Abstract
Here we describe a new hyperpolarization approach for magnetic resonance imaging applications at 1.5 T. Proton signal enhancements of more than 20 were achieved with a newly designed multimode microwave resonator situated inside the bore of the imager and used for Overhauser dynamic nuclear polarization of the water proton signal. Different from other approaches in our setup the hyperpolarization is achieved continuously by liquid water flowing through the polarizer under continuous microwave excitation. With an available flow rate of up to 1.5 ml/min, which should be high enough for DNP MR angiography applications in small animals like mice and rats. The hyperpolarized liquid cooled to physiological temperature can be routed by a mechanical switch to a quartz capillary for injection into the blood vessels of the target object. This new approach allows hyperpolarization of protons without the need of an additional magnet and avoids the losses arising from the transfer of the hyperpolarized solution between magnets. The signal-to-noise improvement of this method is demonstrated on two- and three-dimensional phantoms of blood vessels.
Highlights
We describe a new hyperpolarization approach for magnetic resonance imaging applications at 1.5 T
Polarization transfer from unpaired electron spins of radicals to the proton nuclear spins is achieved by microwave irradiation of the TEMPOL/water solution directly inside the bore of the Magnetic resonance imaging (MRI) magnet
This ‘in-bore’ approach is different from other approaches which hyperpolarize the proton signal outside the MRI magnet at lower magnetic field values
Summary
We describe a new hyperpolarization approach for magnetic resonance imaging applications at 1.5 T. Magnetic resonance imaging (MRI) techniques allow non-invasive visualization of living tissue morphology and functions with a high soft tissue contrast These techniques are for many years routinely used in clinical practice, technological, chemical and methodological research focuses on the improvement of the intrinsically low sensitivity of the method arising from the small polarization of the nuclear spin signal. An alternative way of increasing the NMR signal is to create nuclear spin magnetization higher than the Boltzmann polarization at thermal equilibrium at a given static magnetic field Such hyperpolarization methods have been explored extensively for NMR spectroscopy and MRI. Dissolution DNP is the only commercially available hyperpolarization method of liquids for MRI12,13 It uses metabolites such as 13C-pyruvate, which are polarized external to the MRI magnet at very low temperatures (~1 K) by microwave irradiation. The usage of 1H as imaging nuclei for dissolution DNP has been once demonstrated[16], it is not a wide-spread approach due to the short relaxation times and loss of proton polarization within the transfer step
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