Accuracy of linear measurements obtained from stitched cone beam computed tomography images versus direct skull measurements

Introduction

The use of cone beam computed tomography (CBCT) machines in dentistry started in the second half of the 1990s¹. Now, CBCT is extensively used in the dental field for implant planning, in endodontics, maxillofacial surgeries and orthodontics².

In the field of orthodontics, analysis of cephalometric radiographs requires accurate identification of specific landmarks for precise measurements between these landmarks³. As a consequence, the small field of view (FOV) CBCT systems available in small clinics cannot yet satisfy the needs of maxillofacial surgeons or orthodontists⁴. Thus, visualizing all of these landmarks on the same scan is not always possible⁵.

In order to compensate for this shortcoming, small FOV images can be scanned and then fused together to produce a single large FOV image. However, there are few data to show whether this fused image is as precise as a single image of the whole area of interest^4,6.

Therefore, the aim of the current study was to assess the diagnostic accuracy of stitched CBCT linear measurements versus direct measurements on skulls.

Methods

The current study was conducted on nine dry human skulls obtained from the Anatomy department, Faculty of Medicine, Cairo University to avoid the exposure of living humans to unnecessary radiation doses. 26 anatomical landmarks were identified on each skull (Table 1). Gutta percha cones (GE16121542, META BIOMED) were glued and used as radiopaque markers (Figure 1–Figure 3).

Figure 1. A photograph showing the frontal aspect of the skull.

Figure 2. A photograph showing the lateral aspect of the skull.

Figure 3. A photograph showing the skull base.

22 linear measurements were taken and recorded using a high precision sliding digital caliper (6400192, Allendale Electronics Ltd, Hertfordshire, UK). Ten measurements in the cranio-caudal plane (Table 2), two measurements in the antero-posterior plane (Table 3), and ten measurements in the medio-lateral plane (Table 4). 22 direct linear measurements were measured and were considered to be the gold standard in the study (Figure 4–Figure 6).

Figure 4. A photograph showing direct linear measurement from Nasion to Anterior nasal spine.

Figure 5. A photograph showing direct linear measurement from right Zygomatic foramen to right Mental foramen.

Figure 6. A photograph showing direct linear measurement from right Medial orbital wall to left Medial orbital wall.

For soft tissue simulation, the skulls were covered with 20 layers of pink modelling wax (1mm thick each) (Tenatex eco, Kemdent) to achieve an average of 14–16 mm wax thickness⁷.

The skulls were stabilized in the Planmeca ProMax 3D Mid CBCT machine using a wooden stand passing through the foramen magnum and were oriented using the laser beams (Figure 7). Three consecutive FOVs were scanned: two scans each of the size 160 ×60 mm (single arch) for the mandible and the maxilla separately (Figure 8 and Figure 9) and one scan with a FOV size 200 ×100 mm for the upper third of the face (Figure 10). Each one of the three FOVs was scanned separately using a voxel resolution 0.2 mm, 90 kVp and 10 mA, then stitching was performed using Romexis software (Planmeca Romexis Viewer Launcher 4.5.0.R) creating one large volume (Figure 11). After completion of the stitching procedure the linear measurements were obtained from the stitched CBCT images for a later comparison with the gold standard (Figure 12 and Figure 13).

Figure 7. A photograph showing the skull centralized within the CBCT machine in the proper position.

Figure 8. First field of view showing the mandible.

Figure 9. Second field of view showing the maxilla.

Figure 10. Third field of view showing the upper third of the face, orbits, frontal bone.

Figure 11. The final image showing the stitched three small fields of view into a single large one.

Figure 12. Coronal cut showing linear measurement of Orbital foramen (right)-Mental foramen (right) on a stitched image.

Figure 13. Coronal cut showing linear measurement of Medial orbital wall (right)-Medial orbital wall (left) on a stitched image.

Results

Discussion

The smaller the scan FOV, the higher the spatial resolution of the image⁸. Stitching of small CBCT images to create a large image can be very useful to collect the needed cranio-maxillofacial data with small FOV machines⁹. Increasing the FOV can be done by automatically fusing up to three small FOVs to obtain a larger FOV¹⁰.

CBCT ‘‘Stitching’’ option could be useful but whether it is precise enough to obtain accurate and reliable measurements remains doubtful¹¹. Assessing the accuracy of stitched CBCT measurements is infrequently mentioned in current literature, as in the studies conducted by Kopp and Ottl; 2010, Kim et al.; 2012, Egbert et al.; 2015, and Srimawong et al.; 2015^5,8,10,12.

The results of the current study showed that the relative Dahlberg error ranged from 0.2% to a maximum of 1.8%. Consequently, the error was considered small and clinically non-significant as the measurement error in craniofacial imaging is considered clinically acceptable up till the value of 5%^13,14.

The results of the current study go in agreement with the study performed by Srimawong et al.; 2015 on 10 dry human mandibles¹². Their results showed that, the mean absolute differences between direct measurements and stitched CBCT measurements for vertical and horizontal distances were (0.27±0.24 mm) and (0.34±0.27 mm), respectively. Their results showed that the stitched CBCT measurements were highly accurate comparable to the direct measurements.

In support of the present results, Egbert et al.; 2015⁸ research revealed that the mean difference between the direct linear measurements and the stitched CBCT measurements was 0.34 mm with a 95% confidence interval of (0.24 to 0.44 mm). They concluded that the precision of stitched CBCT measurements allow accurate construction of implant surgical stents.

Moreover, the results of Kopp and Ottl; 2010¹⁰ further agree with those obtained from the current study. They used an automated method to increase the FOV of CBCT images by stitching three small FOV volumes to obtain a larger FOV one. They concluded that, the stitching software was accurate in the obtained linear measurements.

On the same line of agreement, a study was performed by Kim et al.; 2012⁵ to investigate whether images of skulls obtained by both manual and automatic stitching of three CBCT images, provided accurate measurements as those obtained by multidetector computed tomography (MDCT). The results showed that the mean difference between automatically stitched CBCT images and the reference images ranged from (-0.8944 mm to -1.0628 mm).

Conclusion

Stitched CBCT linear measurements were highly comparable to the direct skull measurements. However, a percent of error should be expected from CBCT-derived measurements.

Data availability

Underlying data is available from Open Science Framework

OSF: Dataset 1. Accuracy of Linear Measurements Obtained from Stitched Cone Beam CT Images Versus Direct Skull Measurements https://doi.org/10.17605/OSF.IO/SUTWK¹⁵