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Abstract
Purpose
To compare the orbital volume calculated from various slice thickness facial computed tomography scans using a semi-automated computer program.
Methods
Axial and coronal scans of 2, 2.5, 3 mm slice thickness facial computed tomography scans were used to measure the orbital volume. The cross-sectional area was determined from each slice using a semi-automated computer program (MATLAB 2009a®, MathWorks, Inc., Natick, MA, USA), and then the volume was calculated from serial reconstruction of the cross sections.
Results
The measured value in the 2 mm images was 33.14 ± 2.37 cm3 in the right orbit and 34.32 ± 2.60 cm3 in the left orbit for the axial scans, and 35.54 ± 3.58 cm3 in the right orbit and 34.96 ± 4.05 cm3 in the left orbit for the coronal scans. In the 2.5 mm images, the values were 33.28 ± 3.35 cm3 in the right orbit and 33.73 ± 4.10 cm3 in the left orbit for the axial scans, and 35.24 ±3.98 cm3 in the right orbit and 35.10 ± 3.93 cm3 in the left orbit for the coronal scans. In the 3 mm images, the values were 33.23 ± 2.70 cm3 in the right orbit and 33.39 ± 2.69 cm3 in the left orbit for the axial scans, and 33.20 ± 3.64 cm3 in the right orbit and 32.95 ± 3.45 cm3 in the left orbit for the coronal scans. In the 3 mm image, there was not a significant difference in the calculated volume between the axial and coronal scans (p3mm = 0.62).
Conclusions
Because there is no difference in the results of the orbital volumetric measurements between three other slice thicknesses in the axial scan, using axial scan images with a computer program that semi-automatically calculates orbital volume is useful. In addition, the volume measured by thick slice images has more reproducibility than the volume measured by thin slice images.
References
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Figure 1.
Volume measurement process of the computer program from axial scans images. (A) The original computed tomography (CT) axial scan image. (B) Simplified the photograph by the algorithm of the computer program. (C) Automatic outlining the bony orbit. (D) The white arrows indicate the straight line drawn by examiner. (E) Black arrow mark the seed points and asterisks (*) indicate the automated outlining region. (F) Red solid line in the original CT image indicated the measured region.
Figure 2.
Volume measurement process of the computer program from coronal scans images. (A) The original computed tomography (CT) coronal scan image. (B) Simplified the photograph by the algorithm of the computer program. (C) Automatic outlining the bony orbit. (D) The white arrows indicate the straight line drawn by examiner. (E) Black arrow mark the seed points and asterisks (*) indicate the automated outlining region. (F) Red solid line in the original CT image indicated the measured region.
Figure 3.
The correlation of measured orbital volume from right orbit with from left orbit. There is a strong positive correlation between measured orbital volume from right orbit and left orbit. The correlation coefficients are +0.9. r Axi 2mm = 0.913, r Cor 2mm = 0.965, r Axi 2.5mm = 0.903, r Cor 2.5mm = 0.912, r Axi 3mm = 0.984, rCor 3mm = 0.943.
Figure 4.
Diagram of partial volume effect. (A) Pixel computed tomography (CT) value with thick slice. (B) Pixel CT value with thin slice. The partial volume effect can be defined as the loss of apparent activity in small objects or regions because of the limited resolution of the imaging system.
Table 1.
Comparison of the right and left orbit volume (cm3)
|
2.0 mm thickness |
2.5 mm thickness |
3.0 mm thickness |
||||
|---|---|---|---|---|---|---|
| Axial | Coronal | Axial | Coronal | Axial | Coronal | |
| Right orbit | 33.14 ± 2.37 | 35.54 ± 3.58 | 33.28 ± 3.35 | 35.24 ± 3.98 | 33.23 ± 2.70 | 33.20 ± 3.64 |
| Left orbit | 34.32 ± 2.60 | 34.96 ± 4.05 | 33.73 ± 4.10 | 35.10 ± 3.93 | 33.39 ± 2.69 | 32.95 ± 3.45 |
| p-value* | 0.002 | 0.189 | 0.257 | 0.670 | 0.527 | 0.302 |
Table 2.
Difference in measured volume (cm3) between axial and coronal scan
| 2.0 mm thickness | 2.5 mm thickness | 3.0 mm thickness | p-value† | |
|---|---|---|---|---|
| Axial scan | 33.73 ± 2.51 | 33.50 ± 3.67 | 33.31 ± 2.65 | 0.87 |
| Coronal scan | 35.25 ± 3.76 | 35.17 ± 3.88 | 33.08 ± 3.48 | 0.05 |
| p-value* | 0.025 | 0.013 | 0.62 |
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