Free induction decay (FID) refers 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 magnetization 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*.
A spin echo produced by a 180o pulse following a 90° pulse appears as a back-to-back FID where the signal increase exponentially to the middle of the signal then decrease exponentially like the FID.
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