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Continuous‐wave saturation considerations for efficient xenon depolarization
Author(s) -
Kunth Martin,
Witte Christopher,
Schröder Leif
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.3307
Subject(s) - xenon , saturation (graph theory) , chemistry , nuclear magnetic resonance , depolarization , amplitude , atomic physics , analytical chemistry (journal) , physics , biophysics , optics , mathematics , combinatorics , chromatography , biology
The combination of hyperpolarized Xe with chemical exchange saturation transfer (Hyper‐CEST) is a powerful NMR technique to detect highly dilute concentrations of Xe binding sites using RF saturation pulses. Crucially, that combination of saturation pulse strength and duration that generates the maximal Hyper‐CEST effect is a priori unknown. In contrast to CEST in proton MRI, where the system reaches a steady‐state for long saturation times, Hyper‐CEST has an optimal saturation time, i.e. saturating for shorter or longer reduces the Hyper‐CEST effect. Here, we derive expressions for this optimal saturation pulse length. We also found that a pulse strength, B 1 , corresponding to five times the Xe exchange rate, k BA (i.e. B 1 = 5 k BA / γ with the gyromagnetic ratio of 129 Xe, γ ), generates directly and without further optimization 96 % of the maximal Hyper‐CEST contrast while preserving spectral selectivity. As a measure that optimizes the amplitude and the width of the Hyper‐CEST response simultaneously, we found an optimal saturation pulse strength corresponding to 2 times the Xe exchange rate, i.e. B 1 = 2 k BA / γ . When extremely low host concentration is detected, then the expression for the optimum saturation time simplifies as it approaches the longitudinal relaxation time of free Xe. Copyright © 2015 John Wiley & Sons, Ltd.