What is the immediate decay of the RF signal in the transverse plane after excitation referred to as?

• A. Free Induction Decay (FID)
• B. Signal Loss
• C. Transverse Relaxation
• D. Echo Time

Answer: (A)

The immediate decay of the RF signal in the transverse plane after excitation is referred to as Free Induction Decay (FID). This phenomenon occurs following the application of a radiofrequency pulse, which tips the spins of hydrogen protons into the transverse plane. After the pulse is switched off, these spins begin to relax back to their equilibrium state, leading to a loss of signal intensity in the transverse plane due to the inhomogeneity of the magnetic field and interactions among the spins.

FID is the initial signal that is detected before any additional imaging techniques or sequences are applied. Understanding FID is crucial because it provides the fundamental basis for the acquisition of MR signals and therefore serves as a foundational concept in Magnetic Resonance Imaging.

The other options do not accurately describe this immediate signal decay: Signal Loss refers more generally to the reduction in signal quality or intensity without specifying the immediate context of decay; Transverse Relaxation is the process following the decay which describes how spins return to equilibrium over time, specifically characterized by the time constant T2; Echo Time is the time between the initial excitation pulse and the peak of the detected echo signal, not a term that directly describes the immediate decay itself.