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k-space analysis of point-resolved spectroscopy (PRESS) with regard to spurious echoes in in vivo (1)H MRS.

Journal article
Authors Göran Starck
Arvid Carlsson
Maria Ljungberg
Eva Forssell-Aronsson
Published in NMR in biomedicine
Volume 22
Issue 2
Pages 137-47
ISSN 0952-3480
Publication year 2009
Published at Institute of Clinical Sciences, Department of Radiation Physics
Institute of Clinical Sciences
Pages 137-47
Language en
Links dx.doi.org/10.1002/nbm.1289
Keywords Algorithms, Artifacts, Computer Simulation, Magnetic Resonance Spectroscopy, methods, Models, Chemical, Protons, Reproducibility of Results, Sensitivity and Specificity
Subject categories Radiological physics

Abstract

The spurious echo artefact, not uncommon in (1)H MRS in the brain, comes from refocusing outer volume signal. Application of MRS in small volumes in susceptibility-affected regions often results in large shim gradients. The artefact problem is accentuated when the global effect of the shim gradient shifts the water resonance outside the water suppression band in the outer volume. This scenario brings the issue of spurious echoes once again to the fore. In this paper, spurious signals of the point-resolved spectroscopy (PRESS) sequence are analysed using the concept of k-space. This new approach facilitates a more geometrical view of the problem, well suited for studying the effect of gradient spoiling and refocusing of signal. Several spoiling options are shown, and the probability of the global effects of shimming being a primary cause of the artefact is discussed. Fourier transform analysis of realistic slice profiles, combined with the k-space description of spurious echoes, shows that unsuppressed water signal in outer regions greatly increases the demands on spoiling. Gradient spoiling adequate for artefact suppression at a given size of MRS volume may not be sufficient at a smaller size. Several ways to improve PRESS measurements with regard to suppression of spurious signal are discussed.

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