(IR) The inversion recovery pulse sequence produces signals, which represent the longitudinal magnetization existing after the application of a 180° radio frequency pulse that rotates the magnetization Mz into the negative plane. After an inversion time (TI - time between the starting 180° pulse and the following 90° pulse), a further 90° RF pulse tilts some or all of the z-magnetization into the xy-plane, where the signal is usually rephased with a 180° pulse as in the spin echo sequence. During the initial time period, various tissues relax with their intrinsic T1 relaxation time.
In the pulse sequence timing diagram, the basic inversion recovery sequence is illustrated. The 180° inversion pulse is attached prior to the 90° excitation pulse of a spin echo acquisition. See also the Pulse Sequence Timing Diagram. There you will find a description of the components.
The inversion recovery sequence has the advantage, that it can provide very strong contrast between tissues having different T1 relaxation times or to suppress tissues like fluid or fat. But the disadvantage is, that the additional inversion radio frequency RF pulse makes this sequence less time efficient than the other pulse sequences.

Contrast values:
PD weighted: TE: 10-20 ms, TR: 2000 ms, TI: 1800 ms
T1 weighted: TE: 10-20 ms, TR: 2000 ms, TI: 400-800 ms
T2 weighted: TE: 70 ms, TR: 2000 ms, TI: 400-800 ms

See also Inversion Recovery, Short T1 Inversion Recovery, Fluid Attenuation Inversion Recovery, and Acronyms for 'Inversion Recovery Sequence' from different manufacturers.

(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.

(PS) Excitation technique applying repeated RF pulses in times comparable to or shorter than T1. Incomplete T1 relaxation leads to reduction of the signal amplitude; there is the possibility of generating images with increased contrast between regions with different relaxation times.
Although partial saturation is also commonly referred to as saturation recovery, that term should properly be reserved for the particular case of partial saturation in which recovery after each excitation effectively takes place from true saturation. A GRE sequence where α = 90° is identical to the partial saturation or saturation recovery pulse sequence.
It does not directly produce images of T1. However, since the measured signal will depend on T1, the method generates contrast between regions with different relaxation times. If T2 and/or T2 effects are minimized through the use of a short echo time TE, the result is a T1 weighted image. It is not a T1 image due to the possible presence of spin density and T2 effects as well as the nonlinear dependence on T1.
The change in signal from a region resulting from a change in the interpulse time, TR, can be used to calculate T1 for the region.

Reciprocals of the relaxation times, T1 and T2 (R1 = 1/T1 and R2 = 1/T2). There is often a linear relation between the concentration of MR contrast agents and the resulting change in relaxation rate. The rate of relaxation is influenced by molecules with protons that are tumbling. A slower tumble rate will result in faster relaxation rate (shorter relaxation time). Due to the molecular structure of fat with its larger size than water, fat will tumble slower than water molecules. The slower tumble rate of fat enables a faster relaxation rate.

Relaxation time is a general physics concept for the characteristic time in which a system relaxes under certain changes in external conditions. Relaxometry is the theory of relaxation times (spin lattice (T1) and spin spin relaxation (T2)), and their dependence on physical parameters such as magnetic field strength, molecular structure, temperature, pH, and the presence and type of relaxation agents.