MRI (Erin) Flashcards
1
Q
How is the larmor frequency calculated
A
Magnetic field strength and the gyromagnetic radion
2
Q
What is the gyromagnetic ratio of hydrogen in a 1T machine?
A
42.5 MHz
3
Q
What is the gyromagnetic ration of hydrogen in a 1.5T machine
A
63 MHz
4
Q
Which is longer T1 or T2
A
T1
Apart from in water where T1= T2
5
Q
What is the T1 time constant
A
the time taken for Mz to recover to 63% of its maximal value
6
Q
Fat T1 and T2 numbers?
A
260ms
80ms
7
Q
CSF T1 and T2 time?
A
2400ms
160ms
8
Q
Bone and Teeth: T1 and T2?
A
Long T1
Short T2
9
Q
Water: T1 and T2 numbers?
A
3000 ms for both
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)
11
Q
What does a hydrogen atom consist of?
A
Single proton
12
Q
Do fat and protein have a long or short T1
A
Short
13
Q
Does water have a long or short T1
A
Long
14
Q
Does bone, calcium and metal have long or short T2
A
Short T2
15
Q
Does fat have a long or short T2
A
Short T2
16
Q
Does water have a long or short T2
A
Very long T2
17
Q
Which is shorter T2 or T2*
A
T2*
18
Q
What is T2* due to
A
T2 decay due to spin-spin relaxation and magnetic field inhomogeneity
19
Q
At equilibrium what is Mz equal to
A
Mz = Mo
20
Q
At equilibrium what is Mxy equal to
A
Mxy = 0
21
Q
After 90 pulse what is Mz equal to
A
Mz =0
22
Q
What does T2 represent
A
Signal loss due to spin-spin relaxation only
23
Q
What is T1 also called
A
Spin-lattic relaxation
24
Q
What is the effect of increasing B0 on T1
A
T1 increases with increasing B0
25
Define TR
Time interval in msec between repeating portions of a pulse sequence
26
Define TE
Time interval in msec between a pulse and the measurement of the MR signal
27
Does T1 use short TE or long TE (and why)
Short TE to minimised T2 signal contribution
28
Does T1 use a short or long TR (and why)
Short T1 to emphasise the differences in T1 relaxation
(maximise T1 contrast)
29
Typical TR for T1 sequences
Short TR
300-800ms
30
Typical TR for T2 sequence
Long TR
>2000ms
31
Typical TR for PD sequence
Long TR
>2000ms
32
Typical TE for T1 sequence
Short TE
<30 ms
33
Typical TE for T2 sequence
Long TE
>80ms
34
Typical TE for PD sequence
Short TE
<30 ms
35
Does T2 use a short or long TE (and why)?
Long TE to emphasis e the influence of T2 on the size of the MR signal
Shows best T2 contrast
36
Does T2 use a short or long TR (and why)?
Long TR
To minimise the differences due to T1 relaxation
37
How does CSF appear in a T2 weighted image
Bright
38
Hows does CSF appear in a T1 weighted image
Dark
39
Does PD use a long or short TE (and why)
Short TE to minimise the T2 input on the signal
40
Does PD weighted imaging use a short or long TR (and why)
Long TR
To minimise the T1 input of the signal
41
how does bone appear on T1
Dark
Due to long T1
42
Hows does bone appear on T2
Dark
Due to short T2
43
How does fatty tissue appear on T1
Bright
Fluid - dark
Fat - bright
44
How does fatty tissue appear on T2
Mid range
Fluid bright
Fat - less bright than fluid
45
How is voxel size calculated
Pixel size x slice thickness
46
What is the effect of increasing Bo on SNR
Increases SNR
47
What is the effect of increasing voxel size on SNR
Increases SNR
48
What is the effect of increased proton density on SNR
Increases SNR
49
What is the effect of increasing TR on SNR
Increases SNR
50
What is the effect of increasing TE on SNR
Decreases SNR
51
What is the effect of increasing receiver bandwidth on SNR
Decreases SNR
(increases the range of frequencies detected - increases recorded noise)
52
What is the effect of increasing the number of excitations (no times the MR pulse sequence is repeated) on the SNR
Increases SNR
52
What is the effect of smaller voxels on SR in MRI
Improved SR
53
What is the effect of smaller voxels on SNR
Decreases SNR (more noise)
54
Hows is pixel size calculated
FOV/Matrix size
55
What is the effect of increasing matrix size on SR
Improves SR
56
What is the effect of decreasing FOV on SR
Improves SR
57
What is the effect of decreasing slice thickness on SR
Improves SR
58
What is the effect of increasing B0, TR or TE on SR
No effect
59
When is the SSG switched on?
During each RF pulse
60
What is the effect of a steeper Slice selection gradient on slice thickness
thinner slice
61
What is the effect of a shallower SSG on gradient on slice thickness
Thicker slice
62
When is the PEG applied
Period between the stimulating RF pulses and the generation of the echo signal
63
What is the effect of a larger RF pulse bandwidth on slice thickness
Thicker slice
(larger range of frequencies)
64
What is the effect of a smaller RF pulse bandwidth on slice thickness
Thinner slice
(smaller range of frequencies)
65
In which axis is the phase encoding gradient applied
The Y axis
66
For a NxN matrix size how many phase encoding steps are required
N number
67
When is the FEG applied
When the echo signal is being detected
Hence its sometimes called a readout gradient
68
Which axis does the FEG affect
X axis
69
How does the FEG work?
Creates gradient along X axis
Alters the larmor frequency along X axis
Echo signal will now be detected at different frequencies
70
In FEG what does the frequency of the signal relate to
The position of the voxel along the readout axis
71
What FEG what does amplitude of the signal relate to
The brightness
72
In which encoding direction does chemical shift artefact occur?
Frequency encoding gradient
73
Why does chemical shift occur
Due to molecular structure
Water molecules resonate slightly faster than (more shielded) fat molecules
Can cause misplacement of signal along the FEG
74
Where are low spatial frequencies (but contrast information) stored in K space
Centre
75
Where are high spatial frequencies stored in K space
Periphery
76
What does the centre of K space tell us about
Contrast
77
What does the periphery of K space tell us about
Spatial resoluction
78
What are the two main types of sequences used in MRI
Spin echo
Gradient recalled echo
79
What is the purpose of 180 degree RF pulse in spin echo sequences
To rephase the spins
Compensate for T2* decay
80
Which is shorter T2 or T2*
T2*
81
How is spin echo acquisition time calculated
Number excitations x number phase encoding steps x TR
82
Are spin echo sequences sensitive to B0 inhomogeneities?
No they are insensitive to Bo inhomogeneity
83
How is the effective echo time (T eff) in Turbo spin echo determined
Time interval between the 90 degree RF pulse and the CENTRAL ECHO
84
How does turbo spin echo work
Each 90 degree pulse is followed by a train of 180 degree pulses
85
How does multi-echo sequence work?
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
How is sequence time calculated in a turbo echo sequence
Number phase encoding steps x TR x number of slices/ ETL
87
What is the echo train length (ETL) in turbo/fast spin echo?
the number of 180 echoes applied after the 90 degree RF pulse
88
What is the first RF pulse applied in inversion recovery
180 pulse
89
What does STIR sequence nullify the signal from
Fat
90
What does FLAIR sequence nullify the signal from
CSF
91
How is the TI calculated for each tissues
TI = 0.693 x T1 (of that tissue)
92
Does FLAIR used a short or long TI
Long TI
2200ms
93
Does STIR use a short or long TI
Short TI
TI 160ms
94
What are the disadvantages of turbo spin echo sequences (2)
- 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
What is the main advantage of a turbo spin sequence
Very fast
96
What size flip angles does GRE use
Small flip angles (usually <90)
97
Does GRE have long or short TR
Short TR
98
Is GRE susceptible to B0 inhomogeneities
GRE IS susceptible to B0 inhomogeneties
99
Are T2 weighted images possible for GRE
No
100
What is the effect of increasing matrix size on scan time
Increases scan time
101
What is the effect of increasing matrix size on SR
Increases SR
102
How are spins rephased in gradient echo
Gradient applied to rephase
103
In gradient echo sequence:
Does a small flip angle give T2* or T1 weighted images
T2* weighted
104
In gradient echo sequence:
Does a large flip angle give T2* or T1 weighted images
T1 weighted
105
Describe sequence of events in gradient echo sequence
- 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
What are the disadvantages of GRE (3)
SNR lower than SE
T2 weighting not possible
Susceptibility artefacts can lead to signal loss
107
What are the advantages of GRE (2)
Fast sequence which can create T1, T2* or PD imaging
Sensitive to blood flow (angiography)
108
Which encoding Gradient does motion artefact affect
Phase encoding gradient
109
Why is fat signal suppressed in T2 spin echo sequence but high in T2 fast spin echo sequence?
Spin echo T2 - fat suppressed by J coupling
Fast spin echo T2- fat high signal as J coupling is broken
110
How is risk of RF heating in tissue expressed
By specific absorption rate
111
What is SAR and how is it calculated
Risk of RF heating in tissue
= RF power absorbed per kg tissue
112
In GRE what is SAR proportional to
(Flip angle) squared
113
What is the usual limit of SAR for whole body
<4W/kg
114
What is the usual temperature rise in human tissue caused by MRI (for whole body exposures)
For whole body exposures the patients temperature should not increase by more than 1 degree during the examination
115
What side effects can patient experience due to the static field strength B0
Metallic taste
Dizziness
116
What dangers/risks can Bo static magnetic field pose to people
Projectile risks for ferromagnetic materials
Surgical clips/screws etc may move
117
What side effects can patient experience due to the rapid gradient switching
Can cause peripheral nerve stimulation
118
What dangers/risks can Rapid gradient switching pose to people
Control & programming of pacemakers may be adversely affected
Acoustic noise levels
119
At what fringe field should there be restricted access to an MR environment
>0.5mT
120
What is the effect of increasing matrix size on SNR
Decreases SNR
121
What is the effect of increasing FOV on SNR
Improves SNR
122
What is the effect of increasing FOV on SR
Lower resolution
123
What is the effect of increasing FOV on viewing area
Increased viewing area
124
What is the effect of increasing slice thickness on SNR
Increases SNR
125
What is the effect of increasing slice thickness on SR
Lower resolution
126
What is the effect of increasing slice thickness on the partial volume effect
Increases partial volume effect
127
What is the effect of increasing slice gap on cross talk
Less cross talk
128
What is the effect of increasing NEX on SNR
Increases SNR
129
What is the effect of increasing NEX on artefacts
Fewer artefacts due to signalling average
130
What is the effect of increasing NEX on scan time
Increases scan time
131
Define MRI safe
“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
Define MRI Conditional
“an item with demonstrated safety in the MR environment within defined conditions.”
133
Define MRI Unsafe
“an item that poses unacceptable risks to the patient, medical staff or other persons within the MR environment.”
134
What is the default SAR in normal operating mode
SAR = 2W/Kg
135
What is the default SAR in controlled operating mode
SAR = 4W/kg
136
What is the temperature rise limit for infants, pregnant women and those with cardiocirculatory impairment
0.5 degrees C
137
What is the Tesla limit for whole body exposures normal mode
4 T
138
What is the Tesla limit for whole body exposures controlled mode
8 T
139
What is the Tesla limit for whole body exposures for research purposes
No limit
140
What is the Tesla limit for whole body exposures in pregnant women
<2.5 T
141
What is the staff exposure limit (T) for whole body and limbs
<2T for whole body
<5T for limbs
142
At what dB level is hearing protection needed to prevent reversible damage
At 90dB
143
What causes louder noise in MRI scans (2)
higher field strength
higher gradient amplitudes
144
What is the maximum acoustic noise allowance in MRI
140dB
145
List some absolute contraindications to MRI
Non-MR compatible pacemaker / cochlear implant
Metallic foreign body in eye
Bullets
Non-MR compatible cerebral aneurysm clips
146
List some relative contraindications to MRI
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
During which trimester are pregnant patients usually not scanned
During first trimester
148
What does 1W/kg equal in temperature rise
0.5 degrees C
149
List some techniques which can reduce SAR
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
In DWI:
How will restricted diffusion appear on DWI and ADC
DWI - bright
ADC - dark
151
In DWI:
How will increased diffusion appear on DWI and ADC
DWI - Dark
ADC - bright
152
In DWI:
How will T2 shine through appear on DWI and ADC sequences
DWI - dark
ADC - bright
153
What types of pathology can cause restricted diffusion
Infarcts, timours, infarcts
154
What type of spin echo sequence is used in DWI and why
Echo planar imaging
(very fast)
155
How is DWI imaging produced?
How does this affect stationary and mobile spins?
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
What is meant by a higher B value in DWI
More sensitive the DWI sequence is to diffusion
More noise
Less signal
157
What is meant by B value in DWI
The degree of diffusion weighting is represented as the b value
158
How is B value in DWI increased
Larger diffusion gradients (increase amplitude or duration)
Increased time between dephasing and rephasing gradients
159
What type of weighting do DWI sequences have?
T2 weighting
160
What does the ADC map remove
Removes the effects of inherent T2 signal
161
What is T2 shine through
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
In DWI:
Do higher B values have increased or reduced SNR?
Reduced SNR
More noise
Less signal
163
What is the purpose of the ADC map in DWI
removes the T2 dependence and so can be used to distinguish true restricted diffusion from T2 shine through
164
What types of sequence does time of flight (MR angiography) use?
Gradient echo
165
What type of enhancement does time of flight (MR angiography) use?
Flow related enhancement
166
How does blood appear of time of flight (MRA)?
Bright
167
Is time of flight MRA a contrast or non contrast technique?
Non contrast
168
Describe how time of flight MRA works?
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
What TE/TR is used for time of flight MRA?
Short TE
Short TR
170
What size of flip angle is used for time of flight MRA?
Large flip angle
171
Advantages of time of flight MRA (4)
Contrast agent not required
Can be used for venous or arterial imaging
Sensitive to flow
Saturates out all background signal
172
Describe the phase contrast MRA
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
Does phase contrast MRA have short or long acquisition time?
Long
174
In phase contrast MRA:
A phase change is only induced when the flow is _______ to the gradient
Parallel
175
In phase contrast MRA:
Because the phase change is only induced when the flow is parralell to the gradient what must occur?
Acquisitions must be repeated with gradient in different directions
176
Why do flow voids occur in spin-echo
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
Regarding flow voids:
What happens if the tissue moves faster than TE/2
No signal
178
Advantages of phase contrast (MRA) (3)
Contrast agent not used
Good background suppression
Velocity and blood direction can be quantified
179
Can blood direction and velocity be quantified in time of flight (MRA)
No
180
How does contrast enhanced (MRA) work
Use Gadolinium Chelate agents
Cause shortening of T1 relaxation time of blood
Leading to high signal intensity blood on T1 imaging
181
In contrast enhanced MRA when are images acquired
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
Examples of ferromagnetic materials
Iron
Nickel
Cobalt
183
What type of magnetism does Gadolinium have?
Paramagnetic
184
How does gadolinium contrast affect T1 and T2
Reduces both
However far greater effect on T1 compared to T2
185
Is T1 or T2 used for contrast sequences
T1
186
Does Gd-DPTA cross the blood brain barrier easily
No
187
Why must gadolinium be attached or chelated to other compounds?
As gadolinium on its own is very toxic
188
Is GD-TPA water soluble?
Yes
189
What is meant by diamagnetic
When placed in magnetic field weakly repel the field
But revert to a random distribution if the magnetic field is removed
190
What is meant by ferromagnetic
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
What is meant by paramagnetic
Weakly attracted to alight in the same direction as the magnetic field
e.g gadolinium
192
What is the effect of increased B0 on SNR?
Increases SNR
(reduced noise)
193
What is the effect of increased voxel size on SNR
Increased SNR
194
What is the effect of increased proton density on SNR
Increased SNR
195
What is the effect of increased TR on SNR
Increased SNR
196
What is the effect of increased TE on SNR
Decreases SNR
197
What is the effect of decreased TE on SNR?
Increased SNR
198
What is the effect of increased receiver bandwidth on SNR
Reduced SNR
199
What is the effect of reduced receiver bandwidth on SNR
Improved SNR
200
What is the effect of increased number of excitations on SNR
Improved SNR
201
What is the effect of increased phase encoding steps on SNR
No effect
202
What is the effect of increased matrix size on SR
Improved SR
203
What is the effect of decreased pixel size on SR
Improved SR
204
What is the effect of a decreased FOV on SR?
Improved SR
205
What is the effect of decreased slice thickness on SR?
Improved SR
206
How is noise generated in MR?
From the whole patient through Brownian motion of biological molecules
207
Along which axis is motion artefact typically observed
Phase encoding axis
208
What is the rule for breastfeeding after administration of Gadolinium contrast?
No need to stop
209
At what eGFR is Gadolinium contrast not advised
GFR <30
210
How does the BO magnetic field and RF fields pose risk to metal implants?
BO field - projectile risk
RF field - heating risk
211
Is chemical shift artefact more apparent at higher or lower field strengths?
Higher field strengths
212
What are the 3 types of magnetic susceptibility?
Diamagnetic
Paramagnetic
Ferromagnetic
213
