Magnetic resonance imaging (MRI) generates cross-sectional images of the human body by using nuclear magnetic resonance (NMR). The process begins with positioning the imaged body (a) in a strong, uniform magnetic field, which polarizes the nuclear magnetic moments of water protons by forcing their spins into one of two possible orientations (b). Then an appropriately polarized radio-frequency field, applied at resonant frequency, forces spin transitions between orientations (c). Those transitions create a signal (d) (which is an NMR phenomenon) that can be detected by a receiving coil.

An MRI scanner applies the radio-frequency field as finely crafted pulses, which excite only protons whose resonant frequencies fall within a fairly narrow range. Applying magnetic-field gradients during the radio-frequency pulse creates resonant conditions for only the protons that are located in a thin, predetermined slice of the body. Orientation and thickness of this slice can be selected arbitrarily in the imaged body. The NMR signal encodes positional information across the slice by using a method known as the ``spin warp,'' and a two-dimensional Fourier Transform extracts that positional information. The process creates a data matrix in which each element represents an NMR signal from a single, localized volume element, or voxel, within the imaged slice. A two-dimensional display of this matrix's contents creates a human-readable image of the selected slice. Each image element, or pixel, represents the NMR signal strength that was recorded for its corresponding voxel.

An MRI image provides unmatched soft-tissue contrast. When compared with other medical-imaging techniques, MRI provides several significant advantages: noninvasiveness, safety (because it uses non-ionizing radiation), and superb soft-tissue contrast, generated by an NMR signal's sensitivity to tissue morphology and pathology.

For more information: http://biac.stanford.edu/