All specimens were scanned at four-time points (before root canal preparation, after canal preparation, after canal filling, and after root canal retreatment), the obtained DICOM files of the scanned batches were coregistered in 3D using the Elastix rigid image registration module within 3D Slicer (v4.1.1) software (Harvard SPL; Boston; MA; USA). The acquired registered data were segmented with semiautomatic threshold-based segmentation to form a 3D canal and RFMs model for measurement by 3D Slicer. The RFMs region was set from the cemento-enamel junction to 0.5 mm from the apex of the root (entire canal). The volume of RFMs and the percentage of RFMs-covered areas in the entire canal were measured. The volume of RFMs of the total, apical third, and coronal two-thirds after retreatment was measured by 3D Slicer software. The measurement of the RFMs-covered surface was carried out as follows: both the surface of the region of interest (ROI) of the canal and the surface of intracanal residual filling materials were generated and exported to STL format in 3D slicer. The surface area of the canal wall in contact with RFMs was determined by calculating the area of the canal surface and the Boolean intersection (one voxel = 30 μm) in Geomagic Studio 2012 software (Raindrop Geomagic; Research Triangle Park; NC; USA). The surface was regarded as uncontacted when the distance between the canal surface and RFMs surface exceeded at least one voxel (24 μm); in that case, the Boolean intersection of the canal surface and the contacted surface area of RFMs was zero. All measurement work was performed by the same person. The percentage of the volume of RFMs and the area of RFMs were calculated using the equation below:
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