Proton exchange in aqueous urea solutions measured by water‐exchange (WEX) NMR spectroscopy and chemical exchange saturation transfer (CEST) imaging in vitro

PurposeTo characterize the proton exchange in aqueous urea solutions using a modified version of the WEX II filter at high magnetic field, and to assess the feasibility of performing quantitative urea CEST MRI on a 3T clinical MR system.MethodsIn order to study the dependence of the exchange‐rate co...

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Bibliographic Details
Published in:Magnetic resonance in medicine 2019-09, Vol.82 (3), p.935-947
Main Authors: Stabinska, Julia, Neudecker, Philipp, Ljimani, Alexandra, Wittsack, Hans‐Jörg, Lanzman, Rotem Shlomo, Müller‐Lutz, Anja
Format: Article
Language:English
Subjects:
CEST
Dependence
exchange rate
Feasibility studies
Foreign exchange rates
Magnetic fields
Magnetic resonance imaging
Magnetic resonance spectroscopy
Mathematical analysis
NMR
NMR spectroscopy
Nuclear magnetic resonance
Organic chemistry
pH effects
proton exchange
Protons
Rate constants
Saturation
Spectroscopy
Spectrum analysis
urCEST
Urea
Ureas
WEX
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Summary:PurposeTo characterize the proton exchange in aqueous urea solutions using a modified version of the WEX II filter at high magnetic field, and to assess the feasibility of performing quantitative urea CEST MRI on a 3T clinical MR system.MethodsIn order to study the dependence of the exchange‐rate constant ksw of urea as a function of pH and T, the WEX‐spectra were acquired at 600 MHz from urea solutions in a pH range from 6.4 to 8.0 and a temperature range from T=22∘C to 37∘C. The CEST experiments were performed on a 3T MRI scanner by applying a train of 50 Gaussian‐shaped pulses, each 100‐millisecond long with a spacing of 100 milliseconds, for saturation. Exchange rates of urea were calculated using the (extended) AREX metric.ResultsThe results showed that proton exchange in aqueous urea solutions is acid and base catalyzed with the rate constants: ka=(9.95±1.1)×106 l/(mol·s) and kb=(6.21±0.21)×106 l/(mol·s), respectively. Since the urea protons undergo a slow exchange with water protons, the CEST effect of urea can be observed efficiently at 3T. However, in neutral solutions the exchange rate of urea is minimal and cannot be estimated using the quantitative CEST approach.ConclusionsBy means of the WEX‐spectroscopy, the kinetic parameters of the proton exchange in urea solutions have been determined. It was also possible to estimate the exchange rates of urea in a broad range of pH values using the CEST method at a clinical scanner.
ISSN:0740-3194
1522-2594
DOI:10.1002/mrm.27778