Magnetic resonance imaging (MRI) or tomography (MRT) has developed to a preferred imaging modality in many diagnostic situations due to its unparalleled soft tissue contrast, combined with high spatial resolution, and its capability to generate images of slices in arbitrary orientation or even of entire volumes. P. C. Lauterbur described how magnetic field gradients could be employed to obtain images similar to those generated with X-ray computed tomography. The limits placed on spatial resolution by the wavelength in the imaging process with waves are circumvented in MRI by superposing two fields. With the aid of a radio frequency (rf)-field in the megahertz (MHz) range and a locally variable static magnetic field, the sharp resonance absorption of hydrogen nuclei in biological tissue is used to obtain the spatial distribution of the nuclear magnetization. Contrary to other imaging modalities in medicine such as X-rays or ultrasound, imaging with nuclear magnetic resonance (NMR) employs a nonlinear system.