Plane-wise sensitivity based inhomogeneous excitation fields for magnetorelaxometry imaging of magnetic nanoparticles

Promising biomedical applications of magnetic nanoparticles share the need for a quantitative knowledge of their in vivo distribution. From multichannel magnetorelaxometry measurements with sequential activation of inhomogeneous excitation fields, the distribution can be quantitatively determined. In first studies, single excitation coils were consecutively activated. We aim at further advancing this imaging technology by suitable activation patterns involving multiple excitation coils. In this work, we propose the estimation of these patterns based on the spatial sensitivity in order to reduce the number of required measurements. The sensitivity of a voxel carrying magnetic nanoparticles is determined by its position relative to the sensors and the excitation field. Whereas the position is fixed within a given setup, the excitation is controlled by the currents in the coils. The currents required for a defined target sensitivity are estimated by solving an inverse problem. In our work, two target sensitivity paradigms are presented: (a) plane-wise activation, where only one plane with high sensitivities is sought and moved through the source space and (b) plane-wise non-activation, where all voxels except for one plane should receive high sensitivity. Our approach is investigated in simulation studies using a setup with a cubic region of interest and a planar sensor array. The imaging quality of both activation paradigms is evaluated. Our results demonstrate the principal applicability of this spatial sensitivity based approach for defining inhomogeneous activation patterns. The obtained patterns allow for a similar imaging quality using a lower number of activation sequences compared to the conventional single coil activation.