Korean J Orthod.  2008 Dec;38(6):427-436. 10.4041/kjod.2008.38.6.427.

Comparison of landmark position between conventional cephalometric radiography and CT scans projected to midsagittal plane

Affiliations
  • 1Department of Orthodontics, Kooalldam Dental Hospital, Korea.
  • 2Department of Industrial Engineering, Seoul National University, Korea. cyber7@snu.ac.kr
  • 3Department of Orthodontics, School of Dentistry, Seoul National University, Korea.

Abstract


OBJECTIVE
The purpose of this study is to compare landmark position between cephalometric radiography and midsagittal plane projected images from 3 dimensional (3D) CT.
METHODS
Cephalometric radiographs and CT scans were taken from 20 patients for treatment of mandibular prognathism. After selection of landmarks, CT images were projected to the midsagittal plane and magnified to 110% according to the magnifying power of radiographs. These 2 images were superimposed with frontal and occipital bone. Common coordinate system was established on the base of FH plane. The coordinate value of each landmark was compared by paired t test and mean and standard deviation of difference was calculated.
RESULTS
The difference was from -0.14 +/- 0.65 to -2.12 +/- 2.89 mm in X axis, from 0.34 +/- 0.78 to -2.36 +/- 2.55 mm (6.79 +/- 3.04 mm) in Y axis. There was no significant difference only 9 in X axis, and 7 in Y axis out of 20 landmarks. This might be caused by error from the difference of head positioning, by masking the subtle end structures, identification error from the superimposition and error from the different definition.
CONCLUSIONS
This study revealed innate shortcomings of radiography. For the development of 3D cephalometry, more study was needed.

Keyword

Landmark position; Cephalometric radiography; Projected images from 3D CT

MeSH Terms

Axis, Cervical Vertebra
Cephalometry
Head
Humans
Masks
Occipital Bone
Prognathism

Figure

  • Fig 1. Overall procedure.

  • Fig 2. Acquirement of projected images from CT data. A, Segmentation of skull and skin part using V works; B, projected image to midsagittal plane. By reducing the transparency of the skin part, the projected image looks more like a cephalometric radiograph.

  • Fig 3. Superimposition of projected images with cephalometric tracings and establishment of common Cartesian coordinates. A, Two reference points were created parallel to the FH plane in the projected images; B, cephalometric radiograph was traced; C, projected image and tracing were superimposed on to frontal bone and occipital bone, and then the two reference points were copied to the tracing for reprinting of a common coordinate system.

  • Fig 4. Difference of landmark position; arrow head is the position in the radiograph, base of arrow is the position in the projected image, and the size of arrow corresponds to the amount of error.

  • Fig 5. Errors from differences in body posture. Projected image showed backward downward movement of the mandible.

  • Fig 6. Errors from the discrepancy of midsagittal plane selected between x-ray taking and projected images. Anterior contour of radiograph was changed with increase in projection angle. A, Projected line was perpendicular to the midsagittal plane; B, projected line was tilted to the midsagittal plane.

  • Fig 7. Errors from the masking of bony projection. A, Ray sum images of the entire head made the position of ANS appear more backward; B, ray sum image around point ANS revealed its original position.

  • Fig 8. Errors from the positional differences defined in the radiograph and CT scans. A, Ray sum image of entire head made the position of Ptm appear more downward and backward; B, ray sum image around point Ptm revealed its original position.


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