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Towards quantification of pulsed spinlock and CEST at clinical MR scanners: an analytical interleaved saturation–relaxation (ISAR) approach
Author(s) -
Roeloffs Volkert,
Meyer Christian,
Bachert Peter,
Zaiss Moritz
Publication year - 2015
Publication title -
nmr in biomedicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.278
H-Index - 114
eISSN - 1099-1492
pISSN - 0952-3480
DOI - 10.1002/nbm.3192
Subject(s) - saturation (graph theory) , specific absorption rate , magnetization transfer , nuclear magnetic resonance , computational physics , bloch equations , relaxation (psychology) , chemistry , pulse (music) , waveform , analytical chemistry (journal) , materials science , physics , optics , magnetic resonance imaging , computer science , mathematics , medicine , psychology , telecommunications , social psychology , combinatorics , chromatography , detector , antenna (radio) , radiology , voltage , quantum mechanics
Off‐resonant spinlock (SL) enables an NMR imaging technique that can detect dilute metabolites similar to chemical exchange saturation transfer. However, in clinical MR scanners, RF pulse widths are restricted due to recommended specific absorption rate limits. Therefore, trains of short RF pulses that provide effective saturation during the required irradiation period are commonly employed. Quantitative evaluation of spectra obtained by pulsed saturation schemes is harder to achieve, since the theory of continuous wave saturation cannot be applied directly. In this paper we demonstrate the general feasibility of quantifying proton exchange rates from data obtained in pulsed SL experiments on a clinical 3 T MR scanner. We also propose a theoretical treatment of pulsed SL in the presence of chemical exchange using an interleaved saturation–relaxation approach. We show that modeling magnetization transfer during the pauses between the RF pulses is crucial, especially in the case of exchange rates that are small with respect to the delay times. The dynamics is still governed by a monoexponential decay towards steady state, for which we give the effective rate constant. The derived analytical model agrees well with the full numerical simulation of the Bloch–McConnell equations for a broad range of values of the system parameters. Copyright © 2014 John Wiley & Sons, Ltd.