The signal produced immediately following an RF pulse is:

• A. FID
• B. Echo
• C. Gradient
• D. Noise

Answer: (A)

After the RF pulse tips the net magnetization into the transverse plane, the spins begin to precess together but quickly lose phase coherence due to spin-spin interactions and local field inhomogeneities. The signal detected by the coil at this moment is the free induction decay (FID): the decaying transverse magnetization that carries the initial MR signal. As time passes, the FID fades away because of T2 and T2* effects. An echo, by contrast, is produced later when the spins are rephased—via a 180-degree refocusing pulse in spin-echo sequences or via gradients in gradient-echo sequences—so it is not the immediate signal. Gradient refers to the spatial encoding gradients, not the instantaneous signal type, and noise is random, not the coherent MR signal from precessing spins.