Chapter 3: Spin-echo Pulse Sequences Flashcards
FID decays with about how long after the RF pulse is applied?
About 10ms
What 2 ways are used to rephase the magnetic moments of hydrogen nuclei to produce an echo?
By using an additional 180 degree RF pulse or by using gradients.
Sequences that use a 180 degree RF rephasing pulse to generate an echo are called what?
Spin-echo pulse sequence
Sequences that use a gradient RF rephasing pulse to generate an echo are called what?
Gradient-echo pulse sequence
Gradients are applied to do what two things?
Spatially encode signal
Rephase and dephase magnetic moments of hydrogen nuclei
T or F: All spin-echo pulse sequences are characterized by RF rephasing.
True
In spin-echo pulse sequences, how is T2* dephasing eliminated?
By the 180 degree rephasing pulse, because magnetic field inhomogeneities are largely predictable.
The time taken to rephase after the 180 degree RF rephasing pulse equals the time to dephase when the 90 degree RF excitation pulse is withdrawn. This time is called what?
Tau
Although any number of echoes may be created with spin-echo sequences, typically how many are generated?
One or two
Spin-echo pulse sequences using only one echo are used to produce what type of images?
T1-weighted images
Spin-echo pulse sequences that use two echoes are used to produce what type of images?
PD and T2 weighted images
What type of sequences are considered the gold standard in that the contrast that they produce is understood and predictable.
Spin-echo pulse sequences
What is the main disadvantage of spin-echo sequences?
Long scan time
What are the suggested parameters of a single echo (for T1 weighting)?
TR = 300-700 ms
TE = 10-30 ms
What are the suggested parameters of a dual-echo (for PD/T2 weighting)?
TR = 2000 ms+
TE1 = 20 ms
TE2 = 80 ms
What type of sequences are characterized by 180 degree RF rephasing pulses that refocus the magnetic moments of spins to produce an echo?
Spin-echo
What are FSE/TSE sequences also known as?
RARE (Rapid Acquisition with Relaxation Enhancement)
In TSE/FSE, more than one line of k-space is filled per TR. This is achieved by using several 180 degree rephasing pulses to produce several spin-echoes. This forms what?
An echo train.
What type of amplitude does a very steep phase encoding slope result in? And where are the data from these stored in the lines of k-space?
Low amplitude and they are stored in the outer lines of k-space.
What type of amplitude does a shallow phase-encoding slope result in? And where at in the k-space are these lines stored?
High amplitude and they are stored in the central lines of k-space.
Phase encodings performed at the very beginning and the end of the echo train are _______, and the signal amplitude of these echoes is therefore ______.
Steep; low.
What are the 2 contrast differences between conventional spin-echo and FSE/TSE, both of which are due to the repeated, closely spaced 180 degree RF rephasing pulses in the echo train?
Fat remains bright on T2-weighted images (fat sat techniques can be used to compensate for this)
There is an increase in magnetization transfer effects.
T or F: Multiple 180 degree RF rephasing pulses reduce magnetic susceptibility effects, which can be detrimental when looking for small hemorrhages.
True
T or F: The TR used in TSE is often much longer than that used in conventional spin-echo.
True
What type of imaging involves applying the phase-encoding gradient multiple times in a TR period to varying amplitudes and polarity?
TSE/FSE
How many lines of k-space are filled at a time when the ETL is 16?
16
What type of sequence is it when all the lines of k-space are acquired at once?
Single-shot turbo spin echo (SS-TSE)
What sequence produces increased signal intensity in fluid-based structures such as CSF when using a shorter TR than normal in TSE? A reverse flip angle excitation RF pulse is applied at the end of the echo train.
Driven Equilibrium (DRIVE)
_______ is a spin-echo pulse sequence that uses an RF inverting pulse to suppress signal from certain tissues, although it is also sometimes used to generate heavy T1 contrast.
Inversion recovery
The time from the 180 degree RF inverting pulse to the 90 degree RF excitation pulse is known as what?
TI (time from inversion)
If the 90 degree RF excitation pulse is applied after the NMV has relaxed back through the transverse plane, image contrast depends on the amount of longitudinal recovery of each vector (as in spin echo). The resultant image is ______ weighted.
T1
If the 90 degree RF excitation pulse is not applied until the NMV has reached full recovery, a _______ weighted image is produced, as both fat and water are fully relaxed.
PD
IR is usually used to produce what type of images?
T1-weighted to demonstrate anatomy
T or F: IR pulse sequences produce heavier T1 weighting than conventional spin-echo.
True
What is the most potent controller of contrast in the IR sequences?
The TI (time from inversion)
What is it called when the TE is lengthened in an IR sequence to give tissues with a long T2 decay time a high signal?
Pathology weighting (it produces an image that is predominantly T1-weighted but where pathology is bright)
What are the 2 main sequences in the category of fast inversion recovery?
STIR and FLAIR
______ is a pulse sequence that uses a TI that corresponds to the time it takes the fat vector to recover from full inversion to the transverse plane so that there is no longitudinal magnetization corresponding to fat.
STIR (the is called the null point)
A TI of ______ ms usually achieves fat suppression, although this value varies slightly at different field strengths.
100-175ms
The TI required to null signal from a tissue is always _____ times its T1 relaxation time.
0.69
What sequence would you use in musculoskeletal imaging because normal bone, which contains fatty marrow, is suppressed, and lesions within bone such as bone bruising and tumors are seen more clearly?
STIR
A TI of ______ ms usually achieves CSF suppression in FLAIR sequences.
1700-2200 ms
What sequence is used in brain and spine imaging to see periventricular and cord lesions more clearly because high signal from CSF that lies adjacent is nulled.
FLAIR
FLAIR is especially useful in visualizing what 3 things?
MS
Acute subarachnoid hemorrhage
Meningitis
T or F: The TR does not control T1 contrast in IR because it is always long.
True
Approximately what is the null point of blood?
About 800ms
For spin-echo pulse sequences, what controls the point at which longitudinal magnetization is nulled?
TI
