A 2016 study published by The Journal of Physical Chemistry B titled “Molecular MRI in the Earth’s Magnetic Field Using Continuous Hyperpolarization of a Biomolecule in Water” explores hyperpolarization as a method of increasing magnetic resonance imaging (MRI) sensitivity for aqueous biomolecule imaging.[1]
MRI relies on the evolving spin polarization of atomic nuclei, like hydrogen, when subjected to an external magnetic field. However, since the population difference between nuclear spins’ upper and lower energy levels is very small, MRI’s sensitivity is limited.[2] Continuous hyperpolarization (cHYP) of biomolecules alters this distribution, ensuring more protons induce radio waves detectable with MRI.[3] The article employs the signal amplification by reversible exchange (SABRE), which uses parahydrogen (pH2) to hyperpolarize biomolecules.
Before using cHYP, the study establishes that aqueous nicotinamide (NA) yielded poor MRI signals with a low SNR at 5.6 mT, as NA proton contribution is indistinguishable from surrounding water contribution. To verify that cHYP enhances low-field MRI signal intensity of nicotinamide, a cHYP NA aqueous solution was placed inside an MRI portable unit, alongside a water sample for comparation. Signal acquisition was realized in situ, with a magnetic field cycling between 5.9 mT and Earth’s field (52 μT) for every k-space line. Figure 1 shows the MRI signal intensity in arbitrary units (a.u.), respective to the position (mm) across the acquired image. As expected, the MRI signal intensity was far greater for the cHYP NA solution. In fact, the cHYP NA yielded SNR of 13.4, contrasting water’s 4.7, corresponding to an overall signal intensity enhancement of 4740 for each NA proton due to hyperpolarization.
Although the technique is relatively new, low-field MRI of biomolecules performed in this study represents a significant strive towards cHYP in vivo applications. Work remains to be done on sensitivity limits of cHYP and on its toxicity in vivo.
References :
[1] Rovedo, P., Knecht, S., Bäumlisberger, T., Cremer, A. L., Duckett, S. B., Mewis, R. E., Green, G. G. R., Burns, M., Rayner, P. J., Leibfritz, D., Korvink, J. G., Hennig, J., Pütz, G., von Elverfeldt, D., and Hövener, J.-B. “Molecular MRI in the Earth’s Magnetic Field Using Continuous Hyperpolarization of a Biomolecule in Water”. The Journal of Physical Chemistry B. 2016; 120 (25), 5670-5677 DOI: 10.1021/acs.jpcb.6b02830
[2] Centre for Hyperpolarisation in MR (2020). The sensitivity challenge. University of York. https://www.york.ac.uk/chym/sensitivity-challenge/
[3] Centre for Hyperpolarisation in MR (2020). What is hyperpolarisation?. University of York. https://www.york.ac.uk/chym/hyperpolarisation/