Design and performance of a small bath cryostat with NMR capability for transport of hyperpolarized samples

As of today, dissolution Dynamic Nuclear Polarization (dDNP) is the only clinically available hyperpolarization technique for ¹³C-MRI. Despite the clear path towards personalized medicine that dDNP is paving as an alternative and/or complement to Positron Emission Tomography (PET), the technique struggles to enter everyday clinical practice. Because of the minute-long hyperpolarization lifetime after dissolution, one of the reasons lies in the need and consequent complexities of having the machine that generates the hyperpolarization (i.e. the dDNP polarizer) on site. Since some years, research groups are working to make hyperpolarization transportable. Two different methods have been developed that allow "freezing" of the nuclear spin state prior to samples extraction from the polarizer. Nevertheless, so far, all attempts of transport have been limited to a very small scale and to the level of proof-of-principle experiments. The main reason for that is the lack of adequate hardware, strategy, and control on most of the crucial parameters. To bridge the technical gap with PET and provide MRI facilities with hours long relaxing hyperpolarized compounds at controlled conditions, a new generation of low cost/small footprint liquid He cryostats equipped with a magnetically enforced cryogenic probe is needed. In this paper, we detail the theoretical and practical construction of a hyperpolarized samples transportation device small enough to fit in a car and able to hold a sample at 4.2 K for almost 8 h despite the presence of a cryogenically-demanding purpose-built probe that provides enough magnetic field upon insertion of the sample and NMR quality homogeneity at storage position. Should transportable hyperpolarization via DNP become a reality, we herein provide important details to make it possible.