Spectral lines are split by spin spin coupling into multiplets whose frequencies are separated by an amount depending on the coupling constant, J. The magnitude of J is independent of the strength of the applied magnetic field and is given in units of frequency, Hz.

(DRESS) Depth resolved surface spectroscopy is a localization method that employ gradients to select the region from which spectra are acquired.

Interaction between a spin and its neighbors due to their magnetic dipole moments. This is an important mechanism contributing to relaxation rates. In solids and viscous liquids this can result in broadening of the spectral lines.

Multiplication of the time-dependent signal data by an exponential function, exp(t/TC), where t is time and TC is a parameter called the time constant (in spectroscopy). The time constant can be chosen to either improve the signal to noise ratio (with a negative TC) or decrease the effective spectral line width (with a positive TC) in the resulting spectrum. The use of a negative TC to improve SNR is equivalent to line broadening by convolving the spectrum with a Lorentzian function of corresponding reciprocal width.

Fat suppression is the process of utilizing specific MRI parameters to remove the deleterious effects of fat from the resulting images , e.g. with STIR, FAT SAT sequences, water selective (PROSET WATS - water only selection, also FATS - fat only selection possible) excitation techniques, or pulse sequences based on the Dixon method.
Spin magnetization can be modulated by using special RF pulses. CHESS or its variations like SPIR, SPAIR (Spectral Selection Attenuated Inversion Recovery) and FAT SAT use frequency selective excitation pulses, which produce fat saturation.
Fat suppression techniques are nearly used in all body parts and belong to every standard MRI protocol of joints like knee, shoulder, hips, etc. Imaging of, e.g. the foot can induce bad fat suppression with SPIR/FAT SAT due to the asymmetric volume of this body part. The volume of the foot alters the magnetic field to a different degree than the smaller volume of the lower leg affecting the protons there. There is only a small band of tissue where the fat protons are precessing at the frequency expected, resulting in frequency selective fat saturation working only in that area. This can be corrected by volume shimming or creating a more symmetrical volume being imaged with water bags.
Even with their longer scan time and motion sensitivity, STIR (short T1/tau inversion recovery) sequences are often the better choice to suppress fat. STIR images are also preferred because of the decreased sensitivity to field inhomogeneities, permitting larger fields of views when compared to fat suppressed images and the ability to image away from the isocenter.
See also Knee MRI.
Sequences based on Dixon turbo spin echo (fast spin echo) can deliver a significant better fat suppression than conventional TSE/FSE imaging.