In the literature, a widely adopted approach for assessing the regional lung function in patients undergoing radiation therapy for lung cancer from 4-D X-ray CT data is the Jacobian of deformation. The Jacobian which describes regional lung volume changes however lacks consideration of directional length changes during respiration. Previously, we proposed the use of strain for measurement of regional deformation in the Lung . In this paper, we perform physiologic validation of lung strain and compare the results with those from the Jacobian measure. Lung deformation fields were computed through application of a recently proposed 3-D optical flow technique (MOFID)  to 4-D X-ray CT data sets collected in seven subjects diagnosed with non-small cell primary lung cancer. In addition to 4-D CT data, both SPECT ventilation (VSPECT), and SPECT perfusion (QSPECT data were acquired in all subjects. For each case, voxel-wise statistical correlation of the Jacobian as well as principal strains of deformation (CT-derived pulmonary function images) with both ventilation and perfusion SPECT was performed. The results indicate that the maximum principal strain has a higher correlation with both SPECT ventilation and SPECT perfusion than other indices including the Jacobian.