Susceptibility Anomaly at 50 K in Brain Tissue

The main iron oxide phase that has been identified in brain tissue is a low coercivity phase, which is magnetite/maghemite. Measurement of susceptibility as a function of temperature (χ-T) often does not show a Verwey transition, but may show a perturbation around 50 K. Such an anomaly has been reported in several studies for crystals of multi-domain magnetite, which display a Verwey transition. The anomaly is particularly evident in magnetic after-effect (MAE) spectroscopy. Based on MAE data, Walz (2002, J. Phys. Condens. Matt., 14, R285) suggested that this marks a transition between different mechanisms of electron transfer in magnetite. Above 50 K transfer is accommodated by small-polaron hopping. We have further investigated the magnetic behavior of this 50 K anomaly in brain and tumor tissue with the help of χ-T curves after cooling in zero-field (ZFC) or in a field (FC) and hysteresis loops. Measurements were made on three different Quantum Design MPMS magnetometers: (i) IRM, University of Minnesota, (ii) Institute for Geosciences, University of Bremen, and (iii) Dept. of Chemistry, ETH-Zurich, to assure that the effect was not related to the measurement instrument. The 50 K anomaly is expressed as a bump in χ-T curves over a 10 K temperature range whereby the peak susceptibility lies between 45 and 55 K. It should be noted that oxygen undergoes a transition from liquid to solid state at 53 K and undergoes magnetic ordering at 43 K. Although the magnetic intensity of the samples is small due to a large diamagnetic component from the tissue itself, a phase transition or magnetic ordering does not seem a likely explanation, because both the ZFC and FC curves follow the perturbation, rather than showing a bifurcation at peak susceptibility. This explanation would also preclude ordering of a superparamagnetic component. Hysteresis loops at the peak perturbation temperature show a splitting of the descending and ascending limbs at the maximum starting field. Different mechanism to explain these observations will be discussed.