms

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7
Q

CSF T1 and T2 time?

A

2400ms
160ms

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8
Q

Bone and Teeth: T1 and T2?

A

Long T1
Short T2

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9
Q

Water: T1 and T2 numbers?

A

3000 ms for both

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10
Q

What is the T2 time constant

A

Time taken for signal to decay to 37% of maximum Mxy (i.e losw 63% of maximum energy)

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11
Q

What does a hydrogen atom consist of?

A

Single proton

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12
Q

Do fat and protein have a long or short T1

A

Short

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13
Q

Does water have a long or short T1

A

Long

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14
Q

Does bone, calcium and metal have long or short T2

A

Short T2

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15
Q

Does fat have a long or short T2

A

Short T2

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16
Q

Does water have a long or short T2

A

Very long T2

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17
Q

Which is shorter T2 or T2*

A

T2*

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18
Q

What is T2* due to

A

T2 decay due to spin-spin relaxation and magnetic field inhomogeneity

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19
Q

At equilibrium what is Mz equal to

A

Mz = Mo

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20
Q

At equilibrium what is Mxy equal to

A

Mxy = 0

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21
Q

After 90 pulse what is Mz equal to

A

Mz =0

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22
Q

What does T2 represent

A

Signal loss due to spin-spin relaxation only

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23
Q

What is T1 also called

A

Spin-lattic relaxation

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24
Q

What is the effect of increasing B0 on T1

A

T1 increases with increasing B0

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25
Q

Define TR

A

Time interval in msec between repeating portions of a pulse sequence

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26
Q

Define TE

A

Time interval in msec between a pulse and the measurement of the MR signal

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27
Q

Does T1 use short TE or long TE (and why)

A

Short TE to minimised T2 signal contribution

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28
Q

Does T1 use a short or long TR (and why)

A

Short T1 to emphasise the differences in T1 relaxation
(maximise T1 contrast)

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29
Q

Typical TR for T1 sequences

A

Short TR
300-800ms

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30
Q

Typical TR for T2 sequence

A

Long TR
>2000ms

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31
Q

Typical TR for PD sequence

A

Long TR
>2000ms

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32
Q

Typical TE for T1 sequence

A

Short TE
<30 ms

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33
Q

Typical TE for T2 sequence

A

Long TE
>80ms

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34
Q

Typical TE for PD sequence

A

Short TE
<30 ms

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35
Q

Does T2 use a short or long TE (and why)?

A

Long TE to emphasis e the influence of T2 on the size of the MR signal
Shows best T2 contrast

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36
Q

Does T2 use a short or long TR (and why)?

A

Long TR
To minimise the differences due to T1 relaxation

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37
Q

How does CSF appear in a T2 weighted image

A

Bright

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38
Q

Hows does CSF appear in a T1 weighted image

A

Dark

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39
Q

Does PD use a long or short TE (and why)

A

Short TE to minimise the T2 input on the signal

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40
Q

Does PD weighted imaging use a short or long TR (and why)

A

Long TR
To minimise the T1 input of the signal

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41
Q

how does bone appear on T1

A

Dark
Due to long T1

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42
Q

Hows does bone appear on T2

A

Dark
Due to short T2

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43
Q

How does fatty tissue appear on T1

A

Bright
Fluid - dark
Fat - bright

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44
Q

How does fatty tissue appear on T2

A

Mid range

Fluid bright
Fat - less bright than fluid

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45
Q

How is voxel size calculated

A

Pixel size x slice thickness

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46
Q

What is the effect of increasing Bo on SNR

A

Increases SNR

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47
Q

What is the effect of increasing voxel size on SNR

A

Increases SNR

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48
Q

What is the effect of increased proton density on SNR

A

Increases SNR

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49
Q

What is the effect of increasing TR on SNR

A

Increases SNR

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50
Q

What is the effect of increasing TE on SNR

A

Decreases SNR

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51
Q

What is the effect of increasing receiver bandwidth on SNR

A

Decreases SNR
(increases the range of frequencies detected - increases recorded noise)

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52
Q

What is the effect of increasing the number of excitations (no times the MR pulse sequence is repeated) on the SNR

A

Increases SNR

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52
Q

What is the effect of smaller voxels on SR in MRI

A

Improved SR

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53
Q

What is the effect of smaller voxels on SNR

A

Decreases SNR (more noise)

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54
Q

Hows is pixel size calculated

A

FOV/Matrix size

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55
Q

What is the effect of increasing matrix size on SR

A

Improves SR

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56
Q

What is the effect of decreasing FOV on SR

A

Improves SR

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57
Q

What is the effect of decreasing slice thickness on SR

A

Improves SR

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58
Q

What is the effect of increasing B0, TR or TE on SR

A

No effect

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59
Q

When is the SSG switched on?

A

During each RF pulse

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60
Q

What is the effect of a steeper Slice selection gradient on slice thickness

A

thinner slice

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61
Q

What is the effect of a shallower SSG on gradient on slice thickness

A

Thicker slice

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62
Q

When is the PEG applied

A

Period between the stimulating RF pulses and the generation of the echo signal

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63
Q

What is the effect of a larger RF pulse bandwidth on slice thickness

A

Thicker slice
(larger range of frequencies)

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64
Q

What is the effect of a smaller RF pulse bandwidth on slice thickness

A

Thinner slice
(smaller range of frequencies)

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65
Q

In which axis is the phase encoding gradient applied

A

The Y axis

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66
Q

For a NxN matrix size how many phase encoding steps are required

A

N number

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67
Q

When is the FEG applied

A

When the echo signal is being detected
Hence its sometimes called a readout gradient

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68
Q

Which axis does the FEG affect

A

X axis

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69
Q

How does the FEG work?

A

Creates gradient along X axis
Alters the larmor frequency along X axis
Echo signal will now be detected at different frequencies

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70
Q

In FEG what does the frequency of the signal relate to

A

The position of the voxel along the readout axis

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71
Q

What FEG what does amplitude of the signal relate to

A

The brightness

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72
Q

In which encoding direction does chemical shift artefact occur?

A

Frequency encoding gradient

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73
Q

Why does chemical shift occur

A

Due to molecular structure
Water molecules resonate slightly faster than (more shielded) fat molecules
Can cause misplacement of signal along the FEG

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74
Q

Where are low spatial frequencies (but contrast information) stored in K space

A

Centre

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75
Q

Where are high spatial frequencies stored in K space

A

Periphery

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76
Q

What does the centre of K space tell us about

A

Contrast

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77
Q

What does the periphery of K space tell us about

A

Spatial resoluction

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78
Q

What are the two main types of sequences used in MRI

A

Spin echo
Gradient recalled echo

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79
Q

What is the purpose of 180 degree RF pulse in spin echo sequences

A

To rephase the spins
Compensate for T2* decay

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80
Q

Which is shorter T2 or T2*

A

T2*

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81
Q

How is spin echo acquisition time calculated

A

Number excitations x number phase encoding steps x TR

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82
Q

Are spin echo sequences sensitive to B0 inhomogeneities?

A

No they are insensitive to Bo inhomogeneity

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83
Q

How is the effective echo time (T eff) in Turbo spin echo determined

A

Time interval between the 90 degree RF pulse and the CENTRAL ECHO

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84
Q

How does turbo spin echo work

A

Each 90 degree pulse is followed by a train of 180 degree pulses

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85
Q

How does multi-echo sequence work?

A

90 RF pulse is followed by 2x180 RF pulses
(1 produces short TE and 1 produces long TE)

Short TE - gives PD weighted image
Long TE - gives T2 weighted image

86
Q

How is sequence time calculated in a turbo echo sequence

A

Number phase encoding steps x TR x number of slices/ ETL

87
Q

What is the echo train length (ETL) in turbo/fast spin echo?

A

the number of 180 echoes applied after the 90 degree RF pulse

88
Q

What is the first RF pulse applied in inversion recovery

A

180 pulse

89
Q

What does STIR sequence nullify the signal from

A

Fat

90
Q

What does FLAIR sequence nullify the signal from

A

CSF

91
Q

How is the TI calculated for each tissues

A

TI = 0.693 x T1 (of that tissue)

92
Q

Does FLAIR used a short or long TI

A

Long TI
2200ms

93
Q

Does STIR use a short or long TI

A

Short TI
TI 160ms

94
Q

What are the disadvantages of turbo spin echo sequences (2)

A
  • Only really able to achieve heavily T2 weighted images
  • Mix of contrasts, each echo that fills a different line of K space is at a different time and therefore a different contrast
95
Q

What is the main advantage of a turbo spin sequence

A

Very fast

96
Q

What size flip angles does GRE use

A

Small flip angles (usually <90)

97
Q

Does GRE have long or short TR

A

Short TR

98
Q

Is GRE susceptible to B0 inhomogeneities

A

GRE IS susceptible to B0 inhomogeneties

99
Q

Are T2 weighted images possible for GRE

A

No

100
Q

What is the effect of increasing matrix size on scan time

A

Increases scan time

101
Q

What is the effect of increasing matrix size on SR

A

Increases SR

102
Q

How are spins rephased in gradient echo

A

Gradient applied to rephase

103
Q

In gradient echo sequence:
Does a small flip angle give T2* or T1 weighted images

A

T2* weighted

104
Q

In gradient echo sequence:
Does a large flip angle give T2* or T1 weighted images

A

T1 weighted

105
Q

Describe sequence of events in gradient echo sequence

A
  • RF pulse applied
  • SSG applied
  • PEG applied
  • Negative GFE applied to dephase spins
  • Positive GE applied to rephase spine until they are again in phase
  • Gradient echo created
106
Q

What are the disadvantages of GRE (3)

A

SNR lower than SE
T2 weighting not possible
Susceptibility artefacts can lead to signal loss

107
Q

What are the advantages of GRE (2)

A

Fast sequence which can create T1, T2* or PD imaging
Sensitive to blood flow (angiography)

108
Q

Which encoding Gradient does motion artefact affect

A

Phase encoding gradient

109
Q

Why is fat signal suppressed in T2 spin echo sequence but high in T2 fast spin echo sequence?

A

Spin echo T2 - fat suppressed by J coupling
Fast spin echo T2- fat high signal as J coupling is broken

110
Q

How is risk of RF heating in tissue expressed

A

By specific absorption rate

111
Q

What is SAR and how is it calculated

A

Risk of RF heating in tissue
= RF power absorbed per kg tissue

112
Q

In GRE what is SAR proportional to

A

(Flip angle) squared

113
Q

What is the usual limit of SAR for whole body

A

<4W/kg

114
Q

What is the usual temperature rise in human tissue caused by MRI (for whole body exposures)

A

For whole body exposures the patients temperature should not increase by more than 1 degree during the examination

115
Q

What side effects can patient experience due to the static field strength B0

A

Metallic taste
Dizziness

116
Q

What dangers/risks can Bo static magnetic field pose to people

A

Projectile risks for ferromagnetic materials
Surgical clips/screws etc may move

117
Q

What side effects can patient experience due to the rapid gradient switching

A

Can cause peripheral nerve stimulation

118
Q

What dangers/risks can Rapid gradient switching pose to people

A

Control & programming of pacemakers may be adversely affected
Acoustic noise levels

119
Q

At what fringe field should there be restricted access to an MR environment

A

> 0.5mT

120
Q

What is the effect of increasing matrix size on SNR

A

Decreases SNR

121
Q

What is the effect of increasing FOV on SNR

A

Improves SNR

122
Q

What is the effect of increasing FOV on SR

A

Lower resolution

123
Q

What is the effect of increasing FOV on viewing area

A

Increased viewing area

124
Q

What is the effect of increasing slice thickness on SNR

A

Increases SNR

125
Q

What is the effect of increasing slice thickness on SR

A

Lower resolution

126
Q

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

A

Increases partial volume effect

127
Q

What is the effect of increasing slice gap on cross talk

A

Less cross talk

128
Q

What is the effect of increasing NEX on SNR

A

Increases SNR

129
Q

What is the effect of increasing NEX on artefacts

A

Fewer artefacts due to signalling average

130
Q

What is the effect of increasing NEX on scan time

A

Increases scan time

131
Q

Define MRI safe

A

“an item that poses no known hazards resulting from exposures to any MR environment. MR Safe items are composed of materials that are electrically nonconductive, nonmetallic, and nonmagnetic”

132
Q

Define MRI Conditional

A

“an item with demonstrated safety in the MR environment within defined conditions.”

133
Q

Define MRI Unsafe

A

“an item that poses unacceptable risks to the patient, medical staff or other persons within the MR environment.”

134
Q

What is the default SAR in normal operating mode

A

SAR = 2W/Kg

135
Q

What is the default SAR in controlled operating mode

A

SAR = 4W/kg

136
Q

What is the temperature rise limit for infants, pregnant women and those with cardiocirculatory impairment

A

0.5 degrees C

137
Q

What is the Tesla limit for whole body exposures normal mode

A

4 T

138
Q

What is the Tesla limit for whole body exposures controlled mode

A

8 T

139
Q

What is the Tesla limit for whole body exposures for research purposes

A

No limit

140
Q

What is the Tesla limit for whole body exposures in pregnant women

A

<2.5 T

141
Q

What is the staff exposure limit (T) for whole body and limbs

A

<2T for whole body
<5T for limbs

142
Q

At what dB level is hearing protection needed to prevent reversible damage

A

At 90dB

143
Q

What causes louder noise in MRI scans (2)

A

higher field strength
higher gradient amplitudes

144
Q

What is the maximum acoustic noise allowance in MRI

A

140dB

145
Q

List some absolute contraindications to MRI

A

Non-MR compatible pacemaker / cochlear implant
Metallic foreign body in eye
Bullets
Non-MR compatible cerebral aneurysm clips

146
Q

List some relative contraindications to MRI

A

Surgical clips
Surgery in previous 6 weeks
Joint replacement / prosthesis
Claustrophobic
Large patient
Inability to lie still
Pregnancy – not scanned in first trimester usually

147
Q

During which trimester are pregnant patients usually not scanned

A

During first trimester

148
Q

What does 1W/kg equal in temperature rise

A

0.5 degrees C

149
Q

List some techniques which can reduce SAR

A

Increase the TR (have to apply the RF pulses less)
Reduce the flip angle in GRE
Reduce number of slices in each acquisition
Reduce number of echoes in multi-echo sequences
Reduce room temperature and dress patients in light clothing

150
Q

In DWI:
How will restricted diffusion appear on DWI and ADC

A

DWI - bright
ADC - dark

151
Q

In DWI:
How will increased diffusion appear on DWI and ADC

A

DWI - Dark
ADC - bright

152
Q

In DWI:
How will T2 shine through appear on DWI and ADC sequences

A

DWI - dark
ADC - bright

153
Q

What types of pathology can cause restricted diffusion

A

Infarcts, timours, infarcts

154
Q

What type of spin echo sequence is used in DWI and why

A

Echo planar imaging
(very fast)

155
Q

How is DWI imaging produced?

How does this affect stationary and mobile spins?

A

Utilises SE sequence with two large equal gradient applied either side of the 180 RF pulse
The first gradient causes dephasing of spine
The second gradient will rephase the spins ONLY if they are in the same position

Stationary spins (that have not moved) = will return a high signal as have been exposed to both the dephasing and rephasing gradient

Mobile spins do not return high signal sd they have only been exposed to one or none of the gradients

156
Q

What is meant by a higher B value in DWI

A

More sensitive the DWI sequence is to diffusion
More noise
Less signal

157
Q

What is meant by B value in DWI

A

The degree of diffusion weighting is represented as the b value

158
Q

How is B value in DWI increased

A

Larger diffusion gradients (increase amplitude or duration)

Increased time between dephasing and rephasing gradients

159
Q

What type of weighting do DWI sequences have?

A

T2 weighting

160
Q

What does the ADC map remove

A

Removes the effects of inherent T2 signal

161
Q

What is T2 shine through

A

As DWI is not just dependent on diffusion coefficient of tissue it is also partly T2 weighted
If a tissue has a very long T2 value it can appear bright on DWI and mimic restricted diffusion

162
Q

In DWI:
Do higher B values have increased or reduced SNR?

A

Reduced SNR
More noise
Less signal

163
Q

What is the purpose of the ADC map in DWI

A

removes the T2 dependence and so can be used to distinguish true restricted diffusion from T2 shine through

164
Q

What types of sequence does time of flight (MR angiography) use?

A

Gradient echo

165
Q

What type of enhancement does time of flight (MR angiography) use?

A

Flow related enhancement

166
Q

How does blood appear of time of flight (MRA)?

A

Bright

167
Q

Is time of flight MRA a contrast or non contrast technique?

A

Non contrast

168
Q

Describe how time of flight MRA works?

A

GRE sequence with short TE and TR
And large flip angle
Spins which are stationary become saturated and do not contribute to the image

However blood blood flowing into the slice has not experiences previous RF pulses and so contributes a larger signal to the image appearing bright

169
Q

What TE/TR is used for time of flight MRA?

A

Short TE
Short TR

170
Q

What size of flip angle is used for time of flight MRA?

A

Large flip angle

171
Q

Advantages of time of flight MRA (4)

A

Contrast agent not required
Can be used for venous or arterial imaging
Sensitive to flow
Saturates out all background signal

172
Q

Describe the phase contrast MRA

A

Non contrast technique
Bipolar gradient is applies
Spins which are stationary show no phase change due to opposite and equal gradients

Spins which are moving experience phase change proportional to velocity of flow

173
Q

Does phase contrast MRA have short or long acquisition time?

A

Long

174
Q

In phase contrast MRA:
A phase change is only induced when the flow is _______ to the gradient

A

Parallel

175
Q

In phase contrast MRA:
Because the phase change is only induced when the flow is parralell to the gradient what must occur?

A

Acquisitions must be repeated with gradient in different directions

176
Q

Why do flow voids occur in spin-echo

A

Signal from echo depends on tissue receiving both 90 and 180 pulses
If the blood is exposed to 90 pulse but then moves away it will not be exposed to 180

Similarly blood that wasn’t exposed at the initial 90 pulse might then move towards and be exposed at the 180 RF pulse

If tissue moves faster than TE/2 - no signal

177
Q

Regarding flow voids:
What happens if the tissue moves faster than TE/2

A

No signal

178
Q

Advantages of phase contrast (MRA) (3)

A

Contrast agent not used
Good background suppression
Velocity and blood direction can be quantified

179
Q

Can blood direction and velocity be quantified in time of flight (MRA)

A

No

180
Q

How does contrast enhanced (MRA) work

A

Use Gadolinium Chelate agents
Cause shortening of T1 relaxation time of blood
Leading to high signal intensity blood on T1 imaging

181
Q

In contrast enhanced MRA when are images acquired

A

during the first pass of the bolus through the vessels of interest
Centre of K space is acquired as the peak of the bolus passes through the vessels of interest

182
Q

Examples of ferromagnetic materials

A

Iron
Nickel
Cobalt

183
Q

What type of magnetism does Gadolinium have?

A

Paramagnetic

184
Q

How does gadolinium contrast affect T1 and T2

A

Reduces both
However far greater effect on T1 compared to T2

185
Q

Is T1 or T2 used for contrast sequences

A

T1

186
Q

Does Gd-DPTA cross the blood brain barrier easily

A

No

187
Q

Why must gadolinium be attached or chelated to other compounds?

A

As gadolinium on its own is very toxic

188
Q

Is GD-TPA water soluble?

A

Yes

189
Q

What is meant by diamagnetic

A

When placed in magnetic field weakly repel the field
But revert to a random distribution if the magnetic field is removed

190
Q

What is meant by ferromagnetic

A

Strongly attracted to align in the same direction as the magnetic field
And once the magnetic field is remove remain that way aligned afterwards
Retaining also magnetisation of their own

191
Q

What is meant by paramagnetic

A

Weakly attracted to alight in the same direction as the magnetic field
e.g gadolinium

192
Q

What is the effect of increased B0 on SNR?

A

Increases SNR
(reduced noise)

193
Q

What is the effect of increased voxel size on SNR

A

Increased SNR

194
Q

What is the effect of increased proton density on SNR

A

Increased SNR

195
Q

What is the effect of increased TR on SNR

A

Increased SNR

196
Q

What is the effect of increased TE on SNR

A

Decreases SNR

197
Q

What is the effect of decreased TE on SNR?

A

Increased SNR

198
Q

What is the effect of increased receiver bandwidth on SNR

A

Reduced SNR

199
Q

What is the effect of reduced receiver bandwidth on SNR

A

Improved SNR

200
Q

What is the effect of increased number of excitations on SNR

A

Improved SNR

201
Q

What is the effect of increased phase encoding steps on SNR

A

No effect

202
Q

What is the effect of increased matrix size on SR

A

Improved SR

203
Q

What is the effect of decreased pixel size on SR

A

Improved SR

204
Q

What is the effect of a decreased FOV on SR?

A

Improved SR

205
Q

What is the effect of decreased slice thickness on SR?

A

Improved SR

206
Q

How is noise generated in MR?

A

From the whole patient through Brownian motion of biological molecules

207
Q

Along which axis is motion artefact typically observed

A

Phase encoding axis

208
Q

What is the rule for breastfeeding after administration of Gadolinium contrast?

A

No need to stop

209
Q

At what eGFR is Gadolinium contrast not advised

A

GFR <30

210
Q

How does the BO magnetic field and RF fields pose risk to metal implants?

A

BO field - projectile risk
RF field - heating risk

211
Q

Is chemical shift artefact more apparent at higher or lower field strengths?

A

Higher field strengths

212
Q

What are the 3 types of magnetic susceptibility?

A

Diamagnetic
Paramagnetic
Ferromagnetic

213
Q
A

FRCR Part 1 Physics (20 decks)

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