Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

MRI Physics > chapter 4 > Flashcards

chapter 4 Flashcards

1
Q

How do gradient echo sequences differ from spin echo sequences?

A

Use gradients rather than RF pulses to rephase magnetic moments.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What effect does using a smaller flip angle have on the NMV?

A

Increases longitudinal magnetization recovery.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Which characteristic defines gradient echo (GRE) pulse sequences?

A

Gradient rephasing and variable flip angles.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What causes spins in the vector to rephase in a GRE sequence?

A

Application of a gradient.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is the function of a spoiler in MRI?

A

To dephase residual transverse magnetization.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What parameters are used for T2 weighting in GRE sequences?

A

Small flip angle, long TR, long TE.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Which GRE sequence maintains residual transverse magnetization through rewinding?

A

Coherent GRE.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the purpose of incoherent GRE sequences?

A

Minimize the effects of residual transverse magnetization.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

How does reverse-echo gradient echo differ from traditional GRE sequences?

A

Captures true T2-weighted images.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the main advantage of balanced GRE sequences?

A

Shorter scan times with fewer flow artifacts.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Gradient echo sequences use variable flip angles instead of the 90° flip angle used in spin echo sequences.

A

True

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Residual transverse magnetization is eliminated in coherent GRE sequences.

A

False

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Rewinder gradients are used to rephase transverse magnetization in GRE sequences.

A

True

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Echo planar imaging (EPI) is primarily used to reduce physiological motion artifacts.

A

True

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Spoilers are gradients that rephase the spins in a vector.

A

False

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Reverse-echo gradient echo sequences capture true T2 contrast data.

A

True

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Balanced gradient echo sequences are only used in cardiac imaging.

A

False

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Incoherent GRE sequences use RF spoiling to dephase residual transverse magnetization.

A

True

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Match the terms in Column A with their descriptions in Column B: 1️⃣ Steady State → Condition where energy in equals energy out, maintaining stable magnetization levels.

A

Condition where energy in equals energy out, maintaining stable magnetization levels.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Match the terms in Column A with their descriptions in Column B: 2️⃣ Residual Transverse Magnetization → Magnetization left over from previous RF pulses in steady-state conditions.

A

Magnetization left over from previous RF pulses in steady-state conditions.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Match the terms in Column A with their descriptions in Column B: 3️⃣ Coherent GRE → Maintains residual magnetization coherence through rewinding.

A

Maintains residual magnetization coherence through rewinding.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Match the terms in Column A with their descriptions in Column B: 4️⃣ Incoherent GRE → Dephases residual transverse magnetization to minimize its effect on contrast.

A

Dephases residual transverse magnetization to minimize its effect on contrast.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Match the terms in Column A with their descriptions in Column B: 5️⃣ Reverse-Echo Gradient Echo → Captures true T2 contrast using rewinder gradients.

A

Captures true T2 contrast using rewinder gradients.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Match the terms in Column A with their descriptions in Column B: 6️⃣ Balanced GRE → Reduces flow artifacts using balanced gradient systems.

A

Reduces flow artifacts using balanced gradient systems.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

What are the most common flip angles in a GRE pulse sequence?

A

Less than 90 degrees.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

What can be used to dephase spins in a GRE pulse sequence?

A

A gradient spoiler.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

Why is a variable, less than 90-degree, flip angle used?

A

All of the above (to decrease imaging time, utilize shorter TRs, and permit shorter recovery times).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

What is letter C in the diagram?

A

Dephasing gradient.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

What is the safety issue associated with GR-EPI?

A

Peripheral nerve stimulation.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

What are the disadvantages of GRE pulse sequences?

A

All of the above (reduced SNR, loud gradient noise, magnetic susceptibility).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

When precessing nuclei are exposed to an increased magnetic field, they:

A

Speed up.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

What is an advantage of using gradient echo sequences?

A

Faster scan times.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

Arthrographic means:

A

Visualize joint spaces.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

What type of pulse sequence is shown in the image?

A

GR-EPI.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

Which letter is the FID?

A

F.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
36
Q

The biggest factor in a sequence scan time is:

A

TR.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
37
Q

What is letter E in the diagram?

A

Time to echo.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
38
Q

Which statement is NOT true about FSE T2 pulse sequences?

A

Fat is darker than on SE pulse sequences.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
39
Q

GRE T2*-weighted sequences use:

A

A gradient rewinder.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
40
Q

The steady state is equal amounts of longitudinal and transverse magnetization.

A

True.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
41
Q

What is the conventional scan time formula?

A

TR × Phase Matrix (PM) × NEX.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
42
Q

Imaging parameters in GRE T1-weighted sequences should have:

A

Large flip angle, short TR, short TE.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
43
Q

Gradient echo sequences differ from spin echo sequences by:

A

A & B (use variable RF excitation pulse flip angles, use gradients rather than RF pulses to rephase).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
44
Q

What does ‘bi-polar’ mean in MRI?

A

It consists of two lobes, one negative and one positive.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
45
Q

Uses for incoherent GRE sequences include:

A

T1-weighted imaging.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
46
Q

Which of the following best describes a GR-EPI sequence?

A

A ‘series’ of gradient echoes.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
47
Q

How does using different (variable) flip angles in GRE pulse sequences affect NMV?

A

NMV achieves full relaxation in a much shorter TR than in spin echo sequences.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
48
Q

TR, TE, and flip angle are what type of contrast parameters?

A

Extrinsic.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
49
Q

What type of image would be yielded in the given scenario?

A

T2*.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
50
Q

Although GRE is faster than RF rephasing, what is NOT compensated for in this type of sequence?

A

Inhomogeneities.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
51
Q

Large 70°+ flip angles are used for:

A

T1-weighted imaging.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
52
Q

NMV saturation occurs from what range?

A

91 to 180 degrees.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
53
Q

What weighted image would be created with a GRE pulse sequence using TR 50, TE 5, and FA 70?

A

T1-weighted.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
54
Q

Dynamic enhanced T1-weighted MRA sequences of the renal arteries use:

A

Incoherent gradient echoes.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
55
Q

What is letter B in the diagram?

A

Variable-degree excitation pulse.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
56
Q

What is the Ernst angle?

A

The flip angle that provides optimum signal intensity for a tissue with a given T1 recovery time scanned using a given TR.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
57
Q

Angiographic means:

A

Visualize blood vessels and flow.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
58
Q

What weighted image would be created with a GRE pulse sequence using TR 100, TE 20, and FA 20?

A

T2*-weighted.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
59
Q

T2* decay is due to:

A

Inhomogeneities in the magnetic field.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
60
Q

When precessing nuclei are exposed to a decreased magnetic field, they:

A

Slow down.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
61
Q

Coherent GRE sequences are typically:

A

T2-weighted.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
62
Q

What does EPI stand for?

A

Echo planar imaging.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
63
Q

Gradient echo sequences can yield T1, PD, or T2* contrast.

A

True.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
64
Q

The pulse sequence consisting of a variable flip angle RF excitation pulse followed by a pulse of the magnetic field gradients is called:

A

Gradient echo.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
65
Q

An inversion recovery sequence starts with what RF pulse?

A

180-degree pulse.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
66
Q

TEs in a GRE sequence are typically __________ than CSE.

A

Shorter.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
67
Q

What is letter D in the diagram?

A

Rephasing gradient.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
68
Q

T2* is used to describe spin-spin dephasing.

A

False.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
69
Q

To perform angiographic, myelographic, and arthrographic techniques, what type of GRE is typically used?

A

T2*-weighted.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
70
Q

In a balanced GRE acquisition, the contrast weighting is:

A

Weighted for the ratio of T2/T1.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
71
Q

What is a steady state in MRI?

A

A condition where energy in equals energy out.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
72
Q

Small 5°–20° flip angles are used for:

A

B & C (T2* and PD-weighted).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
73
Q

What is isocenter in an MRI?

A

The exact middle of the magnet.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
74
Q

Gradient echo sequences acquired for the evaluation of hemorrhagic lesions rely on:

A

Susceptibility.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
75
Q

GRE pulse sequences are ____________ than CSE.

A

Louder.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
76
Q

What is EPI used for?

A

Single shot or multi-shot acquisition that fills k-space with data from gradient echoes.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
77
Q

A GRE sequence in which any residual transverse magnetization is removed before the next excitation pulse is known as:

A

Incoherent/spoiled GRE.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
78
Q

What is the opposite of gradient rewinding?

A

Spoiling.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
79
Q

When a GRE sequence with T1 weighting is acquired for dynamic contrast-enhanced imaging of the liver, _____________ is performed.

A

Spoiling.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
80
Q

GRE sequences use ________________ to dephase and rephase spins.

A

Bi-polar gradients.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
81
Q

To maximize T1 weighting in a GRE sequence, the ___ should be ___.

A

TR, short.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
82
Q

Define residual transverse magnetization:

A

Transverse magnetization left over from previous RF pulse in steady state conditions.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
83
Q

Gradient spoilers:

A

Dephase magnetization.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
84
Q

Gradient rewinders:

A

Rephase magnetization.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
85
Q

What is a rapid acquisition technique that starts with RF pulses followed by a series of gradient echoes?

A

Echo planar imaging (EPI).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
86
Q

GRE T1-weighted sequences use:

A

Gradient or RF spoiling.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
87
Q

Incoherent GRE sequences are:

A

T1-weighted.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
88
Q

What does GRASE stand for?

A

Gradient and spin echo pulse sequence.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
89
Q

What determines how long we wait at each slice for an echo?

A

TE.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
90
Q

In the diagram, identify letter ‘B’:

A

Rewinder.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
91
Q

To minimize differences in T2* decay times, the ____________ is short so that neither fat nor water has time to decay.

A

TE.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
92
Q

Any two 90° RF pulses with varying amplitude are called:

A

Hahn echo.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
93
Q

The gradients in the MRI system are used to:

A

Alter the main magnetic field.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
94
Q

Myelographic means:

A

Spinal cord and nerve visualization.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
95
Q

What type of image would this scenario yield?

A

T2*.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
96
Q

GRE sequences acquired for high signal from fluid are known as all of the following EXCEPT:

A

Incoherent gradient echoes.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
97
Q

What type of contrast weighting does a balanced GRE sequence provide?

A

T2/T1 ratio contrast.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
98
Q

What is the primary safety concern associated with GR-EPI?

A

Peripheral nerve stimulation.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
99
Q

Why is a variable, less than 90-degree flip angle used in GRE?

A

To permit shorter recovery times, decrease imaging time, and utilize shorter TRs.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
100
Q

How do gradient-echo pulse sequences differ from spin-echo pulse sequences?

A
  1. They use variable RF excitation pulse flip angles instead of the fixed 90° RF flip angles used in spin-echo sequences.
  2. They use gradients instead of RF pulses to rephase hydrogen nuclei and form an echo.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
101
Q

What is the primary purpose of the two main differences in gradient-echo pulse sequences?

A

To enable shorter TRs and therefore faster scan times compared to spin-echo sequences.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
102
Q

What is the role of variable flip angles in gradient-echo sequences?

A

A variable RF flip angle (typically less than 90°) is used to:
- Reduce NMV movement during excitation.
- Shorten T1 recovery time, allowing for shorter TR and faster scans.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
103
Q

What is the flip angle commonly used in gradient-echo pulse sequences?

A

A flip angle of less than 90° is typically used in gradient-echo sequences.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
104
Q

How does a smaller flip angle impact relaxation in gradient-echo sequences?

A

A smaller flip angle means faster T1 recovery, which allows for shorter TRs and quicker scan times.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
105
Q

Why does a gradient-echo sequence result in shorter scan times than a spin-echo sequence?

A
  • Uses smaller flip angles, reducing the time needed for T1 recovery.
  • Uses gradients instead of 180° RF pulses, enabling shorter TR values.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
106
Q

What causes the immediate occurrence of Free Induction Decay (FID) after an RF excitation pulse is withdrawn?

A
  • Magnetic field inhomogeneities
  • T2* decay
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
107
Q

How is transverse magnetization rephased in spin-echo sequences?

A

A 180° RF pulse is applied to rephase the transverse magnetization, creating a spin echo.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
108
Q

Why can’t RF pulses be used to rephase transverse magnetization in gradient-echo sequences?

A
  • The low flip angles leave a large component of magnetization in the longitudinal plane.
  • A 180° RF pulse would invert magnetization rather than rephase it.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
109
Q

How is transverse magnetization rephased in gradient-echo pulse sequences?

A

A gradient field is applied instead of an RF pulse to rephase the transverse magnetization.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
110
Q

What role do gradients play in MRI?

A

Gradients:
1. Rephase or dephase hydrogen nuclei.
2. Create slice selection, phase encoding, and frequency encoding.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
111
Q

What happens when a gradient is applied to coherent magnetization?

A
  • It alters the magnetic field strength experienced by different hydrogen nuclei.
  • Some nuclei speed up, while others slow down, causing dephasing.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
112
Q

What is the effect of applying a gradient on a coherent magnetization?

A
  • The magnetic moments fan out due to frequency changes caused by the gradient.
  • This leads to dephasing of the signal.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
113
Q

Why do gradient-echo sequences require rephasing gradients?

A

Since RF pulses cannot rephase transverse magnetization, a gradient is used instead.

114
Q

What does the table of gradient-echo acronyms provide?

A

A comparison of gradient-echo sequence names across different MRI manufacturers.

115
Q

What does the acronym GRASS stand for?

A

Gradient Recalled Acquisition in the Steady State (GRASS).

116
Q

What does the acronym FLASH stand for?

A

Fast Low-Angled Shot (FLASH).

117
Q

What is the main advantage of gradient-echo sequences compared to spin-echo sequences?

A

Faster scan times due to shorter TR values.

118
Q

Why does FID occur immediately after an RF excitation pulse is withdrawn?

A

Due to inhomogeneities in the magnetic field and T2* decay.

119
Q

What is steady-state free precession (SSFP)?

A

A type of gradient-echo sequence where magnetization is maintained using rephasing gradients.

120
Q

How does a gradient-echo pulse sequence achieve image contrast?

A

By using variable flip angles and gradient rephasing.

121
Q

Why does a spin-echo sequence use a 180° RF pulse?

A

To rephase transverse magnetization and form a spin echo.

122
Q

What happens to the magnetization in gradient-echo sequences when the RF excitation pulse is turned off?

A

Most of the magnetization remains in the longitudinal plane rather than flipping to transverse.

123
Q

What is the effect of the trailing edge of the fan in gradient-echo sequences?

A

The trailing edge (purple) consists of nuclei that slow down because they are in a lower magnetic field strength relative to the isocenter.

124
Q

What is the effect of the leading edge of the fan in gradient-echo sequences?

A

The leading edge (red) consists of nuclei that speed up because they are in a higher magnetic field strength relative to the isocenter.

125
Q

What happens to the magnetic moments of nuclei when gradients are applied?

A

They are no longer in the same place at the same time, causing dephasing of magnetization.

126
Q

What is the term for gradients that cause dephasing of magnetic moments?

A

They are called spoilers, and the process is called gradient spoiling.

127
Q

How do gradients rephase incoherent magnetization?

A
  • Slow precessing nuclei in the trailing edge (purple) experience increased field strength and speed up.
  • Fast precessing nuclei in the leading edge (red) experience decreased field strength and slow down.
  • Over time, they rephase, creating a gradient-echo.
128
Q

What are gradients that rephase called?

A

They are called rewinders.

129
Q

What determines whether a gradient field adds or subtracts from the main magnetic field?

A

The direction of current passing through the gradient coils, also called gradient polarity.

130
Q

What type of gradient is used to create a gradient-echo?

A

A bipolar gradient, which consists of two lobes (one negative, one positive).

131
Q

How does a bipolar gradient create a gradient-echo?

A
  1. The negative lobe increases dephasing and eliminates the FID.
  2. The positive lobe rephases only those magnetic moments that were dephased by the negative lobe.
132
Q

Why are gradient-echo sequences faster than spin-echo sequences?

A

Gradients rephase faster than RF pulses, generating echoes more quickly, which shortens TE and TR.

133
Q

What is a disadvantage of gradient-echo sequences compared to spin-echo sequences?

A

They do not compensate for magnetic field inhomogeneities, leading to artifacts like magnetic susceptibility.

134
Q

Why is T2 contrast in gradient-echo sequences called T2* contrast?

A

Because magnetic field inhomogeneities are not corrected, causing extra dephasing beyond normal T2 decay.

135
Q

What are the three main mechanisms that affect weighting in gradient-echo sequences?

A
  1. Extrinsic parameters (TR, TE, flip angle)
  2. Steady state
  3. Residual transverse magnetization
136
Q

How does TR affect T1 contrast in gradient-echo sequences?

A

A short TR prevents full longitudinal magnetization recovery, increasing T1 contrast.

137
Q

How does TE affect T2* contrast in gradient-echo sequences?

A

A long TE allows for more T2* dephasing, increasing T2* contrast.

138
Q

How does flip angle affect T1 contrast in gradient-echo sequences?

A

A large flip angle increases T1 contrast by reducing longitudinal recovery before the next RF pulse.

139
Q

What combination of parameters maximizes T1 contrast in gradient-echo sequences?

A
  • Short TR
  • Large flip angle
  • Short TE
140
Q

What combination of parameters maximizes T2* contrast in gradient-echo sequences?

A
  • Long TE
  • Small flip angle
  • Long TR
141
Q

What is the main advantage of using a short TR in gradient-echo imaging?

A

Allows for faster scanning and the acquisition of more slices in a shorter time.

142
Q

What are the key takeaways about gradient-echo pulse sequences?

A
  • Use gradients to rephase instead of RF pulses.
  • Use flip angles <90° to allow shorter TR.
  • Shorter TR and TE enable faster scanning.
  • More susceptible to field inhomogeneities, leading to T2* contrast and magnetic susceptibility artifacts.
143
Q

What strategies allow for shorter TR and TE in gradient-echo sequences?

A
  1. Gradient rephasing instead of RF rephasing.
  2. Low flip angles, reducing relaxation time.
144
Q

Why does gradient rephasing allow for shorter TE?

A

Because gradients rephase magnetic moments faster than RF pulses, reducing wait time for the echo.

145
Q

What is the impact of using a short TR in gradient-echo imaging?

A
  • Faster scanning
  • More slices per TR
  • Increased T1 contrast
146
Q

Why are gradient-echo sequences prone to magnetic susceptibility artifacts?

A

Because inhomogeneities are not corrected, leading to extra T2* dephasing.

147
Q

What conditions maximize T2* contrast in gradient-echo sequences?

A
  • Long TE (to allow dephasing of fat and water)
  • Small flip angle
  • Long TR (to allow full recovery of fat and water before the next RF pulse)
148
Q

Why is T2* weighting in gradient-echo different from T2 weighting in spin-echo?

A

Because gradient rephasing does not compensate for magnetic field inhomogeneities, leading to T2* decay rather than pure T2 decay.

149
Q

How do you obtain a PD-weighted gradient-echo image?

A
  • Short TE (to minimize T2* contrast)
  • Small flip angle
  • Long TR (to allow full recovery of longitudinal magnetization)
150
Q

How do you ‘turn up the heat’ on T1 contrast?

A
  • Short TR (low TR knob)
  • Large flip angle (high flip angle knob)
  • Short TE (low TE knob)
151
Q

How do you ‘turn up the heat’ on T2* contrast?

A
  • Long TE (high TE knob)
  • Long TR (high TR knob)
  • Small flip angle (low flip angle knob)
152
Q

How do you ‘turn down the heat’ on both T1 and T2* contrast for PD-weighting?

A
  • Long TR (high TR knob)
  • Small flip angle (low flip angle knob)
  • Short TE (low TE knob)
153
Q

How do TR, TE, and flip angles compare between spin-echo and gradient-echo sequences?

A

Sequence | TR | TE | Flip Angle |
|————-|——–|——–|—————-|
| Spin-Echo (SE) | Long (2000+ ms) | Long (70+ ms) | 90° |
| Spin-Echo (SE) | Short (300–700 ms) | Short (10–30 ms) | 90° |
| Gradient-Echo (GRE) | Long (100+ ms) | Long (15–25 ms) | Small (5°–20°) |
| Gradient-Echo (GRE) | Short (<50 ms) | Short (<5 ms) | Medium (30°–45°) |
| Gradient-Echo (GRE) | Short (<50 ms) | Short (<5 ms) | Large (70°+)

154
Q

What extrinsic parameters control contrast in gradient-echo pulse sequences?

A
  • TR & Flip Angle → Control whether NMV is saturated (T1 contrast)
  • TE → Controls T2* weighting
155
Q

How do TR, TE, and flip angle influence T1-weighted gradient-echo sequences?

A
  • Large flip angle
  • Short TR (to ensure saturation)
  • Short TE (to minimize T2* contrast)
156
Q

How do TR, TE, and flip angle influence T2*-weighted gradient-echo sequences?

A
  • Small flip angle
  • Long TR (to prevent saturation)
  • Long TE (to maximize T2* contrast)
157
Q

How do TR, TE, and flip angle influence PD-weighted gradient-echo sequences?

A
  • Small flip angle
  • Long TR (to prevent saturation)
  • Short TE (to minimize T2* contrast)
158
Q

What is the steady state in MRI?

A

A stable condition where the energy in = energy out, meaning longitudinal and transverse magnetization remain constant over time.

159
Q

How does the steady state occur in MRI?

A
  • RF pulses continuously excite hydrogen nuclei.
  • Energy is lost through spin-lattice relaxation (T1).
  • If TR is short enough, residual transverse magnetization builds up, forming the steady state.
160
Q

How does the steady state affect image contrast?

A

Contrast depends on the T1/T2 ratio instead of absolute T1 or T2 values.

161
Q

What tissues return high signal intensity in the steady state?

A
  • Fat (short T1 & T2)
  • Water (long T1 & T2)
162
Q

What tissues return low signal intensity in the steady state?

A
  • Muscle (short T2, long T1)
163
Q

What is residual transverse magnetization?

A

Magnetization leftover from previous RF pulses, remaining in the transverse plane over multiple TRs.

164
Q

How does residual transverse magnetization impact contrast?

A

It enhances T2 contrast because long T2 tissues (e.g., water) retain more transverse magnetization.

165
Q

What happens to residual transverse magnetization in gradient-echo sequences?

A

It is rephased by subsequent RF pulses, producing stimulated echoes.

166
Q

What are stimulated echoes?

A

Echoes generated by RF pulses with varying flip angles, rather than the standard 90° and 180° pulses of spin-echo sequences.

167
Q

What are Hahn echoes?

A

Echoes produced by two 90° RF pulses, named after Erwin Hahn.

168
Q

What types of echoes are formed in steady-state gradient-echo sequences?

A
  1. FID (Free Induction Decay)
  2. Stimulated Echoes (Hahn echoes)
169
Q

What is the Ernst angle?

A

The flip angle that provides maximum signal intensity for a tissue with a given T1 relaxation time and TR.

170
Q

How do you calculate the Ernst angle?

A

[\text{Ernst Angle} = \cos^{-1} (e^{-TR/T1})]
Where:
- TR = Repetition time
- T1 = T1 relaxation time of the tissue

171
Q

What is the steady state in MRI?

A

The steady state occurs when the TR is shorter than the T1 and T2 relaxation times, causing residual transverse magnetization to build up over time.

172
Q

What are stimulated echoes in the steady state?

A

Stimulated echoes are echoes produced by residual transverse magnetization, rephased by subsequent RF pulses. These echoes primarily contain T2*/T2 information.

173
Q

What determines contrast in steady-state gradient-echo sequences?

A

Contrast depends on whether the FID, the stimulated echo, or both are used to create the gradient-echo.
- Stimulated echo → T2*/T2 contrast (water = hyperintense)
- FID → T1 and PD contrast (water = hypointense)

174
Q

What are the five types of gradient-echo pulse sequences?

A
  1. Coherent (Rewound) Gradient-Echo
  2. Incoherent (Spoiled) Gradient-Echo
  3. Reverse-Echo Gradient-Echo
  4. Balanced Gradient-Echo
  5. Fast Gradient-Echo
175
Q

What is the mechanism of coherent gradient-echo sequences?

A
  • Uses a variable flip angle RF excitation pulse
  • Uses gradient rephasing to form an echo
  • Maintains steady state by using short TRs
  • Rewinding (phase-encoding gradient reversal) maintains coherence of residual transverse magnetization
176
Q

What contrast does coherent gradient-echo provide?

A

Since both FID and stimulated echoes contribute to the signal, coherent gradient-echo can produce T1, PD, or T2* weighting, but is traditionally used for T2* weighting.

177
Q

What are common uses of coherent gradient-echo sequences?

A
  • Angiographic, myelographic, or arthrographic imaging (because water is hyperintense)
  • Assessing fluid-filled structures and blood flow
  • Single breath-hold acquisitions
178
Q

What are the advantages and disadvantages of coherent gradient-echo?

A

Advantages | Disadvantages |
|—————|——————|
| Very fast scans | Reduced SNR in 2D acquisitions |
| Sensitive to flow (useful for angiography) | Increased magnetic susceptibility artifacts |
| Can be acquired as a 3D volume | Loud gradient noise |

179
Q

What are the suggested parameters for coherent gradient-echo?

A
  • Flip angle: 30°–45°
  • TR: 20–50 ms
  • TE (for T2* weighting): 10–15 ms
  • Gradient moment rephasing to reduce flow artifacts
180
Q

What are some alternative TR, TE, and flip angle settings for coherent gradient-echo?

A
  • T1-weighting: TR = 400 ms / TE = 5 ms / Flip Angle = 90°
  • PD-weighting: TR = 400 ms / TE = 5 ms / Flip Angle = 20°
  • T2*-weighting: TR = 400 ms / TE = 15 ms / Flip Angle = 20°
181
Q

What is the mechanism of incoherent (spoiled) gradient-echo sequences?

A
  • Uses a variable flip angle RF excitation pulse
  • Uses gradient rephasing to form an echo
  • Steady state is maintained, but residual transverse magnetization is eliminated (spoiled)
182
Q

What is the main goal of spoiling in gradient-echo sequences?

A

To dephase residual transverse magnetization, ensuring that only newly created FID signals contribute to image contrast.

183
Q

What are the two methods of spoiling in incoherent gradient-echo?

A
  1. RF Spoiling
    • Alters the phase angle of each RF pulse to eliminate residual transverse magnetization
    • Uses a phase-locked circuit to differentiate newly created transverse magnetization
  2. Gradient Spoiling
    • Uses slice-select, phase-encoding, and frequency-encoding gradients to dephase residual transverse magnetization
184
Q

How does RF spoiling work in incoherent gradient-echo?

A
  • The phase angle of RF pulses changes every TR
  • The receiver coil detects only newly created transverse magnetization, ignoring residual transverse magnetization
185
Q

What contrast does incoherent (spoiled) gradient-echo provide?

A

Since only the FID is sampled, it is mainly used for T1-weighted and proton density (PD) images.

186
Q

How do coherent and incoherent gradient-echo sequences differ?

A

Feature | Coherent (Rewound) GRE | Incoherent (Spoiled) GRE |
|————|————————–|————————–|
| Residual Transverse Magnetization | Maintained | Eliminated (spoiled) |
| Weighting | T1, PD, T2* (mostly T2*) | T1 and PD |
| Flow Sensitivity | Sensitive to flow (useful for angiography) | Less sensitive to flow |
| Artifacts | Increased magnetic susceptibility | Reduced susceptibility |

187
Q

Why would you choose an incoherent gradient-echo sequence instead of a coherent one?

A
  • To reduce T2* effects and flow artifacts
  • To maximize T1 contrast
  • To increase image sharpness in contrast-enhanced imaging
188
Q

What are the key takeaways about coherent (rewound) gradient-echo?

A
  • Short TR & Medium Flip Angle maintain the steady state
  • Rewinding maintains transverse magnetization
  • Both FID and Stimulated Echo contribute to signal
  • Used primarily for T2* weighting
189
Q

What are the key takeaways about incoherent (spoiled) gradient-echo?

A
  • Short TR & Variable Flip Angle maintain the steady state
  • Spoiling eliminates residual transverse magnetization
  • Only the FID is used to create the gradient-echo
  • Used primarily for T1 and PD-weighted imaging
190
Q

What is the main purpose of incoherent (spoiled) gradient-echo sequences?

A

To eliminate residual transverse magnetization and allow only the FID to contribute to image contrast, producing mainly T1- and PD-weighted images.

191
Q

What are the two spoiling techniques used in incoherent gradient-echo sequences?

A
  1. RF Spoiling → Alters the phase angle of each RF excitation pulse every TR to eliminate residual transverse magnetization.
  2. Gradient Spoiling → Uses slice-select, phase-encoding, and frequency-encoding gradients to dephase residual transverse magnetization.
192
Q

What are the common uses of incoherent (spoiled) gradient-echo sequences?

A
  • T1-weighted imaging after gadolinium injection
  • 2D breath-hold imaging
  • 3D volumetric imaging for good anatomical detail
193
Q

What are the advantages and disadvantages of incoherent gradient-echo?

A

Advantages | Disadvantages |
|—————|——————|
| Shorter scan times | Reduced SNR in 2D acquisitions |
| Good for gadolinium-enhanced imaging | Increased magnetic susceptibility artifacts |
| Can be acquired in a volume acquisition | Loud gradient noise |
| Good SNR and anatomical detail in 3D | - |

194
Q

What are the suggested scan parameters for incoherent gradient-echo?

A
  • Flip angle: 30°–45°
  • TR: 20–50 ms
  • TE: 5–10 ms (short for T1 weighting)
195
Q

What is the purpose of reverse-echo gradient-echo sequences?

A

To achieve true T2-weighting in gradient-echo sequences by repositioning the stimulated echo using a rephasing gradient.

196
Q

What are the two TEs used in reverse-echo gradient-echo?

A
  1. Actual TE (TEact) → The time between the peak of the gradient-echo and the next RF excitation pulse.
  2. Effective TE (TEeff) → The time between the peak of the gradient-echo and the RF excitation pulse that created its FID.

Formula: TE_eff = 2 × TR - TE_act

197
Q

Why does reverse-echo gradient-echo achieve better T2 contrast than conventional gradient-echo?

A

Because the effective TE is longer than the TR, allowing for better separation of true T2 contrast rather than T2*.

198
Q

What are the common uses of reverse-echo gradient-echo sequences?

A
  • Brain imaging (T2-weighted sequences)
  • Joint imaging
  • Perfusion imaging (used in rapid data acquisition)
199
Q

What are the advantages and disadvantages of reverse-echo gradient-echo?

A

Advantages | Disadvantages |
|—————|——————|
| Short scan times | Reduced SNR in 2D acquisitions |
| True T2 weighting instead of T2* | Loud gradient noise |
| Can be acquired in a volume acquisition | Increased susceptibility artifacts |
| Good SNR and anatomical detail in 3D | Image quality can be poor |

200
Q

What are the suggested scan parameters for reverse-echo gradient-echo?

A
  • Flip angle: 30°–45°
  • TR: 20–50 ms
  • Short TEact to maximize TEeff for T2 contrast
201
Q

What is the key feature of balanced gradient-echo sequences?

A

A balanced gradient scheme that corrects for phase errors in flowing blood and CSF, enhancing the signal of moving fluids.

202
Q

How does balanced gradient-echo differ from coherent gradient-echo?

A
  • Uses higher flip angles and shorter TRs for better SNR and shorter scan times.
  • Alternating RF phase angles prevent saturation, ensuring high signal from water and fat.
203
Q

What are the common uses of balanced gradient-echo sequences?

A
  • Cardiac and great vessel imaging
  • Spinal imaging (CSF flow visualization)
  • Internal auditory canal and cervical spine imaging
  • Joint and abdominal imaging
204
Q

What are the advantages and disadvantages of balanced gradient-echo?

A

Advantages | Disadvantages |
|—————|——————|
| Shorter scan times | Reduced SNR in 2D acquisitions |
| Reduced flow artifacts | Loud gradient noise |
| Good SNR and anatomical detail in 3D | Requires high-performance gradients |
| High contrast between fat, water, and surrounding tissue | Increased magnetic susceptibility artifacts |

205
Q

What are the suggested scan parameters for balanced gradient-echo?

A
  • Flip angle: Variable (higher flip angles increase signal)
  • TR: Less than 10 ms (reduces scan time and flow artifact)
  • TE: 5–10 ms
206
Q

What is the primary goal of fast gradient-echo sequences?

A

To acquire a volume of images in a single breath-hold, reducing scan time while maintaining image quality.

207
Q

How is the TE reduced in fast gradient-echo sequences?

A
  • Using a partial RF excitation pulse
  • Reading only a portion of the echo (partial echo)
  • Increasing receive bandwidth
  • Using ramped sampling (acquiring data before the frequency-encoding gradient reaches peak)
208
Q

Why are inversion pulses sometimes used in fast gradient-echo sequences?

A

To pre-magnetize the tissue and enhance T1 contrast, allowing for better contrast between organs and tissues.

209
Q

What is echo planar imaging (EPI)?

A

A rapid imaging technique that starts with one or more RF pulses and is followed by a train of gradient-echoes.

210
Q

What are the two types of echo planar imaging (EPI)?

A
  1. Gradient-Echo EPI (GE-EPI): Uses an RF excitation pulse and multiple gradient-echoes.
  2. Spin-Echo EPI (SE-EPI): Uses a 90° RF excitation pulse followed by a 180° rephasing pulse before EPI readout.
211
Q

What are the key differences between steady-state gradient-echo sequences?

A

Sequence | Weighting | Signal Source |
|————-|————-|—————-|
| Coherent GRE | T1, PD, T2* | FID + Stimulated Echo |
| Incoherent GRE | T1, PD | FID only |
| Reverse-Echo GRE | T2 | Stimulated Echo only |
| Balanced GRE | T2/T1 | Both echoes + flow enhancement |

212
Q

How do gradient-echo sequences differ from spin-echo sequences?

A
  1. Use variable flip angles instead of a fixed 90° RF excitation pulse.
  2. Use gradients rather than a 180° RF pulse to rephase spins and form an echo.
213
Q

Why do gradient-echo sequences allow for shorter TR values?

A

Because smaller flip angles allow faster T1 recovery, reducing the required TR for image acquisition.

214
Q

What is a bipolar gradient in a gradient-echo sequence?

A

A gradient consisting of two lobes (one negative, one positive) that:
1. Dephases magnetic moments (negative lobe).
2. Rephases only those affected by the first lobe (positive lobe).

215
Q

Why do gradient-echo sequences suffer from magnetic susceptibility artifacts?

A

Because gradient rephasing does not compensate for magnetic field inhomogeneities, leading to T2* decay rather than true T2 contrast.

216
Q

What are the three mechanisms that affect image weighting in gradient-echo sequences?

A
  1. Extrinsic contrast parameters (TR, TE, flip angle).
  2. The steady state (ratio of T1 recovery to T2 decay).
  3. Residual transverse magnetization (affects signal intensity).
217
Q

How does T1 weighting work in gradient-echo sequences?

A
  • Short TR + large flip angle = Saturation, enhancing T1 contrast.
  • Short TE to minimize T2* decay.
218
Q

How does T2* weighting work in gradient-echo sequences?

A
  • Long TE to allow for T2* decay.
  • Small flip angle + long TR to minimize T1 effects.
219
Q

How does proton density (PD) weighting work in gradient-echo sequences?

A
  • Short TE to minimize T2* decay.
  • Long TR + small flip angle to minimize T1 effects.
220
Q

What is the steady state in gradient-echo imaging?

A

A condition where the TR is shorter than T1 and T2 relaxation times, leading to a build-up of residual transverse magnetization over time.

221
Q

What is residual transverse magnetization, and how does it affect image contrast?

A

Residual transverse magnetization is the remaining magnetization from previous TR periods. It can cause T2* contrast enhancement in steady-state sequences.

222
Q

What is the Ernst angle, and why is it important?

A

The flip angle that provides maximum signal intensity for a given TR and T1 relaxation time.

Formula:
[
\theta_{\text{Ernst}} = \cos^{-1} \left( e^{-\frac{\text{TR}}{\text{T1}}} \right)
]

223
Q

What signals contribute to image contrast in coherent (rewound) gradient-echo?

A

Both the FID and the stimulated echo, allowing for T1-, PD-, or T2*-weighting.

224
Q

What is the primary purpose of incoherent (spoiled) gradient-echo?

A

To eliminate residual transverse magnetization, allowing only the FID to contribute, leading to T1- or PD-weighted images.

225
Q

How does reverse-echo gradient-echo achieve true T2 contrast?

A

By repositioning the stimulated echo using a rewinder gradient, ensuring the effective TE is long enough to generate T2 contrast.

Formula:
[
TE_{\text{eff}} = 2 \times TR - TE_{\text{act}}
]

226
Q

What makes balanced gradient-echo unique?

A
  1. Balanced gradient system minimizes phase errors in flowing blood and CSF.
  2. Alternating RF pulse phase prevents saturation, keeping fat and water bright.
227
Q

What are the advantages of balanced gradient-echo?

A
  • Short scan times.
  • Good contrast between fat, water, and tissues.
  • Reduced flow artifacts.
228
Q

What is the primary goal of fast gradient-echo sequences?

A

To acquire multiple slices in a single breath-hold, reducing motion artifacts.

229
Q

How does echo planar imaging (EPI) work?

A

Uses an initial RF pulse followed by a series of gradient-echoes, enabling ultrafast imaging in one TR period.

230
Q

What are the two types of EPI?

A
  1. Gradient-Echo EPI (GE-EPI) → Uses a single RF excitation pulse followed by gradient-echoes.
  2. Spin-Echo EPI (SE-EPI) → Uses 90° RF pulse + 180° rephasing pulse before EPI readout.
231
Q

What are the advantages of EPI?

A
  • Very fast scan times.
  • Good for functional MRI (fMRI) and diffusion imaging.
  • Minimizes motion artifacts.
232
Q

What are the disadvantages of EPI?

A
  • Lower resolution than conventional imaging.
  • Increased susceptibility artifacts.
  • More distortion due to gradient imperfections.
233
Q

What are the key differences between common gradient-echo sequences?

A

|
Sequence | Weighting | Main Signal Source |
|————-|————-|———————-|
| Coherent GRE | T1, PD, T2* | FID + Stimulated Echo |
| Incoherent GRE | T1, PD | FID only |
| Reverse-Echo GRE | T2 | Stimulated Echo only |
| Balanced GRE | T2/T1 | Flow Enhancement + Both Echoes |

234
Q

What is the main advantage of Echo Planar Imaging (EPI)?

A

EPI provides ultrafast imaging, reducing scan times significantly and minimizing motion artifacts.

235
Q

What is the difference between Spin-Echo EPI (SE-EPI) and Gradient-Echo EPI (GE-EPI)?

A
  • SE-EPI: Begins with a 90° RF pulse, followed by a 180° RF rephasing pulse, reducing susceptibility artifacts.
  • GE-EPI: Begins with a variable flip angle RF pulse, followed by gradient rephasing, making it faster but more prone to artifacts.
236
Q

What is the main disadvantage of EPI?

A

EPI is highly susceptible to artifacts, including:
- Chemical shift artifacts along the phase axis.
- Blurring due to T2* decay.
- Ghosting artifacts (half-FOV ghosts) caused by gradient timing errors.

237
Q

How does Single-Shot EPI (SS-EPI) differ from Multi-Shot EPI (MS-EPI)?

A
  • SS-EPI: Acquires all k-space data in a single TR, making it very fast but increasing artifacts.
  • MS-EPI: Splits k-space acquisition across multiple TRs, reducing artifacts but taking longer.
238
Q

What are common uses of EPI sequences?

A
  1. Functional MRI (fMRI) – Captures brain activation in response to stimuli.
  2. Diffusion-Weighted Imaging (DWI) – Detects stroke and white matter abnormalities.
  3. Perfusion Imaging – Measures blood flow.
  4. Cardiac & Coronary Imaging – Reduces motion artifacts.
239
Q

What is GRASE (Gradient and Spin Echo)?

A

A hybrid sequence that combines gradient echoes for speed and spin echoes for better T2 contrast, reducing T2* susceptibility effects.

240
Q

What is EPI-FLAIR, and why is it useful?

A

EPI-FLAIR uses an inversion recovery pulse (180°) before EPI readout, allowing CSF suppression while maintaining fast scan times.

241
Q

What modifications help speed up fast gradient-echo imaging?

A
  1. Ramped sampling – Starts acquiring data before gradients reach maximum strength.
  2. Partial k-space filling – Uses symmetry to reduce scan time.
  3. Wider bandwidth – Speeds up signal acquisition.
242
Q

What are the advantages of fast gradient-echo sequences?

A
  • Allows breath-hold acquisitions for motion-sensitive imaging.
  • Shortens scan time, making it ideal for contrast-enhanced studies.
  • Useful for dynamic imaging, such as monitoring contrast uptake.
243
Q

What is Double Echo Steady State (DESS)?

A

A sequence that generates two echoes:
1. A coherent gradient-echo for high resolution.
2. A reverse-echo gradient-echo for enhanced T2 contrast.

Common use: Orthopedic imaging (joints, cartilage).

244
Q

What are the primary extrinsic parameters in gradient-echo sequences?

A
  1. TR – Controls T1 contrast (short TR = more T1 weighting).
  2. TE – Controls T2* contrast (long TE = more T2* weighting).
  3. Flip angle – Controls saturation (high flip = more T1 contrast).
245
Q

How does the flip angle affect T1 contrast in gradient-echo sequences?

A
  • Large flip angle (70°+) → Enhances T1 contrast.
  • Medium flip angle (30°–45°) → Balanced contrast.
  • Small flip angle (5°–20°) → Reduces T1 effects (better for T2* or PD weighting).
246
Q

What is the b value, and where is it used?

A

The b value is used in Diffusion-Weighted Imaging (DWI) to determine how much diffusion sensitivity is applied.

Higher b-values = More diffusion weighting.

247
Q

What are the major artifacts in gradient-echo sequences?

A
  1. Magnetic susceptibility artifacts – Worsened by gradient-echo rephasing.
  2. Chemical shift artifacts – Common in EPI along the phase axis.
  3. Blurring – Due to T2* decay during long echo train acquisitions.
  4. Ghosting (half-FOV ghosts) – Caused by gradient timing errors.
248
Q

How can susceptibility artifacts be reduced in gradient-echo imaging?

A
  • Use SE-EPI instead of GE-EPI.
  • Decrease TE (shorter TE = less dephasing).
  • Increase bandwidth to minimize distortions.
249
Q

What is the primary cause of blurring in EPI?

A

T2* decay during k-space filling leads to signal loss, reducing spatial resolution.

Fix: Use shorter echo trains or multi-shot EPI.

250
Q

How do different gradient-echo sequences compare in contrast and weighting?

A

Sequence | Weighting | Key Features |
|————-|————-|——————|
| Coherent GRE | T1, PD, T2* | Uses both FID + Stimulated Echo |
| Incoherent GRE | T1, PD | Uses FID only (RF Spoiling) |
| Reverse-Echo GRE | T2 | Uses Stimulated Echo only |
| Balanced GRE | T2/T1 | Corrects flow artifacts, high SNR |
| EPI | Any | Fastest, functional imaging |

251
Q

Why is balanced GRE preferred for imaging CSF and blood flow?

A

It uses a balanced gradient system, reducing phase errors and improving signal intensity of flowing fluids like CSF and blood.

252
Q

What are the main benefits of gradient-echo sequences?

A
  1. Faster scan times than spin-echo sequences.
  2. More flexible weighting (T1, PD, T2*).
  3. Better for flow imaging (angiography, CSF studies).
253
Q

What is the biggest drawback of gradient-echo sequences?

A

They do not compensate for T2* effects, making them more sensitive to susceptibility artifacts than spin-echo sequences.

254
Q

What is k-space in MRI?

A

K-space is a mathematical space where raw MRI data is stored before image reconstruction using the Fourier Transform.

255
Q

What determines spatial resolution in k-space?

A

High-frequency data, which is stored in the peripheral regions of k-space, determines spatial resolution.

256
Q

What determines image contrast in k-space?

A

Low-frequency data, stored in the center of k-space, determines image contrast.

257
Q

What is the Fourier Transform used for in MRI?

A

The Fourier Transform converts raw k-space data into an actual MRI image.

258
Q

What are the two main k-space filling strategies?

A
  1. Cartesian filling – Standard left-to-right and top-to-bottom filling. 2. Radial filling – Fills k-space in a circular or spiral pattern.
259
Q

What is Partial Fourier Acquisition in k-space filling?

A

Partial Fourier Acquisition fills only part of k-space, using symmetry to reconstruct the missing data, reducing scan time.

260
Q

How does Parallel Imaging (SENSE/GRAPPA) affect k-space filling?

A

Parallel imaging undersamples k-space, reducing scan time while using coil sensitivity profiles to reconstruct missing data.

261
Q

What is Single-Shot Imaging, and how does it work?

A

Single-shot imaging fills all of k-space in one TR, making it extremely fast (e.g., in EPI sequences).

262
Q

What are the benefits of radial k-space filling?

A

Less motion artifacts than Cartesian filling. Better contrast resolution. More robust to flow and susceptibility artifacts.

263
Q

What is Keyhole Imaging in k-space?

A

Keyhole Imaging fills the center of k-space more frequently, improving temporal resolution (e.g., in contrast-enhanced imaging).

264
Q

What is the primary mechanism behind functional MRI (fMRI)?

A

fMRI detects changes in blood oxygenation (BOLD signal) to study brain activity.

265
Q

What does BOLD signal stand for in fMRI?

A

Blood-Oxygen-Level-Dependent (BOLD) signal, which detects brain activity based on changes in oxyhemoglobin vs. deoxyhemoglobin levels.

266
Q

Why does deoxyhemoglobin appear dark in fMRI?

A

Deoxyhemoglobin is paramagnetic, causing signal loss and T2* contrast changes in activated brain regions.

267
Q

What is the temporal resolution of fMRI?

A

fMRI has a temporal resolution of ~2-3 seconds, limited by the hemodynamic response of blood flow changes.

268
Q

What sequence is most commonly used in fMRI?

A

Gradient-Echo EPI (GE-EPI) because it provides fast acquisition and is highly sensitive to BOLD contrast.

269
Q

What are the clinical applications of fMRI?

A
  1. Mapping brain function (e.g., motor, language, vision). 2. Pre-surgical planning for brain tumor resections. 3. Neurological research (e.g., memory, learning, emotions).
270
Q

What are the limitations of fMRI?

A

Low spatial resolution (~2-3 mm voxel size). Susceptibility to motion artifacts. Delayed hemodynamic response (~2 sec lag).

271
Q

What preprocessing steps are applied to fMRI data?

A
  1. Motion correction – Aligns brain volumes over time. 2. Spatial smoothing – Reduces noise by averaging neighboring voxels. 3. Temporal filtering – Removes physiological noise (e.g., breathing, heartbeat).
272
Q

What is Resting-State fMRI (rs-fMRI)?

A

rs-fMRI analyzes spontaneous neural activity when the subject is at rest, used to study functional connectivity between brain regions.

273
Q

What are common fMRI activation paradigms?

A
  1. Block Design – Alternates between task and rest periods. 2. Event-Related Design – Measures response to isolated stimuli. 3. Resting-State Design – No task, just observing spontaneous activity.
274
Q

What is perfusion imaging, and why is it important?

A

Perfusion imaging measures blood flow (CBF), blood volume (CBV), and transit time (MTT) in tissues, commonly used in stroke and tumor evaluation.

275
Q

What are the three main types of perfusion MRI?

A
  1. Dynamic Susceptibility Contrast (DSC) – Uses gadolinium contrast to track T2* signal loss. 2. Dynamic Contrast-Enhanced (DCE) – Measures contrast uptake in tissues, used in tumors. 3. Arterial Spin Labeling (ASL) – Uses blood water as an endogenous contrast agent.
276
Q

How does Arterial Spin Labeling (ASL) work?

A

ASL magnetically labels arterial blood water before it enters the brain, allowing perfusion to be measured without contrast agents.

277
Q

What are the advantages of ASL perfusion imaging?

A

Non-invasive (no contrast required). Good for repeated scans (e.g., pediatrics, kidney patients). Measures absolute cerebral blood flow (CBF).

278
Q

How does DSC perfusion imaging detect stroke?

A

By analyzing T2*-weighted signal drop caused by gadolinium bolus passage, it identifies ischemic areas with low cerebral blood flow (CBF).

279
Q

What does Mean Transit Time (MTT) represent in perfusion MRI?

A

MTT is the average time blood takes to pass through a given brain region, helping detect delayed circulation in stroke.

280
Q

Why is Dynamic Contrast-Enhanced (DCE) perfusion useful in tumors?

A

DCE measures vascular permeability, helping distinguish benign vs. malignant tumors based on contrast leakage patterns.

281
Q

What is CBF vs. CBV in perfusion MRI?

A

CBF (Cerebral Blood Flow) – Blood delivered per unit time (mL/100g/min). CBV (Cerebral Blood Volume) – Total volume of blood in a given tissue region.

282
Q

What are clinical applications of perfusion MRI?

A

Stroke (ischemic vs. hemorrhagic). Brain tumors (angiogenesis and malignancy assessment). Neurodegenerative diseases (Alzheimer’s, dementia).

MRI Physics (6 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2025 Bold Learning Solutions. Terms and Conditions