Following the excitation pulse, how does the FID decay?

• A. Linearly
• B. Quadratically
• C. Constantly
• D. Exponentially

Answer: (D)

The Free Induction Decay (FID) signal in Magnetic Resonance Imaging (MRI) is characterized by its exponential decay, which is linked to the loss of coherence among the spins of excited nuclei. When a sample is placed within a magnetic field and excited by a radiofrequency pulse, the nuclei begin to precess in phase. However, as time progresses, various factors such as magnetic field inhomogeneities, spin interactions, and relaxation processes cause the spins to dephase. This loss of phase coherence results in a decay of the generated FID signal.

The mathematical treatment of this decay can be modeled using an exponential function. This is because the signal diminishes in a manner that proportionally reduces it over time—initially decreasing rapidly and then more slowly as time goes on. This behavior reflects the fundamental nature of relaxation processes in MRI, such as T2 relaxation, which quantifies how quickly the spin coherence is lost.

In contrast to exponential decay, linear, quadratic, or constant decay functions would not accurately describe the physical processes at play, as they do not encompass the influence of the interactions that lead to dephasing and energy loss in a coherent nuclear spin system. Therefore, the correct characterization of FID decay is exponential, matched to