(MRI) Magnetic resonance imaging is a noninvasive medical imaging technique that uses the interaction between radio frequency pulses, a strong magnetic field and body tissue to obtain images of slices/planes from inside the body. These magnets generate fields from approx. 2000 times up to 30000 times stronger than that of the Earth. The use of nuclear magnetic resonance principles produces extremely detailed pictures of the body tissue without the need for x-ray exposure and gives diagnostic information of various organs.
Measured are mobile hydrogen nuclei (protons are the hydrogen atoms of water, the 'H' in H20), the majority of elements in the body. Only a small part of them contribute to the measured signal, caused by their different alignment in the magnetic field. Protons are capable of absorbing energy if exposed to short radio wave pulses (electromagnetic energy) at their resonance frequency. After the absorption of this energy, the nuclei release this energy so that they return to their initial state of equilibrium.
This transmission of energy by the nuclei as they return to their initial state is what is observed as the MRI signal. The subtle differing characteristic of that signal from different tissues combined with complex mathematical formulas analyzed on modern computers is what enables MRI imaging to distinguish between various organs. Any imaging plane, or slice, can be projected, and then stored or printed.
The measured signal intensity depends jointly on the spin density and the relaxation times (T1 time and T2 time), with their relative importance depending on the particular imaging technique and choice of interpulse times. Any motion such as blood flow, respiration, etc. also affects the image brightness.
Magnetic resonance imaging is particularly sensitive in assessing anatomical structures, organs and soft tissues for the detection and diagnosis of a broad range of pathological conditions. MRI pictures can provide contrast between benign and pathological tissues and may be used to stage cancers as well as to evaluate the response to treatment of malignancies. The need for biopsy or exploratory surgery can be eliminated in some cases, and can result in earlier diagnosis of many diseases.

See also MRI History and Functional Magnetic Resonance Imaging (fMRI).

(Mo) Equilibrium value of the magnetization;; directed along the direction of the static magnetic field. Proportional to spin density (N).

The integration of all the individual nuclear magnetic moments, which have a positive magnetization value at equilibrium versus those in a random state.

(MLSI) Variations of sequential line imaging techniques that can be used if selective excitation methods that do not affect adjacent lines are employed. Adjacent lines are imaged while waiting for relaxation of the first line toward equilibrium, which may result in decreased image acquisition time. A different type of MLSI uses simultaneous excitation of two or more lines with different phase encoding followed by suitable decoding.

A usual variation of sequential plane imaging technique that can be used with selective excitation technique that does not affect adjacent slices. Since, in SE imaging TR longer than TE, the machine would be idling most of the time, if a single slice would be acquired; multiple slice imaging was introduced early on. Adjacent slices are imaged while waiting for relaxation of the first slice toward equilibrium, resulting in decreased image acquisition time for the set of slices. The maximum number of slices of a pulse sequence depends on the repetition time.