Molecular imaging with endogenous substances

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Abstract
Dynamic nuclear polarization has enabled hyperpolarization of nuclei such as 13 C and 15 N in endogenous substances. The resulting high nuclear polarization makes it possible to perform subsecond 13 C MRI. By using the dynamic nuclear polarization hyperpolarization technique, 10% polarization was obtained in an aqueous solution of 100 mM 13 C-labeled urea, ready for injection. The in vivo T 1 relaxation time of 13 C in the urea solution was determined to 20 ± 2 s. Due to the long relaxation time, it is possible to use the hyperpolarized substance for medical imaging. A series of high-resolution (≈1-mm) magnetic resonance images were acquired, each with a scan time of 240 ms, 0-5 s after an i.v. injection of the hyperpolarized aqueous [ 13 C]urea solution in a rat. The results show that it is possible to perform 13 C angiography with a signal-to-noise ratio of ≈275 in ≈0.25 s. Perfusion studies with endogenous substances may allow higher spatial and/or temporal resolution than is possible with current proton imaging techniques.