Quantitative imaging of magnetic nanoparticles by magnetorelaxometry with multiple excitation coils

New therapies against cancer based on magnetic nanoparticles (MNPs) require a quantitative spatially resolved imaging of MNPs inside a body. In magnetorelaxometry (MRX), a distribution of nanoparticles can be quantified non-invasively by measuring its relaxation after removal of an external magnetizing field. Conventionally, in MRX the sample is exposed to a homogeneous magnetizing field resulting in a quantitative reconstruction with rather poor spatial resolution. Theoretical work suggests an improvement of spatial resolution may be achieved by a sequential application of inhomogeneous fields magnetizing only parts of a sample. Here, we experimentally demonstrate the feasibility of this approach by reconstructing a nanoparticle distribution inside a compact three-dimensional volume phantom made of 54 gypsum cubes (1 cm(3) cube(-1)), of which 12 gypsum cubes were filled with MNPs. Using 48 small excitation coils surrounding the phantom, a sequence of MRX signals was obtained where only those MNPs near an individual coil contribute. By combined evaluation of these 48 MRX measurements, the positions and content of the 12 MNP-filled cubes could be determined accurately with a deviation below 4%, while by conventional homogeneous MRX only the MNP content was reconstructable with a deviation of about 9%. The results demonstrate the improvement of quantitative MRX imaging by using sequential activation of multiple magnetizing fields.