Figure 5.

Example of an ill-behaved transformation. The plausibility of the resulting transformations was evaluated by examining the spatial Jacobian of the transformation and by visualizing the deformable vector field. Additionally, the deformed images were visually inspected by physicians for noticeable registration errors. Shown is an ill-behaved region of the transformation resulting from deformable image registration using the in-house algorithm. Folding of space by the transformation occurs at the anterior edge of the heart in the axial slices shown, as illustrated in the complementary color overlay of the prone inhale and deformed supine inhale images and overlaid deformation vector field (DVF) (left). The spatial Jacobian of the transformation is shown (center). Values greater than 1.0 indicate expansion, values less than 1.0 indicate compression, and negative values (shown in black) represent nonphysical transformation. Contours propagated from prone inhale to supine inhale are shown for the left lung (green), right side of the lung (blue), heart (red), esophagus (yellow), and tumor (magenta) using the in-house registration result (right). Atelectasis (posterior to the tumor) was contoured but excluded from the analysis. Despite the issues with the transformation, contour propagation using the transformation still yields reasonable accuracy. (A color version of this figure is available at http://doi.dx.10.1016/j.adro.2018.08.023.)