As a functional imaging technique, MPI measures the direct magnetic response of magnetic nanoparticles (MNP) resulting from sinusoidal magnetic excitation without any background signal. The fundamental imaging principle is based on the nonlinear magnetization behavior of MNP and the saturation characteristics at a specific magnetic field strength. Excitation of MNP with a magnetic field results in a nonlinear magnetic response, the magnetization curve (Figure top left).

The resulting magnetization of MNP (Figure top middle) then follows the orthogonal projection of the excitation field along the magnetization curve (green dotted line), which is measured as the induction signal in a receive coil (Figure top right). In MPI the MNP are excited with a sinusoidal magnetic field. Due to the saturation of the magnetic response at a specific field strength, the MNP magnetization over time is no longer pure sinusoidal but gets rectangular distorted . The Fourier-transformed spectrum (Figure middle right) of the signal subsequently comprises not only the excitation frequency but also higher harmonics, which are integer multiples of the sinusoidal excitation frequency. Proper filtering techniques enable the isolation of these higher harmonics for use in image reconstruction. The harmonic spectrum reflects the characteristics of the measured MNP (amount, chemical composition, etc.), but also the state of their microenvironment (temperature, viscosity, etc.) as the environment significantly influences the relaxation behavior of the MNP.

To archive spatial resolution of MNP distributions, a magnetic selection field (Figure middle) is used which generates a field free region (FFR) in the form of a field free point (FFP) or a field free line (FFL) by a gradient field. Through an overlay with the excitation field, only MNP in the FFR generate a MPI signal (green path for signal generation). All other MNP are in a constant saturation state and therefore not used for signal acquisition (blue path for signal suppression).