Hilschenz, Ingo, Sangwon Oh, Seong-Joo Lee, Kwon Kyu Yu, Seong-min Hwang, Kiwoong Kim, and Jeong Hyun Shim. “Dynamic Nuclear Polarisation of Liquids at One Microtesla Using Circularly Polarised RF with Application to Millimetre Resolution MRI.” Journal of Magnetic Resonance 305 (August 2019): 138–45.
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 lT. A birdcage coil was adopted into an 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.