Figure 5-.

Distance between landmark points matched in the expiratory and inspiratory images before and after image registration using different distances for the knots of the B-spline regularization (13, 26, 52 and 130 mm). Landmark distance was used as a measure of overall image registration accuracy. Data correspond to two randomly selected supine sheep (S1 and S2) at the beginning (0h) and end of the experiment (24h). These time points were chosen because they represent the extremes in aeration and atelectasis. A measurement at 6h of a third sheep (S3) was added because it represented the study with the largest gas volume difference between inspiratory and expiratory images. An average of 95.8 points [range 74–122] were used for each animal-time point combination. Landmarks were semi-automatically matched in each pair of images by one observer. Dotted line represents the largest dimension of image voxels (2.5 mm). The B-spline knots’ distance corresponds to the first stage of registration.