Speaker
Description
Nuclear spin hyperpolarization has long been exploited to enable a host of applications, from fundamental physics experiments to enhanced NMR / MRI. In order to take such efforts in new directions, our lab’s collaborative work is investigating the use of two hyperpolarization methods – Spin-Exchange Optical Pumping (SEOP) and SABRE (Signal Amplification By Reversible Exchange, a parahydrogen-based technique). First, we have been investigating the use of SEOP and SABRE to hyperpolarize 131Xe and 117Sn, respectively. These isotopes have gained recent interest as potential targets for sensitive searches for time-reversal violation in neutron-nucleus interactions beyond the Standard Model. Unlike 129Xe (with spin I=1/2, and readily polarized to near-unity values via SEOP), 131Xe is quadrupolar (I=3/2), and the resulting short T1 relaxation times make it very difficult to hyperpolarize 131Xe in bulk. With the help of next-generation spectrally-narrowed lasers and low-field in-situ NMR, we have been able to generate HP 131Xe with P up to 7.6%, corresponding to a ~100-fold improvement in P*N over previous efforts—work that has also helped to enable the first measurement of neutron-Xe pseudomagnetic precession. We have also performed the first studies of SABRE-hyperpolarized 117Sn (I=1/2), with implications for NOPTREX (neutron optics parity- and time-reversal experiments).
Next, we have been investigating the preparation and characterization of metal-organic frameworks (MOFs) as supports for SABRE under heterogeneous conditions (“HET-SABRE”). In the course of these efforts, we found a novel manifestation of a previously observed effect: the creation of hyperpolarized orthohydrogen (o-H2, the triplet spin isomer). Here, our observations were unexpected not only because the HP o-H2 signals were so large – with 1H enhancements up to ~500-fold -- but that they were also anti-phase in nature. Such an observation is seemingly paradoxical, because the resonances from the two transitions from the T0 state should ostensibly cancel. We found that this anomalous effect is attained only when using an intact MOF catalyst and is qualitatively independent of substrate nature. This observation is analogous to the “partial negative line” (PNL) effect recently explained in the context of Parahydrogen Induced Polarization (PHIP) by Ivanov and co-workers. The two-spin order of the o-H2 resonance is manifested by a two-fold higher Rabi frequency, and the lifetime of the antiphase HP o-H2 signal is extended by several-fold. Potential implications of this effect will be discussed.
Finally, the increasingly strong magnets used in most clinical scanners are expensive, bulky, and immobile, and scans can be time-consuming and confining. Low-field (LF) MRI can potentially obviate such limitations; however, sensitivity and contrast can be limited. Thus, the increased sensitivity and contrast provided by HP agents can be highly synergistic with low-field MRI. Our collaboration is working to integrate multiple HP approaches with low-field MRI platforms--including a 64 mT portable "point-of-care" scanner (Hyperfine). We are investigating the potential for adapting this type of clinical scanner for use with HP substances. We have recently demonstrated imaging of HP pyrazine and nicotinamide (via SABRE), with small-volume imaging performed in a head-coil with a resolution of 1x1x3.5 mm3 in <15 s. We are currently working to integrate this platform with a ~30-fold scaled-up continuous-flow SABRE reactor. A longer-term goal is to enable imaging of PHIP-hyperpolarized propane gas with partial spin-lock-induced crossing (SLIC)-modified sequences.
