To visualize these interesting small organic molecules, the large water peak must be suppressed. Methods to accomplish this are described below, the most common being CHESS (CHEmical Shift Selective saturation) or one of its variants.
CHESS was originally developed as a technique for fat suppression in conventional MR imaging, where it is commonly known by the generic name "fat-sat". By tuning the CHESS pulses to the resonant frequency of water instead of fat, water suppression may be obtained.
Advanced Discussion (show/hide)»
At body temperatures the concentration of pure water in water is (¹H20 is 55.6 M. Because there are 2 hydrogens per water molecule, the concentration of ¹H in water is twice this much, or approximately 111 M. The fraction of water in brain tissue is about 70%, giving a water proton concentration of 0.70 x 111 ≈ 80 M.
Even with a superior level of water suppression in a perfectly shimmed magnet, some residual water resonance or secondary ripple effects are always present near δ = 4.65 ppm. Hence the spectral band from about 4.4 to 5.0 ppm remains largely uninterpretable for ¹H spectroscopy of tissues.
de Graaf RA, Nicolay K. Adiabatic water suppression using frequency selective excitation. Magn Reson Med 1998; 40:690–696. (First description of the SWAMP technique that uses adiabatic RF-pulses and is offered as a product by Philips).
Haase A, Frahm J, Hänicke W, Matthaei D. ¹H NMR chemical shift selective (CHESS) imaging. Phys Med Biol 1985; 30:341-344.
Mescher M, Tannus A, O’Neil Johnson M, Garwood M. Solvent suppression using selective echo dephasing. J Magn Reson A 1996; 123:226–229. (First description of the MEGA technique)
Mescher M, Merkle H, Kirsch J, et al. Simultaneous in vivo spectral editing and water suppression. NMR Biomed 1998; 11:266–272. (Further MEGA refinements)
Ogg RJ, Kingsley PB, Taylor JS. WET, a T1- and B1-insensitive water-suppression method for in vivo localized ¹H NMR spectroscopy. J Magn Reson Ser B 1994; 104:1-10.
Star-Lack J, Nelson SJ, Kurhanewicz J, et al. Improved water and lipid suppression for 3D PRESS CSI using RF band selective inversion with gradient dephasing (BASING). Magn Reson Med. 1997; 38:311–321.
Tkac I, Starcuk Z, Choi I-Y, Gruetter R. In vivo ¹H NMR spectroscopy of rat brain at 1 ms echo time. Magn Reson Med 1999; 41:649-656. (First description of the VAPOR technique).
How do Fat-Sat pulses work?
How and why is fat suppression performed in an MRS study?