A method for establishing a local concentration distribution of magnetic particles of at least one particle class within an examination volume or a variable derived from this concentration distribution. The method includes providing at least one system matrix, providing MPI signal data of at least one sample including magnetic particles of at least one particle class within a measurement volume, and reconstructing spatially resolved MPI image data from the provided MPI signal data. At least one spatial projection of at least one part of the system matrix is carried out along a projection direction and a projected system matrix is generated thereby. The reconstruction of the MPI image data is implemented with the at least partly projected system matrix, as a result of which MPI image data of a spatial projection of the local concentration distribution of the magnetic particles are produced along the projection direction.