Voxel-based superimposition of serial craniofacial cone-beam computed tomographies for facial soft tissue assessment: Reproducibility and segmentation effects

Abstract

The aim of this investigation was to evaluate the reproducibility of a voxel-based 3-dimensional superimposition method and the effect of segmentation error on determining soft tissue surface changes. Methods: A total of 15 pairs of serial cone-beam computed tomography images (interval: 1.69 6 0.37 years) from growing subjects (initial age: 11.75 6 0.59 years) were selected from an existing digital database. Each pair was superimposed on the anterior cranial base, in 3 dimensions with Dolphin 3D software (version 2.1.6079.17633; Dolphin Imaging & Management Solutions, Chatsworth, Calif). The reproducibility of superimposition outcomes and surface segmentation were tested with intra-and interoperator comparisons. Results: Median differences in inter-and intrarater measurements at various areas presented a range of 0.08-0.21 mm. In few instances, the differences were larger than 0.5 mm. In areas where T0-T1 changes were increased, the error did not appear to increase. However, the method error increased the farther the measurement area was from the superimposition reference structure. For individual images, the median soft tissue segmentation error ranged from 0.05 to 0.06 at various areas and in no subject exceeded 0.13 mm. Conclusions: The presented voxel-based superimposition method was efficient and well reproducible. The segmentation process was a minimal source of error; however, there were a few cases in which the total error was more than 0.5 mm and could be considered clinically significant. Therefore, this method can be used clinically to assess 3-dimensional soft tissue changes during orthodontic treatment in growing patients.

Valid 3D surface superimposition references to assess facial changes during growth

“…Voxel-based registration of serial CBCT images was performed through Dolphin 3D software © (Version 2.1.6079.17633, Dolphin Imaging and Management Solutions Chatsworth, CA 91311 U.S.A). The pairs of CBCTs (T0 and T1) were superimposed on the anterior cranial base (ACB) using a previously validated voxel-based best fit approach 4 , 19 , 22 . Since the anterior cranial base is morphologically stable already at an early age 23 , 24 , it is located centrally in the craniofacial complex, and it has a steady relation to the natural head position 25 , superimposition in this area is considered a gold standard for the assessment of maxillofacial structures 5 , 9 .…”

“…Since the anterior cranial base is morphologically stable already at an early age 23 , 24 , it is located centrally in the craniofacial complex, and it has a steady relation to the natural head position 25 , superimposition in this area is considered a gold standard for the assessment of maxillofacial structures 5 , 9 . The selection of the reference area was performed on the CBCT T1, in a multiplanar view and consisted of a rectangular box of voxels, which included the anterior cranial base 4 , 19 . CBCT T0 was then relocated to the position of the cranial base of CBCT T1, following a best fit registration of the two models on the selected voxels.…”

“…The surface segmentation was performed manually by one operator (S.H.) by varying the threshold until the facial surface was smooth and with the least amount of noise or distortion 19 . Surface data were exported in an STL format.…”

“…For further assessment, all facial surface models were imported in Viewbox 4 Software (version 4.1.0.1 BETA 64, dHAL software, Kifisia, Greece), which has been shown previously to properly process surface data 4 , 11 , 19 , 26 – 28 . The T1 surface model was cropped to include only the facial areas of interest (Fig.…”

“…Furthermore, there is no study comparing two facial surface superimposition methods to each other or to another method. Therefore, the present investigation assessed various 3D facial superimposition reference areas in growing patients, using CBCT derived data, and compared them to a voxel-based anterior cranial base (ACB) superimposition, which is considered the gold standard approach 4 , 9 , 19 .…”