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

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 (19)F-MRI is that it provides non-invasive and background-free active marker signals in biomedical applications, such as monitoring drugs that contain (19)F. In former NMR spectroscopic experiments, hyperpolarized (19)F nuclei were efficiently generated by using low magnetic field (Earth's field) conditions. In order to apply the method to (19)F-hyperpolarized MRI, we chose an exploratory target molecule, for which a successful transfer of PHIP had already been attested. The transfer of hyperpolarization to (19)F 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 (19)F-MR images were received. Despite the low spin density of the sample (0.045 per thousand of the (1)H density in H(2)O), a sufficient signal-to-noise was obtained within a short acquisition time of 3.2 s.