Geometry and extension of signal voids in MR images induced by aggregations of magnetically labelled cells.

The ability of magnetic resonance imaging (MRI) to visualize magnetically labelled cells has attracted much attention for revealing cellular events. The present study addressed the geometry and the extension of signal voids in static signal dephasing MRI induced by aggregations of magnetically labelled cells by means of a three-dimensional numerical model. The magnetic field distortions around spherical cell aggregations were treated as equivalent to those of a magnetic dipole. Intravoxel signal dephasing and respective signal voids attributed to these field inhomogeneities were computed. Effects of cell concentration on the signal void in the plane of view were evaluated in terms of dipole magnetization. Signal void characteristics were scrutinized systematically for fundamental sequence parameters including echo time, voxel size and plane-of-view orientation. For all variables examined, significant changes in geometry as well as extension of signal voids were demonstrated. The results are of crucial importance to optimize and interpret MR images with regard to spatial accuracy as well as sensitivity to detect aggregations of labelled cells in vitro or even in vivo. It is anticipated that the dependence of the extension of signal voids on the local magnetization may be valuable for quantifying labelled cells.

Effect of spatial distribution of magnetic dipoles on Lamor frequency distribution and MR Signal decay--a numerical approach under static dephasing conditions.

Cells loaded with superparamagnetic iron oxide (SPIO) cause relatively strong magnetic field distortions, implying that field position effects of neighboring SPIO loaded cells have to be accounted for. We treated SPIO loaded cells as magnetic dipoles in a homogeneous magnetic field and computed the 3D frequency distribution and the related signal decay using a numerical approach under static dephasing conditions. The volume fraction of dipoles was kept constant for all simulations. For larger randomly distributed magnetic dipoles we found a non-Lorentzian frequency distribution and a non-monoexponential signal decay whereas, for smaller dipoles, the frequency distribution was more Lorentzian and the signal decay was well fitted monoexponentially. Moreover, based on our numerical and experimental findings, we found the gradient echo signal decay due to a single SPIO labeled cell to be non-monoexponential. The numerical approach provides deeper understanding of how the spatial distribution of SPIO loaded cells affects the MR signal decay. This fact has to be considered for the in vivo quantification of SPIO loaded cells, implying that in tissues with different spatial distributions of identical SPIO concentrations, different signal decays might be observed.