The role of dipolar interaction in the quantitative determination of particulate magnetic carriers in biological tissues.

The use of magnetic ac susceptibility measurements of biological tissues in the quantitative determination of their particulate magnetic carrier content has been investigated. In a first step, an ad hoc series of agar dilutions of the superparamagnetic contrast agent Endorem, used as an example of magnetic carrier, has been characterized to determine the influence of the dipolar interaction. With this result in hand, the quantitative determination of the content of a magnetic carrier in the ex vivo liver and spleen tissues of rats, to which the same compound was previously administered, has been accomplished. It is shown that, by careful interpretation of the temperature dependent out-of-phase susceptibility profiles in the cryogenic range, it is possible to discern between the magnetic contribution of the carrier and that of biomineral iron, being able to detect magnetic carrier iron concentrations of the order of 1 microg Fe g(-1) dry tissue. At the usual dosages in humans, necessarily small to avoid toxicity, the amount of magnetic carrier in terms of elemental iron is small compared to physiological iron. The choice of their most salient property, that is, the magnetic moment, therefore makes the quantification possible even in such a minority proportion. By analysing the magnetic dynamics, through a method that just considers the in-phase and the out-of phase components of the susceptibility at only one frequency, it has been possible to decouple the carrier concentration from eventual local aggregations, opening the possibility of investigating the degree of particle clustering at a larger observation scale compared with transmission electron microscopy, and independently of physiological iron.