Chapter 4: Gradient-echo Pulse Sequences Flashcards
Gradient-echo pulse sequences differ from spin-echo sequences in what two ways?
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.
Full T1 recovery is achieved in a much _______ TR than in spin-echo sequences.
Shorter
What is the process of dephasing magnetic moments with gradients called?
Gradient spoiling
Gradients that dephase magnetic moments are called what?
Spoilers
Gradients that rephase magnetic moments are called what?
Rewinders
Whether a gradient field adds or subtracts from the main magnetic field depends on the ________ of the current that passes through the gradient coils.
Direction
The direction of current that passes through the gradient coils is called the ______ of the gradient.
Polarity
Gradient-echoes are created by what type of gradient?
Bipolar
Which rephases magnetic moments faster, gradients or RF pulses?
Gradients
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 _________.
Maximized
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 ________.
Minimized
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?
Large; Short
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.
Small; Long
For gradient-echo pulse sequences, what 2 things control whether the NMV is saturated or not?
TR and flip angle. Saturation is required for T1 weighting only!
The _________ is the flip angle that provides optimum signal intensity for a tissue with a given T1 recovery time scanned using a given TR.
Ernst angle
The RF excitation pulse gives energy to hydrogen nuclei, and the amount of energy applied is determined by the _______.
Flip angle
Energy is lost by hydrogen nuclei through spin-lattice energy transfer, and the amount of lost energy is determined by the _______.
TR
Typically, TRs less than ______ are considered appropriate to maintain the steady state.
50ms
Flip angles between _____ and _____ are typically used in the steady state.
30 and 45 degrees
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.
steady state
The stimulated echo contains mainly _______ weighted information because it is generated from the residual transverse magnetization.
T2*/T2
The ______ tends to create contrast that relies on T1 and proton density effects.
FID
Coherent or rewound gradient-echo pulse sequences are generally used to create _____-weighted images in a very short scan time.
T2*
T or F: Both FID and stimulated echoes are used in coherent/rewound gradient-echo sequences.
True
What does rewinding consist of?
Reversing the slope of the phase-encoding gradient after readout.
_______ 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.
Rewinding
T or F: Gradient spoiling is the opposite of rewinding.
True
What are the 2 spoiling methods?
RF spoiling (phase-locked circuit) and Gradient spoiling (opposite of rewinding)
Incoherent gradient-echo is a steady state sequence that utilizes a ______ TR and a _______ flip angle.
short; medium
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.
RF spoiling
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.
Reverse-echo gradient-echo
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.
70
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.
Next
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.
Previous
The steady state produces 2 signals. What are these?
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.
Incoherent gradient-echo samples only the _____ to produce images that are mainly ___/___ weighted.
FID; T1/PD
Reverse-echo gradient-echo samples only the ________ to produce images that are _____ weighted.
stimulated echo; T2
Balanced gradient-echo is a steady state sequence in which longitudinal magnetization is maintained during the acquisition, thereby preventing _______.
saturation
