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'

The NMR, MRI relevant nuclear spin is the rotational movement of a subatomic particle (proton or neutron) around its axis. Whether a nucleus has an overall spin, depends on its amount of protons and neutrons. Nuclei with an identical number of protons and neutrons cancel out their overall spins. Nuclei with an odd number of protons or an odd number of neutrons or both have an overall spin. This spin is measured with a nuclear spin quantum number (I). The nuclear spin quantum number of a nuclei depends on the protons/neutrons which are not paired, and is a positive integer multiple of 0.5. 1H, 19F, 13C, 31P and 15N are examples of nuclei with an nuclear spin quantum number of 0.5, 2H and 14N have a nuclear spin quantum number of 1.

See also Spin Quantum Number.

Quick Overview
Please note that there are different common names for this artifact.

During frequency encoding, fat protons precess slower than water protons in the same slice because of their magnetic shielding. Through the difference in resonance frequency between water and fat, protons at the same location are misregistrated (dislocated) by the Fourier transformation, when converting MRI signals from frequency to spatial domain. This chemical shift misregistration cause accentuation of any fat-water interfaces along the frequency axis and may be mistaken for pathology. Where fat and water are in the same location, this artifact can be seen as a bright or dark band at the edge of the anatomy.
Protons in fat and water molecules are separated by a chemical shift of about 3.5 ppm. The actual shift in Hertz (Hz) depends on the magnetic field strength of the magnet being used. Higher field strength increases the misregistration, while in contrast a higher gradient strength has a positive effect. For a 0.3 T system operating at 12.8 MHz the shift will be 44.8 Hz compared with a 223.6 Hz shift for a 1.5 T system operating at 63.9 MHz.

For artifact reduction helps a smaller water fat shift (higher bandwidth), a higher matrix, an in phase TE or a spin echo technique. Since the misregistration offset is present in the read out axis the patient may be rescanned with this axis parallel to the fat-water interface. Steeper gradient may be employed to reduce the chemical shift offset in mm. Another strategy is to employ specialized pulse sequences such as fat saturation or inversion recovery imaging. Fat suppression techniques eliminate chemical shift artifacts caused by the lack of fat signal.

See also Black Boundary Artifact and Magnetic Resonance Spectroscopy.

(PDWI) In density weighted imaging, the contrast is dependent on the density of protons in the tissue. Proton density weighted images are generated by choosing TR greater than T1 (typically ≥ 2 000 ms) and TE less than T2 (typically ≤ 30 ms), the two exponential terms are both close to one and therefore M is relatively independent of T1 and T2, thereby emphasizing Mxy0, which is proportional to the proton density. Also called (Rho) ρ-weighted.

See also Proton Density Weighted Image.

Hydrogen nuclei magnetic moments are randomly oriented in the absence of an external magnetic field and are considered to have a net magnetization of zero. Once hydrogen protons are placed in the presence of an external magnetic field, they align themselves in one of two directions, parallel or anti parallel to the net magnetic field, which is commonly referred to as the vector B0. The parallel and anti parallel protons cancel each other out, only the small number of low energy protons left aligned with the magnetic field create the overall net magnetization, this difference is all that counts. The magnetic moments of these protons are added together and are referred to as net magnetization vector (NMV) or the symbol 'M'.

See also Magnetization Transfer Contrast.