Chapter 4: Gradient-echo Pulse Sequences

Question 1

Gradient-echo pulse sequences differ from spin-echo sequences in what two ways?

Answer 1

They use variable RF excitation pulse flip angles as opposed to 90 degree flip angles.
They use gradients rather than RF pulses to rephase the magnetic moments to form an echo.

Question 2

Full T1 recovery is achieved in a much _______ TR than in spin-echo sequences.

Answer 2

Shorter

Question 3

What is the process of dephasing magnetic moments with gradients called?

Answer 3

Gradient spoiling

Question 4

Gradients that dephase magnetic moments are called what?

Answer 4

Spoilers

Question 5

Gradients that rephase magnetic moments are called what?

Answer 5

Rewinders

Question 6

Whether a gradient field adds or subtracts from the main magnetic field depends on the ________ of the current that passes through the gradient coils.

Answer 6

Direction

Question 7

The direction of current that passes through the gradient coils is called the ______ of the gradient.

Answer 7

Polarity

Question 8

Gradient-echoes are created by what type of gradient?

Answer 8

Bipolar

Question 9

Which rephases magnetic moments faster, gradients or RF pulses?

Answer 9

Gradients

Question 10

If the combination of flip angle and TR causes saturation of the vectors (ie they never fully recover their longitudinal magnetization during the TR period), then T1 contrast is _________.

Answer 10

Maximized

Question 11

If the combination of flip angle and TR does not cause saturation of the vectors (ie they recover most, or all, of their longitudinal magnetization during the TR period), then T1 contrast is ________.

Answer 11

Minimized

Question 12

To avoid full recovery of fat and water vectors longitudinal magnetization, how should your flip angle be in a gradient echo T1 weighting, large or small? And the TR, long or short?

Answer 12

Large; Short

Question 13

To minimize differences in T1 recovery times, the flip angle is _______ and the TR is _______ to permit full recovery of the fat and water vectors before the next RF excitation pulse is applied.

Answer 13

Small; Long

Question 14

For gradient-echo pulse sequences, what 2 things control whether the NMV is saturated or not?

Answer 14

TR and flip angle. Saturation is required for T1 weighting only!

Question 15

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

Answer 15

Ernst angle

Question 16

The RF excitation pulse gives energy to hydrogen nuclei, and the amount of energy applied is determined by the _______.

Answer 16

Flip angle

Question 17

Energy is lost by hydrogen nuclei through spin-lattice energy transfer, and the amount of lost energy is determined by the _______.

Answer 17

TR

Question 18

Typically, TRs less than ______ are considered appropriate to maintain the steady state.

Answer 18

50ms

Question 19

Flip angles between _____ and _____ are typically used in the steady state.

Answer 19

30 and 45 degrees

Question 20

The _______ is created when the TR is shorter than the T1 and T2 relaxation times of tissues. Residual transverse magnetization therefore builds up over time.

Answer 20

steady state

Question 21

The stimulated echo contains mainly _______ weighted information because it is generated from the residual transverse magnetization.

Answer 21

T2*/T2

Question 22

The ______ tends to create contrast that relies on T1 and proton density effects.

Answer 22

FID

Question 23

Coherent or rewound gradient-echo pulse sequences are generally used to create _____-weighted images in a very short scan time.

Answer 23

T2*

Question 24

T or F: Both FID and stimulated echoes are used in coherent/rewound gradient-echo sequences.

Answer 24

True

Question 25

What does rewinding consist of?

Answer 25

Reversing the slope of the phase-encoding gradient after readout.

Question 26

_______ rephases all transverse magnetization regardless of when it is created so that it is in phase or coherent at the beginning of the next TR period.

Answer 26

Rewinding

Question 27

T or F: Gradient spoiling is the opposite of rewinding.

Answer 27

True

Question 28

What are the 2 spoiling methods?

Answer 28

RF spoiling (phase-locked circuit) and Gradient spoiling (opposite of rewinding)

Question 29

Incoherent gradient-echo is a steady state sequence that utilizes a ______ TR and a _______ flip angle.

Answer 29

short; medium

Question 30

What ensures that residual transverse magnetization is not sampled by altering the phase angle of each RF excitation pulse every TR and locking this to the receiver coil.

Answer 30

RF spoiling

Question 31

What pulse sequence allows the combination of a short TR and a long TE so that true T2 weighting is achieved at the same time as a short scan time.

Answer 31

Reverse-echo gradient-echo

Question 32

In gradient-echo sequences, the TE is not long enough to measure the T2 decay times of tissues, as a TE of at least ______ms is required for this.

Answer 32

70

Question 33

The actual TE is the time between the peak of the gradient-echo to the ______ RF excitation pulse. This is the TE selected in the scan protocol in these sequences, but it is not the TE that determines T2 contrast.

Answer 33

Next

Question 34

The effective TE is the time from the peak of the gradient-echo to the ______ RF pulse (i.e. the RF pulse that created its FID). This is the TE that determines T2 contrast, as this is the time allowed for T2 decay in the gradient-echo.

Answer 34

Previous

Question 35

The steady state produces 2 signals. What are these?

Answer 35

A FID made up of transverse magnetization that has just been created by switching off an RF pulse. A stimulated echo made up of the residual transverse magnetization component that builds up over time.

Question 36

Incoherent gradient-echo samples only the _____ to produce images that are mainly ___/___ weighted.

Answer 36

FID; T1/PD

Question 37

Reverse-echo gradient-echo samples only the ________ to produce images that are _____ weighted.

Answer 37

stimulated echo; T2

Question 38

Balanced gradient-echo is a steady state sequence in which longitudinal magnetization is maintained during the acquisition, thereby preventing _______.

Answer 38

saturation