Gradient [Recalled] Echo (GE/ GRE) Flashcards
Define a T1w Spoiled/Incoherent GRE pulse sequence (alpha, TE, TR)
alpha = ~30 to 50 degrees (emphasizes T1)
TE: short TE (~3 ms, prevents dephasing)
TR: TR < T2 (~40 ms)
TR should be short to enhance the effects of T1 and the TE should be short to minimize T2*
The flip angle should be above 45° to allow for T1-weighted contrast between the tissues; tissues with shorter T1 will recover more quickly than those with longer T1
Define a T2w Steady-State Free Precession (SSFP) GRE pulse sequence (alpha, TE, TR)
alpha = ~30-45 degrees
Teff = 2 * TR - TE (~80 ms)
TE: determined by Teff (~20 ms)
TR: TR < < T2 (~50 ms)
What is a Conventional GRE?
No transverse magnetic moment
TR > T2
T1w: TR~T1, TE < < T2*
T2w: TR < < T1, TE~T2
FID_3 + SE_1
What is a Steady-State GRE? What are the three types?
Residual transverse magnetic moment
TR < T2
- Coherent
- Incoherent / Spoiled
- Steady-State Free Precession (SSFP)
** FID and SE components
What do Steady-State GRE FID components contribute to?
T1 and T2*
What do Steady-State GRE SE components contribute to?
Also known as Hahn effects. Contribute to T2 and T2*
How is the larmor frequency different than the 180 degree pulse in SE?
The direction of the spin is not altered – forces not reversed
Define a T1w Conventional GRE pulse sequence (alpha, TE, TR)
alpha = >45 degrees (large)
TE: TE < < T2* (limits dephasing time)
TR: TR > T2 (>200 ms)
Define a T2w Conventional GRE pulse sequence (alpha, TE, TR)
alpha = >5-15 degrees (minimize T1)
TE: TE ~ T2* (minimize T2, ~10-20 ms)
TR: TR > T2 (>200 ms)
What is an Incoherent / Spoiled GRE?
Phase-locked excitation removes SE from the 0-degree phase and blocks the SE signal
FID_3
Used for T1w
What is a SSFP GRE?
Rewind gradient shifts the SE signal earlier in time by TE, placing it before the FID signal. Important: the spin is sped up, but time is restored before the next pulse
SE_1
Used for T2w
