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 :
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 :
T2* is always shorter than T2. T2* signal decay is lower than pure T2 signal decay.