Improving MRI contrast : Imaging water and fat
After reading this chapter, you should be able:
- To present the different fat signal suppression techniques
- Describe the advantages and disadvantages of STIR sequences, fat saturation, selective excitation of water
- Specify the different families of contrast agents
- Explain their action on relaxation time and contrast
- List their contraindications and main indications
- Explain magnetization transfer and how it can improve background suppression
Water and Fat
Inversion-recovery with TI adapted to suppress the fat signal (according to its T1)
Usually combined with fast spin echo sequences
Good fat signal suppression
Low dependency on magnetic field heterogeneities
Not specific to tissue but to T1 (fat, hematoma, lesion enhanced by Gadolinium…)
(Long acquisition times in standard STIR)
Selective RF pulse centered on the resonance frequency of fat + spoiler gradients
+/- selective inversion associated with adapted TI
Fat suppression after Gadolinium injection
No modification in the contrast of the other tissues
Requires homogenous field: less effective in wide FOV or if there are magnetic susceptibility artifacts
TR and acquisition time increased
Combination of RF pulses at intervals adapted to water/fat dephasing to separate magnetization of the water and fat
Only the water supplies the signal
Less sensitive to field heterogeneities
Efficacy increased if the pulse sequence is more complex
The longer the preparation, the longer the acquisition time
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