Magnetic Field Gradients

A magnetic field gradient is a smooth (usually linear) variation in the static magnetic field (B0) from one position to another position. Magnetic field gradients are purposefully applied in MRI as part of the imaging process. This is illustrated in Fig. 5. Application of a magnetic field gradient that varies smoothly in the inferior-superior direction causes B0 in the neck to be relatively smaller compared to that in the brain. In the presence of this field gradient, the Larmor relationship ensures that there will be smooth linear dependence of the MRI signal frequency along the inferior-superior axis. Therefore, it is possible to know where along the superior-inferior axis a particular signal-generating volume element is located through precise frequency measurement. There is no strict requirement that a field gradient be oriented along any particular anatomic axis. Field gradients may be created that cause linear variation of B0 along the left-right and the anterior-posterior axes or any arbitrary oblique axis that lies at any angle between the principal anatomic axes. Furthermore, it should be understood that magnetic field gradients can be turned on and off (i.e., switched) during the pulse sequence. This permits the application of magnetic field gradients on different axes during different parts of the pulse sequence.

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