Spurious Phase Correction in Multi-Shot MRSI
Résumé
Introduction
Spectral multi-shot (MS) MRSI sequences are used essentially to decrease the scan time and enlarge the spectral width. Here, the spectral information is encoded by acquiring several experiments while shifting the readout position relative to the echo time (TE). These techniques are highly exposed to phase errors with the phase evolution being indirectly sampled through the different echo time increments. During this sampling, phase coherency is crucial. Nowadays, MS techniques can lead up to image distortion because of the loss of phase coherency between the shots. The purpose of this work is to present theoretically and experimentally how to overcome this phase problem.
Subjects/Methods
We demonstrated that during the extra delay to shift the echo time for a 2D slice selection, the phase evolves
depending on the chemical shift with respect to the excitation frequency. The latter is generally significantly
higher than the chemical shift, and the phase will mostly depend on it. This phase shift can be corrected on
either the acquired signal or the reconstructed image by subtracting the additional frequency excitation
applied when shifting the slice position. These methods were applied to rapid metabolic imaging (1) of a 13C
phantom and on the particular case of B0 mapping.
Results/Discussion
Results showed phase errors in MS MRSI, leading to image distortion, therefore incorrect and inefficient
studies. However, applying one of the provided correction methods cancels the phase errors, improving the
image quality.
Unwanted phase shifts are susceptible to be accumulated in MS techniques during different echo time
increments, especially when the slice position is shifted from a null resonance frequency. We were able to
correct the phase-shifting through post-processing in the k-space dimension, by image phase correction
according to the Nyquist frequency rule or even during a particular experimental case. The three proposed
correction approaches were able to discard the spurious phase and then to reconstruct parametric maps
accurately.
Domaines
Chimie analytique
Fichier principal
Poster NES ESMRMB 2020.pdf (544.44 Ko)
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2020 Abstract ESMRM.pdf (348.89 Ko)
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ESMRMB 2020 - Nour EL SABBAGH - final version.pptx (3.82 Mo)
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