EUROISMAR 2019

Hyperpolarization via dissolution Dynamic Nuclear Polarization (dDNP) is without doubt the most widespread technique to overcome the low sensitivity in the liquid state Magnetic Resonance.¹ Hyperpolarized water is a versatile tool with possible applications ranging from biomedicine to chemistry. For instance, it can be used to acquire high resolution angiographic and perfusion images without employing any metal-ion contrast agent (e.g. Gd³⁺) in animals,² or probe proton exchange in proteins.³

Nevertheless, exploiting the ¹H nuclei enhanced signal is not as straightforward as for ¹³C or other low-gamma nuclei: not only is the T₁ of pure water intrinsically too short on the dDNP scale (3-4 s),⁴ but the strong interaction between water protons and paramagnetic agents in solution provides a fatal relaxation rate contribution during dissolution and transfer.

UV-induced non-persistent radicals have been employed to efficiently polarize ¹³C and other low-gamma nuclei via dDNP.⁵ Generated by UV-irradiation of a frozen solution containing a fraction of pyruvic acid or its derivatives, these radicals are stable as far as the sample temperature is below 190 K. The UV-radicals natural quenching at the moment of dissolution alleviates from the radical elimination step, and drastically reduces relaxation processes during dissolution and transfer.

Here we show that pyruvic acid derivates UV-irradiated for 10 min at 77 K can generate water solid-state ¹H polarizations higher than 90% within 20 min at 6.7 T and 1.1 K, leading to radical-free liquid-state water ¹H polarizations of ~ 75 % with liquid-state ¹H T₁s higher than 40 s at 9.4 T and 313 K.

¹ J. H. Ardenkjaer-Larsen et. al, Proc Natl Acad Sci, 2003
² K. W. Lipsø et. al, Mag Res Med, 2017
³ Q. Chappuis et. al, J Phys Chem Lett, 2015
⁴ K. Krynicki, Physica, 1966
⁵ A. Capozzi et. al, Nat Com, 2017