  Quantification of image similarity is a common problem in image processing.
For pairs of two images, a variety of options is available and well-understood.
However, some applications such as dynamic imaging or serial sectioning involve
the analysis of image sequences and thus require a simultaneous and unbiased
comparison of many images. This paper proposes a new similarity measure, that
takes a global perspective and involves all images at the same time. The key
idea is to look at Schatten-q-norms of a matrix assembled from normalized
gradient fields of the image sequence. In particular, for q = 0, the measure is
minimized if the gradient information from the image sequence has a low rank.
This global perspective of the novel SqN-measure does not only allow to
register sequences from dynamic imaging, e.g. DCE-MRI, but is also a new
opportunity to simultaneously register serial sections, e.g. in histology. In
this way, an accumulation of small, local registration errors may be avoided.
First numerical experiments show very promising results for a DCE-MRI sequence
of a human kidney as well as for a set of serial sections. The global structure
of the data used for registration with SqN is preserved in all cases.
