title: Towards quantification of pulsed spinlock and CEST at clinical MR scanners: an analytical interleaved saturation-relaxation (ISAR) approach.
abstract: 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.
authors.affiliation: Deutsches Krebsforschungszentrum (DKFZ), German Cancer Research Center, Division of Medical Physics in Radiology, Heidelberg, Germany; Biomedizinische NMR Forschungs GmbH, am Max-Planck-Institut für biophysikalische Chemie, Göttingen, Germany.
authors.forename: Volkert
authors.initials: V
authors.lastname: Roeloffs
authors.forename: Christian
authors.initials: C
authors.lastname: Meyer
authors.forename: Peter
authors.initials: P
authors.lastname: Bachert
authors.forename: Moritz
authors.initials: M
authors.lastname: Zaiss
mesh_terms: D003198:Computer Simulation; D060328:Magnetic Phenomena; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D009716:Numerical Analysis, Computer-Assisted; D019047:Phantoms, Imaging; D011674:Pulse
keywords: CEST; amide proton transfer, APT; chemical exchange; magnetization transfer, MT; pulsed saturation; spinlock