Creating an Image

The preceding description yields a single number: T2*, the decay rate of the net NMR signal. This signal is conventionally called the free induction decay (FID) of the NMR signal because it is elicited when nothing else is done to the protons—that is, if the system were left free to decay at its own rate. However, this signal represents the net effect of inducing an NMR signal from the entire volume of tissue being subjected to the main magnet and the orientation-flipping RF pulse. To create an image in which different NMR signals are measured for different points in the three-dimensional volume, nonuniform magnetic fields are applied intentionally. The basic idea is to use linear magnetic field gradients, applied at various times and in various orientations, to distinguish the NMR signals arising

Outside the magnet Inside the magnet RF pulse applied
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