In a spin echo pulse sequence, what does FID stand for?

In a spin echo pulse sequence, FID stands for Free Induction Decay. This term describes the signal that is emitted by the excited nuclear spins as they relax back to their equilibrium state after being disturbed by a radiofrequency pulse. The FID occurs before any additional sequence of pulses is applied, such as that which creates a spin echo.

During the free induction decay, the signals from the individual spins combine into a detectable electromagnetic signal that decreases in amplitude over time due to various relaxation processes. Understanding FID is essential in magnetic resonance imaging (MRI) and spectroscopy as it forms the basis for subsequent signal processing and image formation. The decay characteristics provide valuable information about the molecular environment and dynamics of the material being examined.

The other terms listed do not correspond to the accepted terminology in magnetic resonance imaging and do not accurately represent the physical process occurring in this scenario.