  The tomographic imaging method magnetic particle imaging (MPI) requires a
multi-patch approach for capturing large field of views. This approach consists
of a continuous or stepwise spatial shift of a small sub-volume of only few
cubic centimeters size, which is scanned using one or multiple excitation
fields in the kHz range. Under the assumption of ideal magnetic fields, the MPI
system matrix is shift invariant and in turn a single matrix suffices for image
reconstruction significantly reducing the calibration time and reconstruction
effort. For large field imperfections, however, the method can lead to severe
image artifacts. In the present work we generalize the efficient multi-patch
reconstruction to work under non-ideal field conditions, where shift invariance
holds only approximately for small shifts of the sub-volume. Patches are
clustered based on a magnetic-field-based metric such that in each cluster the
shift invariance holds in good approximation. The total number of clusters is
the main parameter of our method and allows to trade off calibration time and
image artifacts. The magnetic-field-based metric allows to perform the
clustering without prior knowledge of the system matrices. The developed
reconstruction algorithm is evaluated on a multi-patch measurement sequence
with 15 patches, where efficient multi-patch reconstruction with a single
calibration measurement leads to strong image artifacts. Analysis reveals that
calibration measurements can be decreased from 15 to 11 with no visible image
artifacts. A further reduction to 9 is possible with only slight degradation in
image quality.
