"Free induction decay (FID) refers to a short-lived sinusoidal electromagnetic signal which appears immediately following the 90° pulse. It is induced in the receiver coil by the rotating component of the magnetisation vector in the x-y plane which crosses the coil loops perpendicularly. It does not contribute to MR image but is used for single voxel MR spectroscopy.This sinusoidal signal decays exponentially with a time constant of T2*."