A positively charged particle located in the nucleus of an atom. The number of protons in the nucleus governs the chemical properties of that element.

The concentration of mobile Hydrogen atoms within a sample of tissue.

See also Hydrogen Density.

An image produced by controlling the selection of scan parameters to minimize the effects of T1 and T2, resulting in an image dependent primarily on the density of protons in the imaging volume. Proton density contrast is a quantitative summary of the number of protons per unit tissue. The higher the number of protons in a given unit of tissue, the greater the transverse component of magnetization, and the brighter the signal on the proton density contrast image. Conversely the lower the number of protons in a given unit of tissue, the less the transverse magnetization and the darker the signal on the proton density image. Also called (Rho) ρ-weighted.

See also Density Weighted Imaging and Image Contrast Characteristics.

(MT) Magnetization Transfer was accidentally discovered by Wolff and Balaban in 1989. Conventional MRI is based on the differences in T1, T2 and the proton density (water content and the mobility of water molecules) in tissue; it relies primarily on free (bulk) water protons. The T2 relaxation times are greater than 10 ms and detectable. The T2 relaxation times of protons associated with macromolecules are less then 1 ms and not detectable in MRI.
Magnetization Transfer Imaging (MTI) is based on the magnetization interaction (through dipolar and/or chemical exchange) between bulk water protons and macromolecular protons. By applying an off resonance radio frequency pulse to the macromolecular protons, the saturation of these protons is then transferred to the bulk water protons. The result is a decrease in signal (the net magnetization of visible protons is reduced), depending on the magnitude of MT between tissue macromolecules and bulk water. With MTI, the presence or absence of macromolecules (e.g. in membranes, brain tissue) can be seen.
The magnetization transfer ratio (MTR) is the difference in signal intensity with or without MT.

See also Magnetization Transfer Contrast.

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.
The strength of the external magnetic field and the thermal energy of the atoms are the factors, which affect the direction of alignment of the hydrogen protons. The high-energy protons are strong enough to align themselves against or anti parallel to the magnetic field, whereas the lower energy protons will align themselves with or parallel to the magnetic field.
As the magnetic field increases, there are fewer protons, which are strong enough to align anti parallel to the magnetic field. There are always a larger number of protons aligned parallel with the magnetic field, so once 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 of the patient's body.