Image analysis and superimposition of 3-dimensional cone-beam computed tomography models

Abstract

Three-dimensional (3D) imaging techniques can provide valuable information to clinicians and researchers. But as we move from traditional 2-dimensional (2D) cephalometric analysis to new 3D techniques, it is often necessary to compare 2D with 3D data. Cone-beam computed tomography (CBCT) provides simulation tools that can help bridge the gap between image types. CBCT acquisitions can be made to simulate panoramic, lateral, and posteroanterior cephalometric radioagraphs so that they can be compared with preexisting cephalometric databases. Applications of 3D imaging in orthodontics include initial diagnosis and superimpositions for assessing growth, treatment changes, and stability. Three-dimensional CBCT images show dental root inclination and torque, impacted and supernumerary tooth positions, thickness and morphology of bone at sites of mini-implants for anchorage, and osteotomy sites in surgical planning. Findings such as resorption, hyperplasic growth, displacement, shape anomalies of mandibular condyles, and morphological differences between the right and left sides emphasize the diagnostic value of computed tomography acquisitions. Furthermore, relationships of soft tissues and the airway can be assessed in 3 dimensions.

Fig 1

Dolphin 3D beta version images (Dolphin Imaging and Management, Chatsworth, Calif). A, Lateral view of 3D virtual models with transparency of soft tissue. B, 2D cephalogram generated from 3D models with 0 magnification and in orthogonal projection. C, 2D maximum intensity projection cephalogram. Dolphin 3D interface is user-friendly tool, allowing easy segmentation of anatomic structures, 3D linear measurements, and option of orthogonal or perspective projections to simulate conventional cephalograms.

Fig 2

Axial, lateral (sagittal), and anteroposterior (coronal) cross-sections for each CT image acquisition. Using ITK-SNAP, we can scroll through 330 axial, 360 lateral, and 360 anteroposterior slices of volumetric data. NewTom 3G software also allows panoramic views.

Fig 3

3D virtual models of 2 patients with hemifacial microsomia, showing segmentation of all slices stacked together without smoothing. A, Images acquired with 12-in field of view. Note costocondral graft establishing working condyle. B, Images acquired with 9-in field of view. Note significant asymmetry and missing articular fossa but presence of ramus and condyle on affected side. (Resolution is compromised by patient motion during acquisition; patient must remain still for 30 seconds after final alignment, and even swallowing can cause noise.)

Fig 4

Transparency of bones allows visualization of developing permanent teeth. Panoramic x-ray suggested that surgical pins from graft might be impairing tooth eruption, but CBCT 3D models show that surgeon avoided tooth buds.

Fig 5

Conventional initial records suggested orthodontic treatment in conjunction with maxillary surgery for correction of cross-bite and anterior open-bite. A and B, 3D virtual models and display without posterior cortical bone show lingual tipping of maxillary premolars and molars. Patient was also offered orthodontic correction without surgery.

Fig 6

Superior views of 3D models of mandibular rami of 3 patients with condylar shape anomalies. A, Patient with idiopathic condylar resorption. B, Patient with left hemimandibular hypertrophy. C, Early right condylar fracture with abnormal growth of condyle around articular eminence.

Fig 7

A, Presurgery, 1-week postsurgery, and 1-year postsurgery 3D models of patient treated with maxillary advancement and mandibular setback. B, Superimposition of pre- and postsurgery models showing surface distances between 2 models. Surface of cranial base was used for registration. Cranial base color map is green (0 mm surface distance), showing adequate match of before and after models for cranial base structures. Note that maxilla was brought forward as shown in red. Mandibular setback precisely maintained rami position, sliding mandibular corpus posteriorly, with slight counterclockwise rotation to correct open-bite tendency. C, Surface distances between 1-week and 1-year postsurgery models shows values close to 0 mm and stability of surgical procedures.

Fig 8

Semitransparent overlay of registered 1-week and 1-year postsurgery mandibular models of patient in Fig 7. Other anatomic structures are masked for better visualization of changes in mandible. Red, presurgery model; blue, area where pre- and postsurgery models overlap; green, postsurgery model.