MRI Physics Chapter 4 Gradient-echo pulse sequences

After reading this chapter, you will be able to: Explain how gradient-echo sequences differ from spin-echo. Describe how gradient-echoes are created. Analyze the steady state and why it is important in gradient-echo pulse sequences. Understand the mechanisms of common gradient-echo pulse sequences. Apply what you have learned to understand how images of different weighting are created using gradient-echo pulse sequences.

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/False

True

Residual transverse magnetization is eliminated in coherent GRE sequences.

True/False

False

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

True/False

True

Echo planar imaging (EPI) is primarily used to reduce physiological motion artifacts
True/False

True

Spoilers are gradients that rephase the spins in a vector.

True/False

False

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

True/False

True

Balanced gradient echo sequences are only used in cardiac imaging.

True/False

False

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

True/False

True

Steady State

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

Residual Transverse Magnetization

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

Coherent GRE

Maintains residual magnetization coherence through rewinding

Incoherent GRE

Dephases residual transverse magnetization to minimize its effect on contrast.

Reverse-Echo Gradient Echo

Captures true T2 contrast using rewinder gradients.

Balanced GRE

Reduces flow artifacts using balanced gradient systems.

Actual TE

time between the echo and the next RF pulse in reverse-echo gradient-echo pulse sequences.

Balanced gradient-echo

pulse sequence characterized by a balanced gradient scheme that corrects for flow. Alternating RF pulses with varying phase angles prevent saturation and maintain the steady state.

Bipolar gradient

gradient with two lobes, positive and negative. Often used to dephase and then rephase magnetic moments to produce a gradient-echo.

Blurring

result of T2* decay during the course of the EPI acquisition.

Coherent gradient-echo

pulse sequence characterized by a rewinder gradient. All transverse magnetization (FID and stimulated echo) is rephased so all types of weighting are possible.

Echo-planar imaging

single-shot or multishot acquisition that fills k-space with data from gradient-echoes.

Effective TE

TE selected in TSE. Used to weight the image as accurately as possible given that echoes with different TEs are used to determine image weighting.

Ernst angle

flip angle that generates the highest signal intensity in a tissue with a given T1 recovery time and in a given TR.

Gradient

a linear change of the magnetic field strength.

Gradient-echo

echo produced as a result of gradient rephasing.

Gradient-echo-EPI (GE-EPI)

gradient-echo sequence with EPI readout.

Gradient spoiling

use of gradients to dephase magnetic moments – the opposite of rewinding.

GRASE

gradient-echo and spin-echo.

Hahn echoes

echoes formed when any two 90° RF pulses are used in steady state sequences.

Half FOV ghosts

seen in EPI where a ghost of the real image appears shifted in the phase direction by one half of the FOV.

Hybrid sequences

combination of fast spin-echo and EPI sequences where a series of gradient-echoes are interspersed with spin-echoes.

Incoherent gradient-echo

pulse sequence that spoils the residual transverse magnetization so that the FID makes the most contribution to image contrast. Used for T1 weighting.

Isocenter

center of the bore of the magnet in all planes.

Phase-locked circuit

used in incoherent or spoiled gradient-echo sequences to enable the receiver coil to lock onto the most recently transmitted transverse magnetization. Residual magnetization that has a different phase angle is ignored.

Polarity

direction of a gradient, i.e. which end is greater than B0 and which is lower than B0. Depends on the direction of the current through the gradient coil.

Ramped sampling

where sampling begins before the frequency-encoding gradient reaches its maximum amplitude.

Residual transverse magnetization

transverse magnetization left over from previous RF pulses in steady state conditions.

Reverse-echo gradient-echo

pulse sequence characterized by a rewinder gradient that repositions the stimulated echo so that it is read by the system. True T2 makes a significant contribution to image contrast because the residual transverse magnetization contains mainly T2 contrast data.

Rewinders

gradients that rephase.

RF spoiling

use of a digitized RF phase-lock circuit to transmit and receive at a different phase every TR. Residual transverse magnetization is differentiated from new transverse magnetization by its phase value.

Signal-to-noise ratio (SNR)

ratio of signal relative to noise.

Spin-echo–EPI (SE-EPI)

spin-echo sequence with EPI readout.

Spoilers

gradients that dephase.

Steady state

condition where the TR is less than T1 and T2 relaxation times of tissues. Also defined generically as a stable condition that does not change over time.

T2*

dephasing due to magnetic field inhomogeneities.

T2* weighting

used in gradient-echo sequences to indicate T2 weighting. Recognizes that T2* decay makes a large contribution to image contrast.

T2 decay

loss of transverse magnetization as a result of spin–spin relaxation.

Magnetic isocenter

center of the bore of the magnet in all planes.

As the NMV is moved through a smaller _______in the excitation phase of the pulse sequence, it does not take as long for the NMV to achieve full relaxation once the RF excitation pulse is removed.

angle 

As a relatively large ___________is used in spin-echo pulse sequences, most of the _________is still in the transverse plane when the 180° RF rephasing pulse is applied.

flip angle, magnetization 

A gradient is applied to ________(in phase) magnetization (all the magnetic moments are in the same place at the same time).

coherent 

Is this the correct definition of T2* weighting?

used in gradient-echo sequences to indicate T2 weighting. Recognizes that T2* decay makes a large contribution to image contrast.

yes 

To avoid full ______of their longitudinal _____, the flip ______ is large and the TR short so that the fat and water vectors are still in the process of recovering when the next RF excitation pulse is applied.

recovery, magnetization, angle 

The same is true in gradient-echo pulse sequences except that T2 is termed T2* to reflect the fact that magnetic field _______are not compensated for by gradient rephasing.

inhomogeneities 

To maximize differences in T2* decay times, the TE is long so that fat and water _______have had time to dephase.

vectors 

Time taken for 63% of the transverse magnetization to decay.

T2 decay time 

In practice, echo production is so rapid that the tails of ________signals merge with stimulated echoes resulting in a continuous signal of varying amplitude.

FID

The extrinsic contrast parameters (TR, TE, and flip angle) are selected to generate the ______state and to enhance T2* contrast.

steady 

The effect of ______transverse magnetization is seen from the high signal from water in the stomach.

residual

Pulse sequence characterized by a rewinder gradient. All transverse magnetization (FID and stimulated echo) is rephased so all types of weighting are possible.

Coherent gradient-echo

Incoherent or spoiled gradient-echo pulse sequences begin with a variable _________RF excitation pulse and use gradient ______ to produce a gradient-echo.

flip angle, rephasing 

Reverse-echo gradient-echo pulse sequences allow the combination of a short TR and a long TE so that true T2 ______is achieved at the same time as a short scan time.

weighting 

________ gradient-echo is a sequence that was first used in the 1980s before other fast sequences such as turbo or fast spin-echo were developed.

Coherent

What is the definition of "Incoherent gradient-echo"?

Pulse sequence that spoils the residual transverse magnetization so that the FID makes the most contribution to image contrast. Used for T1 weighting.

In most gradient-echo pulse sequences, the TR and the flip angle are selected to maintain the ______state rather than to control T1 contrast.

steady 

The resultant gradient-echo is dominated by the stimulated echo and therefore demonstrates better T2 ________than conventional gradient-echo sequences.

weighting

As previously described, every RF pulse regardless of its net magnitude contains _______that have sufficient magnitude to rephase spins and produce a stimulated echo.

energies

Higher flip angles and shorter TRs are used than in_______gradient-echo producing a higher SNR and shorter scan times.

coherent 

_______is avoided, and fat and water, which have T1/T2 _______times approaching ________, return a higher signal than tissues that do not.

saturation, relaxation, parity

Sampling begins before the frequency-encoding gradient reaches its _______ amplitude.

maximum

Ramped sampling

where sampling begins before the frequency-encoding gradient reaches its maximum amplitude

As each gradient-echo is acquired rapidly, there is relatively little______shift in the frequency direction.

chemical 

Either PD or T2* _______is achieved by selecting either a short or long effective TE, which corresponds to the time interval between the RF excitation pulse and when the center of k-space is filled.

weighting 

Echo planar imaging (EPI) is a rapid acquisition ______that begins with a sequence of one or more RF pulses and is followed by a series of gradient-echoes.

technique 

Half FOV ghosts

seen in EPI where a ghost of the real image appears shifted in the phase direction by one half of the FOV

What is A

TR

What is B

RF

What is C

Dephase

What is D

Rephase 

What is E

FID

What is F

Echo

What is G

TE

What is A

P

What is B

RF

What is C

Phase 

What is D

Frequency

What is E

Signal

What is A

90

What is B

180

What is C

RF

What is D

Phase