FLIRT with Rigidity‚Image Registration with a Local Non-rigidity Penalty

Registration is a technique nowadays commonly
used in medical imaging. A drawback of most of the cur-
rent registration schemes is that all tissue is being consid-
ered as non-rigid (Staring et al., Proceedings of the SPIE
2006, vol. 6144, pp. 1–10, 2006). Therefore, rigid objects in
an image, such as bony structures or surgical instruments,
may be transformed non-rigidly. In this paper, we integrate
the concept of local rigidity to the FLexible Image Registra-
tion Toolbox (FLIRT) (Haber and Modersitzki, in SIAM J.
Sci. Comput. 27(5):1594–1607, 2006; Modersitzki, Numer-
ical Methods for Image Registration, 2004). The idea is to
add a penalty for local non-rigidity to the cost function and
thus to penalize non-rigid transformations of rigid objects.
As our examples show, the new approach allows the main-
tenance of local rigidity in the desired fashion. For example,
the new scheme can keep bony structures rigid during regis-
tration.
We show, how the concept of local rigidity can be in-
tegrated in the FLIRT approach and present the variational
backbone, a proper discretization, and a multilevel optimiza-
tion scheme. We compare the FLIRT approach to the B-
spline approach. As expected from the more general setting
of the FLIRT approach, our examples demonstrate that the
FLIRT results are superior: much smoother, smaller defor-
mations, visually much more pleasing.