The production of an MR image begins with the alignment of tissue hydrogen nuclei by an external strong magnetic field. Hydrogen nuclei are targeted because they are abundant in tissue and produce a strong signal. Radiofrequency (RF) pulses are directed at the tissues, which excite the nuclei. As these nuclei relax, they emit RF signals. Delineation of tissue types and tissue boundaries is produced by differences in relaxation rates by different tissues. Further localization of the signal is determined by manipulation of the magnetic field. Emitted signals are measured at varying times after the initial RF pulse. A computer algorithm is used to reconstruct the data into 2-dimensional images.The contrast in the MR signal has 2 primary determinants: spin density and relaxation. Density simply refers to the number of hydrogen nuclei per unit volume. Relaxation refers to the process and timing of the return of the excited nuclei to the unexcited state.
MR imaging does not delineate bony anatomy as well as CT scanning. MR imaging is most useful in delineating soft tissue relationships.