# Dynamic nuclear polarisation of liquids at one microtesla using circularly polarised RF with application to millimetre resolution MRI

## TL;DR

This paper demonstrates a method to significantly enhance nuclear polarization in ultra-low field MRI using circularly polarized RF, enabling millimeter resolution imaging with improved signal strength and image quality.

## Contribution

It introduces the use of circularly polarized RF for Overhauser DNP at microtesla fields, achieving unprecedented enhancement factors and enabling high-resolution MRI.

## Key findings

- Achieved a polarization enhancement factor of around 150,000 at 1.2 microtesla.
- Successfully acquired a 1 mm in-plane resolution MR image.
- De-noising with compressive sensing improved image quality.

## Abstract

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.