PHYSICS - MRI Flashcards
(147 cards)
1
Q
Ferromagnetic substances
A
dramatic increase in local magnetic field, large increase in susceptibility; e.g. metal
2
Q
Paramagnetic substances
A
small increase in local magnetic field, small increase in susceptibility; e.g. deoxyhemoglobin, gadolinium
3
Q
Diamagnetic substances
A
small decrease in local magnetic field, small decease in susceptibility; e.g. tissues, calcium
4
Q
Requirement for an atom to have net magnetism
A
odd mass number (protons + neutrons)
5
Q
Net magnetization in the absence of an external magnetic field
A
no net magnetization; protons are randomly oriented
6
Q
Larmor frequency is proportional to…
A
magnetic field strength
7
Q
Net longitudinal magnetization (Mz) is proportional to…
A
magnetic field strength; parallel vs. antiparallel orientation
8
Q
Larmor frequency for H+ at 1 Tesla
A
42 MHz; which means the gyromagnetic ratio of H+ is 42 MHz/Tesla
9
Q
Transverse magnetization (Mxy) immediately after external magnetic field applied
A
none; phase is random
10
Q
Is the parallel or antiparallel orientation a lower energy state?
A
parallel orientation is a lower energy state (preferred)
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
12
Q
Flip angle definition
A
angle of net magnetization vector relative to the Z-axis
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
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
15
Q
Relationship between magnetic field strength and FID signal
A
directly proportional; increased field strength => increased FID signal
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
17
Q
Short T1 relaxation time - bright or dark
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bright; tissue recovers signal quickly
18
Q
Long T1 relaxation time - bright or dark
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dark; tissue recovers signal slowly
19
Q
Short T2 relaxation time - bright or dark
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dark; tissue loses signal quickly
20
Q
Long T2 relaxation time - bright or dark
A
bright; tissue loses signal slowly
21
Q
Causes of loss of phase coherence
A
spin-spin interactions and magnetic field inhomogeneities (may be external or local)
22
Q
T2 relaxation time increases or decreases with increased field strength?
A
neither; T2 relaxation is independent of magnetic field strength
23
Q
T1 or T2 relaxation times are longer? (generally)
A
T1 relaxation times are much longer
24
Q
Faraday’s Law of Induction
A
a moving magnetic field with induce a current within a coil
25
T1 time constant
time at which 63% of Mz has formed
26
T2 time constant
time at which Mxy has decayed to 37% of its original value
27
How many T1's for recovery of full net longitudinal magnetization?
4 T1's (~99%); same for T2 decay (4 T2's)
28
How does an increase in magnetic field strength by 4x affect T1 relaxation?
2x increases in T1 relaxation time
29
T2 signal when longitudinal magnetization has fully recovered
none; not possible to have T2 signal (transverse magnetization) when longitudal magnetization has fully recovered
30
T2* decay
dephasing due to spin-spin interactions and magnetic field inhomogenties (local or external)
31
Formula for T1 contribution to signal
1 - e^(-t/T1), where t is the TR and T1 is tissue specific
32
Formula for T2 contribution to signal
e^(-t/T2), where t is the TE and T2 is tissue specific
33
Short TR
<500 msec
34
Long TR
>2000 msec
35
Short TE
<30 msec
36
Long TE
>80 msec
37
TR and TE for proton density
long TR, short TE
38
Spin density sequence
a.k.a. proton density
39
Spin echo: 180 degree pulse timing
TE/2
40
T2 vs. T2* decay
T2 decay is the result of spin-spin interactions, while T2* decay is the result of spin-spin interactions + field inhomogeneities
41
Relationship between magnetic field strength and SNR
directly proportional; 2x field strength => 2x SNR (noise does not change with field strength)
42
Gradient echo pulse sequence
very short TR; <90 degree pulse => bipolar (dephasing and rephasing) gradients
43
How to: increase T1-weighting on GRE
increase flip angle
44
How to: increase T2-weighting on GRE
increase TE
45
Effect of gadolinium on T1 and T2 relaxation
gad increases T1 and T2 relaxation (shortening) => T1 bright, T2 dark
46
Standard dose of gadolinium
0.1 mmol/kg
47
Risk of gadolinium administration in CKD
nephrogenic systemic fibrosis (widespread tissue fibrosis)
48
Contraindications to gadolinium
pregnancy, GFR <30
49
Gadolinium agents with no known association with NSF
macrocyclic gadolinium agents
50
Timing of slice select gradient
applied during RF pulse (RF pulse determines which "slice" of tissue is excited); during ALL RF pulses, not just initial
51
How to: obtain thinner slices
decrease transmit bandwidth (of RF pulse), increase slice select gradient strength
52
Timing of frequency encoding gradient
applied at TE (during sampling)
53
Echo sampling rate
number of times each echo is sampled; corresponds to frequency encode matrix size
54
Determinants of matrix size
number of times each echo is sampled in the frequency encoding direction, number of phase encoding gradients in the phase encoding direction
55
Timing of phase encoding gradient
between RF pulse and echo; different phase encoding gradient is applied for each acquired echo
56
Center of k-space
center represents low spatial frequencies (large structures/"contrast")
57
Periphery of k-space
periphery represents high spatial frequencies (small features and edges/"details")
58
Gradient applied across the widest dimension (generally)
frequency encoding gradient
59
Fast spin echo (FSE)
fill multiple rows of k-space within a single TR (multiple TEs in each TR); longer TR required
60
Shimming
used to correct small inhomogeneities in the external magnetic field => improved uniformity
61
Transmit bandwidth
range of frequencies emitted in an RF pulse
62
Relationship between receiver bandwidth and noise
directly proportional; greater receiver bandwidth => increased noise
63
Echo train length (ETL)
number of echoes acquired in a single TR; FSE or turbo spin echo sequences
64
256 x 128 matrix - which is FE and which is PE direction?
longer dimension is typically the FE direction (so 256 in this example)
65
TOF in head
3-D TOF GRE
66
TOF in neck
2-D TOF GRE
67
Advantages of GRE
short TR (faster acquisition)
68
Disadvantages of GRE
lower signal (smaller flip angles), more noise; echoes formed by rephasing gradients are relatively weak
69
TI (in inversion recovery)
between 180 and 90 degree pulses; TR in IR sequences includes both TI and TE
70
Effect of increased magnetic field strength on TI
increased field strength => increased TI (T1 relaxation is longer)
71
Inversion recovery + gad
STIR is not used with gad; T1 shortening caused by gad creates a null point similar to fat
72
Dielectric artifact
a.k.a. standing wave artifact; RF pulse wavelength approximates dimension of patient; worse with higher field strengths
73
Dark signal in central abdomen over left lobe of liver
dielectric artifact
74
3-D MRI
2 phase-encoding gradients (y and z-axes); better Z-axis resolution, but longer study time (motion)
75
Echo planar imaging (EPI)
rapidly switching gradients results in numerous echoes generated within a single TR; fast, less motion, but increases susceptibility
76
BOLD (acronym)
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
Contrast in MRI is determined by...
tissue properties (T1, T2, T2*)
78
How to: increase SNR
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
Type 1 chemical shift artifact
differences in precessional frequencies at fat-water interfaces results in misregistration; seen in all sequences
80
How to: fix type 1 chemical shift artifact
increase receiver bandwidth, increase strength of frequency encoding gradient (steeper slope), or use fat sat
81
Effect of increased magnetic field strength on type 1 chemical shift artifact
increased type 1 chemical shift artifact
82
Type 2 chemical shift artifact
in-phase and OOP sequences detetermined by differences in precessional frequencies of fat and water protons; GRE only
83
Pulse sequence for in and out-of-phase images
GRE pulse sequence
84
India ink artifact
voxels at fat-organ interfaces contain portions of both fat and water resulting in signal dropout
85
Dixon W
sum of in and out-of-phase images; result is a fat-saturated T1 image
86
Truncation artifact
a.k.a. Gibbs or ringing; occurs at sharp edges; may mimic syrinx
87
How to: fix truncation artifact
increase matrix size (# of phase-encoding steps), use smoothing filter
88
Wrap around artifact
a.k.a. aliasing; caused by small FOV
89
How to: fix wrap around artifact
increase FOV, phase oversampling, switch PE and FE directions, apply saturation bands outside FOV
90
Magic angle artifact
occurs in tendons; 55 degree angle to Z-axis; disappears on T2
91
Fat saturation techniques for an inhomogeneous field
STIR or Dixon W
92
Fat saturation techniques for post-gad imaging
chemical fat sat (FSFS) or Dixon W
93
Artifact(s) occurring in frequency-encoding direction
type 1 chemical shift
94
Spike artifact
a.k.a. herringbone; electromagnetic spike during filling of k-space
95
How to: fix spike artifact
remove bad data point or re-scan patient
96
Requirements for MR scanning in pregnancy
must document necessity of information, cannot be achieved by ultrasound, and cannot wait until after delivery
97
SAR limits
relates to heating; 3 W/kg per 15 min for head and 4 W/kg per 15 min for body
98
Who sets SAR limits?
FDA
99
Effect of increasing receiver bandwidth
decreased SNR, decreased type 1 chemical shift, decreased TR/TE, decreased scan time
100
Difference between k-space matrix size and image matrix size
no difference in terms of size; both have the same dimensions
101
Effect of partial k-space acquisition
decreased SNR, decreased scan time
102
How to: decrease susceptibility
decrease TE, use SE instead of GRE, decrease field strength, metal suppression sequence (if from metal)
103
How to: prevent muscle twitching or paresthesias
tell patient to not cross legs or join hands; due to rapidly switching gradients (also create acoustic noise)
104
MR conditional
safe for specific MR environments (e.g. magnet strength, SAR)
105
MR zone 1
waiting area
106
MR zone 2
patient survey area
107
MR zone 3
control room; restricted access
108
MR zone 4
magnet room; restricted access
109
Zipper artifact
may appear as an irregular line across image or scattered dots
110
How to: fix zipper artifact
shut scanner room door, remove cell phone, check RF shielding of room
111
How to: fix incomplete fat suppression
shimming, use STIR instead (non-post gad only)
112
Signal of fat on FSE
increased T2 relaxation time of fat => fat is bright on T2 (J-coupling)
113
Relationship between coil size and SNR
increase coil size => more noise => decreased SNR
114
Five-gauss line
defines controlled access area around the MRI scanner; 0.5 mT
115
When to use half-dose of gad?
GFR 30-40
116
Advantages of FSE
shorter study, more time for larger FOV, less motion => higher resolution
117
Disadvantage of FSE
decreased contrast (less signal from each subsequent TE), worse T1 (because longer TR)
118
Chemical fat suppression
a.k.a. frequency-selective fat saturation; prepatory pulse => spoiler gradient => normal sequence; longer acquisition because of extra prepatory pulse
119
Susceptibility artifact based on sequence
EPI > GRE > SE > FSE
120
SAR based on sequence
FSE > SE > GRE; GRE has a small flip angle and single RF pulse
121
Determinants of spatial resolution
slice thickness, FOV, matrix size
122
BLADE or PROPELLER
redundant sampling of the center of k-space => less motion artifact, slower acquisition
123
Half-fourier acquisition
faster acquisition, lower SNR
124
Timings of in and out-of-phase sequences
TE 2.2 ms for out-of-phase, TE 4.4 ms for in-phase (at 1.5 T)
125
How to: decrease SAR
use fewer RF pulses, increase TR, smaller flip angles, lower field strength
126
Fastest MR sequence
EPI (single shot)
127
Readout gradient
a.k.a. frequency encoding gradient
128
Bright blood (cardiac MRI)
SSFP (GRE based); may obtain cine imaging
129
Dark blood (cardiac MRI)
SE based; "new" blood not excited by 90 degree pulse, thus generating no signal
130
Ghosting artifact
due to motion; may also be called smearing, pulsation, or motion artifact; occurs in phase-encode direction
131
Gadolinium should be administered at what temperature?
room temperature (72 degrees F)
132
How to: fix dilelectric artifact
use a lower field strength (1.5T magnet), drain ascites
133
Determinants of SAR
field strength, flip angle, TR, frequency of RF pulses; 2x field strength or flip angle => 4x SAR
134
Increase in core body temperature should not exceed... (adults)
1 C
135
Increase in core body temperature should not exceed... (infants)
0.5 C
136
Sequence with highest SNR
proton density (but has poor tissue contrast)
137
SAR (acronym)
specific absorption rate
138
Short or long inversion times for fat and fluid
fat has a short TI and fluid has a long TI (based on their T1 properties)
139
DWI pulse sequence
90 degree pulse => dephasing gradient => 180 degree pulse => rephasing gradient; EPI-based pulse sequence
140
Difference between b-0 and b-1000 (DWI)
b-0 has no diffusion gradients; b-1000 has diffusion gradients; b-0 and b-1000 are used to compute the ADC
141
Flow-related enhancement
a.k.a. TOF; GRE-based; fresh blood in plane is not saturated by short TRs => high signal
142
Causes of signal loss on TOF
slow flow, turbulent flow, no flow, flow parallel to imaging plane
143
Phase contrast MRA
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
Benefit of phase contrast MRA
quantitive measurement of velocity
145
Majority of MRI-related adverse events
RF burns (bone screws, tattoos, EKG leads)
146
Diffusion tensor imaging
quantifies extent to which water molecules are restricted in various directions; can infer path of white matter tracts
147
2D Fourier transform
k-space => image; 3D images required a 3D Fourier transform
