The principle of NMR is based on the magnetic moment of the nucleus, which is linked to the nuclear spin. A anologon from classical physics is to interpret these magnetic moments as small bar magnets, which align according to an external magnetic field similar to a compass needle. In contrast to classical physics, in which any orientation of the bar magnetic in the magnetic field is possible with smooth transitions between energy states, in quantum physics only discrete orientations and energy levels are allowed. By irradiation with an electromagnetic field at resonance frequency (Larmor frequency, for 1H 42.6 MHz/Tesla) transitions between energy niveaus can be induced. The precession of transverse magnetization after a pulsed irradiation causes a small electrical signal in a coil (a special kind of antenna), which can be measured with an amplifier and an analog-digital-converter (ADC).
The spatial encoding is performed by super-imposition of linear magnetic field gradients on the static background field resulting in spatially different resonance frequencies. In 2D imaging, the spin excitation of an imaging slice is performed by irradiation with an electromagnetic field at the resonance frequency of the slice during application of a gradient in z-direction ("slice-selection"). The second dimension is encoded with a gradient in x-direction applied during signal readout ("frequency selection"). Finally, by multiple repetition of the excitation-readout sequence with application of gradients of varying strength in y-direction, the spins exhibit different phases depending on their spatial location ("phase encoding"). From the distribution of frequencies and phases ("k-space") in the MRI signal, the images in the position space can be calculated by Fast Fourier transformation.
Department of Radiology
University Hospital of Zürich