Proton double irradiation field-cycling nuclear magnetic resonance imaging: Testing new concepts and calibration methods

Abstract

Double resonance experiments in field-cycling nuclear magnetic resonance (NMR) are powerful tools for the indirect detection of quadrupolar and electron resonances, dynamic nuclear polarization-induced signal amplification, within other possibilities. In the context of magnetic resonance imaging, double irradiation at high and low magnetic fields in field-cycling experiments can be conveniently used for instrumental calibration purposes and contrast manipulation. The basic goal of this article is to present a practical method for the measurement and calibration of the magnetic field in field-cycling magnetic resonance imaging instruments using proton resonances at high and low fields. We discuss an experimental protocol to adjust the optimal position of the gradient unit respect to the sample and the magnet system, and to measure the effective magnetic field gradient across the sample. A new modality of slice selection by irradiating the sample in the presence of a gradient field at low magnet-field conditions is also discussed. As a practical matter, we show how the sample irradiation at low fields can be used for contrast-enhancement, while handling the NMR signal intensity with spatial selectivity. To the best of our knowledge, this is the first calibration and adjustment package for a field-cycled magnetic resonance imaging instrument and the first example of double irradiation direct manipulation of contrasts in a proton image.