Using hyperpolarized water as a magnetic resonance contrast agent
Accurately tracing water’s movement through already water-logged substances can prove challenging, especially because most contrast agents alter the liquid’s physical, chemical and biological behavior. However, hyperpolarized water may serve as a contrast agent without the addition of other compounds — a concept that has potential applications in MRI-based blood perfusion analysis, vortex visualization in flow reactors, and flow dispersion measurement in separation columns, among other areas.
Scientists led by Songi Han of the University of California, Santa Barbara, confirmed this potential through studies on the movement of radical-free water in a 1H spin-polarized state through model reactors and molecular sieves.
To change the interaction between electron and proton spins to induce hyperpolarization, the researchers used a dynamic nuclear polarization setup. Within the device, water passed over agarose gel beads firmly peptide-bound with radicals, called spin labels. Microwave radiation ensured the transfer of energy from the spin labels to the protons of the water. As the water moved on, the scientists used 2-D spin warp imaging with a Bruker nuclear magnetic resonance probe to visualize macroscopic flow patterns in the reactors and dispersion through the sieves.
In contrast flow images, the agent yielded 10-fold signal enhancement, which directly translated to a contrast 10 times greater than that feasible with conventional techniques. The ability to polarize water in continuous flow and the long 6-s lifetime of the polarization allowed sufficient time to acquire high-quality images. The study appeared in the Feb. 6 issue of PNAS.
Because of the hyperpolarized water’s authenticity and lack of toxic radicals, the researchers stress that it could be used to portray bulk transport of water through single cells. Additionally, they noted that the water can move across the blood-brain barrier, unlike gadolinium-containing agents. They plan to undertake MRI-based projects in both of these areas.
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