Cone Beam CT (CBCT) in ENT

What Is CBCT?

• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol 2 - CBCT radiation dose reported at ~60% of conventional CT
• Grainger & Allison's Diagnostic Radiology - provides accurate high-resolution volumetric visualisation of osseous structures

Technical Principles

Applications in ENT

1. Paranasal Sinuses and Rhinology

• Uncinate process
• Ostiomeatal unit (OMU)
• Skull base and cribriform plate
• Lamina papyracea

• Scott-Brown's Otorhinolaryngology Vol 1 - potential role includes intraoperative sinonasal imaging and assessment of resection margins
• Cummings Otolaryngology - CT is the imaging standard for rhinosinusitis; CBCT increasingly filling this role at lower dose

2. Temporal Bone Imaging

• Excellent visualisation of the ossicular chain (better than conventional CT in some studies)
• Identification of ossicular fixation, e.g. malleus head fixation, incus fusion (Figure 89.11)
• Detection of superior semicircular canal dehiscence (SSCD)

• CBCT can resolve slices down to 0.3 mm with lower radiation than HRCT
• Sensitivity similar to HRCT for fenestral otosclerosis (>90% with modern techniques)
• Useful when cochlear implantation is being considered

• Sufficient to diagnose labyrinthine malformations and dysplasia
• Detection of traumatic lesions, labyrinthine wall erosion/dehiscence

• Imaging of choice for postoperative follow-up due to markedly reduced metal artefacts
• Can determine electrode position (scala tympani vs vestibuli) - critical for programming
• Accurate visualisation of electrode array and its relationship to the modiolus

• CBCT offers bony detail, but lacks soft tissue contrast; MRI (especially non-EPI DW-MRI) remains gold standard for soft tissue assessment
• Scott-Brown's Vol 2 - "low sensitivity to metallic artefacts makes it the imaging of choice in follow-up of cochlear implants"
• Grainger & Allison - HRCT including CBCT most common technique for middle ear cleft assessment in conductive/mixed hearing loss

3. Skull Base

• High isotropic resolution for bony skull base detail
• Assessment of tumour involvement of bony structures (though MRI still needed for soft tissue extension)
• Pre-operative mapping for skull base surgery

• Scott-Brown's Vol 2 - "CBCT is being increasingly used in anterior and lateral skull base imaging... radiation dose reported to be 60% of conventional CT scans"

4. Facial Trauma

• Rosen's Emergency Medicine - cone beam CT improves surgical planning and esthetic outcomes in complex orbital fractures

5. Paediatric ENT

6. Dental/Maxillofacial Applications Relevant to ENT

• Pre-surgical dental implant planning
• Orthodontic assessment
• Relationship of tooth roots to maxillary sinus floor (important in odontogenic sinusitis)
• Assessment of tissue spaces around the jaws

Advantages Summary

1. Radiation dose reduction - up to 60-fold less than MDCT in some protocols; effective dose 0.04-0.19 mSv
2. High spatial/bony resolution - isotropic voxels, sub-mm slices (0.3 mm), excellent for fine bony structures
3. Reduced metal artefact - superior to MDCT for cochlear implant imaging
4. Point-of-care capability - compact units suitable for ENT outpatient clinics; FDA-approved, IAC-accredited
5. Cost-effectiveness - benefit-cost ratio reported as ~16:1 in one analysis
6. 3D reconstruction - reliable for surgical planning and anatomical mapping

Limitations

• Poor soft tissue contrast - cannot adequately assess soft tissue masses, tumour infiltration, or differentiate cholesteatoma from fluid/granulation tissue. MRI is mandatory in these settings.
• Beam hardening artefacts - from dental restorations; limits utility in some maxillofacial cases
• No contrast enhancement - cannot assess vascularity or permeability
• Interpretation requires training - wide FOV scans may include skull base and spine, requiring radiologist co-reporting
• Still hospital/clinic dependent - though point-of-care models exist

CBCT vs MRI vs MDCT: When to Use What

Key Points

• CBCT is AAO-HNS endorsed as appropriate for paranasal sinus, skull base, and temporal bone imaging - physicians have discretion to choose between CBCT and conventional CT.
• It does not replace MRI where soft tissue characterisation is needed.
• Future directions include improved image quality, better soft tissue algorithms, and expanded intraoperative use.
• Scott-Brown's Otorhinolaryngology Vol 1 notes: "Anticipated developments in cone beam CT are expected to include improved image quality and potentially intra-operative use."