Measurements of DIR spatial accuracy for each case were obtained using manually identified sets of prominent anatomical landmark feature pairs identified across multiple consecutive respiratory phase images, from the maximum inhalation phase (designated T00) to the maximum exhalation phase (designated T50). A Matlab-based software interface named APRIL (Assisted Point Registration of Internal Landmarks), previously described (Castillo et al., 2009b (link)), was utilized to facilitate manual selection of landmark feature pairs between volumetric images. Basic features of the software include separate window and level settings for each display, visualization of equivalent voxel locations in the orthogonal plains, and interactive tools for segmentation of lung voxels from the image data. To determine corresponding feature points the user must manually designate the feature correspondence via mouse click on the target image. For all cases, a reference set of pulmonary landmark feature pairs was generated using the maximum inhale/exhale component phase images from the 4DCT set. No implanted fiducials or added contrast agents were used to aid in the selection of landmark features, which typically included vessel and bronchial bifurcations. Source feature points were selected systematically on the 10 test image pairs by an expert in thoracic imaging, beginning at the apex of the lung.
For the first 5 test image pairs the expert selected >10 features points for each lung per axial slice, these images were described in our prior publication (Castillo et al., 2009b (link)) and are available on the Internet (www.dir-lab.com). For the second 5 image pairs, points were selected with an initial goal of >3 feature points for each lung per axial image slice. This approach ensured the collection of >1100 validation point pairs for the first 5 cases and >400 for the subsequent 5 cases. Following feature selection for a given case, all landmark pairs were visually reviewed by the primary reader a second time and the location adjusted on the exhale image if necessary. The verification step was required before the initial registration process, performed by the primary reader, was considered complete. The points were then used to test the spatial accuracy of DIR algorithms for this study. For each of the 10 cases a subset of 75 landmark features were propagated across the expiratory phases T00 to T50, as shown in the example in Figure 2.