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FID modulus: a simple and efficient technique to phase and align MR spectra

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Abstract

Object

The post-processing of MR spectroscopic data requires several steps more or less easy to automate, including the phase correction and the chemical shift assignment. First, since the absolute phase is unknown, one of the difficulties the MR spectroscopist has to face is the determination of the correct phase correction. When only a few spectra have to be processed, this is usually performed manually. However, this correction needs to be automated as soon as a large number of spectra is involved, like in the case of phase coherent averaging or when the signals collected with phased array coils have to be combined. A second post-processing requirement is the frequency axis assignment. In standard mono-voxel MR spectroscopy, this can also be easily performed manually, by simply assigning a frequency value to a well-known resonance (e.g. the water or NAA resonance in the case of brain spectroscopy). However, when the correction of a frequency shift is required before averaging a large amount of spectra (due to B 0 spatial inhomogeneities in chemical shift imaging, or resulting from motion for example), this post-processing definitely needs to be performed automatically.

Materials and methods

Zero-order phase and frequency shift of a MR spectrum are linked respectively to zero-order and first-order phase variations in the corresponding free induction decay (FID) signal. One of the simplest ways to remove the phase component of a signal is to calculate the modulus of this signal: this approach is the basis of the correction technique presented here.

Results

We show that selecting the modulus of the FID allows, under certain conditions that are detailed, to automatically phase correct and frequency align the spectra. This correction technique can be for example applied to the summation of signals acquired from combined phased array coils, to phase coherent averaging and to B 0 shift correction.

Conclusion

We demonstrate that working on the modulus of the FID signal is a simple and efficient way to both phase correct and frequency align MR spectra automatically. This approach is particularly well suited to brain proton MR spectroscopy.

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Abbreviations

Composite signal:

A simulated signal composed of one or several FIDs, and eventually a noise signal

Conventional processing:

Refers to the conventional post-processing performed on the original FID signal: this is the conventional technique, compared in this study to the modulus post-processing (see below)

CSI:

Chemical shift imaging

FID:

Free induction decay

FWHM:

Full Width at Half the Maximum

LB:

Line broadening in Hz. LB is the full width at half maximum (FWHM) of the peak obtained after fast Fourier transformation of a time decaying exponential function e (−πLBt) [1]

Modulus processing:

Refers to the post-processing performed on the modulus of the FID signal as presented in this study and compared to the conventional technique (see above)

MRS:

Magnetic resonance spectroscopy

MRI:

Magnetic resonance imaging

SNR:

Signal-to-noise ratio, calculated as the maximum of the peak over the standard deviation of the noise in a frequency range selected in a region of the spectrum devoid of other signals and/or artifacts

SD:

Standard deviation

SW:

Spectral width

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Authors and Affiliations

  1. Aix-Marseille Université, CNRS, CRMBM UMR 7339, 13385, Marseille, France

    Yann Le Fur & Patrick J. Cozzone

  2. APHM, Hôpitaux de la Timone, CEMEREM, 13385, Marseille, France

    Patrick J. Cozzone

Authors
  1. Yann Le Fur
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  2. Patrick J. Cozzone
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Correspondence to Yann Le Fur.

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Le Fur, Y., Cozzone, P.J. FID modulus: a simple and efficient technique to phase and align MR spectra. Magn Reson Mater Phy 27, 131–148 (2014). https://doi.org/10.1007/s10334-013-0381-8

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  • DOI: https://doi.org/10.1007/s10334-013-0381-8

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