chapter 4 Flashcards
How do gradient echo sequences differ from spin echo sequences?
Use gradients rather than RF pulses to rephase magnetic moments.
What effect does using a smaller flip angle have on the NMV?
Increases longitudinal magnetization recovery.
Which characteristic defines gradient echo (GRE) pulse sequences?
Gradient rephasing and variable flip angles.
What causes spins in the vector to rephase in a GRE sequence?
Application of a gradient.
What is the function of a spoiler in MRI?
To dephase residual transverse magnetization.
What parameters are used for T2 weighting in GRE sequences?
Small flip angle, long TR, long TE.
Which GRE sequence maintains residual transverse magnetization through rewinding?
Coherent GRE.
What is the purpose of incoherent GRE sequences?
Minimize the effects of residual transverse magnetization.
How does reverse-echo gradient echo differ from traditional GRE sequences?
Captures true T2-weighted images.
What is the main advantage of balanced GRE sequences?
Shorter scan times with fewer flow artifacts.
Gradient echo sequences use variable flip angles instead of the 90° flip angle used in spin echo sequences.
True
Residual transverse magnetization is eliminated in coherent GRE sequences.
False
Rewinder gradients are used to rephase transverse magnetization in GRE sequences.
True
Echo planar imaging (EPI) is primarily used to reduce physiological motion artifacts.
True
Spoilers are gradients that rephase the spins in a vector.
False
Reverse-echo gradient echo sequences capture true T2 contrast data.
True
Balanced gradient echo sequences are only used in cardiac imaging.
False
Incoherent GRE sequences use RF spoiling to dephase residual transverse magnetization.
True
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.
Condition where energy in equals energy out, maintaining stable magnetization levels.
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.
Magnetization left over from previous RF pulses in steady-state conditions.
Match the terms in Column A with their descriptions in Column B: 3️⃣ Coherent GRE → Maintains residual magnetization coherence through rewinding.
Maintains residual magnetization coherence through rewinding.
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.
Dephases residual transverse magnetization to minimize its effect on contrast.
Match the terms in Column A with their descriptions in Column B: 5️⃣ Reverse-Echo Gradient Echo → Captures true T2 contrast using rewinder gradients.
Captures true T2 contrast using rewinder gradients.
Match the terms in Column A with their descriptions in Column B: 6️⃣ Balanced GRE → Reduces flow artifacts using balanced gradient systems.
Reduces flow artifacts using balanced gradient systems.
What are the most common flip angles in a GRE pulse sequence?
Less than 90 degrees.
What can be used to dephase spins in a GRE pulse sequence?
A gradient spoiler.
Why is a variable, less than 90-degree, flip angle used?
All of the above (to decrease imaging time, utilize shorter TRs, and permit shorter recovery times).
What is letter C in the diagram?
Dephasing gradient.
What is the safety issue associated with GR-EPI?
Peripheral nerve stimulation.
What are the disadvantages of GRE pulse sequences?
All of the above (reduced SNR, loud gradient noise, magnetic susceptibility).
When precessing nuclei are exposed to an increased magnetic field, they:
Speed up.
What is an advantage of using gradient echo sequences?
Faster scan times.
Arthrographic means:
Visualize joint spaces.
What type of pulse sequence is shown in the image?
GR-EPI.
Which letter is the FID?
F.
The biggest factor in a sequence scan time is:
TR.
What is letter E in the diagram?
Time to echo.
Which statement is NOT true about FSE T2 pulse sequences?
Fat is darker than on SE pulse sequences.
GRE T2*-weighted sequences use:
A gradient rewinder.
The steady state is equal amounts of longitudinal and transverse magnetization.
True.
What is the conventional scan time formula?
TR × Phase Matrix (PM) × NEX.
Imaging parameters in GRE T1-weighted sequences should have:
Large flip angle, short TR, short TE.
Gradient echo sequences differ from spin echo sequences by:
A & B (use variable RF excitation pulse flip angles, use gradients rather than RF pulses to rephase).
What does ‘bi-polar’ mean in MRI?
It consists of two lobes, one negative and one positive.
Uses for incoherent GRE sequences include:
T1-weighted imaging.
Which of the following best describes a GR-EPI sequence?
A ‘series’ of gradient echoes.
How does using different (variable) flip angles in GRE pulse sequences affect NMV?
NMV achieves full relaxation in a much shorter TR than in spin echo sequences.
TR, TE, and flip angle are what type of contrast parameters?
Extrinsic.
What type of image would be yielded in the given scenario?
T2*.
Although GRE is faster than RF rephasing, what is NOT compensated for in this type of sequence?
Inhomogeneities.
Large 70°+ flip angles are used for:
T1-weighted imaging.
NMV saturation occurs from what range?
91 to 180 degrees.
What weighted image would be created with a GRE pulse sequence using TR 50, TE 5, and FA 70?
T1-weighted.
Dynamic enhanced T1-weighted MRA sequences of the renal arteries use:
Incoherent gradient echoes.
What is letter B in the diagram?
Variable-degree excitation pulse.
What is the Ernst angle?
The flip angle that provides optimum signal intensity for a tissue with a given T1 recovery time scanned using a given TR.
Angiographic means:
Visualize blood vessels and flow.
What weighted image would be created with a GRE pulse sequence using TR 100, TE 20, and FA 20?
T2*-weighted.
T2* decay is due to:
Inhomogeneities in the magnetic field.
When precessing nuclei are exposed to a decreased magnetic field, they:
Slow down.
Coherent GRE sequences are typically:
T2-weighted.
What does EPI stand for?
Echo planar imaging.
Gradient echo sequences can yield T1, PD, or T2* contrast.
True.
The pulse sequence consisting of a variable flip angle RF excitation pulse followed by a pulse of the magnetic field gradients is called:
Gradient echo.
An inversion recovery sequence starts with what RF pulse?
180-degree pulse.
TEs in a GRE sequence are typically __________ than CSE.
Shorter.
What is letter D in the diagram?
Rephasing gradient.
T2* is used to describe spin-spin dephasing.
False.
To perform angiographic, myelographic, and arthrographic techniques, what type of GRE is typically used?
T2*-weighted.
In a balanced GRE acquisition, the contrast weighting is:
Weighted for the ratio of T2/T1.
What is a steady state in MRI?
A condition where energy in equals energy out.
Small 5°–20° flip angles are used for:
B & C (T2* and PD-weighted).
What is isocenter in an MRI?
The exact middle of the magnet.
Gradient echo sequences acquired for the evaluation of hemorrhagic lesions rely on:
Susceptibility.
GRE pulse sequences are ____________ than CSE.
Louder.
What is EPI used for?
Single shot or multi-shot acquisition that fills k-space with data from gradient echoes.
A GRE sequence in which any residual transverse magnetization is removed before the next excitation pulse is known as:
Incoherent/spoiled GRE.
What is the opposite of gradient rewinding?
Spoiling.
When a GRE sequence with T1 weighting is acquired for dynamic contrast-enhanced imaging of the liver, _____________ is performed.
Spoiling.
GRE sequences use ________________ to dephase and rephase spins.
Bi-polar gradients.
To maximize T1 weighting in a GRE sequence, the ___ should be ___.
TR, short.
Define residual transverse magnetization:
Transverse magnetization left over from previous RF pulse in steady state conditions.
Gradient spoilers:
Dephase magnetization.
Gradient rewinders:
Rephase magnetization.
What is a rapid acquisition technique that starts with RF pulses followed by a series of gradient echoes?
Echo planar imaging (EPI).
GRE T1-weighted sequences use:
Gradient or RF spoiling.
Incoherent GRE sequences are:
T1-weighted.
What does GRASE stand for?
Gradient and spin echo pulse sequence.
What determines how long we wait at each slice for an echo?
TE.
In the diagram, identify letter ‘B’:
Rewinder.
To minimize differences in T2* decay times, the ____________ is short so that neither fat nor water has time to decay.
TE.
Any two 90° RF pulses with varying amplitude are called:
Hahn echo.
The gradients in the MRI system are used to:
Alter the main magnetic field.
Myelographic means:
Spinal cord and nerve visualization.
What type of image would this scenario yield?
T2*.
GRE sequences acquired for high signal from fluid are known as all of the following EXCEPT:
Incoherent gradient echoes.
What type of contrast weighting does a balanced GRE sequence provide?
T2/T1 ratio contrast.
What is the primary safety concern associated with GR-EPI?
Peripheral nerve stimulation.
Why is a variable, less than 90-degree flip angle used in GRE?
To permit shorter recovery times, decrease imaging time, and utilize shorter TRs.
How do gradient-echo pulse sequences differ from spin-echo pulse sequences?
- They use variable RF excitation pulse flip angles instead of the fixed 90° RF flip angles used in spin-echo sequences.
- They use gradients instead of RF pulses to rephase hydrogen nuclei and form an echo.
What is the primary purpose of the two main differences in gradient-echo pulse sequences?
To enable shorter TRs and therefore faster scan times compared to spin-echo sequences.
What is the role of variable flip angles in gradient-echo sequences?
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.
What is the flip angle commonly used in gradient-echo pulse sequences?
A flip angle of less than 90° is typically used in gradient-echo sequences.
How does a smaller flip angle impact relaxation in gradient-echo sequences?
A smaller flip angle means faster T1 recovery, which allows for shorter TRs and quicker scan times.
Why does a gradient-echo sequence result in shorter scan times than a spin-echo sequence?
- Uses smaller flip angles, reducing the time needed for T1 recovery.
- Uses gradients instead of 180° RF pulses, enabling shorter TR values.
What causes the immediate occurrence of Free Induction Decay (FID) after an RF excitation pulse is withdrawn?
- Magnetic field inhomogeneities
- T2* decay
How is transverse magnetization rephased in spin-echo sequences?
A 180° RF pulse is applied to rephase the transverse magnetization, creating a spin echo.
Why can’t RF pulses be used to rephase transverse magnetization in gradient-echo sequences?
- 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 is transverse magnetization rephased in gradient-echo pulse sequences?
A gradient field is applied instead of an RF pulse to rephase the transverse magnetization.
What role do gradients play in MRI?
Gradients:
1. Rephase or dephase hydrogen nuclei.
2. Create slice selection, phase encoding, and frequency encoding.
What happens when a gradient is applied to coherent magnetization?
- It alters the magnetic field strength experienced by different hydrogen nuclei.
- Some nuclei speed up, while others slow down, causing dephasing.
What is the effect of applying a gradient on a coherent magnetization?
- The magnetic moments fan out due to frequency changes caused by the gradient.
- This leads to dephasing of the signal.
Why do gradient-echo sequences require rephasing gradients?
Since RF pulses cannot rephase transverse magnetization, a gradient is used instead.
What does the table of gradient-echo acronyms provide?
A comparison of gradient-echo sequence names across different MRI manufacturers.
What does the acronym GRASS stand for?
Gradient Recalled Acquisition in the Steady State (GRASS).
What does the acronym FLASH stand for?
Fast Low-Angled Shot (FLASH).
What is the main advantage of gradient-echo sequences compared to spin-echo sequences?
Faster scan times due to shorter TR values.
Why does FID occur immediately after an RF excitation pulse is withdrawn?
Due to inhomogeneities in the magnetic field and T2* decay.
What is steady-state free precession (SSFP)?
A type of gradient-echo sequence where magnetization is maintained using rephasing gradients.
How does a gradient-echo pulse sequence achieve image contrast?
By using variable flip angles and gradient rephasing.
Why does a spin-echo sequence use a 180° RF pulse?
To rephase transverse magnetization and form a spin echo.
What happens to the magnetization in gradient-echo sequences when the RF excitation pulse is turned off?
Most of the magnetization remains in the longitudinal plane rather than flipping to transverse.
What is the effect of the trailing edge of the fan in gradient-echo sequences?
The trailing edge (purple) consists of nuclei that slow down because they are in a lower magnetic field strength relative to the isocenter.
What is the effect of the leading edge of the fan in gradient-echo sequences?
The leading edge (red) consists of nuclei that speed up because they are in a higher magnetic field strength relative to the isocenter.
What happens to the magnetic moments of nuclei when gradients are applied?
They are no longer in the same place at the same time, causing dephasing of magnetization.
What is the term for gradients that cause dephasing of magnetic moments?
They are called spoilers, and the process is called gradient spoiling.
How do gradients rephase incoherent magnetization?
- 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.
What are gradients that rephase called?
They are called rewinders.
What determines whether a gradient field adds or subtracts from the main magnetic field?
The direction of current passing through the gradient coils, also called gradient polarity.
What type of gradient is used to create a gradient-echo?
A bipolar gradient, which consists of two lobes (one negative, one positive).
How does a bipolar gradient create a gradient-echo?
- The negative lobe increases dephasing and eliminates the FID.
- The positive lobe rephases only those magnetic moments that were dephased by the negative lobe.
Why are gradient-echo sequences faster than spin-echo sequences?
Gradients rephase faster than RF pulses, generating echoes more quickly, which shortens TE and TR.
What is a disadvantage of gradient-echo sequences compared to spin-echo sequences?
They do not compensate for magnetic field inhomogeneities, leading to artifacts like magnetic susceptibility.
Why is T2 contrast in gradient-echo sequences called T2* contrast?
Because magnetic field inhomogeneities are not corrected, causing extra dephasing beyond normal T2 decay.
What are the three main mechanisms that affect weighting in gradient-echo sequences?
- Extrinsic parameters (TR, TE, flip angle)
- Steady state
- Residual transverse magnetization
How does TR affect T1 contrast in gradient-echo sequences?
A short TR prevents full longitudinal magnetization recovery, increasing T1 contrast.
How does TE affect T2* contrast in gradient-echo sequences?
A long TE allows for more T2* dephasing, increasing T2* contrast.
How does flip angle affect T1 contrast in gradient-echo sequences?
A large flip angle increases T1 contrast by reducing longitudinal recovery before the next RF pulse.
What combination of parameters maximizes T1 contrast in gradient-echo sequences?
- Short TR
- Large flip angle
- Short TE
What combination of parameters maximizes T2* contrast in gradient-echo sequences?
- Long TE
- Small flip angle
- Long TR
What is the main advantage of using a short TR in gradient-echo imaging?
Allows for faster scanning and the acquisition of more slices in a shorter time.
What are the key takeaways about gradient-echo pulse sequences?
- 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.
What strategies allow for shorter TR and TE in gradient-echo sequences?
- Gradient rephasing instead of RF rephasing.
- Low flip angles, reducing relaxation time.
Why does gradient rephasing allow for shorter TE?
Because gradients rephase magnetic moments faster than RF pulses, reducing wait time for the echo.
What is the impact of using a short TR in gradient-echo imaging?
- Faster scanning
- More slices per TR
- Increased T1 contrast
Why are gradient-echo sequences prone to magnetic susceptibility artifacts?
Because inhomogeneities are not corrected, leading to extra T2* dephasing.
What conditions maximize T2* contrast in gradient-echo sequences?
- 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)
Why is T2* weighting in gradient-echo different from T2 weighting in spin-echo?
Because gradient rephasing does not compensate for magnetic field inhomogeneities, leading to T2* decay rather than pure T2 decay.
How do you obtain a PD-weighted gradient-echo image?
- Short TE (to minimize T2* contrast)
- Small flip angle
- Long TR (to allow full recovery of longitudinal magnetization)
How do you ‘turn up the heat’ on T1 contrast?
- Short TR (low TR knob)
- Large flip angle (high flip angle knob)
- Short TE (low TE knob)
How do you ‘turn up the heat’ on T2* contrast?
- Long TE (high TE knob)
- Long TR (high TR knob)
- Small flip angle (low flip angle knob)
How do you ‘turn down the heat’ on both T1 and T2* contrast for PD-weighting?
- Long TR (high TR knob)
- Small flip angle (low flip angle knob)
- Short TE (low TE knob)
How do TR, TE, and flip angles compare between spin-echo and gradient-echo sequences?
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°+)
What extrinsic parameters control contrast in gradient-echo pulse sequences?
- TR & Flip Angle → Control whether NMV is saturated (T1 contrast)
- TE → Controls T2* weighting
How do TR, TE, and flip angle influence T1-weighted gradient-echo sequences?
- Large flip angle
- Short TR (to ensure saturation)
- Short TE (to minimize T2* contrast)
How do TR, TE, and flip angle influence T2*-weighted gradient-echo sequences?
- Small flip angle
- Long TR (to prevent saturation)
- Long TE (to maximize T2* contrast)
How do TR, TE, and flip angle influence PD-weighted gradient-echo sequences?
- Small flip angle
- Long TR (to prevent saturation)
- Short TE (to minimize T2* contrast)
What is the steady state in MRI?
A stable condition where the energy in = energy out, meaning longitudinal and transverse magnetization remain constant over time.
How does the steady state occur in MRI?
- 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.
How does the steady state affect image contrast?
Contrast depends on the T1/T2 ratio instead of absolute T1 or T2 values.
What tissues return high signal intensity in the steady state?
- Fat (short T1 & T2)
- Water (long T1 & T2)
What tissues return low signal intensity in the steady state?
- Muscle (short T2, long T1)
What is residual transverse magnetization?
Magnetization leftover from previous RF pulses, remaining in the transverse plane over multiple TRs.
How does residual transverse magnetization impact contrast?
It enhances T2 contrast because long T2 tissues (e.g., water) retain more transverse magnetization.
What happens to residual transverse magnetization in gradient-echo sequences?
It is rephased by subsequent RF pulses, producing stimulated echoes.
What are stimulated echoes?
Echoes generated by RF pulses with varying flip angles, rather than the standard 90° and 180° pulses of spin-echo sequences.
What are Hahn echoes?
Echoes produced by two 90° RF pulses, named after Erwin Hahn.
What types of echoes are formed in steady-state gradient-echo sequences?
- FID (Free Induction Decay)
- Stimulated Echoes (Hahn echoes)
What is the Ernst angle?
The flip angle that provides maximum signal intensity for a tissue with a given T1 relaxation time and TR.
How do you calculate the Ernst angle?
[\text{Ernst Angle} = \cos^{-1} (e^{-TR/T1})]
Where:
- TR = Repetition time
- T1 = T1 relaxation time of the tissue
What is the steady state in MRI?
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.
What are stimulated echoes in the steady state?
Stimulated echoes are echoes produced by residual transverse magnetization, rephased by subsequent RF pulses. These echoes primarily contain T2*/T2 information.
What determines contrast in steady-state gradient-echo sequences?
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)
What are the five types of gradient-echo pulse sequences?
- Coherent (Rewound) Gradient-Echo
- Incoherent (Spoiled) Gradient-Echo
- Reverse-Echo Gradient-Echo
- Balanced Gradient-Echo
- Fast Gradient-Echo
What is the mechanism of coherent gradient-echo sequences?
- 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
What contrast does coherent gradient-echo provide?
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.
What are common uses of coherent gradient-echo sequences?
- Angiographic, myelographic, or arthrographic imaging (because water is hyperintense)
- Assessing fluid-filled structures and blood flow
- Single breath-hold acquisitions
What are the advantages and disadvantages of coherent gradient-echo?
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 |
What are the suggested parameters for coherent gradient-echo?
- Flip angle: 30°–45°
- TR: 20–50 ms
- TE (for T2* weighting): 10–15 ms
- Gradient moment rephasing to reduce flow artifacts
What are some alternative TR, TE, and flip angle settings for coherent gradient-echo?
- 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°
What is the mechanism of incoherent (spoiled) gradient-echo sequences?
- 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)
What is the main goal of spoiling in gradient-echo sequences?
To dephase residual transverse magnetization, ensuring that only newly created FID signals contribute to image contrast.
What are the two methods of spoiling in incoherent gradient-echo?
-
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
-
Gradient Spoiling
- Uses slice-select, phase-encoding, and frequency-encoding gradients to dephase residual transverse magnetization
How does RF spoiling work in incoherent gradient-echo?
- The phase angle of RF pulses changes every TR
- The receiver coil detects only newly created transverse magnetization, ignoring residual transverse magnetization
What contrast does incoherent (spoiled) gradient-echo provide?
Since only the FID is sampled, it is mainly used for T1-weighted and proton density (PD) images.
How do coherent and incoherent gradient-echo sequences differ?
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 |
Why would you choose an incoherent gradient-echo sequence instead of a coherent one?
- To reduce T2* effects and flow artifacts
- To maximize T1 contrast
- To increase image sharpness in contrast-enhanced imaging
What are the key takeaways about coherent (rewound) gradient-echo?
- 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
What are the key takeaways about incoherent (spoiled) gradient-echo?
- 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
What is the main purpose of incoherent (spoiled) gradient-echo sequences?
To eliminate residual transverse magnetization and allow only the FID to contribute to image contrast, producing mainly T1- and PD-weighted images.
What are the two spoiling techniques used in incoherent gradient-echo sequences?
- RF Spoiling → Alters the phase angle of each RF excitation pulse every TR to eliminate residual transverse magnetization.
- Gradient Spoiling → Uses slice-select, phase-encoding, and frequency-encoding gradients to dephase residual transverse magnetization.
What are the common uses of incoherent (spoiled) gradient-echo sequences?
- T1-weighted imaging after gadolinium injection
- 2D breath-hold imaging
- 3D volumetric imaging for good anatomical detail
What are the advantages and disadvantages of incoherent gradient-echo?
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 | - |
What are the suggested scan parameters for incoherent gradient-echo?
- Flip angle: 30°–45°
- TR: 20–50 ms
- TE: 5–10 ms (short for T1 weighting)
What is the purpose of reverse-echo gradient-echo sequences?
To achieve true T2-weighting in gradient-echo sequences by repositioning the stimulated echo using a rephasing gradient.
What are the two TEs used in reverse-echo gradient-echo?
- Actual TE (TEact) → The time between the peak of the gradient-echo and the next RF excitation pulse.
- 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
Why does reverse-echo gradient-echo achieve better T2 contrast than conventional gradient-echo?
Because the effective TE is longer than the TR, allowing for better separation of true T2 contrast rather than T2*.
What are the common uses of reverse-echo gradient-echo sequences?
- Brain imaging (T2-weighted sequences)
- Joint imaging
- Perfusion imaging (used in rapid data acquisition)
What are the advantages and disadvantages of reverse-echo gradient-echo?
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 |
What are the suggested scan parameters for reverse-echo gradient-echo?
- Flip angle: 30°–45°
- TR: 20–50 ms
- Short TEact to maximize TEeff for T2 contrast
What is the key feature of balanced gradient-echo sequences?
A balanced gradient scheme that corrects for phase errors in flowing blood and CSF, enhancing the signal of moving fluids.
How does balanced gradient-echo differ from coherent gradient-echo?
- 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.
What are the common uses of balanced gradient-echo sequences?
- Cardiac and great vessel imaging
- Spinal imaging (CSF flow visualization)
- Internal auditory canal and cervical spine imaging
- Joint and abdominal imaging
What are the advantages and disadvantages of balanced gradient-echo?
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 |
What are the suggested scan parameters for balanced gradient-echo?
- Flip angle: Variable (higher flip angles increase signal)
- TR: Less than 10 ms (reduces scan time and flow artifact)
- TE: 5–10 ms
What is the primary goal of fast gradient-echo sequences?
To acquire a volume of images in a single breath-hold, reducing scan time while maintaining image quality.
How is the TE reduced in fast gradient-echo sequences?
- 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)
Why are inversion pulses sometimes used in fast gradient-echo sequences?
To pre-magnetize the tissue and enhance T1 contrast, allowing for better contrast between organs and tissues.
What is echo planar imaging (EPI)?
A rapid imaging technique that starts with one or more RF pulses and is followed by a train of gradient-echoes.
What are the two types of echo planar imaging (EPI)?
- Gradient-Echo EPI (GE-EPI): Uses an RF excitation pulse and multiple gradient-echoes.
- Spin-Echo EPI (SE-EPI): Uses a 90° RF excitation pulse followed by a 180° rephasing pulse before EPI readout.
What are the key differences between steady-state gradient-echo sequences?
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 |
How do gradient-echo sequences differ from spin-echo sequences?
- Use variable flip angles instead of a fixed 90° RF excitation pulse.
- Use gradients rather than a 180° RF pulse to rephase spins and form an echo.
Why do gradient-echo sequences allow for shorter TR values?
Because smaller flip angles allow faster T1 recovery, reducing the required TR for image acquisition.
What is a bipolar gradient in a gradient-echo sequence?
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).
Why do gradient-echo sequences suffer from magnetic susceptibility artifacts?
Because gradient rephasing does not compensate for magnetic field inhomogeneities, leading to T2* decay rather than true T2 contrast.
What are the three mechanisms that affect image weighting in gradient-echo sequences?
- Extrinsic contrast parameters (TR, TE, flip angle).
- The steady state (ratio of T1 recovery to T2 decay).
- Residual transverse magnetization (affects signal intensity).
How does T1 weighting work in gradient-echo sequences?
- Short TR + large flip angle = Saturation, enhancing T1 contrast.
- Short TE to minimize T2* decay.
How does T2* weighting work in gradient-echo sequences?
- Long TE to allow for T2* decay.
- Small flip angle + long TR to minimize T1 effects.
How does proton density (PD) weighting work in gradient-echo sequences?
- Short TE to minimize T2* decay.
- Long TR + small flip angle to minimize T1 effects.
What is the steady state in gradient-echo imaging?
A condition where the TR is shorter than T1 and T2 relaxation times, leading to a build-up of residual transverse magnetization over time.
What is residual transverse magnetization, and how does it affect image contrast?
Residual transverse magnetization is the remaining magnetization from previous TR periods. It can cause T2* contrast enhancement in steady-state sequences.
What is the Ernst angle, and why is it important?
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)
]
What signals contribute to image contrast in coherent (rewound) gradient-echo?
Both the FID and the stimulated echo, allowing for T1-, PD-, or T2*-weighting.
What is the primary purpose of incoherent (spoiled) gradient-echo?
To eliminate residual transverse magnetization, allowing only the FID to contribute, leading to T1- or PD-weighted images.
How does reverse-echo gradient-echo achieve true T2 contrast?
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}}
]
What makes balanced gradient-echo unique?
- Balanced gradient system minimizes phase errors in flowing blood and CSF.
- Alternating RF pulse phase prevents saturation, keeping fat and water bright.
What are the advantages of balanced gradient-echo?
- Short scan times.
- Good contrast between fat, water, and tissues.
- Reduced flow artifacts.
What is the primary goal of fast gradient-echo sequences?
To acquire multiple slices in a single breath-hold, reducing motion artifacts.
How does echo planar imaging (EPI) work?
Uses an initial RF pulse followed by a series of gradient-echoes, enabling ultrafast imaging in one TR period.
What are the two types of EPI?
- Gradient-Echo EPI (GE-EPI) → Uses a single RF excitation pulse followed by gradient-echoes.
- Spin-Echo EPI (SE-EPI) → Uses 90° RF pulse + 180° rephasing pulse before EPI readout.
What are the advantages of EPI?
- Very fast scan times.
- Good for functional MRI (fMRI) and diffusion imaging.
- Minimizes motion artifacts.
What are the disadvantages of EPI?
- Lower resolution than conventional imaging.
- Increased susceptibility artifacts.
- More distortion due to gradient imperfections.
What are the key differences between common gradient-echo sequences?
|
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 |
What is the main advantage of Echo Planar Imaging (EPI)?
EPI provides ultrafast imaging, reducing scan times significantly and minimizing motion artifacts.
What is the difference between Spin-Echo EPI (SE-EPI) and Gradient-Echo EPI (GE-EPI)?
- 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.
What is the main disadvantage of EPI?
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.
How does Single-Shot EPI (SS-EPI) differ from Multi-Shot EPI (MS-EPI)?
- 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.
What are common uses of EPI sequences?
- Functional MRI (fMRI) – Captures brain activation in response to stimuli.
- Diffusion-Weighted Imaging (DWI) – Detects stroke and white matter abnormalities.
- Perfusion Imaging – Measures blood flow.
- Cardiac & Coronary Imaging – Reduces motion artifacts.
What is GRASE (Gradient and Spin Echo)?
A hybrid sequence that combines gradient echoes for speed and spin echoes for better T2 contrast, reducing T2* susceptibility effects.
What is EPI-FLAIR, and why is it useful?
EPI-FLAIR uses an inversion recovery pulse (180°) before EPI readout, allowing CSF suppression while maintaining fast scan times.
What modifications help speed up fast gradient-echo imaging?
- Ramped sampling – Starts acquiring data before gradients reach maximum strength.
- Partial k-space filling – Uses symmetry to reduce scan time.
- Wider bandwidth – Speeds up signal acquisition.
What are the advantages of fast gradient-echo sequences?
- 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.
What is Double Echo Steady State (DESS)?
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).
What are the primary extrinsic parameters in gradient-echo sequences?
- TR – Controls T1 contrast (short TR = more T1 weighting).
- TE – Controls T2* contrast (long TE = more T2* weighting).
- Flip angle – Controls saturation (high flip = more T1 contrast).
How does the flip angle affect T1 contrast in gradient-echo sequences?
- 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).
What is the b value, and where is it used?
The b value is used in Diffusion-Weighted Imaging (DWI) to determine how much diffusion sensitivity is applied.
Higher b-values = More diffusion weighting.
What are the major artifacts in gradient-echo sequences?
- Magnetic susceptibility artifacts – Worsened by gradient-echo rephasing.
- Chemical shift artifacts – Common in EPI along the phase axis.
- Blurring – Due to T2* decay during long echo train acquisitions.
- Ghosting (half-FOV ghosts) – Caused by gradient timing errors.
How can susceptibility artifacts be reduced in gradient-echo imaging?
- Use SE-EPI instead of GE-EPI.
- Decrease TE (shorter TE = less dephasing).
- Increase bandwidth to minimize distortions.
What is the primary cause of blurring in EPI?
T2* decay during k-space filling leads to signal loss, reducing spatial resolution.
Fix: Use shorter echo trains or multi-shot EPI.
How do different gradient-echo sequences compare in contrast and weighting?
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 |
Why is balanced GRE preferred for imaging CSF and blood flow?
It uses a balanced gradient system, reducing phase errors and improving signal intensity of flowing fluids like CSF and blood.
What are the main benefits of gradient-echo sequences?
- Faster scan times than spin-echo sequences.
- More flexible weighting (T1, PD, T2*).
- Better for flow imaging (angiography, CSF studies).
What is the biggest drawback of gradient-echo sequences?
They do not compensate for T2* effects, making them more sensitive to susceptibility artifacts than spin-echo sequences.
What is k-space in MRI?
K-space is a mathematical space where raw MRI data is stored before image reconstruction using the Fourier Transform.
What determines spatial resolution in k-space?
High-frequency data, which is stored in the peripheral regions of k-space, determines spatial resolution.
What determines image contrast in k-space?
Low-frequency data, stored in the center of k-space, determines image contrast.
What is the Fourier Transform used for in MRI?
The Fourier Transform converts raw k-space data into an actual MRI image.
What are the two main k-space filling strategies?
- Cartesian filling – Standard left-to-right and top-to-bottom filling. 2. Radial filling – Fills k-space in a circular or spiral pattern.
What is Partial Fourier Acquisition in k-space filling?
Partial Fourier Acquisition fills only part of k-space, using symmetry to reconstruct the missing data, reducing scan time.
How does Parallel Imaging (SENSE/GRAPPA) affect k-space filling?
Parallel imaging undersamples k-space, reducing scan time while using coil sensitivity profiles to reconstruct missing data.
What is Single-Shot Imaging, and how does it work?
Single-shot imaging fills all of k-space in one TR, making it extremely fast (e.g., in EPI sequences).
What are the benefits of radial k-space filling?
Less motion artifacts than Cartesian filling. Better contrast resolution. More robust to flow and susceptibility artifacts.
What is Keyhole Imaging in k-space?
Keyhole Imaging fills the center of k-space more frequently, improving temporal resolution (e.g., in contrast-enhanced imaging).
What is the primary mechanism behind functional MRI (fMRI)?
fMRI detects changes in blood oxygenation (BOLD signal) to study brain activity.
What does BOLD signal stand for in fMRI?
Blood-Oxygen-Level-Dependent (BOLD) signal, which detects brain activity based on changes in oxyhemoglobin vs. deoxyhemoglobin levels.
Why does deoxyhemoglobin appear dark in fMRI?
Deoxyhemoglobin is paramagnetic, causing signal loss and T2* contrast changes in activated brain regions.
What is the temporal resolution of fMRI?
fMRI has a temporal resolution of ~2-3 seconds, limited by the hemodynamic response of blood flow changes.
What sequence is most commonly used in fMRI?
Gradient-Echo EPI (GE-EPI) because it provides fast acquisition and is highly sensitive to BOLD contrast.
What are the clinical applications of fMRI?
- Mapping brain function (e.g., motor, language, vision). 2. Pre-surgical planning for brain tumor resections. 3. Neurological research (e.g., memory, learning, emotions).
What are the limitations of fMRI?
Low spatial resolution (~2-3 mm voxel size). Susceptibility to motion artifacts. Delayed hemodynamic response (~2 sec lag).
What preprocessing steps are applied to fMRI data?
- 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).
What is Resting-State fMRI (rs-fMRI)?
rs-fMRI analyzes spontaneous neural activity when the subject is at rest, used to study functional connectivity between brain regions.
What are common fMRI activation paradigms?
- 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.
What is perfusion imaging, and why is it important?
Perfusion imaging measures blood flow (CBF), blood volume (CBV), and transit time (MTT) in tissues, commonly used in stroke and tumor evaluation.
What are the three main types of perfusion MRI?
- 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.
How does Arterial Spin Labeling (ASL) work?
ASL magnetically labels arterial blood water before it enters the brain, allowing perfusion to be measured without contrast agents.
What are the advantages of ASL perfusion imaging?
Non-invasive (no contrast required). Good for repeated scans (e.g., pediatrics, kidney patients). Measures absolute cerebral blood flow (CBF).
How does DSC perfusion imaging detect stroke?
By analyzing T2*-weighted signal drop caused by gadolinium bolus passage, it identifies ischemic areas with low cerebral blood flow (CBF).
What does Mean Transit Time (MTT) represent in perfusion MRI?
MTT is the average time blood takes to pass through a given brain region, helping detect delayed circulation in stroke.
Why is Dynamic Contrast-Enhanced (DCE) perfusion useful in tumors?
DCE measures vascular permeability, helping distinguish benign vs. malignant tumors based on contrast leakage patterns.
What is CBF vs. CBV in perfusion MRI?
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.
What are clinical applications of perfusion MRI?
Stroke (ischemic vs. hemorrhagic). Brain tumors (angiogenesis and malignancy assessment). Neurodegenerative diseases (Alzheimer’s, dementia).
