Chapter 3 Flashcards
(230 cards)
1
Q
What is the purpose of pulse sequences in MRI?
A
Pulse sequences control how RF pulses and gradients are applied to determine image weighting and contrast.
2
Q
What causes dephasing of hydrogen nuclei in MRI?
A
Dephasing is caused by magnetic field inhomogeneities, leading to a rapid loss of coherent transverse magnetization and signal.
3
Q
Why is dephasing problematic in MRI?
A
Dephasing causes signal loss before most tissues can reach their T1 or T2 relaxation times, making it difficult to measure relaxation accurately.
4
Q
What is Free Induction Decay (FID)?
A
FID is the rapid decay of transverse magnetization following an RF pulse, typically within 10 ms.
5
Q
Why is FID too fast for measuring relaxation times?
A
FID decays within about 10 ms, which is too short to capture significant T1 or T2 relaxation data from tissues.
6
Q
What do pulse sequences do to counteract FID?
A
Pulse sequences rephase the magnetic moments of hydrogen nuclei to generate a measurable signal called an echo.
7
Q
Why is rephasing important in MRI?
A
Rephasing restores transverse magnetization at a later time, allowing differentiation of tissues based on their T1, T2, or proton density properties.
8
Q
What are the two main ways to rephase magnetic moments in MRI?
A
- Using a 180° RF rephasing pulse
- Using gradients
9
Q
What is a spin-echo pulse sequence?
A
A sequence that uses a 180° RF rephasing pulse to generate an echo.
10
Q
What is a gradient-echo pulse sequence?
A
A sequence that uses magnetic field gradients instead of a 180° RF pulse to generate an echo.
11
Q
What is the main difference between spin-echo and gradient-echo sequences?
A
- Spin-echo sequences use a 180° RF pulse for rephasing.
- Gradient-echo sequences use a gradient reversal to rephase spins.
12
Q
How do spin-echo sequences affect T2 weighting?
A
Spin-echo sequences provide true T2 weighting by eliminating field inhomogeneity effects with the 180° RF pulse.
13
Q
How do gradient-echo sequences affect T2* weighting?
A
Gradient-echo sequences create T2*-weighted images because they do not correct for magnetic field inhomogeneities.
14
Q
What three factors determine MRI image contrast?
A
- T1 recovery times
- T2 decay times
- Proton density (PD) differences
15
Q
How does T1 recovery affect image contrast?
A
Tissues with shorter T1 recovery times appear brighter, while tissues with longer T1 times appear darker.
16
Q
How does T2 decay affect image contrast?
A
Tissues with longer T2 decay times (e.g., fluid) appear brighter, while tissues with shorter T2 times (e.g., muscle) appear darker.
17
Q
How does proton density (PD) affect image contrast?
A
Tissues with higher proton density return a stronger signal and appear brighter, while tissues with lower PD appear darker.
18
Q
What is a pulse sequence?
A
A pulse sequence is a timed sequence of RF pulses and gradient applications used to generate different MRI image weightings.
19
Q
Why are pulse sequences necessary in MRI?
A
Pulse sequences rephase the magnetic moments of hydrogen nuclei, allowing them to generate a measurable signal (spin-echo or gradient-echo) for imaging.
20
Q
What happens to transverse magnetization without rephasing?
A
Without rephasing, magnetic field inhomogeneities cause rapid T2* dephasing, leading to a rapid loss of signal before significant relaxation can occur.
21
Q
What is a pulse sequence diagram?
A
A schematic representation of RF pulses, gradient applications, and signals over time, illustrating the sequence of MRI events.
22
Q
What do the five horizontal lines in a pulse sequence diagram represent?
A
- RF pulses
- Signal collection
- Slice selection gradient
- Phase encoding gradient
- Frequency encoding gradient
23
Q
How are gradients represented in pulse sequence diagrams?
A
- Above the line → Positive polarity gradient
- Below the line → Negative polarity gradient
- Amplitude deviation → Gradient strength
24
Q
How is a pulse sequence similar to a dance?
A
Just as dances involve coordinated arm and leg movements, pulse sequences involve coordinated RF pulses and gradient applications that determine image weighting.
25
What are the two main types of pulse sequences?
1. Spin-echo pulse sequences – use a 180° RF rephasing pulse.
2. Gradient-echo pulse sequences – use magnetic field gradients for rephasing.
26
What characterizes spin-echo pulse sequences?
Spin-echo sequences are defined by a 90° RF excitation pulse followed by a 180° RF rephasing pulse to refocus the signal.
27
What happens immediately after a 90° RF excitation pulse in a spin-echo sequence?
A free induction decay (FID) occurs due to T2* dephasing caused by magnetic field inhomogeneities.
28
What is the purpose of the 180° RF rephasing pulse?
The 180° RF pulse flips the magnetic moments, reversing their phase shifts and allowing them to rephase, forming a spin-echo.
29
How does T2* dephasing affect hydrogen nuclei?
Magnetic moments become out of phase due to field inhomogeneities, causing the signal to decay rapidly.
30
How does the 180° RF pulse correct T2* dephasing?
The leading edge of dephased spins becomes the trailing edge after the 180° RF flip, allowing faster spins to catch up and rephase.
31
At what point does full rephasing occur in a spin-echo sequence?
At TE (Echo Time), when all magnetic moments are back in phase, generating maximum signal.
32
What is T2* dephasing?
A rapid signal loss due to magnetic field inhomogeneities and spin-spin interactions.
33
How does a 180° RF pulse eliminate T2* dephasing?
It flips spins 180°, allowing faster spins to catch up and rephase, restoring signal coherence.
34
Does the 180° RF pulse affect T2 decay?
No, T2 decay continues because it is caused by random spin-spin interactions, which cannot be reversed.
35
What is TR (Repetition Time) in a spin-echo sequence?
TR is the time between successive 90° RF excitation pulses for each slice.
36
What is TE (Echo Time) in a spin-echo sequence?
TE is the time from the 90° RF excitation pulse to the peak of the spin-echo signal.
37
What is Tau in a spin-echo sequence?
Tau is the time taken to dephase after the 90° RF pulse, which equals the time required to rephase after the 180° RF pulse.
38
How is TE related to Tau?
TE = 2 × Tau (since rephasing takes the same amount of time as dephasing).
39
Why is time allowed between RF pulses in spin-echo sequences?
To allow tissues to reach their T1 and T2 relaxation times, affecting image contrast.
40
How do spin-echo and gradient-echo sequences differ in rephasing?
- Spin-echo uses a 180° RF rephasing pulse.
- Gradient-echo uses a gradient reversal for rephasing.
41
Which sequence provides true T2 weighting: spin-echo or gradient-echo?
Spin-echo provides true T2 weighting, as the 180° RF pulse compensates for field inhomogeneities.
42
Which sequence is more susceptible to magnetic field inhomogeneities: spin-echo or gradient-echo?
Gradient-echo, because it lacks a 180° RF rephasing pulse, making it sensitive to T2* effects.
43
Why is the spin-echo symmetrical?
As the magnetic moments of hydrogen nuclei come into phase, signal builds to a peak at TE. After TE, dephasing occurs again, leading to a gradual loss of signal, mirroring the initial signal growth.
44
What causes dephasing after the peak of the spin-echo?
Magnetic moments that are precessing rapidly overtake those that are precessing slowly, causing dephasing and signal loss.
45
What are the three main types of spin-echo pulse sequences?
1. Conventional spin-echo
2. Fast or turbo spin-echo (FSE/TSE)
3. Inversion recovery (including STIR and FLAIR)
46
What is the basic mechanism of a conventional spin-echo sequence?
A 90° RF excitation pulse is followed by one or more 180° RF rephasing pulses, which generate spin-echoes for imaging.
47
How do multiple spin-echoes form in a spin-echo sequence?
Each 180° RF rephasing pulse generates a separate spin-echo that contributes to the final image.
48
What determines image contrast in spin-echo sequences?
Image contrast is determined by:
- The spin-echo signal
- Negative polarity gradients applied for rephasing
- Spoiler gradients ensuring no residual transverse magnetization
49
How is a T1-weighted image generated using a spin-echo sequence?
A single-echo spin-echo sequence is used with:
- Short TR (300–700 ms)
- Short TE (10–30 ms)
50
Why is a short TR used for T1 weighting?
A short TR ensures that fat and water do not fully recover, making T1 differences the dominant contrast factor.
51
Why is a short TE used for T1 weighting?
A short TE minimizes T2 decay effects, reducing T2 contrast.
52
How is a proton density (PD) and T2-weighted image generated using a spin-echo sequence?
A dual-echo spin-echo sequence is used with:
- Long TR (2000+ ms)
- Short TE1 (~20 ms) for PD-weighting
- Long TE2 (~80 ms) for T2-weighting
53
Why is a long TR used in dual-echo spin-echo sequences?
A long TR minimizes T1 recovery differences, allowing PD and T2 contrast to dominate.
54
Why does the first spin-echo in a dual-echo sequence minimize T2 contrast?
The first spin-echo is collected at a short TE, reducing T2 decay effects and emphasizing proton density differences.
55
Why does the second spin-echo in a dual-echo sequence maximize T2 contrast?
The second spin-echo is collected at a long TE, allowing significant T2 decay and enhancing T2 contrast.
56
Why is the first spin-echo considered 'free' in dual-echo spin-echo sequences?
The first spin-echo is acquired without increasing scan time, as the system must wait for the second spin-echo to form.
57
Does omitting the first spin-echo reduce scan time?
No, because the system must still wait for the second spin-echo, so the first is collected for free.
58
How does this concept apply if multiple spin-echoes are acquired?
If four spin-echoes are acquired, the first three are considered free.
59
What are the advantages of spin-echo sequences?
- Good image quality
- Very versatile
- True T2 weighting
- Available on all systems
- Gold standard for image contrast and weighting
60
What is the main disadvantage of spin-echo sequences?
- Long scan times
61
What are the suggested parameters for a T1-weighted spin-echo sequence?
- TR: 300–700 ms
- TE: 10–30 ms
62
What are the suggested parameters for a dual-echo PD/T2-weighted spin-echo sequence?
- TR: 2000+ ms
- TE1 (PD): 20 ms
- TE2 (T2): 80 ms
63
How does TR affect image contrast in spin-echo sequences?
- Short TR → Increases T1 contrast
- Long TR → Minimizes T1 contrast, allowing PD and T2 contrast to dominate
64
How does TE affect image contrast in spin-echo sequences?
- Short TE → Minimizes T2 decay, reducing T2 contrast
- Long TE → Maximizes T2 decay, increasing T2 contrast
65
How does TE selection control the signal-to-noise ratio (SNR)?
A short TE improves SNR, while a long TE reduces SNR due to increased T2 decay.
66
What is the purpose of spoiler gradients in spin-echo sequences?
Spoiler gradients ensure no residual transverse magnetization remains before the next TR cycle begins.
67
How do negative polarity gradients contribute to rephasing?
Negative polarity gradients rephase hydrogen nuclei, contributing to stronger spin-echo formation.
68
What characterizes all spin-echo pulse sequences?
They include 90° RF excitation pulses and 180° RF rephasing pulses to refocus spins and generate spin-echoes.
69
What are the three main types of image weighting achievable with spin-echo?
1. T1-weighted
2. T2-weighted
3. Proton density (PD)-weighted
70
Why is conventional spin-echo considered the gold standard?
It provides true T2 weighting, excellent contrast, and is highly predictable across all MRI systems.
71
What is Fast or Turbo Spin-Echo (FSE/TSE)?
FSE/TSE is a modified spin-echo pulse sequence that significantly reduces scan time compared to conventional spin-echo by using multiple 180° RF rephasing pulses per TR.
72
What is another name for FSE/TSE?
FSE/TSE is also known as RARE (Rapid Acquisition with Relaxation Enhancement).
73
How does FSE/TSE reduce scan time?
By increasing the number of phase-encoding steps per TR, which allows more k-space lines to be filled per TR instead of one line per TR.
74
What are the main factors affecting scan time in MRI?
1. Repetition Time (TR) 2. Number of Signal Averages (NSA) 3. Phase Matrix 4. Echo Train Length (ETL) / Turbo Factor
75
How does k-space filling differ between conventional spin-echo and FSE/TSE?
Conventional spin-echo: One phase-encoding step per TR → one k-space line filled per TR. FSE/TSE: Multiple phase-encoding steps per TR → multiple k-space lines filled per TR.
76
What is the turbo factor (echo train length, ETL) in FSE/TSE?
The number of 180° RF pulses per TR, which determines the number of spin-echoes produced per TR and the number of k-space lines filled per TR.
77
How does the turbo factor affect scan time?
A higher turbo factor means more phase-encoding steps per TR, reducing the total scan time.
78
How does increasing the turbo factor impact image contrast?
Higher turbo factors shorten scan time, but increase mixture of different weightings in the final image.
79
How is k-space filling in FSE/TSE similar to a chest of drawers?
Conventional spin-echo: One drawer is opened per TR to store data. FSE/TSE: Multiple drawers are opened per TR to store more data at once, reducing scan time.
80
What is the 'effective TE' in TSE?
The selected TE that determines the primary image contrast, while other echoes contribute secondary contrast.
81
How does the system organize k-space data in TSE?
The system orders phase-encoding gradients so that spin-echoes occurring near the effective TE are placed in the central k-space lines, which determine contrast.
82
Where are echoes with the strongest signal placed in k-space?
Echoes with high signal amplitude are placed in the central k-space lines, affecting image contrast.
83
Where are echoes with weak signal placed in k-space?
Echoes with low signal amplitude are placed in the outer k-space lines, affecting image resolution.
84
How does contrast in TSE compare to conventional spin-echo?
TSE contrast is similar to spin-echo, but with two key differences: 1. Fat remains bright on T2-weighted images due to reduced spin-spin interactions (J coupling). 2. Muscle appears darker due to increased magnetization transfer effects.
85
How does FSE/TSE reduce susceptibility artifacts?
The repeated 180° RF pulses help compensate for magnetic field inhomogeneities, reducing artifacts near metal implants.
86
Why does fat remain bright on T2-weighted TSE images?
Repeated 180° RF pulses reduce spin-spin interactions (J coupling), preventing fat signal loss.
87
How can fat signal be suppressed in TSE?
Using fat saturation techniques to counteract J coupling effects.
88
What is a disadvantage of using TSE for detecting hemorrhage?
TSE reduces magnetic susceptibility effects, making it harder to detect small hemorrhages.
89
Why does image blurring occur in TSE?
Blurring occurs when long echo trains include weak later echoes, which contribute low-resolution data to k-space.
90
How can image blurring be reduced in TSE?
By reducing echo spacing and using a lower turbo factor.
91
Why do flow artifacts increase in TSE?
Multiple echoes are collected per TR, increasing sensitivity to motion and flow artifacts.
92
What are the advantages of TSE?
Shorter scan times, high-resolution imaging, true T2 weighting, reduced magnetic susceptibility artifacts.
93
What are the disadvantages of TSE?
Increased flow artifacts, potential image blurring, fat remains bright on T2-weighted images, some contrast interpretation issues.
94
What role does the turbo factor play in TSE parameter selection?
Higher turbo factor → Shorter scan time, but more mixed contrast. Lower turbo factor → More accurate weighting but longer scan time.
95
How does turbo factor selection affect T1, PD, and T2-weighting in TSE?
T1-weighting: Short TR, low turbo factor (2–8). PD-weighting: Long TR, moderate turbo factor. T2-weighting: Long TR, higher turbo factor.
96
What are the three strategies for acquiring PD- and T2-weighted images in TSE?
1. Full echo train – PD and T2 images acquired separately. 2. Split echo train – First half for PD, second half for T2. 3. Shared echo train – Early echoes for PD, later echoes for T2.
97
Why are the first echoes not 'free' in TSE?
Each echo fills separate k-space lines, meaning a dual-echo TSE scan requires twice the scan time.
98
What are typical scan parameters for T1-weighted TSE?
TR: 300–700 ms, Effective TE: Minimum, Turbo factor: 2–8.
99
What are the suggested parameters for Proton Density (PD) weighting in TSE?
TR: 3000–10,000 ms (depending on slice number)
Effective TE: Minimum
Turbo factor: 4–12
100
What are the suggested parameters for T2-weighting in TSE?
TR: 3000–10,000 ms (depending on slice number)
Effective TE: 80–140 ms
Turbo factor: 12–30
101
How does TR selection in TSE differ from conventional spin-echo?
TSE typically uses a longer TR because multiple 180° RF pulses take time to perform, allowing fewer slices per TR.
102
How does turbo factor selection in TSE affect scan time?
Long turbo factor → More k-space lines filled per TR, shorter scan time.
Short turbo factor → Fewer k-space lines filled per TR, longer scan time.
103
How does turbo factor selection in TSE affect image contrast?
Long turbo factor → Increases T2 contrast.
Short turbo factor → Retains T1 contrast.
104
How does echo spacing impact the number of slices per TR in TSE?
Long echo spacing → Fewer slices per TR.
Short echo spacing → More slices per TR.
105
Why are TRs longer in T2-weighted TSE scans compared to T1-weighted scans?
Long TRs allow enough time for the system to acquire all necessary echoes and fill k-space for multiple slices.
106
How does TSE fill k-space?
TSE applies the phase-encoding gradient multiple times per TR, filling multiple k-space lines per TR based on the turbo factor (ETL).
107
Why does TSE require phase reordering?
Phase reordering ensures that data from spin-echoes with effective TE are placed in the central k-space lines, controlling contrast.
108
What happens when the turbo factor is too high in T1- and PD-weighted TSE?
A long turbo factor introduces T2 contrast, making it difficult to maintain true T1 or PD weighting.
109
What is Single-Shot Turbo Spin-Echo (SS-TSE)?
SS-TSE is a faster version of TSE, acquiring all k-space lines at once using a partial Fourier technique.
110
How does SS-TSE reduce scan time?
SS-TSE acquires half the k-space lines in one TR and transposes the other half, reducing imaging time significantly.
111
What is Driven Equilibrium (DRIVE) in TSE?
A modification of TSE where a reverse flip angle RF pulse is applied at the end of the echo train to drive transverse magnetization back into the longitudinal plane.
112
Why is DRIVE useful?
DRIVE shortens T1 relaxation time, increasing signal intensity in fluid-based structures like CSF.
113
How do manufacturers modify DRIVE sequences?
Some manufacturers apply a 180° RF pulse before a 90° restoration pulse to rephase magnetization before restoring it.
114
How does the RF rephasing pulse angle impact TSE sequences?
Reducing the rephasing angle lowers SAR (Specific Absorption Rate) and allows for more slices per TR.
115
Why is SAR higher in TSE?
Because multiple RF rephasing pulses are applied in quick succession, leading to more energy absorption.
116
How can SAR be reduced in TSE?
By using rephasing angles less than 180° (e.g., 150° or 120°).
117
What is Inversion Recovery (IR)?
IR is a spin-echo pulse sequence that starts with a 180° RF inverting pulse to suppress certain tissue signals.
118
What is the purpose of the 180° RF inverting pulse in IR?
It inverts the NMV through 180°, allowing tissues to relax back at different rates to create contrast.
119
What is the Time from Inversion (TI) in IR?
TI is the time between the 180° inverting pulse and the 90° RF excitation pulse.
120
How does TI affect contrast in IR sequences?
Short TI → Heavy T1 contrast.
Long TI → More PD contrast.
121
What are the primary uses of IR sequences?
IR is used for heavily T1-weighted imaging, particularly for anatomical detail and contrast-enhanced imaging.
122
Why does IR produce stronger T1 contrast than conventional spin-echo?
Because tissues recover from full inversion rather than the transverse plane, creating a larger T1 contrast difference.
123
What are the parameters for T1-weighted IR imaging?
TI: 400–800 ms
TE: 10–20 ms
TR: 3000+ ms
124
What are the parameters for PD-weighted IR imaging?
TI: 1800 ms
TE: 10–20 ms
TR: 3000+ ms
125
What are the parameters for Pathology-weighted IR imaging?
TI: 400–800 ms
TE: 70+ ms
TR: 3000+ ms
126
How does Fast Inversion Recovery (FIR) reduce scan time?
By combining IR with TSE, allowing multiple k-space lines to be filled per TR.
127
Why is Fast IR mainly used for T2-weighted imaging?
Because it can suppress certain tissues while enhancing water and pathology signals.
128
What are the two main sequences in Fast IR?
1. STIR (Short Tau Inversion Recovery) → Fat suppression.
2. FLAIR (Fluid Attenuated Inversion Recovery) → CSF suppression.
129
What is the purpose of STIR?
STIR is an inversion recovery (IR) pulse sequence designed to null signal from fat by selecting a TI at the fat null point.
130
How does STIR null fat signal?
STIR uses a TI (Time from Inversion) that corresponds to 0.69 × T1 relaxation time of fat, ensuring no longitudinal magnetization remains when the 90° RF pulse is applied.
131
What is the typical TI value for STIR?
A TI of 100–175 ms is used to null fat signal, depending on the field strength.
132
What is the mathematical formula for calculating the TI required to null a tissue?
SI = 1 - 2e^{-t/T1}
## Footnote
Where: SI = signal intensity, t = TI (ms), T1 = T1 relaxation time of the tissue.
133
What are the suggested parameters for STIR?
- Short TI (tau): 150–175 ms (for fat suppression)
- Long TE: 50+ ms (to enhance pathology signal)
- Long TR: 4000+ ms (for full longitudinal recovery)
- Turbo Factor: 16–20 (to enhance pathology signal)
134
Why is STIR useful in musculoskeletal imaging?
Because it suppresses fatty bone marrow, making bone bruises, tumors, and other lesions more visible.
135
Why should STIR not be used with contrast enhancement?
Because contrast-enhanced tissues shorten T1 relaxation times, potentially causing them to be nulled along with fat.
136
What is the purpose of FLAIR?
FLAIR is an inversion recovery (IR) pulse sequence designed to null signal from CSF, making pathology adjacent to fluid spaces more visible.
137
How does FLAIR null CSF signal?
FLAIR uses a TI corresponding to the T1 relaxation time of CSF, ensuring no longitudinal magnetization remains when the 90° RF pulse is applied.
138
What is the typical TI value for FLAIR?
A TI of 1700–2200 ms is used to null CSF signal, depending on the field strength.
139
What are the suggested parameters for FLAIR?
- Long TI: 1700–2200 ms (for CSF suppression)
- Long TE: 70+ ms (to enhance pathology signal)
- Long TR: 6000+ ms (for full longitudinal recovery)
- Turbo Factor: 16–20 (to enhance pathology signal)
140
Why is FLAIR useful in brain and spine imaging?
Because it enhances periventricular and spinal cord lesions by removing the bright CSF signal.
141
Which conditions are best visualized with FLAIR?
- Multiple sclerosis plaques
- Acute subarachnoid hemorrhage
- Meningitis
- White matter abnormalities
142
How does modifying the TI in FLAIR help detect white matter lesions?
Selecting a TI of ~300 ms nulls normal white matter, making lesions appear brighter.
143
Why does gadolinium sometimes enhance pathology in FLAIR sequences?
Gadolinium shortens T1 relaxation times of enhancing tissues, making them brighter than expected on FLAIR.
144
Why does fat remain bright in T2-weighted FLAIR images?
The long echo trains used in FLAIR reduce fat signal loss due to J-coupling.
145
What is the purpose of Inversion Recovery (IR)?
IR is a spin-echo pulse sequence that uses a 180° RF inverting pulse to suppress certain tissues or enhance T1 contrast.
146
What is the function of the 180° RF inverting pulse in IR?
It inverts the NMV (Net Magnetization Vector) to the opposite longitudinal plane before allowing T1 recovery.
147
What parameter controls contrast in IR sequences?
The TI (Time from Inversion) determines which tissue signals are nulled or enhanced.
148
What are the suggested parameters for T1-weighted IR imaging?
- Medium TI: 400–800 ms
- Short TE: 10–20 ms
- Long TR: 3000+ ms
149
What are the suggested parameters for PD-weighted IR imaging?
- Long TI: 1800 ms
- Short TE: 10–20 ms
- Long TR: 3000+ ms
150
What are the suggested parameters for Pathology-weighted IR imaging?
- Medium TI: 400–800 ms
- Long TE: 70+ ms
- Long TR: 3000+ ms
151
What is a Double IR Prep Sequence?
A double 180° RF inversion pulse is applied: 1. Non-slice selective pulse inverts all spins. 2. Slice-selective pulse re-inverts spins within a slice.
## Footnote
This is used for black blood imaging in cardiac MRI.
152
What is a Triple IR Prep Sequence?
A third inverting pulse at ~150 ms is added to null both fat and blood, useful for detecting fatty infiltration of the heart walls.
153
What are the three main types of IR sequences?
1. STIR – Suppresses fat signal.
2. FLAIR – Suppresses CSF signal.
3. T1-weighted IR – Enhances T1 contrast.
154
What is the primary function of TI in IR sequences?
It controls the point at which longitudinal magnetization is nulled, determining tissue suppression.
155
What is the role of TE in IR sequences?
TE controls T2 contrast by allowing different levels of T2 decay.
156
What is the role of the Echo Train Length (ETL) in IR sequences?
ETL determines:
- How many k-space lines are filled per TR.
- How many 180° RF pulses are applied per TR.
- How many phase-encoding steps are performed per TR.
157
In FLAIR sequences, which tissue is being suppressed?
CSF
158
In STIR sequences, which tissue signal is being suppressed?
Fat
159
At the beginning of a spin echo sequence, a 90° excitation pulse is applied.
True
160
Which combination of TR and TE would create a T2-weighted image?
TE 90ms, TR 4000ms
161
A pulse sequence with a 90⁰ RF excitation pulse followed by a 180⁰ RF rephasing pulse is known as a(an):
Spin Echo
162
What is a possible disadvantage of longer ETL’s?
Image blurring
163
In the multi-echo spin-echo sequences shown, the number of SHORT TE images created with a 20-slice sequence will be:
20
164
Artifact from metal implants is significantly reduced when using TSE because:
180° RF rephasing pulses compensate for magnetic field inhomogeneity
165
The time from the application of the RF excitation pulse to the peak of the signal induced in the coil is termed the:
Echo time (TE)
166
The TI required to null the signal from tissue is always 0.69 times its T1 relaxation time.
True
167
The time between the ETL’s of the 180-degree rephasing pulse is?
Echo Spacing
168
Short tau inversion recovery images are:
T2-weighted, fat nulled images
169
In IR sequences, the resultant contrast depends primarily on the:
TI
170
IR pulse sequences are characterized by:
A 180⁰ inversion RF pulse followed by a 90⁰ excitation RF pulse
171
The definition of a pulse sequence is a series of RF pulses, gradient applications, and intervening time periods.
True
172
The time between a 180⁰ inversion pulse and the 90⁰ excitation pulse is known as:
Inversion time
173
The time from the application to one RF excitation pulse to the application of the next RF excitation pulse is called the:
Repetition time (TR)
174
What is a typical value for ETL on T1 weighted images?
2-8
175
The advantage of the driven equilibrium modification of TSE/FSE is that images, where there is a high signal in water, are permitted in short TRs.
True
176
When we say that the first echoes are “free” in conventional spin-echo sequences, what does this mean?
Free echoes do not cost additional scan time.
177
Which letter represents the slice select gradient?
b
178
Which letter represents the phase gradient?
c
179
Energy is most effectively transferred from one system to another when the systems are at:
Resonance
180
When utilizing ETL’s for T2 contrast; Fat signal will appear:
Bright
181
What is “DRIVE”?
A reverse 90-degree pulse
182
The 180⁰ RF pulse in a CSE is the rephasing pulse that creates the spin echo.
True
183
What is k-space?
Temporary storage place for data
184
What is a typical TI time for T2 FLAIR?
1700-2200 ms
185
STIR sequences can suppress the signal from all of the following EXCEPT:
Fluid (CSF)
186
FSE uses _________, to fill multiple lines of ___________ per TR.
ETL, k-space
187
Artifacts from metal implants is significantly reduced when using TSE because:
180-degree RF rephasing pulses compensate for magnetic field inhomogeneity
188
T2 weighted CSE images are characterized by:
Dark fat and bright water
189
What is a possible disadvantage of longer ETL's?
Image blurring
190
A good use for the T2 FLAIR sequence would be:
Evaluate the brain for multiple sclerosis
191
T1 weighted CSE images are characterized by:
Bright fat and dark water
192
The time between 180⁰ inversion pulse and the 90⁰ excitation pulse is known as:
Inversion time
193
In a fast spin echo sequence with an echo train of 12, how many RF rephasing pulses are applied for a given slice, during one TR period?
12
194
Which combination of CSE TE and TR create a T1-weighted image?
Short TE, short TR
195
In STIR sequences, which tissue is being suppressed?
Fat
196
Which of the following is the correct scan time for a spin echo sequence with the following parameters: TR = 2200ms, TE = 20, 90ms, matrix 256 x 256, 1 excitation, FOV = 230mm?
9 minutes and 23 seconds
197
TR, TE, TI, and flip angle determine what in a pulse sequence?
Image contrast weighting and pulse timing
198
What is the extrinsic contrast parameter unique to TSE/FSE?
Turbo factor or echo train length (ETL)
199
The time in-between the ETL’s of 180-degree rephasing pulses is?
Echo spacing
200
The purpose of a pulse sequence is to rephase the magnetic moments of hydrogen nuclei at a point in time when the signal from these nuclei can be read.
True
201
The more ETL, the more _____________ is needed.
TR
202
In the multi-echo spin-echo sequences shown in Figure C.1, the TOTAL images created with a 20-slice sequence will be:
40
203
In FLAIR sequences, which tissue signal is being suppressed?
CSF
204
What is effective TE?
The calculated TE value
205
What is k-space?
Temporary storage place for image data
206
When utilizing ETL’s for T2 contrast, FAT signal is:
Bright
207
What is a typical TI time for STIR?
150-175 ms
208
The following parameters are suitable in a T2 FLAIR sequence EXCEPT:
Short TE to enhance T2 weighting
209
Which letter is the frequency gradient?
d
210
The application of an RF pulse that causes resonance to occur is termed:
Excitation
211
The point in a tissue’s longitudinal recovery where there is no component of magnetization and therefore no signal created on an IR pulse sequence is called:
Null point
212
Which combination of CSE TE and TR create a T2-weighted image?
Long TE, long TR
213
A patient is suspected of having meningitis. What pulse sequence is extremely useful to evaluate for this?
T2 FLAIR
214
The consequences of increasing the turbo factor or ETL include:
True
215
What pulse sequence is useful when evaluating a patient’s brain for multiple sclerosis, meningitis, or subarachnoid hemorrhage?
T2 FLAIR pulse sequence
216
Spin echo sequences:
Have a 90-degree excitation pulse followed by a 180-degree rephasing pulse
217
How many RF pulses are in this pulse sequence?
1
218
T2 Fluid attenuated inversion recovery (FLAIR) sequences are typically used for the evaluation of:
Periventricular white matter disease
219
In the multi-echo spin-echo sequences shown in Figure C.1, the number of LONG TE images created with a 20-slice sequence will be:
20
220
A technique used to visualize the morphology of the heart and great vessels is:
Double IR
221
Which statement is NOT true about FSE pulse sequences?
Fat is darker than on SE pulse sequences
222
What is 'J' coupling?
Fat signal is bright on FSE/TSE
223
T2 weighted, Fluid attenuated inversion recovery (FLAIR) sequences are typically used for the evaluation of:
Periventricular white matter disease such as multiple sclerosis
224
STIR sequences should NOT be used after administering gadolinium because the T1 recovery times of enhancing structures are shortened by gadolinium so that they approach the T1 recovery time of fat. In a STIR sequence, therefore, enhancing tissue may also be nulled.
True
225
Spin echo pulse sequences:
Have a 90-degree RF excitation pulse followed by a 180-degree RF rephasing pulse
226
Which combination of CSE TR and TE creates a T1-weighted image?
TE 15ms, TR 400ms
227
Which combination of CSE TR and TE creates a T2-weighted image?
TE 90ms, TR 4000ms
228
What is a typical value for ETL on T2 weighted images?
12-30
229
What is a typical TI time for T1 weighted IR?
400-800 ms
230
A long ETL will have what effect on image quality?
Increased blurring
