MR spectroscopy is a method to examine what kind of chemical compounds are present in the body. It takes a lot more time than a typicl 1H-MRI scan, but it adds an important dimension to MR. The most widely used type of MR spectroscopy is 1H-MRS. There are other types of spectroscopy using other nuclei such as 31P-MRS and 13C MRS.
An important part of 1H-MR spectroscopy is the suppression of water and fat signals. The water and fat signals are much stronger that the metabolite signals that we are interested in. This creates problems with the dynamic range of the MR receiver system and the intense water and fat peaks can distort the baseline of the spectrum. Worse than that: imperfections in the gradient waveforms can produce false metabolite signals*.
There are several ways to suppress the unwanted signals. In all methods we use some sort of frequency selective pulse we can manipulate the magnetization of the water and/or fat signals to reduce or eliminate their contribution to the spectrum.
1) We can saturate the signals by selective excitation on water or fat followed by a gradient pulse to completely dephase (destroy) the signal.
2) We can invert the unwanted signals and wait until the exponential recovery of the signal from negative to positive is halfway and the unwanted signals are (close to) zero.
3) We can create a selective double spin echo sequence within the 1H-MRS echo sequence (this can be the PRESS or STEAM sequence) to selectively defocus the water and fat. The suppressed signal will be experiencing two additional 180 pulses, each flanked by gradients. With a balanced set of spoil gradients the selected water and or fat signals will be defocused and the metabolites will be refocused (for those metabolites the sequence adds a little diffusion gradient pair).
*Bolan PJ, DelaBarre L, Baker EH, Merkle H, Everson LI, Yee D, Garwood M. Eliminating spurious lipid sidebands in H-1 MRS of breast lesions. Magnetic Resonance in Medicine 2002;48(2):215-222.