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MRI Physics > Chapter 2 > Flashcards

Chapter 2 Flashcards

1
Q

Why is contrast important in MRI?

A

Contrast is essential for distinguishing between normal anatomy and pathology in diagnostic imaging.

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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.

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3
Q

What are intrinsic contrast parameters?

A

Intrinsic contrast parameters are inherent to the body’s tissues and cannot be changed.

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4
Q

List the intrinsic contrast parameters in MRI.

A
  1. T1 recovery time
  2. T2 decay time
  3. Proton density (PD)
  4. Flow
  5. Apparent diffusion coefficient (ADC)
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5
Q

What are extrinsic contrast parameters?

A

Extrinsic contrast parameters can be controlled by adjusting the scan protocol.

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6
Q

List the extrinsic contrast parameters in MRI.

A
  1. TR (Repetition Time)
  2. TE (Echo Time)
  3. Flip angle
  4. TI (Inversion Time)
  5. Turbo Factor / Echo Train Length
  6. b-value (used in diffusion imaging)
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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.

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8
Q

What are the two types of relaxation?

A
  1. T1 recovery (spin-lattice relaxation)
  2. T2 decay (spin-spin relaxation)
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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.

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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.

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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

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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 |

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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.

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14
Q

What type of tissues recover longitudinal magnetization faster?

A

Fat recovers faster than water, meaning fat appears brighter on T1-weighted images.

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15
Q

What is T2 decay?

A

T2 decay is the loss of coherent transverse magnetization due to spin-spin interactions between hydrogen nuclei.

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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).

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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

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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 |

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19
Q

Why does T2 decay occur?

A

Due to magnetic field interactions between hydrogen nuclei causing small frequency differences, leading to dephasing.

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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).
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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.

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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

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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.

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24
Q

What three factors determine MRI image contrast?

A
  1. T1 recovery
  2. T2 decay
  3. Proton density (PD)
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25
Q

What is proton density (PD)?

A

PD refers to the number of mobile hydrogen protons per unit volume, affecting signal intensity.

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26
Q

How does T1 recovery contribute to contrast?

A

Tissues with short T1 recovery times (e.g., fat) appear bright on T1-weighted images.

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27
Q

How does T2 decay contribute to contrast?

A

Tissues with long T2 decay times (e.g., fluid) appear bright on T2-weighted images.

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28
Q

How does proton density (PD) contribute to contrast?

A

Tissues with higher proton density return higher signal and appear brighter on PD-weighted images.

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29
Q

What two factors influence T1 and T2 relaxation times?

A
  1. How closely molecular tumbling rate matches the Larmor frequency.
  2. How closely packed molecules are in the tissue.
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30
Q

How does molecular tumbling rate affect relaxation?

A
  • Fat tumbles slowly, matching the Larmor frequency, leading to fast T1 recovery.
  • Water tumbles rapidly, leading to slow T1 recovery.
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31
Q

What is spin-lattice energy transfer?

A

The process where hydrogen nuclei transfer energy to surrounding tissues, causing T1 recovery.

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32
Q

What is spin-spin interaction?

A

The process where hydrogen nuclei transfer energy to each other, causing T2 decay.

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33
Q

What are the molecular structures of fat and water?

A

Fat: Hydrogen is arranged with carbon and oxygen, forming large lipid molecules.
Water: Consists of two hydrogen atoms and one oxygen (H₂O).

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34
Q

Why does fat recover faster than water in MRI?

A

Fat molecules are closely packed, and their slow molecular tumbling matches the Larmor frequency, making energy exchange efficient.

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35
Q

Why does water take longer to recover in MRI?

A

Water molecules are spaced apart and have fast molecular tumbling, which does not match the Larmor frequency, making energy exchange inefficient.

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36
Q

What is T1 recovery?

A

T1 recovery is the realignment of hydrogen nuclei along the longitudinal axis after the RF excitation pulse is removed.

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37
Q

How does T1 recovery occur in fat?

A

Fat has a low inherent energy and absorbs energy efficiently. The slow molecular tumbling in fat allows rapid T1 recovery.

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38
Q

How does T1 recovery occur in water?

A

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.

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39
Q

How does magnetic field strength (B0) affect T1 recovery?

A

As B0 increases, T1 recovery takes longer because fewer molecules move at relaxation-causing frequencies.

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40
Q

What is T2 decay?

A

T2 decay is the loss of transverse magnetization due to spin-spin interactions between hydrogen nuclei.

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41
Q

Why does fat experience faster T2 decay?

A

Fat molecules are closely packed, leading to strong spin-spin interactions. Magnetic moments dephase quickly, resulting in short T2 decay time.

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42
Q

Why does water experience slower T2 decay?

A

Water molecules are spaced apart, leading to fewer spin-spin interactions. Magnetic moments dephase slowly, resulting in long T2 decay time.

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43
Q

How does magnetic field strength (B0) affect T2 decay?

A

T2 decay is slightly prolonged as B0 increases, but the effect is less significant than in T1 recovery.

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44
Q

What is T1 contrast in MRI?

A

T1 contrast is derived from the differences in T1 recovery times between tissues.

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45
Q

How does TR affect T1 contrast?

A

Short TR → Increased T1 contrast
Long TR → Reduced T1 contrast (Tissues fully recover, minimizing contrast differences).

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46
Q

What is the T1 recovery time difference between fat and water?

A

Fat: Short T1 recovery time (realigns quickly with B0).
Water: Long T1 recovery time (takes longer to realign).

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47
Q

What happens if TR is shorter than tissue relaxation times?

A

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).

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48
Q

What is partial saturation?

A

When the NMV is flipped beyond 90° but not fully to 180°, affecting signal intensity.

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49
Q

What is full saturation?

A

When the NMV is flipped to 180°, meaning there is no remaining longitudinal magnetization before the next RF pulse.

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50
Q

What happens if TR is shorter than T1 relaxation times?

A

Fat and water do not fully recover before the next RF pulse. This results in partial saturation, increasing T1 contrast.

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51
Q

What happens if TR is longer than T1 relaxation times?

A

Fat and water fully recover before the next RF pulse. T1 contrast is lost, and contrast depends only on proton density (PD).

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52
Q

What is the steady-state in MRI?

A

A condition where vectors recover the same amount of longitudinal magnetization every TR, ensuring consistent signal generation.

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53
Q

What are preparatory or dummy pulses?

A

The first few RF pulses applied in a sequence that do not contribute to image formation but help achieve steady-state magnetization.

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54
Q

What factors influence how long it takes to reach the steady-state?

A
  1. Magnetic field strength (B0)
  2. Proton density (PD)
  3. Flip angle
  4. T1 relaxation time
  5. RF pulse duration
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55
Q

How does T1 recovery differ between fat and water?

A

Fat: Short T1 recovery time (realigns quickly).
Water: Long T1 recovery time (realigns slowly).

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56
Q

How does B0 affect T1 recovery?

A

As B0 increases, T1 recovery takes longer due to fewer molecules tumbling at the relaxation-causing frequency.

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57
Q

How does B0 affect T2 decay?

A

T2 decay is slightly longer at higher field strengths, but the effect is minor compared to T1.

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58
Q

What scan parameter controls T1 contrast?

A

TR (Repetition Time) determines T1 contrast.

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59
Q

What TR values are needed for good T1 contrast?

A

Short TR → Strong T1 contrast.
Long TR → Poor T1 contrast (all tissues fully recover).

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60
Q

What are the key differences between fat and water in MRI?

A

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)

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61
Q

What is T2 contrast in MRI?

A

T2 contrast occurs when image contrast is based on differences in the T2 decay times of tissues.

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62
Q

How does TE affect T2 contrast?

A
  • Long TE → Increases T2 contrast (more dephasing between tissues).
  • Short TE → Reduces T2 contrast (less dephasing).
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63
Q

Why does water appear bright on T2-weighted images?

A

Water has a long T2 decay time, meaning it retains more coherent transverse magnetization and appears hyperintense (bright).

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64
Q

Why does fat appear dark on T2-weighted images?

A

Fat has a short T2 decay time, so it loses transverse magnetization quickly and appears hypointense (dark).

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65
Q

What are the key characteristics of T2 decay?

A
  • 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.
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66
Q

What is proton density (PD) contrast?

A

PD contrast refers to differences in signal intensity between tissues based on their hydrogen proton density per unit volume.

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67
Q

How does proton density affect MRI contrast?

A
  • Tissues with high proton density → Hyperintense (bright) signal.
  • Tissues with low proton density → Hypointense (dark) signal.
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68
Q

What intrinsic contrast properties affect signal intensity?

A
  1. Proton density (PD)
  2. T1 recovery time
  3. T2 decay time
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69
Q

What extrinsic contrast parameters affect signal intensity?

A
  1. Repetition Time (TR)
  2. Echo Time (TE)
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70
Q

What is weighting in MRI?

A

Weighting refers to selecting extrinsic contrast parameters (TR, TE) to emphasize one intrinsic contrast property (T1, T2, PD) over others.

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71
Q

What is a T1-weighted image?

A

A T1-weighted image is one where contrast is based on differences in T1 recovery times between tissues.

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72
Q

How is T1 weighting achieved?

A
  • Short TR (minimizes full recovery of magnetization).
  • Short TE (minimizes T2 effects).
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73
Q

What happens if TR is too long?

A

Both fat and water fully recover before the next RF pulse, reducing T1 contrast.

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74
Q

Why is fat bright and water dark on T1-weighted images?

A
  • 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).
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75
Q

What tissues have a high signal (bright) on T1-weighted images?

A
  • Fat
  • Methemoglobin
  • Hemangiomas
  • Fatty degeneration
  • Cysts with proteinaceous fluid
  • Slow-flowing blood
  • Paramagnetic contrast agents
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76
Q

What tissues have a low signal (dark) on T1-weighted images?

A
  • Cortical bone
  • Avascular necrosis
  • Infarction
  • Tumors
  • Sclerosis
  • Calcifications
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77
Q

What tissues produce no signal (black) on T1-weighted images?

A
  • Air
  • Fast-flowing blood
  • Tendons
  • Scar tissue
  • Cortical bone
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78
Q

What is the equation for T1 contrast?

A

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.

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79
Q

What are the key characteristics of T1 weighting?

A
  • 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.
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80
Q

What is a T2-weighted image?

A

A T2-weighted image is one where contrast depends on differences in T2 decay times between tissues.

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81
Q

How is T2 weighting achieved?

A
  • Long TR (minimizes T1 effects).
  • Long TE (maximizes T2 decay contrast).
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82
Q

Why is water bright and fat dark on T2-weighted images?

A
  • 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).
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83
Q

What tissues have a high signal (bright) on T2-weighted images?

A
  • Water
  • Synovial fluid
  • Infection
  • Inflammation
  • Edema
  • Tumors
  • Hemorrhage
  • Cysts
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84
Q

What tissues have a low signal (dark) on T2-weighted images?

A
  • Cortical bone
  • Bone islands
  • Deoxyhemoglobin
  • Calcifications
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85
Q

What tissues produce no signal (black) on T2-weighted images?

A
  • Air
  • Fast-flowing blood
  • Tendons
  • Scar tissue
  • Cortical bone
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86
Q

What is the equation for T2 contrast?

A

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.

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87
Q

What are the key characteristics of T2 weighting?

A
  • 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.
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88
Q

What is the final comparison of T1 vs. T2 weighting?

A

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 |

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89
Q

What is diffusion in MRI?

A

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.

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90
Q

What is the apparent diffusion coefficient (ADC)?

A

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.

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91
Q

What does a high ADC value indicate?

A

A high ADC means free diffusion due to large extracellular spaces, seen in normal gray matter and normal liver tissue.

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92
Q

What does a low ADC value indicate?

A

A low ADC means restricted diffusion due to small extracellular spaces, seen in ligaments and pathology.

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93
Q

How is diffusion-weighted imaging (DWI) achieved?

A

By applying two diffusion-sensitizing gradients: The first dephases magnetic moments. The second rephases them if diffusion is restricted.

94
Q

How does signal intensity relate to ADC in DWI?

A

Low ADC → High signal (bright) (e.g., stroke, tumors). High ADC → Low signal (dark) (e.g., normal brain tissue).

95
Q

What is the b factor in DWI?

A

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
Q

Equation for b factor in DWI

A

b = γ² × G² × δ² × (Δ - δ/3)

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
Q

What is functional MRI (fMRI)?

A

A technique that acquires rapid MRI images of the brain during activity and at rest to measure brain function rather than anatomy.

98
Q

What is the main physiological mechanism of fMRI?

A

fMRI relies on Blood Oxygenation Level Dependent (BOLD) contrast, which detects changes in oxyhemoglobin vs. deoxyhemoglobin levels.

99
Q

How does deoxyhemoglobin affect fMRI signal?

A

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
Q

How is an fMRI image processed?

A

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
Q

What is magnetization transfer contrast (MTC)?

A

MTC is an MRI technique that enhances contrast by manipulating the exchange of magnetization between bound and free nuclei.

102
Q

What is the difference between bound and free nuclei?

A

Bound nuclei → Restricted movement, very short T2 decay, normally not imaged. Free nuclei → Observable, have longer T2 decay times.

103
Q

How does MTC affect signal intensity?

A

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
Q

What is susceptibility-weighted imaging (SWI)?

A

SWI exploits magnetic susceptibility differences between tissues to enhance contrast, especially for detecting blood products and mineral deposits.

105
Q

Which pulse sequence is commonly used for SWI?

A

Gradient-echo (GRE) sequences with a long TE enhance susceptibility differences.

106
Q

Why are contrast agents used in MRI?

A

They alter T1 and T2 relaxation times of certain tissues, improving contrast between pathology and normal tissues.

107
Q

What is relaxivity?

A

Relaxivity refers to the ability of a contrast agent to selectively shorten T1 or T2 relaxation times of nearby water molecules.

108
Q

What are the two main types of MRI contrast agents?

A

T1 agents (shorten T1 recovery times, make tissues hyperintense). T2 agents (shorten T2 decay times, make tissues hypointense).

109
Q

What is the most commonly used T1 contrast agent?

A

Gadolinium (Gd) – a paramagnetic rare-earth metal, made safe by chelation with diethylene triaminepentaacetic acid (DTPA).

110
Q

Why is gadolinium chelated?

A

In its natural form, gadolinium is toxic, so it is bound to chelating molecules to allow safe excretion from the body.

111
Q

How does gadolinium enhance MRI contrast?

A

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
Q

What are T2 contrast agents?

A

Typically superparamagnetic iron oxide (SPIO) particles. Distort local magnetic fields, leading to T2 shortening and hypointense signals.

113
Q

What are key features of DWI?

A

Uses diffusion gradients to differentiate restricted vs. free diffusion.

114
Q

What are key features of ADC?

A

Measures net diffusion of molecules per second in tissue.

115
Q

What are key features of fMRI?

A

Uses BOLD contrast to measure brain activity.

116
Q

What are key features of MTC?

A

Enhances contrast by transferring energy from bound to free nuclei.

117
Q

What are key features of SWI?

A

Exploits magnetic susceptibility differences, using GRE sequences.

118
Q

What are key features of Gadolinium (Gd)?

A

Shortens T1, making enhancing tissues hyperintense.

119
Q

What are key features of iron-based contrast?

A

Shortens T2, making enhancing tissues hypointense.

120
Q

What are intrinsic contrast parameters?

A

Factors that cannot be changed because they are inherent in body tissues. Examples: T1 recovery, T2 decay, proton density (PD), flow, ADC.

121
Q

What are extrinsic contrast parameters?

A

Factors that can be changed and selected at the console. Examples: TR, TE, TI, flip angle, ETL, b-value.

122
Q

What is relaxation in MRI?

A

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
Q

What is T1 recovery?

A

Growth of longitudinal magnetization as a result of spin-lattice relaxation.

124
Q

What percentage of total energy must be regained for T1 relaxation, and in which plane does it occur?

A

63% of the total magnetization must be regained in the longitudinal plane.

125
Q

What is T2 decay?

A

Loss of transverse magnetization as a result of spin-spin relaxation.

126
Q

What percentage of total energy must be lost for T2 relaxation, and in which plane does it occur?

A

63% of the total magnetization must be lost in the transverse plane.

127
Q

What is proton density?

A

The number of mobile hydrogen protons per unit volume of the tissue.

128
Q

How does proton density affect signal intensity?

A

The higher the proton density, the more signal available from that tissue.

129
Q

What does T1 and T2 relaxation depend on?

A
  • T1: Molecular tumbling rate matching the Larmor frequency of hydrogen.
  • T2: Molecules closely packed together increasing spin-spin interactions.
130
Q

What is the T1 recovery time of fat and why?

A

Short; fat has low inherent energy, allowing it to quickly regain longitudinal magnetization.

131
Q

What is the T1 recovery time of water and why?

A

Long; water has high inherent energy and takes longer to regain longitudinal magnetization.

132
Q

What is the T2 decay time of fat and why?

A

Short; fat molecules are packed closely together, leading to frequent spin-spin interactions.

133
Q

What is the T2 decay time of water and why?

A

Long; water molecules are more spread apart, making spin-spin interactions less likely.

134
Q

Which tissue appears bright on a T1-weighted image?

A

Fat.

135
Q

Which tissue appears dark on a T1-weighted image?

A

Water.

136
Q

Which tissue appears bright on a T2-weighted image?

A

Water.

137
Q

Which tissue appears dark on a T2-weighted image?

A

Fat.

138
Q

How does tissue with high and low proton density appear on MRI?

A
  • High PD: Hyperintense (bright).
  • Low PD: Hypointense (dark).
139
Q

What TR and TE values create a T1-weighted image?

A

Short TR, short TE.

140
Q

What TR and TE values create a T2-weighted image?

A

Long TR, long TE.

141
Q

What TR and TE values create a PD-weighted image?

A

Long TR, short TE.

142
Q

What is diffusion in MRI?

A

Movement of molecules in the extracellular space due to random thermal motion.

143
Q

What is ADC (apparent diffusion coefficient)?

A

The net displacement of molecules diffusing across an area of tissue per second.

144
Q

What is diffusion-weighted imaging (DWI)?

A

A technique that produces images based on differences in ADC between tissues. Damaged tissue appears bright due to restricted diffusion.

145
Q

What is the b-value in diffusion-weighted imaging?

A

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
Q

What is functional MRI (fMRI)?

A

A rapid imaging technique that assesses brain function by detecting changes in blood oxygenation levels.

147
Q

What is BOLD imaging in fMRI?

A

Blood Oxygen Level Dependent imaging, which detects differences in magnetic susceptibility between oxyhemoglobin and deoxyhemoglobin in active brain areas.

148
Q

What is magnetization transfer contrast (MTC)?

A

A technique that enhances contrast by exchanging energy between bound and free water molecules, often used to suppress background tissue.

149
Q

What is susceptibility-weighted imaging (SWI)?

A

A technique that enhances contrast based on differences in magnetic susceptibility between tissues.

150
Q

How do MRI contrast agents work?

A

They indirectly affect the relaxation times of water nuclei to enhance imaging contrast.

151
Q

What type of contrast agent is gadolinium, and what does it do?

A

A T1 contrast agent that shortens T1 relaxation time, making enhancing tissues appear bright on T1-weighted images.

152
Q

What are some common gadolinium-based contrast agents?

A

Gadavist, Magnevist, MultiHance, Eovist, Ablavar.

153
Q

What causes T2 decay?

A

Spin-spin relaxation.

154
Q

How does CSF appear on T2-weighted images?

A

Hyperintense (bright).

155
Q

What does SWI stand for?

A

Susceptibility Weighted Imaging.

156
Q

Is ADC an extrinsic contrast parameter?

A

False.

157
Q

What kind of T1 recovery time does water have?

A

Long.

158
Q

What causes the growth of longitudinal magnetization?

A

Spin-lattice relaxation.

159
Q

Gadolinium-based contrast agents are considered _______________.

A

T1 agents.

160
Q

What determines the flip angle?

A

The amplitude and duration of the excitation RF pulse.

161
Q

What are intrinsic contrast parameters?

A

Factors inherent to body tissues that cannot be changed.

162
Q

What imaging technique uses magnetic susceptibility differences to generate contrast?

A

SWI (Susceptibility Weighted Imaging).

163
Q

Most pathology is associated with what change in fluid levels?

A

An increase in fluids.

164
Q

When is T2 relaxation time reached?

A

When 63% of the transverse magnetization has decayed.

165
Q

How is gadolinium made safe for injection?

A

By chelating and binding to other molecules.

166
Q

On which type of imaging is water hyperintense?

A

T2-weighted imaging.

167
Q

What MRI technique involves the fast exchange of energy between bound and free nuclei?

A

MTC (Magnetization Transfer Contrast).

168
Q

What factors determine MRI signal intensity?

A

Proton density and the amount of transverse magnetization at time TE.

169
Q

What TR and TE settings produce a T1-weighted image?

A

Short TR and short TE.

170
Q

Does water have a shorter T2 time than fat?

A

False.

171
Q

How do tissues with high proton density appear on MRI?

A

Hyperintense (bright).

172
Q

How does fat appear on a T1-weighted image?

A

Hyperintense (bright).

173
Q

When is T1 relaxation time reached?

A

When 63% of the longitudinal magnetization has recovered.

174
Q

What occurs due to hydrogen nuclei giving up their energy to the surrounding molecular lattice?

A

T1 recovery.

175
Q

How does water appear on a T1-weighted image?

A

Hypointense (dark).

176
Q

What TR and TE settings create a T1-weighted image?

A

Short TR and short TE.

177
Q

What kind of T1 recovery time does fat have?

A

Short.

178
Q

Is TR an extrinsic contrast parameter?

A

True.

179
Q

Which type of MRI weighting is most useful for identifying pathology?

A

T2-weighted imaging.

180
Q

What TR value should be used to produce a T1-weighted image?

A

Short TR.

181
Q

Where is ADC low due to small extracellular space?

A

Areas of restricted diffusion.

182
Q

What effect does gadolinium-based contrast have on tissues?

A

It shortens T1 time.

183
Q

What MRI technique images activated cerebral cortex using oxyhemoglobin and deoxyhemoglobin differences?

A

BOLD (Blood Oxygen Level Dependent Imaging).

184
Q

A long TR maximizes _______________.

A

T2 contrast.

185
Q

Does fat have a shorter T1 time than water?

A

True.

186
Q

How does fat and water appear on T1-weighted images?

A

Bright fat and dark water.

187
Q

What is another term for T2 relaxation?

A

T2 decay.

188
Q

What primarily determines contrast in a T1-weighted image?

A

Differences in T1 recovery times between fat and water.

189
Q

How do gadolinium-based contrast agents affect T1 time?

A

They shorten it.

190
Q

What imaging technique uses the b-value?

A

DWI (Diffusion Weighted Imaging).

191
Q

A tissue has high signal if it has:

A

A large transverse component of coherent magnetization at time TE.

192
Q

How do T2 contrast agents affect T2 time?

A

They shorten it.

193
Q

How does cortical bone appear on MRI?

A

Dark on both T1 and T2 images.

194
Q

How does fat appear on a T2 contrast image?

A

Dark.

195
Q

What factors determine T1 and T2 relaxation times?

A

Molecular tumbling rate, molecular spacing, and density.

196
Q

What is not an extrinsic contrast parameter?

A

Proton Density (PD).

197
Q

How do you maximize proton density contrast?

A

Use a long TR and a short TE.

198
Q

What type of imaging is performed after gadolinium contrast?

A

T1-weighted imaging.

199
Q

How do you maximize T2 contrast?

A

Use a long TR and a long TE.

200
Q

What tissue appears bright on diffusion-weighted imaging?

A

Abnormal tissue with low ADC.

201
Q

What does repetition time (TR) measure?

A

The time from one RF pulse to the next.

202
Q

How do you maximize T1 contrast?

A

Use a short TR and a short TE.

203
Q

What parameter determines how much T2 decay occurs?

A

Echo Time (TE).

204
Q

What does DWI use to generate image weighting?

A

The b-value.

205
Q

What term refers to the number of mobile hydrogen protons per unit volume?

A

Proton Density (PD).

206
Q

What images are used for anatomy and post-gadolinium contrast enhancement?

A

T1-weighted images.

207
Q

What does T2* represent?

A

Dephasing due to inhomogeneities.

208
Q

What term describes molecular movement due to random thermal motion?

A

Diffusion.

209
Q

What kind of contrast parameters are extrinsic?

A

Those that can be changed, such as TR, TE, and flip angle.

210
Q

What is the MRI technique that images brain activity?

A

Functional MRI (fMRI).

211
Q

What does ADC measure?

A

The net displacement of molecules diffusing across a tissue per second.

212
Q

What is the name of the advanced MRI technique that images brain activity?

A

Functional MRI (fMRI).

213
Q

What does ADC stand for?

A

Apparent Diffusion Coefficient.

214
Q

What does the T1 recovery curve diagram represent?

A

The process of T1 recovery over time.

215
Q

How does water appear on a T2 contrast image?

A

Hyperintense (bright).

216
Q

What does the image of magnetic field inhomogeneities represent?

A

Variations in the magnetic field affecting signal uniformity.

217
Q

What does the T2 spin-spin relaxation diagram illustrate?

A

The decay of transverse magnetization due to spin-spin interactions.

218
Q

What substance has the same T1 recovery and T2 decay times?

A

Water.

219
Q

What is caused by magnetic field inhomogeneities?

A

T2* dephasing.

220
Q

What does it mean when 37% of transverse magnetization remains?

A

T2 decay time has been reached.

221
Q

The higher the ______________, the more potential signal available from a tissue.

A

Proton Density.

222
Q

How does water appear on a T1 contrast image?

A

Hypointense (dark).

223
Q

What does the Free Induction Decay (FID) diagram show?

A

The signal decay after an RF pulse is removed.

224
Q

Which relaxation process occurs first, spin-lattice or spin-spin?

A

They occur simultaneously.

225
Q

What causes T1 recovery?

A

Spin-lattice relaxation.

226
Q

What TR and TE settings produce a T2-weighted image?

A

Long TR and long TE.

227
Q

How are T2-weighted images characterized?

A

Dark fat and bright water.

228
Q

What happens when a long TR and a long TE are used?

A

A T2-weighted image is produced.

229
Q

What does T2* represent?

A

Dephasing due to inhomogeneities.

230
Q

What term describes the movement of molecules due to random thermal motion?

A

Diffusion.

231
Q

What type of contrast parameters are extrinsic?

A

Those that can be changed, such as TR, TE, and flip angle.

MRI Physics (6 decks)

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