(RE MRI) There are several approaches to speeding up the MRI data acquisition process by repeating the excitation by RF pulses in times short compared to T1, typically using small flip angles and gradient echo refocusing. When TR is also on the order of or shorter than T2, the repeated RF pulses will tend to refocus transverse magnetization remaining from prior excitations, setting up a condition of steady state free precession, and a dependence of signal strength (and image contrast) on both T1 and T2.
This can be modified in various ways, particularly:
1) to spoil the tendency to build up a steady state by reducing coherence between excitations, e.g. by variation of the phase or timing of consecutive RF pulses or of the strength of spoiler gradient pulses, thus increasing the relative dependence of signal strength on T1 or
2) acquire the signal when it is refocusing immediately prior to the next RF pulse, thus increasing the relative dependence of signal strength on T2.

See also Ultrafast Gradient Echo Sequence.

(ROAST) A gradient echo sequence.
See Refocused Gradient Echo Sequence and Steady State Free Precession.

Edward Purcell and Felix Bloch discovered the basic of spectroscopy in 1946 (see MRI History). Nuclear magnetic resonance spectroscopy (NMR Spectroscopy or MRS) is an analytical tool, based on nuclei that have a spin (nuclei with an odd number of neutrons and/or protons) like 1H, 13C, 17O, 19F, 31P etc.
Through nuclear magnetic principles as precession, chemical shift, spin spin coupling etc., the analysis of the content, purity, and molecular structure of a sample is possible. The spectrum produced by this process contains a number of peaks; the highs and the positions of these peaks allow the exact analysis. Unknown compounds can be matched against spectral libraries. Even very complex organic compounds as enzymes and proteins can be determined. For the wide uses of NMR spectroscopy (from mineralogy to medicine) there is a variety of different techniques available.
See Spectroscopic Imaging Techniques.

(E-SHORT) A gradient echo sequence in which a non-zero steady state develops for transverse and longitudinal magnetization. The TR is shorter than the T1 and T2 times of the tissue.
See Gradient Echo Sequence and Steady State Free Precession.

The xy component of the net magnetization vector at right angles to the main magnetic field. The precession of the transverse magnetization at the Larmor frequency is responsible for the detectable MRI signal. In the absence of externally applied RF energy, the transverse magnetization will decay to zero with a characteristic time constant of T2, or more strictly T2*.