With interest, we read the Techno Bytes article on analyzing airway volume by using cone-beam computed tomography (CBCT) in the May 2012 issue of the AJO-DO . The authors used the data from CBCT scans to obtain resin-negative prototypes of the airway. The resin prototypes were scanned again in CBCT, and the airway volumes of the prototypes were measured with Dolphin software with different threshold values. These Dolphin measurements were then compared with the physical volumes of the prototypes to locate the most appropriate threshold value to use in the Dolphin software for airway volume measurement. However, we have the following concerns about this study.
The prepared negative prototypes were expected to mimic the soft tissues surrounding the airway. To enhance the contrast to the air, resin materials were used to print the negative prototypes. However, resin materials have a much higher density than the soft tissues; this might lead to a higher threshold value setting in the software. Consequently, the threshold value suitable for resin prototype measurements might not be comfortably applied directly to the measurement of the in-vivo CBCT scan.
The authors’ choice of the 8 threshold values in the Dolphin software was made by referring to previous research. However, unlike conventional computed tomography, the tube voltage varies widely among different CBCT devices. Therefore, the computed tomography values of the same tissue can have significant deviations when scanned by different CBCT machines. In addition, voxel size and artifacts might also affect the accuracy of the CBCT images. Thus, the threshold values chosen in the Dolphin software might have little comparability when it comes to different CBCT machines and different scan parameters.
We recommend that the authors use a wax material for the prototype to simulate the in-vitro conditions, because wax has a similar density to the soft tissues.