  Magnetic resonance imaging in ultra-low fields is often limited by mediocre
signal-to-noise ratio hindering a higher resolution. Overhauser dynamic nuclear
polarisation (O-DNP) using nitroxide radicals has been an efficient solution
for enhancing the thermal nuclear polarisation. However, the concurrence of
positive and negative polarisation enhancements arises in ultra-low fields
resulting in a significantly reduced net enhancement, making O-DNP far less
attractive. Here, we address this issue by applying circularly polarised RF.
O-DNP with circularly polarised RF renders a considerably improved enhancement
factor of around 150,000 at 1.2 microtesla. A birdcage coil was adopted into a
ultra-low field MRI system to generate the circularly polarised RF field
homogeneously over a large volume. We acquired an MR image of a nitroxide
radical solution with an average in-plane resolution of 1 mm. De-noising
through compressive sensing further improved the image quality.
