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Abstract

Ultralow-field (B0 < 10 mT) magnetic resonance imaging (ULF MRI) is driven by the idea of making MRI cheaper and less dangerous to accidents, but it has other strengths as well: Imaging in the vicinity of metals is possible and for some tissue types the contrast is different compared to high field MRI. The poor signal-to-noise ratio at small B0 field strengths is here countered by hyperpolarization using Overhauser Dynamic Nuclear Polarization (ODNP). ODNP transfers spin order from unpaired electrons to nearby protons via an external RF field. At ULF the RF frequency is in the range of 100 MHz, which is a frequency range capable of penetrating large specimen. As a source for electrons, free radicals can be used. The efficacy of those radicals can be expressed via the parameters Emax (maximum enhancement) and P1/2 (RF power needed to reach half of Emax). For in vivo applications it is important to have as much enhancement as possible at a low RF power level to prevent excessive tissue heating. For in vivo ODNP experiments, stable and biocompatible free radicals or spin probes are required. Alternatively, efficient, but potentially unstable and toxic spin probes can be embedded in biocompatible carrier systems. Such a carrier system is e.g. human serum albumin (HSA), which we tested within this study for its suitability for future in vivo experiments. Various nitroxide-modified biocompatible human serum albumin (HSA-NIT)[1] molecules were tested for their ODNP properties. The ODNP effect was used to track the cleavage process of the spin probes from the HSA-NIT, while exposed to the enzyme trypsin. Since the spin probes in HSA-NIT are shielded from surrounding protons, only a weak ODNP effect can be observed. The trypsin cleaves the spin probes from HSA whereupon a strong ODNP effect appears on a time scale of ~ 5 hours. HSA-NIT was shown to have a long shelf life in the intact state.