Hyperpolarized 19F-MRI: parahydrogen-induced polarization and field variation enable 19F-MRI at low spin density

Abstract

The use of parahydrogen-induced polarization (PHIP) for signal enhancement in nuclear magnetic resonance spectroscopy (NMR) is well established. Recently, this method has been adopted to increase the sensitivity of magnetic resonance imaging (MRI). The transfer of non-thermal spin hyperpolarization—from parahydrogen to a heteronucleus—provides better contrast, thus enabling new imaging agents. The unique advantage of ¹⁹F-MRI is that it provides non-invasive and background-free active marker signals in biomedical applications, such as monitoring drugs that contain ¹⁹F. In former NMR spectroscopic experiments, hyperpolarized ¹⁹F nuclei were efficiently generated by using low magnetic field (Earth's field) conditions. In order to apply the method to ¹⁹F-hyperpolarized MRI, we chose an exploratory target molecule, for which a successful transfer of PHIP had already been attested. The transfer of hyperpolarization to ¹⁹F was further optimized by adequate field manipulations below Earth's magnetic field. This technique, called field cycling, led to a signal enhancement of about 60. For the first time, hyperpolarized ¹⁹F-MR images were received. Despite the low spin density of the sample (0.045‰ of the ¹H density in H₂O), a sufficient signal-to-noise was obtained within a short acquisition time of 3.2 s.