Molecular imaging with endogenous substances

Abstract

Dynamic nuclear polarization has enabled hyperpolarization of nuclei such as 13C and 15N in endogenous substances. The resulting high nuclear polarization makes it possible to perform subsecond 13C MRI. By using the dynamic nuclear polarization hyperpolarization technique, 10% polarization was obtained in an aqueous solution of 100 mM 13C-labeled urea, ready for injection. The in vivo T1 relaxation time of 13C 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 ( approximately 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 [13C]urea solution in a rat. The results show that it is possible to perform 13C angiography with a signal-to-noise ratio of approximately 275 in approximately 0.25 s. Perfusion studies with endogenous substances may allow higher spatial and/or temporal resolution than is possible with current proton imaging techniques.

Fig. 1.

The fully balanced true fast imaging with steady-state precession sequence used in all ¹³C-imaging experiments. The sequence starts with a prepulse and is followed by an imaging loop, which is repeated 64 or 128 times depending on the number of phase-encoding steps. The flip-back pulse brings the remaining xy magnetization back to the longitudinal direction.

Fig. 2.

Peak amplitude of the NMR spectrum from mouse as a function of time after injection. Circles show the measured values after correction for the signal decay caused by the excitation pulses. The line indicates a monoexponential decay with a T₁ of 20 s.

Fig. 3.

¹³C coronal projection images of a rat. The image acquisitions were started immediately (a) and 2 s after completing the injection of the contrast agent (b). The scan time of each image was 0.24 s. For other pulse-sequence parameters see Imaging Experiments.

Fig. 4.

A 2D ¹H coronal image obtained from a rat. The total scan time was 1.7 s. A slice thickness of 10 mm was used. For other pulse-sequence parameters see Imaging Experiments.

Fig. 5.

A coronal MIP of a rat. The MIP was calculated from a 3D ¹H data set. The total scan time was 53 s. For other pulse-sequence parameters see Imaging Experiments.