In: Physics
Explain how Free Induction Decay contributes to creating a MRI image.
A free induction decay curve is generated as excited nuclei
relax. The amplitude of the FID signal becomes smaller over time as
net magnetization returns to equilibrium. If transverse
magnetization of the spins is produced, e.g. by a 90° pulse, a
transient MR signal will result that will decay toward zero with a
characteristic time constant T2 (or T2*); this decaying signal is
the free induction decay.
The signal peaks of the echoes fall onto this T2 decay curve, while
at each echo the signals arise and decay with T2*. The typical T2
relaxation times being of the order of 5-200 ms in the human body.
The first part of the FID is not observable (named the 'receiver
dead time') caused by residual effects of the powerful exciting
radio frequency pulse on the electronics of the receiver.