Chapter 2 Flashcards
(231 cards)
1
Q
Why is contrast important in MRI?
A
Contrast is essential for distinguishing between normal anatomy and pathology in diagnostic imaging.
2
Q
What is a major advantage of MRI over other imaging modalities?
A
MRI provides excellent soft tissue discrimination due to its high contrast resolution.
3
Q
What are intrinsic contrast parameters?
A
Intrinsic contrast parameters are inherent to the body’s tissues and cannot be changed.
4
Q
List the intrinsic contrast parameters in MRI.
A
- T1 recovery time
- T2 decay time
- Proton density (PD)
- Flow
- Apparent diffusion coefficient (ADC)
5
Q
What are extrinsic contrast parameters?
A
Extrinsic contrast parameters can be controlled by adjusting the scan protocol.
6
Q
List the extrinsic contrast parameters in MRI.
A
- TR (Repetition Time)
- TE (Echo Time)
- Flip angle
- TI (Inversion Time)
- Turbo Factor / Echo Train Length
- b-value (used in diffusion imaging)
7
Q
What is relaxation in MRI?
A
Relaxation is the process of energy loss after the RF excitation pulse is turned off, causing hydrogen nuclei to return to equilibrium.
8
Q
What are the two types of relaxation?
A
- T1 recovery (spin-lattice relaxation)
- T2 decay (spin-spin relaxation)
9
Q
What is T1 recovery?
A
T1 recovery is the return of longitudinal magnetization as hydrogen nuclei release energy to the surrounding molecular lattice.
10
Q
What does T1 recovery time measure?
A
T1 recovery time is the time it takes for 63% of the longitudinal magnetization to recover.
11
Q
What is the equation for T1 recovery?
A
M_z(t) = M_z (1 - e^{-t/T1})
Where:
- Mzt = Longitudinal magnetization at time t
- Mz = Fully recovered longitudinal magnetization
- T1 = T1 recovery time
12
Q
What are the T1 recovery times of brain tissues at 1T?
A
Tissue | T1 Recovery Time (ms) |
|————-|————————-|
| Water | 2500 ms |
| Fat | 200 ms |
| CSF | 2000 ms |
| White Matter | 500 ms |
13
Q
What determines how much T1 recovery occurs in a tissue?
A
The Repetition Time (TR) controls how much T1 recovery takes place before the next RF pulse is applied.
14
Q
What type of tissues recover longitudinal magnetization faster?
A
Fat recovers faster than water, meaning fat appears brighter on T1-weighted images.
15
Q
What is T2 decay?
A
T2 decay is the loss of coherent transverse magnetization due to spin-spin interactions between hydrogen nuclei.
16
Q
What does T2 decay time measure?
A
T2 decay time is the time it takes for 63% of the transverse magnetization to dephase (only 37% remains in phase).
17
Q
What is the equation for T2 decay?
A
M_{xy}(t) = M_{xy} e^{-t/T2}
Where:
- Mxy(t) = Transverse magnetization at time t
- Mxy = Fully coherent transverse magnetization
- T2 = T2 decay time
18
Q
What are the T2 decay times of brain tissues at 1T?
A
Tissue | T2 Decay Time (ms) |
|————-|———————-|
| Water | 2500 ms |
| Fat | 100 ms |
| CSF | 300 ms |
| White Matter | 100 ms |
19
Q
Why does T2 decay occur?
A
Due to magnetic field interactions between hydrogen nuclei causing small frequency differences, leading to dephasing.
20
Q
What is the difference between T1 and T2 decay?
A
- T1 recovery involves energy transfer to the molecular lattice.
- T2 decay involves energy transfer between hydrogen nuclei (spin-spin interactions).
21
Q
What is T2* decay?
A
T2* decay is the combined effect of T2 decay and magnetic field inhomogeneities, leading to faster dephasing than T2 alone.
22
Q
What equation describes the relationship between T2 and T2*?
A
[ \frac{1}{T2^*} = \frac{1}{T2} + \frac{1}{2\gamma \Delta B0} ]
Where:
- T2* = Observed transverse decay time
- T2 = True spin-spin relaxation time
- γ = Gyromagnetic ratio
- ΔB0 = Magnetic field inhomogeneity
23
Q
How can T2* decay be reduced?
A
By using pulse sequences with refocusing pulses, like spin-echo sequences, to correct for field inhomogeneities.
24
Q
What three factors determine MRI image contrast?
A
- T1 recovery
- T2 decay
- Proton density (PD)
25
What is proton density (PD)?
PD refers to the number of mobile hydrogen protons per unit volume, affecting signal intensity.
26
How does T1 recovery contribute to contrast?
Tissues with short T1 recovery times (e.g., fat) appear bright on T1-weighted images.
27
How does T2 decay contribute to contrast?
Tissues with long T2 decay times (e.g., fluid) appear bright on T2-weighted images.
28
How does proton density (PD) contribute to contrast?
Tissues with higher proton density return higher signal and appear brighter on PD-weighted images.
29
What two factors influence T1 and T2 relaxation times?
1. How closely molecular tumbling rate matches the Larmor frequency.
2. How closely packed molecules are in the tissue.
30
How does molecular tumbling rate affect relaxation?
- Fat tumbles slowly, matching the Larmor frequency, leading to fast T1 recovery.
- Water tumbles rapidly, leading to slow T1 recovery.
31
What is spin-lattice energy transfer?
The process where hydrogen nuclei transfer energy to surrounding tissues, causing T1 recovery.
32
What is spin-spin interaction?
The process where hydrogen nuclei transfer energy to each other, causing T2 decay.
33
What are the molecular structures of fat and water?
Fat: Hydrogen is arranged with carbon and oxygen, forming large lipid molecules.
Water: Consists of two hydrogen atoms and one oxygen (H₂O).
34
Why does fat recover faster than water in MRI?
Fat molecules are closely packed, and their slow molecular tumbling matches the Larmor frequency, making energy exchange efficient.
35
Why does water take longer to recover in MRI?
Water molecules are spaced apart and have fast molecular tumbling, which does not match the Larmor frequency, making energy exchange inefficient.
36
What is T1 recovery?
T1 recovery is the realignment of hydrogen nuclei along the longitudinal axis after the RF excitation pulse is removed.
37
How does T1 recovery occur in fat?
Fat has a low inherent energy and absorbs energy efficiently. The slow molecular tumbling in fat allows rapid T1 recovery.
38
How does T1 recovery occur in water?
Water has a high inherent energy and does not easily absorb energy. The fast molecular motion does not match the Larmor frequency, causing slower T1 recovery.
39
How does magnetic field strength (B0) affect T1 recovery?
As B0 increases, T1 recovery takes longer because fewer molecules move at relaxation-causing frequencies.
40
What is T2 decay?
T2 decay is the loss of transverse magnetization due to spin-spin interactions between hydrogen nuclei.
41
Why does fat experience faster T2 decay?
Fat molecules are closely packed, leading to strong spin-spin interactions. Magnetic moments dephase quickly, resulting in short T2 decay time.
42
Why does water experience slower T2 decay?
Water molecules are spaced apart, leading to fewer spin-spin interactions. Magnetic moments dephase slowly, resulting in long T2 decay time.
43
How does magnetic field strength (B0) affect T2 decay?
T2 decay is slightly prolonged as B0 increases, but the effect is less significant than in T1 recovery.
44
What is T1 contrast in MRI?
T1 contrast is derived from the differences in T1 recovery times between tissues.
45
How does TR affect T1 contrast?
Short TR → Increased T1 contrast
Long TR → Reduced T1 contrast (Tissues fully recover, minimizing contrast differences).
46
What is the T1 recovery time difference between fat and water?
Fat: Short T1 recovery time (realigns quickly with B0).
Water: Long T1 recovery time (takes longer to realign).
47
What happens if TR is shorter than tissue relaxation times?
Fat will have more longitudinal magnetization before the next RF pulse, resulting in high signal (bright/hyperintense).
Water will have less longitudinal magnetization, resulting in low signal (dark/hypointense).
48
What is partial saturation?
When the NMV is flipped beyond 90° but not fully to 180°, affecting signal intensity.
49
What is full saturation?
When the NMV is flipped to 180°, meaning there is no remaining longitudinal magnetization before the next RF pulse.
50
What happens if TR is shorter than T1 relaxation times?
Fat and water do not fully recover before the next RF pulse. This results in partial saturation, increasing T1 contrast.
51
What happens if TR is longer than T1 relaxation times?
Fat and water fully recover before the next RF pulse. T1 contrast is lost, and contrast depends only on proton density (PD).
52
What is the steady-state in MRI?
A condition where vectors recover the same amount of longitudinal magnetization every TR, ensuring consistent signal generation.
53
What are preparatory or dummy pulses?
The first few RF pulses applied in a sequence that do not contribute to image formation but help achieve steady-state magnetization.
54
What factors influence how long it takes to reach the steady-state?
1. Magnetic field strength (B0)
2. Proton density (PD)
3. Flip angle
4. T1 relaxation time
5. RF pulse duration
55
How does T1 recovery differ between fat and water?
Fat: Short T1 recovery time (realigns quickly).
Water: Long T1 recovery time (realigns slowly).
56
How does B0 affect T1 recovery?
As B0 increases, T1 recovery takes longer due to fewer molecules tumbling at the relaxation-causing frequency.
57
How does B0 affect T2 decay?
T2 decay is slightly longer at higher field strengths, but the effect is minor compared to T1.
58
What scan parameter controls T1 contrast?
TR (Repetition Time) determines T1 contrast.
59
What TR values are needed for good T1 contrast?
Short TR → Strong T1 contrast.
Long TR → Poor T1 contrast (all tissues fully recover).
60
What are the key differences between fat and water in MRI?
Property | Fat | Water
T1 Recovery | Short | Long
T2 Decay | Short | Long
Molecular Packing | Tight | Loose
Energy Exchange | Efficient | Inefficient
Signal on T1-Weighted Images | Bright (hyperintense) | Dark (hypointense)
Signal on T2-Weighted Images | Dark (hypointense) | Bright (hyperintense)
61
What is T2 contrast in MRI?
T2 contrast occurs when image contrast is based on differences in the T2 decay times of tissues.
62
How does TE affect T2 contrast?
- Long TE → Increases T2 contrast (more dephasing between tissues).
- Short TE → Reduces T2 contrast (less dephasing).
63
Why does water appear bright on T2-weighted images?
Water has a long T2 decay time, meaning it retains more coherent transverse magnetization and appears hyperintense (bright).
64
Why does fat appear dark on T2-weighted images?
Fat has a short T2 decay time, so it loses transverse magnetization quickly and appears hypointense (dark).
65
What are the key characteristics of T2 decay?
- Fat has a short T2 decay time.
- Water has a long T2 decay time.
- T2 decay is caused by spin-spin interactions.
- T2 decay time increases with magnetic field strength (B0).
- For good T2 contrast, TE must be long.
66
What is proton density (PD) contrast?
PD contrast refers to differences in signal intensity between tissues based on their hydrogen proton density per unit volume.
67
How does proton density affect MRI contrast?
- Tissues with high proton density → Hyperintense (bright) signal.
- Tissues with low proton density → Hypointense (dark) signal.
68
What intrinsic contrast properties affect signal intensity?
1. Proton density (PD)
2. T1 recovery time
3. T2 decay time
69
What extrinsic contrast parameters affect signal intensity?
1. Repetition Time (TR)
2. Echo Time (TE)
70
What is weighting in MRI?
Weighting refers to selecting extrinsic contrast parameters (TR, TE) to emphasize one intrinsic contrast property (T1, T2, PD) over others.
71
What is a T1-weighted image?
A T1-weighted image is one where contrast is based on differences in T1 recovery times between tissues.
72
How is T1 weighting achieved?
- Short TR (minimizes full recovery of magnetization).
- Short TE (minimizes T2 effects).
73
What happens if TR is too long?
Both fat and water fully recover before the next RF pulse, reducing T1 contrast.
74
Why is fat bright and water dark on T1-weighted images?
- Fat has a short T1 recovery time → Realigns with B0 quickly → Bright signal (hyperintense).
- Water has a long T1 recovery time → Takes longer to realign → Dark signal (hypointense).
75
What tissues have a high signal (bright) on T1-weighted images?
- Fat
- Methemoglobin
- Hemangiomas
- Fatty degeneration
- Cysts with proteinaceous fluid
- Slow-flowing blood
- Paramagnetic contrast agents
76
What tissues have a low signal (dark) on T1-weighted images?
- Cortical bone
- Avascular necrosis
- Infarction
- Tumors
- Sclerosis
- Calcifications
77
What tissues produce no signal (black) on T1-weighted images?
- Air
- Fast-flowing blood
- Tendons
- Scar tissue
- Cortical bone
78
What is the equation for T1 contrast?
SI = PD e^(-TE/T2) (1 - e^(-TR/T1))
- e^(-TE/T2) → T2 component
- (1 - e^(-TR/T1)) → T1 component
If TE is very short, T2 contrast is minimized, making T1 contrast dominant.
79
What are the key characteristics of T1 weighting?
- TR controls T1 contrast (Short TR enhances T1 contrast).
- TE controls T2 contrast (Short TE minimizes T2 contrast).
- Short TR + Short TE → Maximizes T1 contrast.
- T1-weighted images show anatomy and post-contrast pathology.
80
What is a T2-weighted image?
A T2-weighted image is one where contrast depends on differences in T2 decay times between tissues.
81
How is T2 weighting achieved?
- Long TR (minimizes T1 effects).
- Long TE (maximizes T2 decay contrast).
82
Why is water bright and fat dark on T2-weighted images?
- Water has a long T2 decay time → Retains transverse magnetization → Bright signal (hyperintense).
- Fat has a short T2 decay time → Dephases quickly → Dark signal (hypointense).
83
What tissues have a high signal (bright) on T2-weighted images?
- Water
- Synovial fluid
- Infection
- Inflammation
- Edema
- Tumors
- Hemorrhage
- Cysts
84
What tissues have a low signal (dark) on T2-weighted images?
- Cortical bone
- Bone islands
- Deoxyhemoglobin
- Calcifications
85
What tissues produce no signal (black) on T2-weighted images?
- Air
- Fast-flowing blood
- Tendons
- Scar tissue
- Cortical bone
86
What is the equation for T2 contrast?
SI = PD e^(-TE/T2) (1 - e^(-TR/T1))
- e^(-TE/T2) → T2 component
- (1 - e^(-TR/T1)) → T1 component
If TR is very long, T1 contrast is minimized, making T2 contrast dominant.
87
What are the key characteristics of T2 weighting?
- TR controls T1 contrast (Long TR minimizes T1 contrast).
- TE controls T2 contrast (Long TE maximizes T2 contrast).
- Long TR + Long TE → Maximizes T2 contrast.
- T2-weighted images are used for pathology imaging.
88
What is the final comparison of T1 vs. T2 weighting?
| Feature | T1-Weighted Image | T2-Weighted Image |
|------------------|-------------------|-------------------|
| TR | Short | Long |
| TE | Short | Long |
| Contrast Type | T1 recovery time | T2 decay time |
| Fat Signal | Bright | Dark |
| Water Signal | Dark | Bright |
89
What is diffusion in MRI?
Diffusion refers to the movement of molecules in the extracellular space due to random thermal motion, which can be restricted by membranes, ligaments, and macromolecules.
90
What is the apparent diffusion coefficient (ADC)?
ADC is a measure of net displacement of molecules in tissue per second, affecting image contrast. It is an intrinsic contrast parameter that cannot be controlled.
91
What does a high ADC value indicate?
A high ADC means free diffusion due to large extracellular spaces, seen in normal gray matter and normal liver tissue.
92
What does a low ADC value indicate?
A low ADC means restricted diffusion due to small extracellular spaces, seen in ligaments and pathology.
93
How is diffusion-weighted imaging (DWI) achieved?
By applying two diffusion-sensitizing gradients: The first dephases magnetic moments. The second rephases them if diffusion is restricted.
94
How does signal intensity relate to ADC in DWI?
Low ADC → High signal (bright) (e.g., stroke, tumors). High ADC → Low signal (dark) (e.g., normal brain tissue).
95
What is the b factor in DWI?
The b value (b factor) is an extrinsic contrast parameter that controls the degree of diffusion weighting by altering the gradient amplitude, duration, and interval.
96
Equation for b factor in DWI
b = γ² × G² × δ² × (Δ - δ/3)
## Footnote
b = b value (s/mm²), γ = gyromagnetic ratio (MHz/T), G = gradient amplitude (mT/m), δ = gradient duration (ms), Δ = time between two gradient pulses (ms).
97
What is functional MRI (fMRI)?
A technique that acquires rapid MRI images of the brain during activity and at rest to measure brain function rather than anatomy.
98
What is the main physiological mechanism of fMRI?
fMRI relies on Blood Oxygenation Level Dependent (BOLD) contrast, which detects changes in oxyhemoglobin vs. deoxyhemoglobin levels.
99
How does deoxyhemoglobin affect fMRI signal?
Deoxyhemoglobin is paramagnetic, causing local field inhomogeneities that lead to dephasing and signal loss. During brain activity, blood flow increases, reducing deoxyhemoglobin and increasing signal intensity.
100
How is an fMRI image processed?
Images are acquired during a task ('on') and at rest ('off'). 'Off' images are subtracted from 'on' images. Statistical analysis identifies brain activation regions.
101
What is magnetization transfer contrast (MTC)?
MTC is an MRI technique that enhances contrast by manipulating the exchange of magnetization between bound and free nuclei.
102
What is the difference between bound and free nuclei?
Bound nuclei → Restricted movement, very short T2 decay, normally not imaged. Free nuclei → Observable, have longer T2 decay times.
103
How does MTC affect signal intensity?
Energy is transferred from bound to free nuclei, reducing the signal intensity of free nuclei. This creates additional contrast in tissues like white matter, cartilage, and fibrosis.
104
What is susceptibility-weighted imaging (SWI)?
SWI exploits magnetic susceptibility differences between tissues to enhance contrast, especially for detecting blood products and mineral deposits.
105
Which pulse sequence is commonly used for SWI?
Gradient-echo (GRE) sequences with a long TE enhance susceptibility differences.
106
Why are contrast agents used in MRI?
They alter T1 and T2 relaxation times of certain tissues, improving contrast between pathology and normal tissues.
107
What is relaxivity?
Relaxivity refers to the ability of a contrast agent to selectively shorten T1 or T2 relaxation times of nearby water molecules.
108
What are the two main types of MRI contrast agents?
T1 agents (shorten T1 recovery times, make tissues hyperintense). T2 agents (shorten T2 decay times, make tissues hypointense).
109
What is the most commonly used T1 contrast agent?
Gadolinium (Gd) – a paramagnetic rare-earth metal, made safe by chelation with diethylene triaminepentaacetic acid (DTPA).
110
Why is gadolinium chelated?
In its natural form, gadolinium is toxic, so it is bound to chelating molecules to allow safe excretion from the body.
111
How does gadolinium enhance MRI contrast?
Gadolinium has seven unpaired electrons, creating large magnetic moments. It reduces T1 recovery times of nearby water molecules, making enhancing tissues appear hyperintense on T1-weighted images.
112
What are T2 contrast agents?
Typically superparamagnetic iron oxide (SPIO) particles. Distort local magnetic fields, leading to T2 shortening and hypointense signals.
113
What are key features of DWI?
Uses diffusion gradients to differentiate restricted vs. free diffusion.
114
What are key features of ADC?
Measures net diffusion of molecules per second in tissue.
115
What are key features of fMRI?
Uses BOLD contrast to measure brain activity.
116
What are key features of MTC?
Enhances contrast by transferring energy from bound to free nuclei.
117
What are key features of SWI?
Exploits magnetic susceptibility differences, using GRE sequences.
118
What are key features of Gadolinium (Gd)?
Shortens T1, making enhancing tissues hyperintense.
119
What are key features of iron-based contrast?
Shortens T2, making enhancing tissues hypointense.
120
What are intrinsic contrast parameters?
Factors that cannot be changed because they are inherent in body tissues. Examples: T1 recovery, T2 decay, proton density (PD), flow, ADC.
121
What are extrinsic contrast parameters?
Factors that can be changed and selected at the console. Examples: TR, TE, TI, flip angle, ETL, b-value.
122
What is relaxation in MRI?
Process by which spins lose energy, leading to the recovery of magnetization in the longitudinal plane and decay of coherent magnetization in the transverse plane.
123
What is T1 recovery?
Growth of longitudinal magnetization as a result of spin-lattice relaxation.
124
What percentage of total energy must be regained for T1 relaxation, and in which plane does it occur?
63% of the total magnetization must be regained in the longitudinal plane.
125
What is T2 decay?
Loss of transverse magnetization as a result of spin-spin relaxation.
126
What percentage of total energy must be lost for T2 relaxation, and in which plane does it occur?
63% of the total magnetization must be lost in the transverse plane.
127
What is proton density?
The number of mobile hydrogen protons per unit volume of the tissue.
128
How does proton density affect signal intensity?
The higher the proton density, the more signal available from that tissue.
129
What does T1 and T2 relaxation depend on?
- **T1**: Molecular tumbling rate matching the Larmor frequency of hydrogen.
- **T2**: Molecules closely packed together increasing spin-spin interactions.
130
What is the T1 recovery time of fat and why?
Short; fat has low inherent energy, allowing it to quickly regain longitudinal magnetization.
131
What is the T1 recovery time of water and why?
Long; water has high inherent energy and takes longer to regain longitudinal magnetization.
132
What is the T2 decay time of fat and why?
Short; fat molecules are packed closely together, leading to frequent spin-spin interactions.
133
What is the T2 decay time of water and why?
Long; water molecules are more spread apart, making spin-spin interactions less likely.
134
Which tissue appears bright on a T1-weighted image?
Fat.
135
Which tissue appears dark on a T1-weighted image?
Water.
136
Which tissue appears bright on a T2-weighted image?
Water.
137
Which tissue appears dark on a T2-weighted image?
Fat.
138
How does tissue with high and low proton density appear on MRI?
- High PD: Hyperintense (bright).
- Low PD: Hypointense (dark).
139
What TR and TE values create a T1-weighted image?
Short TR, short TE.
140
What TR and TE values create a T2-weighted image?
Long TR, long TE.
141
What TR and TE values create a PD-weighted image?
Long TR, short TE.
142
What is diffusion in MRI?
Movement of molecules in the extracellular space due to random thermal motion.
143
What is ADC (apparent diffusion coefficient)?
The net displacement of molecules diffusing across an area of tissue per second.
144
What is diffusion-weighted imaging (DWI)?
A technique that produces images based on differences in ADC between tissues. Damaged tissue appears bright due to restricted diffusion.
145
What is the b-value in diffusion-weighted imaging?
A factor that controls how much a tissue’s ADC contributes to image weighting, based on the strength, interval, and duration of the gradients.
146
What is functional MRI (fMRI)?
A rapid imaging technique that assesses brain function by detecting changes in blood oxygenation levels.
147
What is BOLD imaging in fMRI?
Blood Oxygen Level Dependent imaging, which detects differences in magnetic susceptibility between oxyhemoglobin and deoxyhemoglobin in active brain areas.
148
What is magnetization transfer contrast (MTC)?
A technique that enhances contrast by exchanging energy between bound and free water molecules, often used to suppress background tissue.
149
What is susceptibility-weighted imaging (SWI)?
A technique that enhances contrast based on differences in magnetic susceptibility between tissues.
150
How do MRI contrast agents work?
They indirectly affect the relaxation times of water nuclei to enhance imaging contrast.
151
What type of contrast agent is gadolinium, and what does it do?
A **T1 contrast agent** that shortens T1 relaxation time, making enhancing tissues appear **bright** on T1-weighted images.
152
What are some common gadolinium-based contrast agents?
Gadavist, Magnevist, MultiHance, Eovist, Ablavar.
153
What causes T2 decay?
Spin-spin relaxation.
154
How does CSF appear on T2-weighted images?
Hyperintense (bright).
155
What does SWI stand for?
Susceptibility Weighted Imaging.
156
Is ADC an extrinsic contrast parameter?
False.
157
What kind of T1 recovery time does water have?
Long.
158
What causes the growth of longitudinal magnetization?
Spin-lattice relaxation.
159
Gadolinium-based contrast agents are considered _______________.
T1 agents.
160
What determines the flip angle?
The amplitude and duration of the excitation RF pulse.
161
What are intrinsic contrast parameters?
Factors inherent to body tissues that cannot be changed.
162
What imaging technique uses magnetic susceptibility differences to generate contrast?
SWI (Susceptibility Weighted Imaging).
163
Most pathology is associated with what change in fluid levels?
An increase in fluids.
164
When is T2 relaxation time reached?
When 63% of the transverse magnetization has decayed.
165
How is gadolinium made safe for injection?
By chelating and binding to other molecules.
166
On which type of imaging is water hyperintense?
T2-weighted imaging.
167
What MRI technique involves the fast exchange of energy between bound and free nuclei?
MTC (Magnetization Transfer Contrast).
168
What factors determine MRI signal intensity?
Proton density and the amount of transverse magnetization at time TE.
169
What TR and TE settings produce a T1-weighted image?
Short TR and short TE.
170
Does water have a shorter T2 time than fat?
False.
171
How do tissues with high proton density appear on MRI?
Hyperintense (bright).
172
How does fat appear on a T1-weighted image?
Hyperintense (bright).
173
When is T1 relaxation time reached?
When 63% of the longitudinal magnetization has recovered.
174
What occurs due to hydrogen nuclei giving up their energy to the surrounding molecular lattice?
T1 recovery.
175
How does water appear on a T1-weighted image?
Hypointense (dark).
176
What TR and TE settings create a T1-weighted image?
Short TR and short TE.
177
What kind of T1 recovery time does fat have?
Short.
178
Is TR an extrinsic contrast parameter?
True.
179
Which type of MRI weighting is most useful for identifying pathology?
T2-weighted imaging.
180
What TR value should be used to produce a T1-weighted image?
Short TR.
181
Where is ADC low due to small extracellular space?
Areas of restricted diffusion.
182
What effect does gadolinium-based contrast have on tissues?
It shortens T1 time.
183
What MRI technique images activated cerebral cortex using oxyhemoglobin and deoxyhemoglobin differences?
BOLD (Blood Oxygen Level Dependent Imaging).
184
A long TR maximizes _______________.
T2 contrast.
185
Does fat have a shorter T1 time than water?
True.
186
How does fat and water appear on T1-weighted images?
Bright fat and dark water.
187
What is another term for T2 relaxation?
T2 decay.
188
What primarily determines contrast in a T1-weighted image?
Differences in T1 recovery times between fat and water.
189
How do gadolinium-based contrast agents affect T1 time?
They shorten it.
190
What imaging technique uses the b-value?
DWI (Diffusion Weighted Imaging).
191
A tissue has high signal if it has:
A large transverse component of coherent magnetization at time TE.
192
How do T2 contrast agents affect T2 time?
They shorten it.
193
How does cortical bone appear on MRI?
Dark on both T1 and T2 images.
194
How does fat appear on a T2 contrast image?
Dark.
195
What factors determine T1 and T2 relaxation times?
Molecular tumbling rate, molecular spacing, and density.
196
What is not an extrinsic contrast parameter?
Proton Density (PD).
197
How do you maximize proton density contrast?
Use a long TR and a short TE.
198
What type of imaging is performed after gadolinium contrast?
T1-weighted imaging.
199
How do you maximize T2 contrast?
Use a long TR and a long TE.
200
What tissue appears bright on diffusion-weighted imaging?
Abnormal tissue with low ADC.
201
What does repetition time (TR) measure?
The time from one RF pulse to the next.
202
How do you maximize T1 contrast?
Use a short TR and a short TE.
203
What parameter determines how much T2 decay occurs?
Echo Time (TE).
204
What does DWI use to generate image weighting?
The b-value.
205
What term refers to the number of mobile hydrogen protons per unit volume?
Proton Density (PD).
206
What images are used for anatomy and post-gadolinium contrast enhancement?
T1-weighted images.
207
What does T2* represent?
Dephasing due to inhomogeneities.
208
What term describes molecular movement due to random thermal motion?
Diffusion.
209
What kind of contrast parameters are extrinsic?
Those that can be changed, such as TR, TE, and flip angle.
210
What is the MRI technique that images brain activity?
Functional MRI (fMRI).
211
What does ADC measure?
The net displacement of molecules diffusing across a tissue per second.
212
What is the name of the advanced MRI technique that images brain activity?
Functional MRI (fMRI).
213
What does ADC stand for?
Apparent Diffusion Coefficient.
214
What does the T1 recovery curve diagram represent?
The process of T1 recovery over time.
215
How does water appear on a T2 contrast image?
Hyperintense (bright).
216
What does the image of magnetic field inhomogeneities represent?
Variations in the magnetic field affecting signal uniformity.
217
What does the T2 spin-spin relaxation diagram illustrate?
The decay of transverse magnetization due to spin-spin interactions.
218
What substance has the same T1 recovery and T2 decay times?
Water.
219
What is caused by magnetic field inhomogeneities?
T2* dephasing.
220
What does it mean when 37% of transverse magnetization remains?
T2 decay time has been reached.
221
The higher the ______________, the more potential signal available from a tissue.
Proton Density.
222
How does water appear on a T1 contrast image?
Hypointense (dark).
223
What does the Free Induction Decay (FID) diagram show?
The signal decay after an RF pulse is removed.
224
Which relaxation process occurs first, spin-lattice or spin-spin?
They occur simultaneously.
225
What causes T1 recovery?
Spin-lattice relaxation.
226
What TR and TE settings produce a T2-weighted image?
Long TR and long TE.
227
How are T2-weighted images characterized?
Dark fat and bright water.
228
What happens when a long TR and a long TE are used?
A T2-weighted image is produced.
229
What does T2* represent?
Dephasing due to inhomogeneities.
230
What term describes the movement of molecules due to random thermal motion?
Diffusion.
231
What type of contrast parameters are extrinsic?
Those that can be changed, such as TR, TE, and flip angle.
