Types
Type of MR signal | Method of formation |
Free induction decay (FID) | 1 RF pulse |
Gradient echo (GRE) | 1 RF pulse + gradient reversal |
Spin echo (SE) | 2 RF pulses |
Stimulated echo | 3 or more RF pulses |
Free induction decay
- MR (NMR) signals are due to small currents that are generated when the receiver coil is induced by the precession of the net magnetization (M)
- However in normal conditions the NMR signals decay over time exponentially due to the following (called T2* decay). This is called Free induction decay
- Magnetic field inhomogeneities
- Intrinsic T2 mechanisms
- Fat and water vs protein
- See MRI basics
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