Abstract
To develop a method that fits a multipool model to z-spectra acquired from non-steady state sequences, taking into account the effects of variations in T1 or B1 amplitude and the results estimating the parameters for a four-pool model to describe the z-spectrum from the healthy brain. We compared measured spectra with a look-up table (LUT) of possible spectra and investigated the potential advantages of simultaneously considering spectra acquired at different saturation powers (coupled spectra) to provide sensitivity to a range of different physicochemical phenomena. The LUT method provided reproducible results in healthy controls. The average values of the macromolecular pool sizes measured in white matter (WM) and gray matter (GM) of 10 healthy volunteers were 8.9% ± 0.3% (intersubject standard deviation) and 4.4% ± 0.4%, respectively, whereas the average nuclear Overhauser effect pool sizes in WM and GM were 5% ± 0.1% and 3% ± 0.1%, respectively, and average amide proton transfer pool sizes in WM and GM were 0.21% ± 0.03% and 0.20% ± 0.02%, respectively. The proposed method demonstrated increased robustness when compared with existing methods (such as Lorentzian fitting and asymmetry analysis) while yielding fully quantitative results. The method can be adjusted to measure other parameters relevant to the z-spectrum. Magn Reson Med 78:645-655, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Highlights
Magnetization transfer (MT), chemical exchange saturation transfer (CEST), and nuclear Overhauser effect (NOE) phenomena use the transfer of magnetization, or exchange of protons or molecules, to sensitize the visible water proton pool to macromolecules or certain moieties [1]
Fitting the amide proton transfer (APT) signal benefitted from all the acquisitions being made at lower powers, whereas this increased the noise in the fit for the MT pool
The results show that if RF power is well controlled, medium power yields a reasonable fit across all three pools considered here (APT, NOE, and MT)
Summary
Magnetization transfer (MT), chemical exchange saturation transfer (CEST), and nuclear Overhauser effect (NOE) phenomena use the transfer of magnetization, or exchange of protons or molecules, to sensitize the visible water proton pool to macromolecules or certain moieties [1]. More recent studies have used Lorentzian fitting [3,7] and Lorentzian difference methods of analysis [8]. These methods are relatively simple and may be adequate in some circumstances (e.g., where the RF power is well controlled) but are not quantitative; this is because the effects of the different pools do not add linearly, at high saturation powers, and because changes in T1 and RF power [9,10] will disrupt the results. Other methods attempt to suppress the MT contribution by applying double frequency irradiation [11,12], but the CEST effects are still diluted by MT and DS, even if isolated from them
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