  Purpose: Regional lung volume change as a function of lung inflation serves
as an index of parenchymal and airway status as well as an index of regional
ventilation and can be used to detect pathologic changes over time. In this
article, we propose a new regional measure of lung mechanics --- the specific
air volume change by corrected Jacobian.
  Methods: 4DCT and Xe-CT data sets from four adult sheep are used in this
study. Nonlinear, 3D image registration is applied to register an image
acquired near end inspiration to an image acquired near end expiration.
Approximately 200 annotated anatomical points are used as landmarks to evaluate
registration accuracy. Three different registration-based measures of regional
lung mechanics are derived and compared: the specific air volume change
calculated from the Jacobian (SAJ); the specific air volume change calculated
by the corrected Jacobian (SACJ); and the specific air volume change by
intensity change (SAI).
  Results: After registration, the mean registration error is on the order of 1
mm. For cubical ROIs in cubes with size 20 mm $\times$ 20 mm $\times$ 20 mm,
the SAJ and SACJ measures show significantly higher correlation (linear
regression, average $r^2=0.75$ and $r^2=0.82$) with the Xe-CT based measure of
specific ventilation (sV) than the SAI measure. For ROIs in slabs along the
ventral-dorsal vertical direction with size of 150 mm $\times$ 8 mm $\times$ 40
mm, the SAJ, SACJ, and SAI all show high correlation (linear regression,
average $r^2=0.88$, $r^2=0.92$ and $r^2=0.87$) with the Xe-CT based sV without
significant differences when comparing between the three methods.
  Conclusion: Given a deformation field by an image registration algorithm,
significant differences between the SAJ, SACJ, and SAI measures were found at a
regional level compared to the Xe-CT sV in four sheep that were studied.

Medical Physics