Sequences
Learning objectives
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
Key points
| 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 effective 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 |
References
- Elster. Questions and answers in magnetic resonance imaging. 1994:ix, 278 p.
- McRobbie. MRI from picture to proton. 2003:xi, 359 p.
- NessAiver. All you really need to know about MRI physics. 1997.
- Kastler. Comprendre l'IRM. 2006.
- Gibby. Basic principles of magnetic resonance imaging. Neurosurgery clinics of North America. 2005 Jan;16(1):1-64.
- Poustchi-Amin, Mirowitz. Principles and applications of echo-planar imaging: a review for the general radiologist. Radiographics. 2001 May-Jun;21(3):767-79.
- Boyle, Ahern. An interactive taxonomy of MR imaging sequences. Radiographics. 2006 Nov-Dec;26(6):e24; quiz e.
- Bitar, Leung. MR pulse sequences: what every radiologist wants to know but is afraid to ask. Radiographics. 2006 Mar-Apr;26(2):513-37.