Objetivos de aprendizaje
After reading this chapter, you should be able:
- To present the members of the spin echo and gradient echo sequence families
- Describe the principles of signal acquisition for each type of sequence
- Explain the contrast obtained, the advantages and disadvantages of each sequence
- Present the sequence acceleration techniques in spin echo
- Examine the effect of inversion-recovery on contrast and its applications
- Define the relationship between TR, flip angle and longitudinal magnetization in gradient echo
- Describe the notion of steady state transverse magnetization in gradient echo, the conditions in which it occurs and its impact on the sequences
- Explain the methods of echoplanar imaging and compensations for the gain in speed
|Type of sequence||Principles||Advantages||Disadvantages|
|Spin echo (SE)||
T1, T2, DP contrast
|Contrast||Slow (especially in T2)|
|Multiecho SE||SE several TE, several images||DP + T2 images||Slow, even if acquisition of the 2nd image does not lengthen acquisition|
SE, echo train |
Faster than simple SE simple|
|Fat shown as a hypersignal|
|Ultrafast SE||SE, long echo train, half-Fourier||Even faster||Low signal to noise ratio|
|IR||RF 180°, TI + ES/ESR/EG||
T1 weighting |
Tissue suppression signal if TI is adapted to T1
|Longer TR / acquisition time|
|STIR||IR, short TI 150 ms||Fat signal suppression||Longer TR / acquisition time|
|FLAIR||IR, long TI 2200 ms||CSF signal suppression||Longer TR / acquisition time|
|Gradient echo (GE)||
< 90° α and short TR |
No rephasing pulse
|+ speed||T2* not T2|
|GE with spoiled residual transverse magnetization||
TR < T2 |
Gradients / RF dephasers
|T1, DP weighting|
small α and very short TR |
Gradients / RF dephasers
|Poor T1 weighting|
|Ultrafast GE with magnetization preparation||
+ preparation pulse:|
- IR (T1weighted)
- T2 sensibilization
Cardiac perfusion / viability
|Steady state GE||
TR < T2 |
|Contrast enhanced steady state GE||
Hahn echo ( trueT2)
Not much signal|
steady state GE
Balanced gradients in all 3 directions|
++ signal, ++ speed|
Single GE or multi shot|
Preparation by SE (T2), GE (T2*), IR (T1), DW
Exacting for gradients
Limited resolution |
+ intermediary GE
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