Slice selection in low-temperature, DNP-enhanced magnetic resonance imaging by Lee-Goldburg spin-locking and phase modulation.

Large enhancements in nuclear magnetic resonance (NMR) signals provided by dynamic nuclear polarization (DNP) at low temperatures have the potential to enable inductively-detected H magnetic resonance imaging (MRI) with isotropic spatial resolution on the order of one micron, especially when low temperatures and DNP are combined with microcoils, three-dimensional (3D) phase encoding of image information, pulsed spin locking during NMR signal detection, and homonuclear dipolar decoupling by Lee-Goldburg (LG) irradiation or similar methods. However, the relatively slow build-up of nuclear magnetization under DNP leads to very long acquisition times for high-resolution 3D images unless the sample volume or field of view (FOV) is restricted. We have therefore developed a method for slice selection in low-temperature, DNP-enhanced MRI that limits the FOV to about 50 μm in one or more dimensions. This method uses small-amplitude phase modulation of LG irradiation in the presence of a strong magnetic field gradient to invert spin-locked H magnetization in the selected slice. Experimental results are reported, including effects of radio-frequency field inhomogeneity, variations in the amplitude of phase modulation, and shaped phase modulation.