Robust, susceptibility-matched NMR probes for compensation of magnetic field imperfections in magnetic resonance imaging (MRI)

The presented paper describes a magnetic field sensor system for the real-time monitoring of magnetic resonance (MR) image encoding in the presence of gradient and main magnetic field imperfections. For this purpose, a novel simplified design for nuclear magnetic resonance (NMR) probes has been developed. The design fulfills the requirements of providing below ppm measurement accuracy in combination with long readout times. To this end, NMR is performed with a nanoliter water volume, which together with the radiofrequency (RF) solenoid coil is cast into an Epoxy holder. Based on the Larmor relation (i.e. the proportionality between the NMR resonance frequency and the local B₀), time varying magnetic field information is extracted from the phase evolution of the detected RF signals. Susceptibility matching techniques are applied to homogenize the B₀ field across the NMR sample and in this way to extend the readout time of the NMR probes. The presented design of NMR probe is significantly simpler compared to previous presented ones by De Zanche et al. in the sense that it omits the glass capillary plus additional matching fluidics. The optimization of the susceptibility for the Epoxy is described and results of the probe characterization measurements are presented. The feasibility of magnetic field monitored MRI is demonstrated for ultra-fast single- and multishot data acquisition schemes.