The motion of mass points within a moving organ was estimated using non-rigid registration between the volumes of the
reconstructed 4D sequences. The resulting deformations define the trajectories of mass points during respiration.
For the non-rigid 3D registration, the open source
VTK CISG Toolkit  was used,
which implements an intensity-based non-rigid registration algorithm proposed by Rueckert  and Schnabel .
|Example trajectories defined by a sequence of deformations resulting from
non-rigid registration. The manually segmented liver is animated accordingly.
All acquired data sets showed variations in the range of minutes such as drifts and deformations,
which changed both the exhalation position of the liver and the breathing pattern. These changes motivate further
studies on the variability that has to be expected during treatment.
 Hartkens, T. et al.: 2002,
"VTK CISG Registration Toolkit: An open source software package for affine and non-rigid
registration of single- and multimodal 3D images"
Proc. Bildverarbeitung für die Medizin, Springer-Verlag, 409-412
 Rueckert D. et al.: 1999,
"Nonrigid registration using free-form deformations: application to breast MR images",
Medical Imaging, IEEE Transactions on, 18(8), 712-721
 Schnabel, J.A. et al.: 2003,
"Validation of nonrigid image registration using finite-element methods:
application to breast MR images", Medical Imaging, IEEE Transactions on, 22(2), 238-247