Water SuppressionWhy and how do you suppress water signal in MRS?
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Whole brain spectrum without water suppression.
Water is present in tissues at concentrations 10,000 times higher than metabolites of interest (like NAA, Cho, Cr). An unprepared MR spectrum would thus be dominated by a giant water peak, while small organic molecules would be virtually undetectable above background noise.
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.
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.
A CHESS pulse selectively rotates water magnetization into the transverse plane where it is immediately dephased by application of a strong spoiler gradient. For MR spectroscopy a single CHESS pulse provides insufficient water suppression, so 3 CHESS pulses are used in the typical clinical implementation. To insure frequency selectivity, CHESS pulses are relatively long (20-30 ms). The optimal flip angle for each pulse depends on the interpulse spacing as well as local T1 and B1 effects. MR vendors offer automated water suppression procedures that iteratively evaluate and optimize flip angles based on the residual water signal.
References
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).
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).
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