In the presence of a strong magnetic field, the hydrogen nucleus can be likened to a top precessing about the gravitational field; the nucleus precesses about the applied magnetic field of the magnet. This precession has a frequency determined by the strength of the magnet and the magnetic moment of the nucleus that is characteristic of a particular element. For magnets used in medical imaging, this frequency is typically in the range of FM radio waves. By controlling the magnetic field and the frequency and phase of the radio frequency (RF) pulses, images can be obtained in any desired orientation. Information regarding a particular part of an object is obtained by applying magnetic field gradients (which are small relative to the main field) so that the hydrogen in different slices will process at different, but known frequencies. By applying RF pulses at a particular frequency, the nuclei in a particular slice can be excited. As the energy from this slice is dissipated, a receiver measures the resulting signal to obtain information unique to this region of the object. Image contrast is achieved by varying the sequence and types of RF pulses. For each image, a series of pulses are required (typically 256) with a period between the pulses (typically 1 second) to allow the system to return to its original state.
The decay characteristics of the MRI signal are determined by the chemical and physical state of hydrogen, which will distinguish between various tissues such as fat, muscle and spinal discs. By adjusting the imaging parameters the sequence for the tissue of particular interest can be optimized.