Ebert, S., et al., A Mobile DNP Polarizer for Continuous Flow Applications. Appl. Magn. Reson., 2012: p. 1-12.
Despite its wide applicability in natural sciences, nuclear magnetic resonance (NMR) still suffers from its inherently low sensitivity. This could be overcome by hyperpolarization of molecules via dynamic nuclear polarization (DNP). Here, we introduce a substantial improvement of a mobile Overhauser DNP polarizer, based on an inexpensive Halbach magnet operating at 0.35 T. It shows an almost vanishing magnetic flux at its outer side and does not disturb other instruments. It can be placed directly next to a superconducting magnet, thus reducing the transport time of the hyperpolarized liquid. However, two problems for DNP applications remain. Firstly, radicals are needed which are often toxic. This problem becomes crucial with regard to medical applications. Secondly, the sample must be transported from the polarization magnet to the place of detection like a magnetic resonance imaging (MRI) scanner, and polarization losses due to T 1 relaxation may occur. We have implemented a flow system into the mobile DNP polarizer, which overcomes both obstacles. The radicals are immobilized in a gel matrix and the hyperpolarized radical-free fluid is subsequently directly pumped into the MRI scanner. It is shown that even at flow conditions, the NMR signal is enhanced due to Overhauser DNP in the Halbach magnet as well as in the MRI scanner (4.7 T) at a distance of 1.4 m. Acquired images demonstrate the use of enhanced and, due to dipolar coupling, inverted NMR signals, which provide an excellent MRI contrast even for small enhancements.