  Magnetic resonance imaging at microtesla fields is a promising imaging method
that combines the pre-polarization technique and broadband signal reception by
SQUID sensors to enable in vivo MRI at microtesla-range magnetic fields.
Despite significant advances, the potential of microtesla MRI for biomedical
imaging is limited by its insufficient signal-to-noise ratio due to a
relatively low sample polarization. Dynamic nuclear polarization (DNP) is a
widely used approach that allows polarization enhancement by two-four orders of
magnitude without an increase in the polarizing field strength. In this work,
the first implementation of microtesla MRI with Overhauser DNP and SQUID signal
detection is described. The first measurements of carbon-13 NMR spectra at
microtesla fields are also reported. The experiments were performed at the
measurement field of 96 microtesla with Overhauser enhancement at 3.5 - 5.7 mT
field. The Overhauser DNP enabled nuclear polarization enhancement by factor as
high as -95 for protons and as high as -200 for carbon-13, corresponding to
thermal polarizations at 0.33 T and 1.1 T fields, respectively. These results
demonstrate that SQUID-based microtesla MRI can be naturally combined with
Overhauser DNP in one system, and that its signal-to-noise performance is
greatly improved in this case. They also suggest that microtesla MRI can become
an efficient tool for in vivo imaging of hyperpolarized carbon-13, produced by
the low-temperature dissolution DNP.
