Dental Cone-Beam CT

Dental cone beam computed tomography (CBCT) uses a cone shaped X-ray beam and a flat panel detector that rotate once around the head (180 to 360 degrees in 10 to 40 seconds).

It acquires a volume of the teeth, jaws and adjacent structures, reconstructed with isotropic voxels of 0.08 to 0.4 mm.

The field of view can be small (4 to 6 cm, a few teeth), medium (8 to 10 cm, a whole arch), or large (15 to 23 cm, the maxillofacial region and sinuses), and is selected to include only the area of interest. The patient sits or stands with the head stabilised, or lies supine in some units.

Effective dose varies from about 0.01 to 0.1 mSv for a small field, 0.05 to 0.3 mSv for a medium field, and up to 0.1 to 0.5 mSv or more for large fields.

This is higher than a panoramic radiograph (0.01 to 0.03 mSv) but lower than a medical multidetector CT of the facial bones (0.5 to 2 mSv).

CBCT gives three dimensional images with high spatial resolution of bone and teeth, with lower dose than conventional CT, and is widely used in implant planning, endodontics, oral surgery and orthodontics.

Intraoral radiographs miss periapical lesions in 20 to 30% that CBCT detects, and CBCT shows the inferior alveolar nerve canal relation to third molars. Soft tissue contrast is poor, and Hounsfield units are not reliable.

Indications. Dental implant planning: bone height, width, density, and anatomy (inferior alveolar canal, mental foramen, maxillary sinus floor, nasopalatine canal). Impacted teeth, including canines and third molars, with relation to the mandibular canal and adjacent roots. Endodontic problems: complex root canal anatomy, missed canals, periapical lesions not seen on radiographs, vertical root fractures, root resorption, and perforations.

Jaw pathology: cysts and tumours (odontogenic keratocyst, ameloblastoma), fibro osseous lesions, osteomyelitis, osteonecrosis (medication related), and trauma (jaw and dentoalveolar fractures). Temporomandibular joint bony changes. Orthodontic assessment of impacted teeth, airway, and bone for anchorage devices, in selected cases.

Maxillary sinus pathology of dental origin. Pre-surgical planning for orthognathic surgery and cleft alveolar graft. Not for routine screening, caries diagnosis, periodontal soft tissue assessment, or when a lower dose radiograph answers the question.

Contraindications and precautions. Pregnancy: avoid unless essential, though the foetal dose is negligible for a dental field. Children: small fields only with clear justification, since children are more radiosensitive; low dose and child specific settings are needed. Patients unable to remain still, for example because of tremor, make motion degraded images.

Justification, optimisation of field size, voxel size and exposure, and thyroid collar where it does not obscure the area (some collars interfere) follow radiation protection principles. Metal restorations cause artefacts.

The referring clinician must be trained in interpretation, or the images are reported by an oral and maxillofacial radiologist, and the whole volume is reviewed, not only the area of interest.

Preparation and technique. The patient removes jewellery, glasses, hearing aids and removable dental appliances. The head is positioned with the occlusal plane horizontal or as the unit requires and stabilised with a chin rest and head support, and the patient is told to stay still and not swallow.

Positioning lights set the field of view. A scout view may be taken. The exposure is chosen to suit patient size, field of view and voxel size, with the lowest settings that answer the question.

Reconstructed data are reviewed in axial, coronal and sagittal planes, with curved panoramic reconstruction and cross-sectional images through the jaws. For implant planning, a radiographic stent with radio opaque markers may be worn, and software is used to place virtual implants and show the nerve canal.

Normal appearances. Teeth with enamel, dentine and pulp chambers. Roots with canals visible, a continuous lamina dura, and periodontal ligament space of 0.2 to 0.4 mm. Alveolar bone with trabeculae and a cortical outline.

The mandibular canal as a tubular lucency with a thin cortical wall running from the mandibular foramen to the mental foramen, in the lingual side of the molar roots. The maxillary sinus with thin walls and a smooth floor. The nasopalatine canal.

The temporomandibular joint with a smooth condyle and glenoid fossa. And normal air spaces.

Alveolar bone height is 1 to 2 mm apical to the cemento enamel junction.

Interpretation.

• Periapical lesions: widened periodontal space or periapical radiolucency at the root apex (granuloma, abscess, cyst), loss of the lamina dura, and associated cortical expansion or perforation; healing is confirmed by reduction in size.
• Root fractures and resorption: vertical root fracture (radiolucent line with a halo like bone loss along the root, narrow and deep defect), external and internal resorption, and cervical resorption.
• Endodontic anatomy: additional canals (second mesiobuccal canal in maxillary first molars in over 60%), curved canals, calcified canals, perforation, and separated instruments.
• Impacted teeth: position, angulation, and relation to adjacent roots (resorption of lateral incisors by impacted canines), and mandibular canal (loss of the cortical line, darkening of the root, and canal deviation indicate a close relation and a higher risk of nerve injury).
• Implant planning: bone volume (height, width), quality (trabecular pattern, cortical thickness), distance to the inferior alveolar canal (a safety margin of 2 mm), mental foramen and anterior loop.
• Also assess sinus floor and septa, nasopalatine canal, and concavities in the lingual mandible (submandibular fossa) with perforation risk.
• Post-implant assessment of position, bone loss, and sinus complications.
• Cysts and tumours: well defined unilocular radiolucency (radicular, dentigerous, odontogenic keratocyst: tends to grow along the bone with little expansion), multilocular lesion with "soap bubble" or honeycomb pattern (ameloblastoma, odontogenic myxoma), calcification (calcifying odontogenic lesions), root resorption and displacement, cortical thinning or perforation.
• Aggressive features include ill defined borders and soft tissue extension, needing biopsy and MR or CT with contrast.
• Fibro osseous lesions: fibrous dysplasia (ground glass matrix), cemento osseous dysplasia (mixed lesions at the apices of lower teeth in middle aged women), and ossifying fibroma.
• Osteomyelitis and osteonecrosis: patchy sclerosis and lysis with sequestra and periosteal reaction; medication related osteonecrosis of the jaw shows non-healing extraction sockets, sequestrum, and sclerosis in patients on antiresorptive or antiangiogenic drugs; osteoradionecrosis in irradiated jaws.
• Trauma: fractures of the mandible, maxilla and alveolar process, tooth luxation and avulsion, and root fractures.
• Temporomandibular joint: condylar flattening, erosion, osteophytes, subchondral cysts, ankylosis and fractures; the disc is not seen.
• Sinus: odontogenic sinusitis (periapical lesion associated with mucosal thickening), oroantral communication, root or implant in the sinus, and polyps or retention cysts.
• Airway: volume and minimum cross-sectional area of the pharyngeal airway in sleep disordered breathing assessment (limited by supine versus upright position).
• Incidental findings: carotid artery calcification, tonsilloliths, calcified lymph nodes, sinus opacification, and idiopathic osteosclerosis, which sometimes need follow up.

Critical values and findings needing immediate action.

• Spreading odontogenic infection (deep neck space abscess, airway threatened): urgent maxillofacial referral, antibiotics and drainage.
• Aggressive jaw lesion (ill defined, destructive, with soft tissue mass): urgent biopsy and head and neck oncology referral.
• Displaced jaw fracture with airway or occlusion compromise: urgent maxillofacial surgery.
• Osteomyelitis or osteonecrosis with systemic features: antibiotics and surgical review.
• Suspected malignancy on incidental findings (destructive bone lesion, sinus mass): referral to ENT or maxillofacial services.
• Close relation of an impacted tooth or implant site to the inferior alveolar canal: plan modified approach to avoid nerve injury.

Limitations and artefacts.

• Metal restorations, implants and root fillings cause streaking and beam hardening, hiding adjacent roots and bone (especially close to metal) and creating false lucent lines that mimic root fractures; the image is read with caution, and scatter reduction algorithms may help.
• Poor soft tissue contrast: CBCT cannot differentiate soft tissue pathology, tumour from inflammation, or show the disc of the temporomandibular joint; MR or multidetector CT with contrast is needed for soft tissue.
• Grey values vary between scanners and positions, so bone density cannot be quantified in Hounsfield units; bone quality assessment is subjective.
• Patient motion produces blur and double contours, and limited fields of view miss disease outside the volume; the whole field must be reviewed for incidental pathology.
• Radiation exposure is higher than for conventional dental radiographs, cumulative in children and young patients; CBCT must be justified for each patient, not used routinely.
• Voxel size limits detection of thin features: very small root fractures and fine periodontal ligament changes are missed at 0.3 to 0.4 mm voxels.
• Software measurements depend on correct calibration and operator marking; errors of 0.5 to 1 mm are possible, and a safety margin is needed near vital structures.
• Training and reporting standards vary, and non-radiologist clinicians may not recognise incidental pathology outside the dental region.

Special groups.

• Children and adolescents: use only when conventional imaging is insufficient, with smallest field and lowest dose; common indications are impacted canines, cleft alveolus and trauma.
• Pregnant patients: defer elective scans.
• Patients on bisphosphonates, denosumab or after head and neck radiotherapy: risk of osteonecrosis; plan extractions and implants with imaging and specialist advice.
• Patients with sleep apnoea: airway analysis is adjunctive.
• Patients with cleft lip and palate: bone graft planning.
• Older adults: reduced bone height and sinus pneumatisation; implant planning.
• Regions: dental caries and periapical infection with facial space abscess are common, and jaw tumours such as ameloblastoma, Burkitt lymphoma (children) and oral cancer are frequent; CBCT is available in few centres, so panoramic and intraoral radiographs remain the main tools.

Follow up tests.

• Intraoral periapical and panoramic radiographs.
• Multidetector CT or MR for soft tissue extension and malignant lesions.
• Biopsy and histology of jaw lesions, pulp vitality tests and clinical periodontal examination.
• Microbiology and blood tests (full blood count, inflammatory markers, glucose, calcium, alkaline phosphatase) in infection and fibro osseous disease.
• Bone density measurement and review of antiresorptive therapy, and sleep study for airway disease.
• Repeat CBCT after implant placement or graft (selected).
• Referral to oral and maxillofacial surgery, endodontics, periodontics, orthodontics or head and neck oncology.