- Continuing Education
There are two ways of reading the CBCT data set. The first is utilizing DICOM files (Digital Imaging and Communications in Medicine), which is both a communication protocol and a file format standard for handling, storing and transmitting information in medical imaging.4 The format ensures all the patient data and information stay together, as well as provides for transfer of the information between DICOM-supported devices from multiple manufacturers. The DICOM files are the 150-600 axial images that form the volume when they are merged together. Most implant companies offer their own DICOM file reading software, with different capabilities such as treatment planning or 3D printing of surgical guides. If the clinician does not have viewing software to allow uploading of the DICOM data, or the case does not require a 3D printed guide, the clinician can utilize a simple viewer to assess the images. The viewer will allow the clinician to evaluate the MPR images, make linear measurements and recreate a panoramic reformatted image. Other options typically found are zoom, magnification, brightness/contrast, rotation, and mandibular canal tracing.
The first step before analyzing the scan is to virtually reorient the patient data set until the arch is in a suitable position and the head is not tilted. The occlusal plane should be leveled with the horizontal plane. In the axial view, the midline should be coincidental with the patient’s midline. This will help to evaluate symmetry of structures and avoid misinterpreting the images.
Once the patient reorientation is done, the MPR images can be read. It is very important to systematically read the entire comprehensive data volume. Key ideas to remember are to assess for symmetry and continuity of osseous borders. If the clinician does not feel comfortable assessing for abnormalities or pathoses in the skull base or cervical vertebrae, should they be within the field of view of the CBCT scan, consultation or scan interpretation by an oral and maxillofacial radiologist is paramount. The coronal MPR images are helpful to analyze anatomic structures with an anteroposterior orientation such as paranasal sinuses, nasal cavity, and some structures of the skull base (i.e., foramen rotundum). The axial images are helpful to analyze anatomical structures in a vertical path, with the sagittal images analyzing structures latero-medially.
The panoramic reformatted image is a useful tool because all teeth can be assessed in one image without superimposition and magnification that limits a 2D panoramic projection. The CBCT image is reconstructed based on a focal layer or trough, similar to a panoramic projection. The clinician can select the width and the extent of a focal trough in CBCT reformatting. However, an inappropriate choice of an image layer may lead to misdiagnosis. For example, an impacted maxillary canine may not be shown in the panoramic reformatted image if the selected curved image slice is not wide enough to include it. Some clinicians describe this reformat “pseudopanoramic.”
The most important feature of the panoramic rendering is that according to the path of its curved image slice, the cross-sectional image will be displayed. The cross-sectional image is a perpendicular image through the mandibular or maxillary arch. This image will assist in evaluating the height and width of the alveolar ridge for potential implant site measurements, or the buccolingual extension of a pathosis and its relationship with the cortical bone.12