MRI Sequences

Sequences

出版元 Denis Hoa

学習目的

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

キーポイント

Type of sequence Principles Advantages Disadvantages
Spin echo (SE) simple, 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
Fast SE SE, echo train
effctive TE
Faster than simple SE simple
ES contrast
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  
Ultrafast GE small α and very short TR
Gradients / RF dephasers
k-space optimization
++ speed
cardiac perfusion
Poor T1 weighting
Ultrafast GE with magnetization preparation + preparation pulse:
- IR (T1weighted)
- T2 sensibilization
++ speed
AngioMRI Gado
Cardiac perfusion / viability
 
Steady state GE TR < T2
Rephasing gradients
FID
+ signal
++ speed
Complex contrast
Contrast enhanced steady state GE Rephasing gradients
Hahn echo ( trueT2)
Not much signal
T2 weighted
 
Balanced
steady state GE
Balanced gradients in all 3 directions
T2/T1contrast
++ signal, ++ speed
Flow correction
 
Echoplanar Single GE or multi shot
Preparation by SE (T2), GE (T2*), IR (T1), DW
Exacting for gradients
++++ speed
Perfusion
MRIf BOLD
Diffusion
Limited resolution
Artifacts
Hybrid echo Fast SE
+ intermediary GE
++ speed
SAR reduction
 

参照

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