# Cone-Beam Computed Tomography Interpretation for OMFS

## Introduction

Cone-beam computed tomography (CBCT) has become an indispensable imaging modality in OMFS, providing three-dimensional visualization of the maxillofacial skeleton with lower radiation doses and higher spatial resolution than conventional CT for dental and osseous structures. The OMFS surgeon must be proficient in CBCT interpretation, including systematic review of all structures within the field of view and recognition of incidental pathology.

## Principles of CBCT Technology

### How CBCT Works

CBCT uses a cone-shaped X-ray beam that rotates 180 to 360 degrees around the patient. A single rotation acquires a volumetric dataset that is reconstructed into axial, coronal, sagittal, and panoramic views. A flat panel detector captures projection data that is reconstructed using algorithms, typically the Feldkamp-Davis-Kress algorithm. Isotropic voxels ranging from 0.076 to 0.4 mm provide high spatial resolution for osseous detail.

### CBCT vs. Medical CT

| Parameter | CBCT | Medical CT |
|-----------|------|-----------|
| Radiation dose | 20-200 microSv | 400-1,000 microSv |
| Spatial resolution | Higher (0.076-0.4 mm voxels) | Lower (0.5-1.0 mm) |
| Soft tissue contrast | Poor | Excellent |
| Hounsfield units | Not calibrated (approximation only) | True HU values |
| Scan time | 10-40 seconds | 2-10 seconds |
| Cost | Lower | Higher |
| Best for | Dental structures, cortical bone, thin bony structures | Soft tissue masses, lymph nodes, deep space infections, vascular assessment |
| Contrast enhancement | Not available | Available (IV contrast) |

CBCT delivers a lower radiation dose of 20 to 200 microSv depending on the field of view, compared to 400 to 1,000 microSv for medical CT of the head. It offers higher spatial resolution, making it superior for dental structures, cortical bone detail, and thin bony structures. However, CBCT has lower soft tissue contrast than medical CT and should not be used to evaluate soft tissue masses, lymph nodes, or deep space infections. CBCT grey values are not calibrated to true Hounsfield units, so bone density measurements are approximations only. Artifacts from scatter caused by metallic restorations and dental implants, beam hardening, and motion can degrade image quality.

### Field of View (FOV)

| FOV Size | Dimensions | Indications | Radiation Dose |
|----------|-----------|-------------|----------------|
| Small | 5x5 cm | Individual teeth, periapical pathology, implant site evaluation | Lowest |
| Medium | 10x10 cm | Single arch, TMJ evaluation, sinus assessment | Intermediate |
| Large | 16x16 cm or larger | Full maxillofacial skeleton, trauma, orthognathic planning | Highest |

A small FOV (5x5 cm) is appropriate for individual teeth, periapical pathology, and implant site evaluation. A medium FOV (10x10 cm) covers a single arch, the TMJ, or sinus evaluation. A large FOV (16x16 cm or larger) captures the full maxillofacial skeleton and is used for trauma assessment and orthognathic planning. The smallest FOV necessary to answer the clinical question should always be selected to reduce radiation dose and artifact.

![Comparison of CBCT and medical CT images demonstrating differences in spatial resolution and soft tissue contrast](images/cbct-vs-medical-ct.jpg)

## Systematic Interpretation Approach

### ABCDE Method

A systematic interpretation follows the ABCDE method. A stands for Airway, assessing nasopharyngeal and oropharyngeal airway volume, patency, and soft tissue encroachment. B stands for Bone, evaluating cortical and trabecular integrity, the presence of lytic or sclerotic lesions, fractures, and bone density. C stands for Condyles and TMJ, examining condylar morphology, erosions, osteophytes, joint space, and eminence shape. D stands for Dentition, reviewing root morphology, periapical pathology, impacted teeth, root resorption, and caries. E stands for Everything else, including sinuses, cervical spine, soft tissue calcifications, and incidental findings.

### Systematic Review Protocol

Interpretation begins with the axial view, scrolling through the entire volume from superior to inferior. Coronal and sagittal reformats are then evaluated for each anatomic region. The panoramic reconstruction is assessed for an overall dental and skeletal overview. All findings, including incidental pathology outside the region of interest, should be documented.

## OMFS-Specific Applications

### Dentoalveolar Surgery

CBCT is invaluable for assessing the relationship of third molars to the inferior alveolar canal (IAN), evaluating proximity, deviation, or direct contact. CBCT changes management in 12 to 20% of cases where the panoramic radiograph suggests IAN proximity. It is also used for impacted canine localization, determining buccal versus palatal position and proximity to adjacent roots. Supernumerary teeth are assessed for number, position, and relationship to permanent teeth. Periapical pathology is evaluated for extent of lesions and relationship to anatomic structures such as the sinus, nasal floor, and IAN.

### Implant Planning

Bone volume assessment includes height, width, and density at proposed implant sites. Proximity to vital structures including the IAN canal, mental foramen, maxillary sinus, nasal floor, and incisive canal is determined. Cross-sectional views with measurement tools are essential for implant planning. Virtual implant placement is performed with planning software such as BlueSky Plan, Simplant, or Nobel Clinician, and surgical guides are fabricated from the CBCT data.

### Orthognathic Surgery

Three-dimensional cephalometric analysis enables skeletal measurements and asymmetry quantification. Virtual surgical planning incorporates segmentation, osteotomy simulation, and splint design. Airway volumetric analysis is performed for OSA patients, and condylar position and morphology are assessed. Post-surgical evaluation confirms fixation hardware position and bony healing.

### Trauma

CBCT identifies and classifies fractures of the mandible, midface, orbit, nose, and zygoma. Three-dimensional reconstruction aids visualization of complex fracture patterns. Occult fractures not visible on plain radiographs can be detected, and postoperative imaging assesses reduction accuracy and hardware position. However, medical CT with contrast is preferred for initial polytrauma assessment and soft tissue injury evaluation.

### TMJ Evaluation

Condylar morphology is assessed for erosion, flattening, osteophyte formation, and bifid condyle. Osteoarthritis manifests as subchondral sclerosis, cortical irregularity, flattening, and osteophytes. Condylar hyperplasia shows asymmetric condylar size and is correlated with bone scan findings for active growth. Ankylosis presents as bony fusion of the condyle to the temporal bone and guides classification and surgical planning. Disc position cannot be reliably assessed on CBCT, and MRI remains the standard for disc evaluation.

### Pathology

Odontogenic cysts and tumors are evaluated for extent, cortical perforation, tooth involvement, and internal structure. Ameloblastoma appears as a multilocular radiolucency with cortical expansion and root resorption. Odontogenic keratocyst presents as a well-defined unilocular or multilocular radiolucency, often in the posterior mandible. Other osseous lesions such as fibrous dysplasia (ground-glass appearance), central giant cell granuloma, and osteomyelitis are characterized by their imaging features. Medication-related osteonecrosis (MRONJ) demonstrates sequestra, sclerosis, and periosteal reaction.

![CBCT cross-sectional views showing the relationship of an impacted third molar root to the inferior alveolar canal](images/cbct-ian-relationship.jpg)

## Incidental Findings

The OMFS surgeon is obligated to review all structures within the field of view. Common incidental findings in the maxillary sinus include mucous retention cysts, mucosal thickening, polyps, antral pseudocyst, sinusitis, and osteomas. Cervical spine findings include degenerative changes, calcified stylohyoid ligament (Eagle syndrome), and ponticulus posticus. Soft tissue calcifications may represent carotid artery atherosclerosis (calcified plaque), calcified lymph nodes, sialoliths, tonsilloliths, or phleboliths. Airway findings include adenoid hypertrophy, nasal septal deviation, and turbinate hypertrophy. Carotid artery calcification is identified on panoramic or CBCT imaging as a calcified mass adjacent to the C3-C4 vertebrae and warrants referral for vascular evaluation due to increased stroke risk. All incidental findings must be documented and communicated to the patient and referring provider.

## Radiation Safety

The ALARA principle (As Low As Reasonably Achievable) guides all imaging decisions. The smallest FOV, lowest mA, and shortest exposure time that provides diagnostic information should be used. CBCT is not a screening tool and should be obtained only when two-dimensional imaging is insufficient to answer the clinical question. For children, pediatric protocols with reduced exposure parameters are used due to their greater radiation sensitivity. Selection criteria should follow ADA/AAOMR guidelines for appropriate CBCT use in specific clinical situations.

![Examples of common incidental findings on CBCT including carotid calcification, cervical spine abnormality, and maxillary sinus pathology](images/cbct-incidental-findings.jpg)

## Clinical Pearls

CBCT provides superior osseous detail but inferior soft tissue contrast compared to medical CT, and it should not be used to evaluate soft tissue pathology. The entire volume must always be reviewed systematically, as incidental findings such as carotid artery calcification may have life-saving implications. CBCT changes surgical planning in 12 to 20% of third molar cases with suspected IAN proximity. The smallest field of view necessary to answer the clinical question should be selected to minimize radiation dose. CBCT grey values are not true Hounsfield units, so bone density measurements are approximations only.

## References

1. American Academy of Oral and Maxillofacial Radiology. "Clinical Recommendations Regarding Use of Cone Beam Computed Tomography in Orthodontics." *Oral Surgery, Oral Medicine, Oral Pathology and Oral Radiology*. 2013;116(3):238-257.
2. Scarfe WC, Farman AG. "What Is Cone-Beam CT and How Does It Work?" *Dental Clinics of North America*. 2008;52(4):707-730.
3. Garib DG, et al. "Cone-Beam Computed Tomography in Orthodontics, Orthopedics, and Dentofacial Orthopedics." *Dental Press Journal of Orthodontics*. 2014;19(6):94-108.
4. Friedlander AH, et al. "Detection of Carotid Artery Calcification on Panoramic Radiographs." *Oral Surgery, Oral Medicine, Oral Pathology*. 1994;78(4):547-551.