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90° RF pulse

After a 90° RF pulse, net magnetization tips down so that longitudinal magnetization has disappeared and transverse magnetization has appeared.
Once the RF transmitter is turned off, relaxation happens :

• transverse magnetization decays
• longitudinal magnetization recovers
• protons re-radiate the absorbed energy

Coils can receive the signal in the transverse plane due to variations of transverse magnetization vector. This signal is oscillating at resonance frequency and signal enveloppe is a decay curve described as an exponential curve.

In absence of any magnetic gradient, this signal is called Free Induction Decay (FID). FID signal decays faster than T₂ would predict and decreases exponentially at characteristic time constant T₂^*.
T₂^* takes into account :

• tissue specific spin-spin relaxation (random interactions between spins) responsible for pure T₂ decay
• static inhomogeneities in magnetic fields which accelerate spins dephasing