Cephalometry in Orthodontics. Katherine Kula

criteria

      CBCT can provide a wealth of information regarding the maxillofacial regions. The ability to generate 3D images greatly enhances the treatment-planning process for many patients but may be unnecessary for some patients. The decision to scan or not to scan will be dependent on the patient’s condition. The delineation of whom to scan is called selection criteria. In 2013, the American Academy of Oral and Maxillofacial Radiology published clinical recommendations for the use of CBCT in orthodontics.¹² The first recommendation is to use the appropriate imaging modality based on the patient’s clinical presentation and history.¹² The second recommendation pertains to radiation risk assessment, and the third recommendation addresses ways to keep the dose to the patient as low as reasonably achievable (ALARA),¹² such as focusing on resolution, scan time, and field of view (FOV).

       Scanning protocol

      Because the goal in orthodontic imaging is to get the best data with the lowest dose to the patient, several factors should be included in the scanning protocol. The first consideration is the volume of the scan. Volume will be dictated by the choice of the FOV. The larger the FOV, the higher the dose to the patient. Keeping the FOV as small as possible will minimize the dose to the patient. Determining resolution requirements is another way to diminish dose to the patient. Generally, the higher the resolution, the higher the dose. Using the lowest resolution that still provides adequate diagnostic information is a good way to decrease the dose to the patient. Finally, the last consideration is exposure time. As with any radiographic imaging, the longer the exposure time, the higher the dose. Therefore, the shortest scan time that provides adequate diagnostic information should be used. The imaging protocol should address all of these parameters.

       Patient positioning and preparation

      The patient should be draped with a lap apron for image acquisition. Patient positioning differs with every commercially available scanner. Some systems capture the image with the patient standing, while others capture the image with the patient seated. Regardless of manufacturer, all units provide some form of head stabilization. It is important to position the patient’s head with the Frankfort plane parallel to the floor and the midsagittal plane perpendicular to the floor. While the position of the head can be altered during the image-reconstruction process, the same cannot be said for the cervical spine. Many CBCT scanners provide a bite stick for patient positioning. The bite stick produces an end-to-end occlusion that can alter the width of the airway and the condyle/fossa relationship. The use of the bite stick should therefore be avoided.

       Image reconstruction

      Once the appropriate scan is captured, the acquisition computer will generate data as a multiplanar reformation (MPR) providing images in the sagittal, coronal, and axial planes. While this data is captured in a 3D matrix, the MPR images are sequential 2D images. Further reconstruction is needed to generate useful 3D data. Additionally, conventional 2D images—cephalograms and panoramic radiographs—can be derived from the 3D data. As previously stated, there is no magnification in the CBCT-derived cephalograms as opposed to conventionally acquired cephalograms. Numerous studies have quantified the differences between landmark identification in conventional versus CBCT-derived cephalograms.^13–15 For the most part, the identification of landmarks has been comparable between the two. Some of the outcomes of these studies suggest that some “landmarks” visualized in two dimensions are not necessarily point landmarks in 3D data. Research is ongoing to better elucidate cephalometric landmarks in 3D data sets.

       References

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      2.Jacobson A, Jacobson RL (eds). Radiographic Cephalometry: From Basics to 3-D Imaging, ed 2. Chicago: Quintessence, 2006.

      3.American Dental Association. Dental Radiographic Examinations: Recommendations For Patient Selection And Limiting Radiation Exposure [PDF]. http://www.ada.org/en/∼/media/ADA/Member%20Center/FIles/Dental_Radiographic_Examinations_2012. Accessed 22 May 2017.

      4.Brand JW, Gibbs SJ, Edwards M, et al. Radiation Protection in Dentistry [Report 145]. Bethesda, MD: National Council on Radiation Protection and Measurements, 2003.

      5.Ludlow JB, Davies-Ludlow LE, White SC. Patient risk related to common dental radiographic examinations: The impact of 2007 International Commission on Radiological Protection recommendations regarding dose calculation. J Am Dent Assoc 2008;139:1237–1243.

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      8.Chadwick J, Prentice RN, Major PW, Lam EW. Image distortion and magnification of 3 digital CCD cephalometric systems. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2009;107:105–112.

      9.McClure SR, Sadowsky PL, Ferreira A, Jacobson A. Reliability of digital versus conventional cephalometric radiology: A comparative evaluation of landmark identification error. Semin Orthod 2005;11:98–110.

      10.White SC, Pharoah MJ. Oral Radiology Principles and Interpretation, ed 7. St Louis: Mosby/Elsevier, 2014.

      11.Swennen G, Schutyser F, Hausamen J. Three-Dimensional Cephalometry: A Color Atlas and Manual. Berlin: Springer-Verlag, 2006.

      12.Clinical recommendations regarding use of cone beam computed tomography in orthodontics. Position statement by the American Academy of Oral and Maxillofacial Radiology. Oral Surg Oral Med Oral Pathol Oral Radiol 2013;116:238–257 [erratum 2013;116:661].

      13.Park JW, Kim N, Chang YI. Comparison of landmark position between conventional cephalometric radiography and CT scans projected to midsagittal plane. Korean J Orthod 2008;38:426–436.

      14.Chien PC, Parks ET, Eraso F, Hartsfield JK, Roberts WE, Ofner S. Comparison of reliability in anatomical landmark identification using two-dimensional digital cephalometrics and three-dimensional cone beam computed tomography in vivo. Dentomaxillofac Radiol 2009;38:262–273.

      15.Zamora N, Llamas JM, Cibrián R, Gandia JL, Paredes V. Cephalometric measurements from 3D reconstructed images compared with conventional 2D images. Angle Orthod 2011;81:856–864.