Continuous-flow DNP polarizer for MRI applications at 1.5 T

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.

Increased flow rate of hyperpolarized aqueous solution for DNP-enhanced MRI achieved by an open Fabry-Pérot type microwave resonator

“…Uniformity of the RF magnetic field over the sample volume is also an important parameter beside the B1max value (Mett et al, 2019) that can be evaluated by calculating the mean value of B1. In the Fabry-Pérot resonator the B1 mean value over the sample volume is about 1.55 G/W ½ that is slightly higher in comparison to the TE013 cylindrical cavity (1.4 G/W ½ ) ( Figure 4b) used in our MRI DNP experiments previously (Denysenkov et al, 2017).…”

“…polarizer equipped with the new resonator was tested and compared to the cylindrical TE013 cavity described before (Denysenkov et al, 2017). Typically, Fabry-Pérot resonators use concave mirrors of elliptical or parabolic geometry.…”

“…According to the "in-bore" concept, the polarization transfer from the TEMPOL radical electron spins to the water protons as well as the MRI detection are accomplished at ambient temperature in the same 1.5 T magnetic field. This corresponds to an EPR excitation frequency of 42 GHz [Denysenkov et al, 2017].…”

“…In the DNP experiments the temperature of the sample is between 65 °C and 95 °C due to the heating by the much higher applied mw power of 10 W. The higher temperature of the water solution in the DNP resonator shortens the rotational and translational correlations times improving the maximum achievable signal enhancements. After leaving the mw resonator the temperature of the hyperpolarized solution in the outlet capillary can be brought back to any desired temperature by a heat-sink chiller as described before (Denysenkov et al, 2017) to be physiologically compatible with in vivo experiments.…”

Increased flow rate of hyperpolarized aqueous solution for dynamic nuclear polarization-enhanced magnetic resonance imaging achieved by an open Fabry–Pérot type microwave resonator

“…Different strategies have been used so far to hyperpolarize an agent for MRI applications. Optically polarized noble gases as well as para-hydrogen have been used as hyperpolarized molecules for MRI (Salerno et al, 2001;Albert and Balamore, 1998;Limburn et al, 2013;Golman et al, 2001;Duckett and Mewis, 2012;Cavallari et al, 2018). An-Published by Copernicus Publications on behalf of the Groupement AMPERE.…”