title: Continuous-wave saturation considerations for efficient xenon depolarization.
abstract: 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, B1, corresponding to five times the Xe exchange rate, k(BA) (i.e. B1 = 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. B1=√2k(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.
authors.affiliation: ERC Project BiosensorImaging, Leibniz-Institut für Molekulare Pharmakologie (FMP), 13125, Berlin, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Kunth
authors.affiliation: ERC Project BiosensorImaging, Leibniz-Institut für Molekulare Pharmakologie (FMP), 13125, Berlin, Germany.
authors.forename: Christopher
authors.initials: C
authors.lastname: Witte
authors.affiliation: ERC Project BiosensorImaging, Leibniz-Institut für Molekulare Pharmakologie (FMP), 13125, Berlin, Germany.
authors.forename: Leif
authors.initials: L
authors.lastname: Schröder
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008956:Models, Chemical; D015336:Molecular Probe Techniques; D011083:Polycyclic Compounds / Q000032:analysis* / Q000737:chemistry*; D012815:Signal Processing, Computer-Assisted; D014979:Xenon Isotopes / Q000032:analysis* / Q000737:chemistry*
chemical_list: D011083:Polycyclic Compounds; D014979:Xenon Isotopes; C545484:cryptophane A
keywords: Bloch-McConnell; CEST; Hyper-CEST; biosensor; hyperpolarization; quantification; xenon