Magnetic Field Gradient Pulses and Spatial Encoding of MR Signal
Now that we have examined the origin of the NMR signal, we proceed to a description of the use of magnetic field gradient (MFG) pulses. Briefly, MFG pulses are utilized during an MRI scan to generate spatially encoded NMR signals, which serve in the reconstruction of an MR image. In contrast to Chapter 1, it is assumed here that the sample being exposed to the resonance effect is an object of finite size rather than a small collection of spin vectors. The static magnetic field B o exerted by the magnet is uniform over the entire volume of the sample. The MFG refers to an additional magnetic field applied using a separate set of current-carrying coils. The MFG is usually applied as a series of pulses during an imaging study. The direction of the MFG is along the same direction as the B o field (i.e., along the z axis). However, unlike the B o field, the field from an MFG is not uniform in the entire volume of the sample. In fact, the amplitude of the field varies linearly with distance. The amplitude can be made to vary along the x, y, or z axis independently and is generally expressed in millitesla per meter (mT/m). Figure 2.1 shows the magnetic field generated as a result of two values of MFG: 5 and 10mT/m, along the z direction. For example, in the case of a 10 mT/m gradient pulse, the field changes by 2.5 mT at a position 25 cm from the center. It is to be noted that the gradient coils are designed to generate a symmetric change about the isocenter of the magnet.
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