Water & Lipid Suppression - VAPOR, WET, OVS, IR, Novel Approaches (MC, Crushers)
Vincent Oltman Boer1

1Danish Research Center for Magnetic Resonance, Copenhagen University Hospital Hvidovre, Hvidovre, Denmark

Synopsis

In this presentation, the need for water and lipid suppression, as well as the most widely used approaches to achieve this are explained.

Highlights

  • How can we see signals that are more than a thousand times smaller than the lipid and water signal?
  • How can we be sure the small peaks we’re looking at are not an artifact or unsuppressed residual?
  • Which method do I choose without too much of an impact on the scan time?

Target Audience

MR physicists, clinicians or technologists interested in setting up or developing new 1H MRS methods.

Outcome/objective

  • Appreciation of the range of pulse sequences available for water and lipid suppression.
  • Understanding of the different approaches that each method can be used for, and the best ways to choose an appropriate method for the scan you are interested in.

Introduction

In 1H MR spectroscopy we are looking at metabolic signals that are orders of magnitude smaller than the abundant water and lipid signal that we have in the body. Many methods exist that can be used to suppress the water and lipid signal, which off-course should not disturb the magnetization of the small signals of interest. In this lecture several of the most used methods, as well as some advanced newly developed methods will be described from basic principles.

Methods

When looking at the most widely used methods for both water and lipid suppression, we can discriminate basically three different types of sequences.

The first group of sequences is based on IR, or inversion recovery (1), where a relaxation delay after an inversion pulse can be chosen such that the confounding signals are nulled.

A second group of sequences to remove water and lipid disturbances is based on pre-saturation, such as the CHESS sequence (2), or outer volume suppression (OVS). In these sequences an pre-pulse is used for excitation. This can be either frequency selective to the water or lipid peak (in case of the CHESS), or spatially selective on areas as for example the skull (in case of OVS). In combination with crusher gradients, the signal is dephased, resulting in effective water and/or lipid suppression. An important part is compensation of B1 variations which is done in the WET sequence (3) by choosing multiple pre-pulses with optimized flip angles. Also more advanced with combination of inversion recovery and pre-saturation, such as the VAPOR sequence (4), will be described in detail.

A third group of sequences is based on selective spin-echo dephasing, where the unwanted signal is removed by a combination of selective refocusing pulses and dephasing gradients, for example in the MEGA (5) and BASING (6) sequences.

Lastly, newly emerging methods that are typically used at high field, such as shaping an OVS region with multi-transmit solutions (7,8), and dephasing coils to shape a dephasing region (9) will be described.

Acknowledgements

No acknowledgement found.

References

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2. Haase A, Frahm J, Hänicke W, Matthaei D. 1H NMR chemical shift selective (CHESS) imaging. Phys. Med. Biol. 1985;30:341–4.

3. 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. B 1994;104:1–10.

4. Tkác I, Starcuk Z, Choi IY, Gruetter R. In vivo 1H NMR spectroscopy of rat brain at 1 ms echo time. Magn. Reson. Med. 1999;41:649–56.

5. Mescher M, Tannus A, Johnson MO, Garwood M. Solvent Suppression Using Selective Echo Dephasing. J. Magn. Reson. Ser. A 1996;123:226–229. doi: 10.1006/jmra.1996.0242.

6. Star-Lack J, Nelson SJ, Kurhanewicz J, Huang LR, Vigneron DB. Improved water and lipid suppression for 3D PRESS CSI using RF band selective inversion with gradient dephasing (BASING). Magn. Reson. Med. 1997;38:311–21.

7. Hetherington HP, Avdievich NI, Kuznetsov AM, Pan JW. RF shimming for spectroscopic localization in the human brain at 7 T. Magn. Reson. Med. 2010;63:9–19. doi: 10.1002/mrm.22182.

8. Boer VO, Klomp DWJ, Juchem C, Luijten PR, de Graaf RA. Multislice 1H MRSI of the human brain at 7 T using dynamic B0 and B1 shimming. Magn. Reson. Med. 2012;68:662–70. doi: 10.1002/mrm.23288.

9. Boer VO, van de Lindt T, Luijten PR, Klomp DWJ. Lipid suppression for brain MRI and MRSI by means of a dedicated crusher coil. Magn. Reson. Med. 2015;73:2062–8. doi: 10.1002/mrm.25331.

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)