Magnetic microparticle aggregation for viscosity determination by MR

Abstract

Micron-sized magnetic particles were induced to aggregate when placed in homogeneous magnetic fields, like those of MR imagers and relaxometers, and then spontaneously returned to their dispersed state when removed from the field. Associated with the aggregation and dispersion of the magnetic particles were time-dependent increases and decreases in the spin-spin relaxation time (T 2 ) of the water. Magnetic nanoparticles, with far smaller magnetic moments per particle, did not undergo magnetically induced aggregation and exhibited time-independent values of T 2 . The rate of T 2 change associated with magnetic microparticle aggregation was used to determine the viscosity of liquid samples, providing a method that can be of Key words: magnetic particle; nanoparticle; aggregation; relaxivity; viscosity Magnetic nanoparticles (Ϸ10 -100 nm) and magnetic microparticles (Ͼ100 nm) serve as important classes of materials for use as MR contrast agents and in cell sorting applications. Polysaccharide-coated magnetic nanoparticles (NPs), like the MION-47 used in the current study, are used as intravenously administered MRI contrast agents due to their high relaxivities, low toxicity, and ready degradation in vivo. Due to their successful development as clinically useful MR contrast agents, NP effects on water relaxation have often been described (1-8).Larger polystyrene-based micron-sized magnetic particles (MPs) lack the biodegradability of NPs and have been used as gastrointestinal contrast agents where elimination of the intact MP prevents absorption and toxicity (9 -11). MPs are widely used for applications requiring magnetic manipulation, such as immunoassays or cell sorting, since NPs cannot be manipulated by hand-held magnets and without the assistance of magnetic grids (12,13). Although NPs and MPs are available from many commercial sources, these two types of materials are currently used principally in different applications, and there has been little motivation to compare their effects on water relaxation rates. However, the realization that the aggregation of magnetic NPs (and potentially MPs) can be sensed by MR, and used to assay molecular targets in vitro (14 -16), or as components of implantable and removable sensors (17), provides a motivation for understanding the effects of diverse types of materials on water relaxation, since toxicity and biodegradability have minimal relevance for these applications. In comparing the effects of the MION-47 NP and the MP (Dynal MyOne beads) on water relaxation, we noted a pronounced time dependence of the latter material in magnetic fields.When placed in a homogenous magnetic field, solutions of nonsettling, micron-sized superparamagnetic MPs like the MyOne underwent time-dependent increases in the water proton spin-spin relaxation time, T 2 , due to magnetic field-induced MP aggregation. This observation gives MPs certain advantages over NPs in sensor applications, which will be reported on in due course. Here we describe the features of the magnetic field-induced MP ag...

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