ECVDI- physics > MRI > Flashcards

MRI Flashcards

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1
Q

what is the larmor equation?

A

frequency
Y= gyromagnetic ratio
b0= tesla of the magnet. (or the strength of the field at the level in the gradient- slice selection)

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1
Q

what is the gyromagnetic ratio of hydrogren at 1 tesla?

A

43MHz/T (42.5). 43x 1.5 for frequency at 1.5T

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2
Q

which of these will give good t1 contrast?
a) TE 15-20ms, TR 2000ms
b) TE 140ms, TR 2000ms
c) TE 10ms, TR 400ms
d) TE 1000ms, TR 300ms

A

C short TR and TE

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3
Q

which of these will give PD contrast?
a) TE 15-20ms, TR 2000ms
b) TE 140ms, TR 2000ms
c) TE 10ms, TR 400ms
d) TE 1000ms, TR 300ms

A

A short TE long TR

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4
Q

which of these will give good T2 contrast?
a) TE 15-20ms, TR 2000ms
b) TE 140ms, TR 2000ms
c) TE 10ms, TR 400ms
d) TE 1000ms, TR 300ms

A

B long TE, long TR

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5
Q

which of these will give good T2 contrast?
a) TE 15-20ms, TR 2000ms
b) TE 10ms, TR 400ms
c) TE 100ms, TR 2000ms
d) TE 1000ms, TR 600ms

A

C.

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6
Q

which of these will give good t1 contrast?
a) TE 30ms, TR 500ms
b) TE 1000ms, TR 300ms
c) TE 15-20ms, TR 2000ms
d) TE 140ms, TR 2000ms

A

A

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7
Q

define T1 recovery

A

the recovery of longitudinal magnetisation due to sin lattice relaxation after the RF excitation pulse is switched off. also called longitudinal recovery.

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8
Q

what is the T1 recovery time

A

time for 63% of the longitudinal magnetization to RECOVER in a tissue

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9
Q

what is t2 decay

A

the decay of coherent transverse magnetisation due to spin-spin relaxation after the RF pulse is switched off

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10
Q

what is the T2 decay time

A

the time it takes for 63% of coherent transverse magnetisation to decay

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11
Q

a T2 weighted sequence will have a…
a) long TE, long TR
b) short TE, short TR
c) short TE, long TR
d) long TE, short TR

A

A.
b is T1
c is PD
d is nothing

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12
Q

a PD weighted sequence will have a…
a) long TE, short TR
b) short TE, long TR
c) long TE, long TR
d) short TE, short TR

A

B

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13
Q

a traditional t1w spin echo sequence is run in a 1.5T MRI machine and has a TR of 500ms and a TE of 30ms. You have selected a 256 x 256 matrix. You are only doing 1 nex. How long will it take to acquire one image slice?
a) 256s
b) 256ms
c) 128us
d) 128s

A

500ms is 0.5s
0.5x 256= 128s

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14
Q

why does the urinary bladder have pseudolayering after injection of gadolinium IV?

A

Gadolinium-based agents chelated to protective ligands are typically used. At low concentration, such as used in clinical practice, the predominant effect is a shortening of the T1-relaxation time.10 Thus, tissues accumulating this substance generate greater signal—contrast-enhance—on a T1-weighted pulse sequence. At high gadolinium concentration, T2 shortening may result in a significant decrease in signal intensity noticeable on both T1- and T2-weighted images. This effect can be observed in the bladder as a triple-layer intensity pattern, or pseudolayering

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15
Q

At 1.5T, what is the magnitude of the difference in precessional frequency of fat compared to water?
a) 20Hz
b) 40MHz
c) 220Hz
d)230MHz

A

C
this difference is used in chemical fat suppression techniques

fat precesses at a slightly lower frequency than water, more pronounced at higher field strengths.

The difference in precessional frequency becomes smaller at lower field strengths; therefore, chemical fat suppression is limited to field strengths of 1.0 Tesla or greater.

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16
Q

which types of mri sequences is magic angle artefact seen in?

A

T1w, PD and gradient echo images and others with short TEs

It is confined to regions of tightly bound collagen at 54.74° from the main magnetic field (B0), and appears hyperintense, thus potentially being mistaken for tendinopathy.

normally in tightly-bound collagen, water molecules are restricted usually causing very short T2 times, accounting for the lack of signal. When molecules lie at 54.74°, there is lengthening of T2 times with corresponding increase in signal. Thus in short TE sequences, the T2 signal does not decay significantly before the scanner picks up the signal. On the other hand, in long TE sequences (like T2 weighted sequences), by the time the scanner picks up the signal, the T2 signal has already decayed.

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17
Q

what angle does the magic angle artifact occur at?
a) 45 deg
b) 54 deg
c) 78 deg
d) 87 deg

A

B 54.74

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18
Q

what gradient is on at the same time as the RF pulse?
a) slice select
b) frequency
c) phase

A

A

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19
Q

what gradient is on at the same time as the returning signal is being read?
a) slice select
b) frequency
c) phase

A

B

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20
Q

frequency encoding separates out signal along the …
x axis
y axis

A

x axis
a column of data in the x axis

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21
Q

chemical shift occurs along the
a) slice selection direction
b) frequency encoding direction
c) phase encoding direction

A

B

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22
Q

phase encoding separates our signal in the
x axis
y axis

A

y axis

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23
Q

phase encoding gradients are applied…
a) at the same time as the rf pulse
b) at the same time as slice selection
c) between the 90 and 180 degree rf pulse
d) when the signal is read out

A

C

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24
what is the larmor frequency?
The Larmor frequency is the frequency at which protons precess when place in an external magnetic field.
25
why is the b0 net mag vector higher in a 3T than a 1T MRI?
in a field strength of 1T, at any given time the ratio between antiparallel and parallel states is 1,000,000:1,000,006. The small but significant excess of 6 gives a net magnetisation vector (NMV) in the parallel direction. The stronger the field strength of B0 the more energy is required for a proton to switch from a parallel to antiparallel state therefore the excess of parallel protons will be greater, as will the NMV as the field strength of the MRI system increases, the NMV increases, and more signal can be generated from the same volume of tissue
26
at equilibrium(with no RF pulse), what is the NMV in the M(xy) direction?
the precession is random and cancels each other out, M(xy) is zero
27
in order to bring protons in phase, with a NMV in the X,Y plane, the frequency of the RF pulse must be ___ to the larmor frequency
equal to as such, it is possible to selectively excite atoms of different elements by altering the frequency of the RF pulse
28
what is the T2 time of a tissue? the time when M(xy) is __% of its initial value.
37% Caused by interactions between neighbouring spins, not the external lattice- dephasing of spins. not dependent on magnetic field strength. it dependent on tissue composition.
29
which time is shorter, T2 or T2*
T2* T2* = T2 decay + dephasing from magnetic field inhomogeneities.
30
what is spin-lattice decay?
AS the nuclei precess in the transverse plane they are jostled by the surrounding molecules (i.e. surrounding lattice) and they give up their energy to these molecules. As they do they return to the longitudinal magnetization (Mz) exponentially. This is the spin-lattice or longitudinal relaxation. The rate at which this happens is governed by the time constant T1 also called T1 or longitudinal relaxtion
31
T1 is the time it takes for Mz to recover to ___% of its maximum value
63% T1 depends on the surrounding molecules and lattice It is always longer than T2 (except water in which T1 is very similar to T2- both are long).
32
Fat and protein have a ______ T1 time. The molecules are large with low innate energy. This makes them very effective at absorbing energy causing a quick loss of Mxy and, therefore, quick recovery of Mz
short Water: long T1. The molecules are small and move quickly making them inefficient at jostling the nuclei and absorbing energy. This causes a long T1. Fast food (fat causes short T1) and long drink of water (water causes long T1)
33
Bone / calcium / metal have a very _____ T1 time.
long The macromolecules are fixed and rigid and are the least effective at removing energy from the precessing nuclei.
34
Bone / calcium / metal have a very _____ T2 time.
short. The local variation of magnetic field is greatest in solids and macromolecules that are rigid.
35
what is the tau time in a spin echo pulse?
In a spin echo sequence, τ is the time between the 90° pulse and the 180° rephasing pulse, and from 180° pulse to the signal echo. So, TE = 2 × τ
36
which type of sequence is this? 90° excitation pulse followed by 180° rephasing pulse. Produces strong signal, reduces artefacts.
spin echo
37
which type of sequence is this? Starts with 180° inversion pulse, then 90° pulse after a delay (TI) to null specific tissues
Inversion recovery
38
which type of sequence is this? Uses a smaller flip angle (<90°) and gradient reversal instead of a 180° pulse
Gradient echo
39
when is chemical shift artefact most pronounced?
Always occurs in the frequency encoding direction only. Most visible when bandwidth is narrow and field strength is high. If pixels are smaller will also be more prominent- less bandwidth per pixel at fat water interfaces
40
what property makes substances paramagnetic?
unpaired electrons have positive magnetic susceptibility Not strongly magnetised themselves, but they enhance local field strength and speed up relaxation gadolinium- 7 unpaired electrons- t1 shortening at high conc- t2 shortening- in bladder
41
what is the 5 gauss line? what does it mean and what is it for?
gaus- old measuring system 5G= 0.5mT sets the controlled zone for which only trained personelle and patients can cross- no magnetic material allowed Five gauss and below are considered 'safe' levels of static magnetic field exposure for the general public. At 5 gauss and above: cardiac pacemakers and other implanted electronic devices are at risk of being interfered with by the static magnetic field ferromagnetic materials may become projectile (and are thus prohibited from crossing the 5 gauss line) Note that the distance at which a 5 gauss line is drawn around a particular MRI scanner will depend on magnet strength and the level of magnetic shielding used.
42
3) Which part of the MRI machine is important for slice selection? Main magnet Gradient coil RF coil Shim bars
B
43
Which of the below is false? Permanent MRI magnets are heavy, but cheap and can be turned off Superconductor magnets are a type of electromagnet. They are expensive and require helium coolant systems, but have low power requirements. They are left permanently on. Resistive electromagnets can be horizontal or vertical, require high power if you want higher Tesla, and can be turned on and off.
false- Permanent MRI magnets are heavy, but cheap and can be turned off permanently on
44
what is the TR time?
Definition: time interval between successive pulse sequences applied to the same slice (i.e. period between RF (B1) excitation pulses) Role in image formation Determines the time required to acquire an image (Longer TR -> longer scan) T2 decay and T1 relaxation occur during this time How much longitudinal magnetization has recovered before the next RF pulse Influences T1 contrast - Short TR -> Strong T1 weighting A short TR prevents full longitudinal recovery -> tissues with shorter T1 recover more -> appear brighter A long TR allows full recovery -> minimizes T1 contrast
45
what is the TE time?
Definition: the time between the RF excitation pulse and the peak amplitude of an induced echo (either spin or gradient depending on sequence). Determined by applying a 180 degree RF inversion pulse or gradient reversal at a time equal TE/2. Role in image formation Controls the amount of T2 relaxation that has occurred when the signal is read Influences T2 contrast – Long TE -> Stronger T2 weighting Short TE: less decay -> minimal T2 contrast Long TE: more decay -> tissues with longer T2 remain bright
46
what slice gap is used in 2d mri?
Needed to reduce cross-talk (caused by imperfect slice profiles); usually a gap of 10–20% of slice thickness. No slice gap in 3D imaging Can avoid cross-talk artefact via imaging non-contiguous slices (e.g slice 1, 3, 5 and then 2, 4, 6.)
47
If NEX doubled, results in a SNR by a factor of ...
square root of 2 = 1.4 but longer acquisition times Scan times increases linearly with NEX Avoid unnecessarily high NEX — better to optimize coil position or adjust TE/TR for SNR
48
1. What does the central portion of k-space primarily contribute to? A. Image resolution B. Image contrast and signal C. Spatial orientation D. Noise reduction
B
49
2. What happens to scan time if the TR is doubled? A. It is halved B. It remains the same C. It doubles D. It quadruples
C
50
3. In MRI, what does the frequency encoding gradient do? A. Selects the axial slice B. Encodes spatial information along the Y-axis C. Encodes spatial information along the X-axis D. Changes the phase of protons
C
51
4. Which parameter change would most directly improve spatial resolution? A. Increasing matrix size B. Increasing NEX C. Increasing slice thickness D. Decreasing bandwidth
A
52
6. If the number of phase encoding steps is doubled, what happens to scan time? A. It remains the same B. It is halved C. It doubles D. It increases slightly
C
53
9. Which of the following would result in a thinner slice during slice selection? A. Shallow gradient + broad RF bandwidth B. Steep gradient + narrow RF bandwidth C. Steep gradient + broad RF bandwidth D. Shallow gradient + narrow RF bandwidth
B
54
What is the effect of a long TE? A. Maximizes T1 contrast B. Reduces SNR but enhances T2 weighting C. Shortens scan time D. Reduces partial volume effect
B
55
In gradient echo sequences, what is a typical flip angle used? A. 90° B. 180° C. 5°–60° D. 120°
C
56
What is the purpose of using a slice gap in 2D imaging? A. Minimize cross-talk artefacts B. Reduce scan time C. Improve resolution D. Increase contrast
A
57
for T1w gradient echo imaging, what is required? a) small flip angle, short TE, short TR b) small flip angle, long TE, short TR c) large flip angle, long TE, short TR d) large flip angle, short TE, short TR
D
58
for a T2* W GRE image, what is needed? a) small flip angle, short TE, short TR b) small flip angle, long TE, long TR c) small flip angle, long TE, short TR d) large flip angle, short TE, short TR
C
59
how do you calculate the time taken to acquire one slice of an mri sequence
TR x NEX x no of phase encoding steps
60
8. What is a known disadvantage of FSE compared to SE? A. Inability to suppress fat B. Less effective for detecting subtle haemorrhage C. Greater motion artefact D. Increased scan time
B
61
11. In a gradient echo sequence, how is signal rephasing achieved? A. Gradient reversal B. 180° RF refocusing pulse C. T2 refocusing loop D. Spin-echo train
A
62
TOF MRA provides information about blood flow in which direction in relation to the slice phase?
perpendicular
63
Is TOF MRA dark blood or bright blood
bright
64
Is phase contrast MRA a dark blood or bright blood sequence
bright
65
what is the main difference between TOF MRA and phase contrast MRA? a) PC gives directional velocities, but TOF does not b) PC can only be acquired on 3D, where as TOF can be 2D or 3D c) PC required no gadolinium, where as TOF required gadolinium
A PC gives info on directionality, velocity quanification and can be used for lots of different flow speeds (where as TOF is for slow to moderate flow). neither need contrast TOF good for high res vascular mapping.
66
Which of the following best describes the principle of Time-of-Flight (TOF) MRA? A. Enhancement of signal from inflowing, unsaturated spins B. Flow quantification using bipolar gradients C. Suppression of signal from moving spins D. Use of gadolinium to improve vessel visibility
A
67
What is the primary mechanism by which gadolinium enhances MRI signal? A. Shortens T2 relaxation only B. Enhances proton density C. Matches water tumbling frequency to Larmor frequency D. Causes chemical shift
C
68
Why can’t gadolinium be used with STIR sequences? A. It can be B. Its T1-shortening effect is also suppressed C. It shortens TE excessively D. It suppresses fat too aggressively
B
69
why is bandwidth important in the frequency encoding direction for aliasing artefact?
bandwidth determines the range of relative frequencies that can be accurately represented. any signal beyond that bandwidth (outside the FOV) will not be accurately plotted and cause aliasing (wrap) nyquist limit= samling rate/2 bandwidth= sampling rate usually sampling interval (dwell time)= 1/sampling rate
70
what is frequency bandwidth proprtional to in mri?
FOV and gradient. BW = k(FOV x Gradient) k- constant
71
as FOV gets bigger, what happens to the bandwidth in the frequency encoding direction?
increases
72
how does a narrow bandwidth (BW) affect the SNR and sampling time?
increased SNR longer sampling time needed- bad if the TE is short blue is signal, orange is noise SNR= 1/(square root of the BW)
73
how does a broad bandwidth affect the SNR and sampling time?
reduced SNR quicker sampling time SNR= 1/(square root of the BW) may be needed in sequences with very short TEs, as sampling can't take too long
74
In which scenario is chemical shift artefact more pronounced in MRI imaging, with other factors remaining the same? a) narrow bandwidth b) broad bandwidth c) parallel imaging d) using a 1.5T machine instead of a 3T machine e) increased matrix size
A and E A- as only very small differences in frequency across different pixels with broader bandwidths--> larger range of frequencies within one pixel, so both fat and water could be plotted correctly parallel imaging- helps with phase wrap- reduced phase encoding steps and adds images from different coils together, removing abnormal signals. the higher the tesla the larger difference in precessional frequency of fat and water, so CS worse at 3T increased matrix, smaller pixels, reduced frequency difference between pixels, more chemical shift- more chance of CS
75
in mri imaging, if all other factors remain constant, an increased bandwidth (in the frequency encoding direction)... a) increased SNR, reduced chemical shift b) reduced snr, reduced chemical shift c) increased snr, increased chemical shift d) reduced snr, increased chemical shift
B
76
when is the slice select gradient applied in a spin echo sequence? a) during the 90 degree excitation pulse b) during read out c) during 180 degree refocussing pulse d) during both the 90 degree and 180 degree pulse
D
77
when is the slice select gradient applied in a gradient echo sequence? a) during the excitation pulse b) during read out c) during 180 degree refocussing pulse d) during both the excitation pulse and rephasing gradient
A
78
how does chemical fat saturation work?
exploits differences in chemical shift/ larmor frequency Apply 90° pre-saturation pulse at precessional frequency of fat across FOV Excitation RF pulse is then applied to slice and the magnetic moments of fat nuclei are flipped into saturation (nulled) -> produces a signal void Water nuclei are excited, rephased -> signal
79
can a SPIR sequence be used post contrast and show contrast enhancement?
yes Combines fat saturation and inverting mechanisms similar to STIR Advantages: Less affected by inhomogeneity of the main magnetic field than fat saturation but still more than STIR Does not null the signal of Gd Usage - MSK image of normal bone (suppressing fat) allowing lesions within the bone to be seen more clearly
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