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AJT Physics > PHYSICS - MRI > Flashcards

PHYSICS - MRI Flashcards

1
Q

Ferromagnetic substances

A

dramatic increase in local magnetic field, large increase in susceptibility; e.g. metal

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2
Q

Paramagnetic substances

A

small increase in local magnetic field, small increase in susceptibility; e.g. deoxyhemoglobin, gadolinium

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3
Q

Diamagnetic substances

A

small decrease in local magnetic field, small decease in susceptibility; e.g. tissues, calcium

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4
Q

Requirement for an atom to have net magnetism

A

odd mass number (protons + neutrons)

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5
Q

Net magnetization in the absence of an external magnetic field

A

no net magnetization; protons are randomly oriented

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6
Q

Larmor frequency is proportional to…

A

magnetic field strength

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7
Q

Net longitudinal magnetization (Mz) is proportional to…

A

magnetic field strength; parallel vs. antiparallel orientation

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8
Q

Larmor frequency for H+ at 1 Tesla

A

42 MHz; which means the gyromagnetic ratio of H+ is 42 MHz/Tesla

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9
Q

Transverse magnetization (Mxy) immediately after external magnetic field applied

A

none; phase is random

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10
Q

Is the parallel or antiparallel orientation a lower energy state?

A

parallel orientation is a lower energy state (preferred)

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11
Q

Susceptibility definition

A

extent to which matter becomes magnetized when placed in an external magnetic field; causes spin dephasing resulting in signal loss

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12
Q

Flip angle definition

A

angle of net magnetization vector relative to the Z-axis

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13
Q

For resonance to occur, the RF pulse must be…

A

RF pulse must be at the Larmor frequency and perpendicular to the Z-axis

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14
Q

Free induction decay signal

A

voltage detected by coils, which is induced by the rotating transverse magnetization vector; voltage oscillates at the Larmor frequency

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15
Q

Relationship between magnetic field strength and FID signal

A

directly proportional; increased field strength => increased FID signal

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16
Q

T1 relaxation time increases or decreases with increased field strength?

A

increases (longer T1 relaxation time); energy exchange with the lattice is less efficient

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17
Q

Short T1 relaxation time - bright or dark

A

bright; tissue recovers signal quickly

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18
Q

Long T1 relaxation time - bright or dark

A

dark; tissue recovers signal slowly

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19
Q

Short T2 relaxation time - bright or dark

A

dark; tissue loses signal quickly

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20
Q

Long T2 relaxation time - bright or dark

A

bright; tissue loses signal slowly

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21
Q

Causes of loss of phase coherence

A

spin-spin interactions and magnetic field inhomogeneities (may be external or local)

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22
Q

T2 relaxation time increases or decreases with increased field strength?

A

neither; T2 relaxation is independent of magnetic field strength

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23
Q

T1 or T2 relaxation times are longer? (generally)

A

T1 relaxation times are much longer

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24
Q

Faraday’s Law of Induction

A

a moving magnetic field with induce a current within a coil

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25
Q

T1 time constant

A

time at which 63% of Mz has formed

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26
Q

T2 time constant

A

time at which Mxy has decayed to 37% of its original value

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27
Q

How many T1’s for recovery of full net longitudinal magnetization?

A

4 T1’s (~99%); same for T2 decay (4 T2’s)

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28
Q

How does an increase in magnetic field strength by 4x affect T1 relaxation?

A

2x increases in T1 relaxation time

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29
Q

T2 signal when longitudinal magnetization has fully recovered

A

none; not possible to have T2 signal (transverse magnetization) when longitudal magnetization has fully recovered

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30
Q

T2* decay

A

dephasing due to spin-spin interactions and magnetic field inhomogenties (local or external)

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31
Q

Formula for T1 contribution to signal

A

1 - e^(-t/T1), where t is the TR and T1 is tissue specific

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32
Q

Formula for T2 contribution to signal

A

e^(-t/T2), where t is the TE and T2 is tissue specific

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33
Q

Short TR

A

<500 msec

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34
Q

Long TR

A

> 2000 msec

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35
Q

Short TE

A

<30 msec

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36
Q

Long TE

A

> 80 msec

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37
Q

TR and TE for proton density

A

long TR, short TE

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38
Q

Spin density sequence

A

a.k.a. proton density

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39
Q

Spin echo: 180 degree pulse timing

A

TE/2

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40
Q

T2 vs. T2* decay

A

T2 decay is the result of spin-spin interactions, while T2* decay is the result of spin-spin interactions + field inhomogeneities

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41
Q

Relationship between magnetic field strength and SNR

A

directly proportional; 2x field strength => 2x SNR (noise does not change with field strength)

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42
Q

Gradient echo pulse sequence

A

very short TR; <90 degree pulse => bipolar (dephasing and rephasing) gradients

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43
Q

How to: increase T1-weighting on GRE

A

increase flip angle

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44
Q

How to: increase T2-weighting on GRE

A

increase TE

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45
Q

Effect of gadolinium on T1 and T2 relaxation

A

gad increases T1 and T2 relaxation (shortening) => T1 bright, T2 dark

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46
Q

Standard dose of gadolinium

A

0.1 mmol/kg

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47
Q

Risk of gadolinium administration in CKD

A

nephrogenic systemic fibrosis (widespread tissue fibrosis)

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48
Q

Contraindications to gadolinium

A

pregnancy, GFR <30

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49
Q

Gadolinium agents with no known association with NSF

A

macrocyclic gadolinium agents

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50
Q

Timing of slice select gradient

A

applied during RF pulse (RF pulse determines which “slice” of tissue is excited); during ALL RF pulses, not just initial

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51
Q

How to: obtain thinner slices

A

decrease transmit bandwidth (of RF pulse), increase slice select gradient strength

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52
Q

Timing of frequency encoding gradient

A

applied at TE (during sampling)

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53
Q

Echo sampling rate

A

number of times each echo is sampled; corresponds to frequency encode matrix size

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54
Q

Determinants of matrix size

A

number of times each echo is sampled in the frequency encoding direction, number of phase encoding gradients in the phase encoding direction

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55
Q

Timing of phase encoding gradient

A

between RF pulse and echo; different phase encoding gradient is applied for each acquired echo

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56
Q

Center of k-space

A

center represents low spatial frequencies (large structures/”contrast”)

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57
Q

Periphery of k-space

A

periphery represents high spatial frequencies (small features and edges/”details”)

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58
Q

Gradient applied across the widest dimension (generally)

A

frequency encoding gradient

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59
Q

Fast spin echo (FSE)

A

fill multiple rows of k-space within a single TR (multiple TEs in each TR); longer TR required

60
Q

Shimming

A

used to correct small inhomogeneities in the external magnetic field => improved uniformity

61
Q

Transmit bandwidth

A

range of frequencies emitted in an RF pulse

62
Q

Relationship between receiver bandwidth and noise

A

directly proportional; greater receiver bandwidth => increased noise

63
Q

Echo train length (ETL)

A

number of echoes acquired in a single TR; FSE or turbo spin echo sequences

64
Q

256 x 128 matrix - which is FE and which is PE direction?

A

longer dimension is typically the FE direction (so 256 in this example)

65
Q

TOF in head

A

3-D TOF GRE

66
Q

TOF in neck

A

2-D TOF GRE

67
Q

Advantages of GRE

A

short TR (faster acquisition)

68
Q

Disadvantages of GRE

A

lower signal (smaller flip angles), more noise; echoes formed by rephasing gradients are relatively weak

69
Q

TI (in inversion recovery)

A

between 180 and 90 degree pulses; TR in IR sequences includes both TI and TE

70
Q

Effect of increased magnetic field strength on TI

A

increased field strength => increased TI (T1 relaxation is longer)

71
Q

Inversion recovery + gad

A

STIR is not used with gad; T1 shortening caused by gad creates a null point similar to fat

72
Q

Dielectric artifact

A

a.k.a. standing wave artifact; RF pulse wavelength approximates dimension of patient; worse with higher field strengths

73
Q

Dark signal in central abdomen over left lobe of liver

A

dielectric artifact

74
Q

3-D MRI

A

2 phase-encoding gradients (y and z-axes); better Z-axis resolution, but longer study time (motion)

75
Q

Echo planar imaging (EPI)

A

rapidly switching gradients results in numerous echoes generated within a single TR; fast, less motion, but increases susceptibility

76
Q

BOLD (acronym)

A

blood oxygen level dependent (oxy-Hb); fMRI technique to detect areas of increased blood flow related to localized brain activity; heavily T2*-weighted EPI sequence

77
Q

Contrast in MRI is determined by…

A

tissue properties (T1, T2, T2*)

78
Q

How to: increase SNR

A

increase voxel size, increase magnetic field strength, decrease receiver bandwidth, increase number of acquisitions/excitations per slice, smaller coil (surface coil), increase TR/decrease TE

79
Q

Type 1 chemical shift artifact

A

differences in precessional frequencies at fat-water interfaces results in misregistration; seen in all sequences

80
Q

How to: fix type 1 chemical shift artifact

A

increase receiver bandwidth, increase strength of frequency encoding gradient (steeper slope), or use fat sat

81
Q

Effect of increased magnetic field strength on type 1 chemical shift artifact

A

increased type 1 chemical shift artifact

82
Q

Type 2 chemical shift artifact

A

in-phase and OOP sequences detetermined by differences in precessional frequencies of fat and water protons; GRE only

83
Q

Pulse sequence for in and out-of-phase images

A

GRE pulse sequence

84
Q

India ink artifact

A

voxels at fat-organ interfaces contain portions of both fat and water resulting in signal dropout

85
Q

Dixon W

A

sum of in and out-of-phase images; result is a fat-saturated T1 image

86
Q

Truncation artifact

A

a.k.a. Gibbs or ringing; occurs at sharp edges; may mimic syrinx

87
Q

How to: fix truncation artifact

A

increase matrix size (# of phase-encoding steps), use smoothing filter

88
Q

Wrap around artifact

A

a.k.a. aliasing; caused by small FOV

89
Q

How to: fix wrap around artifact

A

increase FOV, phase oversampling, switch PE and FE directions, apply saturation bands outside FOV

90
Q

Magic angle artifact

A

occurs in tendons; 55 degree angle to Z-axis; disappears on T2

91
Q

Fat saturation techniques for an inhomogeneous field

A

STIR or Dixon W

92
Q

Fat saturation techniques for post-gad imaging

A

chemical fat sat (FSFS) or Dixon W

93
Q

Artifact(s) occurring in frequency-encoding direction

A

type 1 chemical shift

94
Q

Spike artifact

A

a.k.a. herringbone; electromagnetic spike during filling of k-space

95
Q

How to: fix spike artifact

A

remove bad data point or re-scan patient

96
Q

Requirements for MR scanning in pregnancy

A

must document necessity of information, cannot be achieved by ultrasound, and cannot wait until after delivery

97
Q

SAR limits

A

relates to heating; 3 W/kg per 15 min for head and 4 W/kg per 15 min for body

98
Q

Who sets SAR limits?

A

FDA

99
Q

Effect of increasing receiver bandwidth

A

decreased SNR, decreased type 1 chemical shift, decreased TR/TE, decreased scan time

100
Q

Difference between k-space matrix size and image matrix size

A

no difference in terms of size; both have the same dimensions

101
Q

Effect of partial k-space acquisition

A

decreased SNR, decreased scan time

102
Q

How to: decrease susceptibility

A

decrease TE, use SE instead of GRE, decrease field strength, metal suppression sequence (if from metal)

103
Q

How to: prevent muscle twitching or paresthesias

A

tell patient to not cross legs or join hands; due to rapidly switching gradients (also create acoustic noise)

104
Q

MR conditional

A

safe for specific MR environments (e.g. magnet strength, SAR)

105
Q

MR zone 1

A

waiting area

106
Q

MR zone 2

A

patient survey area

107
Q

MR zone 3

A

control room; restricted access

108
Q

MR zone 4

A

magnet room; restricted access

109
Q

Zipper artifact

A

may appear as an irregular line across image or scattered dots

110
Q

How to: fix zipper artifact

A

shut scanner room door, remove cell phone, check RF shielding of room

111
Q

How to: fix incomplete fat suppression

A

shimming, use STIR instead (non-post gad only)

112
Q

Signal of fat on FSE

A

increased T2 relaxation time of fat => fat is bright on T2 (J-coupling)

113
Q

Relationship between coil size and SNR

A

increase coil size => more noise => decreased SNR

114
Q

Five-gauss line

A

defines controlled access area around the MRI scanner; 0.5 mT

115
Q

When to use half-dose of gad?

A

GFR 30-40

116
Q

Advantages of FSE

A

shorter study, more time for larger FOV, less motion => higher resolution

117
Q

Disadvantage of FSE

A

decreased contrast (less signal from each subsequent TE), worse T1 (because longer TR)

118
Q

Chemical fat suppression

A

a.k.a. frequency-selective fat saturation; prepatory pulse => spoiler gradient => normal sequence; longer acquisition because of extra prepatory pulse

119
Q

Susceptibility artifact based on sequence

A

EPI > GRE > SE > FSE

120
Q

SAR based on sequence

A

FSE > SE > GRE; GRE has a small flip angle and single RF pulse

121
Q

Determinants of spatial resolution

A

slice thickness, FOV, matrix size

122
Q

BLADE or PROPELLER

A

redundant sampling of the center of k-space => less motion artifact, slower acquisition

123
Q

Half-fourier acquisition

A

faster acquisition, lower SNR

124
Q

Timings of in and out-of-phase sequences

A

TE 2.2 ms for out-of-phase, TE 4.4 ms for in-phase (at 1.5 T)

125
Q

How to: decrease SAR

A

use fewer RF pulses, increase TR, smaller flip angles, lower field strength

126
Q

Fastest MR sequence

A

EPI (single shot)

127
Q

Readout gradient

A

a.k.a. frequency encoding gradient

128
Q

Bright blood (cardiac MRI)

A

SSFP (GRE based); may obtain cine imaging

129
Q

Dark blood (cardiac MRI)

A

SE based; “new” blood not excited by 90 degree pulse, thus generating no signal

130
Q

Ghosting artifact

A

due to motion; may also be called smearing, pulsation, or motion artifact; occurs in phase-encode direction

131
Q

Gadolinium should be administered at what temperature?

A

room temperature (72 degrees F)

132
Q

How to: fix dilelectric artifact

A

use a lower field strength (1.5T magnet), drain ascites

133
Q

Determinants of SAR

A

field strength, flip angle, TR, frequency of RF pulses; 2x field strength or flip angle => 4x SAR

134
Q

Increase in core body temperature should not exceed… (adults)

A

1 C

135
Q

Increase in core body temperature should not exceed… (infants)

A

0.5 C

136
Q

Sequence with highest SNR

A

proton density (but has poor tissue contrast)

137
Q

SAR (acronym)

A

specific absorption rate

138
Q

Short or long inversion times for fat and fluid

A

fat has a short TI and fluid has a long TI (based on their T1 properties)

139
Q

DWI pulse sequence

A

90 degree pulse => dephasing gradient => 180 degree pulse => rephasing gradient; EPI-based pulse sequence

140
Q

Difference between b-0 and b-1000 (DWI)

A

b-0 has no diffusion gradients; b-1000 has diffusion gradients; b-0 and b-1000 are used to compute the ADC

141
Q

Flow-related enhancement

A

a.k.a. TOF; GRE-based; fresh blood in plane is not saturated by short TRs => high signal

142
Q

Causes of signal loss on TOF

A

slow flow, turbulent flow, no flow, flow parallel to imaging plane

143
Q

Phase contrast MRA

A

bipolar gradients (positive and negative) applied between excitation and readout; stationary spins are cancelled out, while mobile spins only experience one of the gradients which generates signal; gray = stationary tissues

144
Q

Benefit of phase contrast MRA

A

quantitive measurement of velocity

145
Q

Majority of MRI-related adverse events

A

RF burns (bone screws, tattoos, EKG leads)

146
Q

Diffusion tensor imaging

A

quantifies extent to which water molecules are restricted in various directions; can infer path of white matter tracts

147
Q

2D Fourier transform

A

k-space => image; 3D images required a 3D Fourier transform

AJT Physics (7 decks)

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