See Macroscopic Magnetization Vector.

(LMR) A particular technique for obtaining NMR spectra, for example, of phosphorus, from a limited region by creating a sensitive volume with inhomogeneous applied gradient magnetic fields, which may be enhanced with the use of surface coils.

Central Moiety: Mr 2+/Mr 3+
A preclinical RES-directed contrast agent under development, for the detection of liver and spleen lesions.

Magnetic relaxation in tissues can be enhanced using contrast agents. The most commonly used for MRI are the paramagnetic contrast agents, which have their strongest effect on the T1, by increasing T1 signal intensity in tissues where they have accumulated.
MRI collects signal from the water protons, but the presence of these contrast agents enhances the relaxation of water protons in their vicinity. Paramagnetic contrast agents contain magnetic centers that create magnetic fields approximately one thousand times stronger than those corresponding to water protons. These magnetic centers interact with water protons in exactly the same way as the neighboring protons, but with much stronger magnetic fields, and therefore, have a much greater impact on relaxation rates, particularly on T1. In MRI, contrast agents are routinely injected intravenously to help identify areas of hypervascularity, as in malignant tumors.

See also Contrast Agents, Gadovist®, MultiHance®, Omniscan®, OptiMARK®.

See also the related poll result: 'The development of contrast agents in MRI is'

(SPIO) Relatively new types of MRI contrast agents are superparamagnetic iron oxide-based colloids (median diameter greater than 50nm). These compounds consist of nonstoichiometric microcrystalline magnetite cores, which are coated with dextrans (in ferumoxide) or siloxanes (in ferumoxsil). After injection they accumulate in the reticuloendothelial system (RES) of the liver (Kupffer cells) and the spleen. At low doses circulating iron decreases the T1 time of blood, at higher doses predominates the T2* effect.
SPIO agents are much more effective in MR relaxation than paramagnetic agents. Since hepatic tumors either do not contain RES cells or their activity is reduced, the contrast between liver and lesion is improved. Superparamagnetic iron oxides cause noticeable shorter T2 relaxation times with signal loss in the targeted tissue (e.g., liver and spleen) with all standard pulse sequences. Magnetite, a mixture of FeO and Fe2O3, is one of the used iron oxides. FeO can be replaced by Fe3O4.
Use of these colloids as tissue specific contrast agents is now a well-established area of pharmaceutical development. Feridex®, Endorem™, GastroMARK®, Lumirem®, Sinerem®, Resovist® and more patents pending tell us that the last word in this area is not said.
Some remarkable points using SPIO:

See also Superparamagnetism, Superparamagnetic Contrast Agents and Classifications, Characteristics, etc..