OCT and CBCT in ENT

MS ENT University Exam Format - 10 Marks Each

OPTICAL COHERENCE TOMOGRAPHY (OCT) IN ENT

Introduction

• Scott-Brown's Otorhinolaryngology Head & Neck Surgery

Principle

• A beam of low-coherence light is directed at the tissue
• Back-reflected light from components with different optical properties at varying depths is detected
• Because light velocity is several magnitudes greater than sound, conventional electronics cannot measure echo time delays; instead a technique called Michelson interferometry is used
• Interferometry measures echo time delay and intensity of back-reflected light by comparing it with light that has traveled a known reference path length
• Produces cross-sectional images at a resolution of 1-15 μm (some systems achieve 7-10 μm)
• Imaging depth is up to 2-3 mm into tissue
• The image is similar to a vertical histological section - hence the term "optical biopsy"
• Fiber-optically based, allowing integration with endoscopes, catheters, and operating microscopes

1. Time-domain OCT (TD-OCT) - older, slower frame rate (~1 fps)
2. Fourier-domain OCT (FD-OCT) - faster, improved sensitivity
3. Long-range OCT (LR-OCT) - extended depth imaging
4. Polarization-Sensitive OCT (PS-OCT) - sensitive to organized linear structures like collagen fibre bundles; particularly useful for vocal fold assessment

Advantages of OCT

Applications in ENT

• Used in conjunction with microlaryngoscopy
• Detects spatial changes in thickness and transparency of laryngeal epithelium
• Visualizes content of connective tissue including glands and vessels
• Assesses integrity of the basement membrane - critical for staging malignancy
• Significantly increases sensitivity of determining benign vs. malignant lesions compared to microlaryngoscopy alone
• Helps grade precancerous lesions (dysplasia grading)
• PS-OCT provides imaging of collagen bundle orientation in vocal folds - useful for vocal fold health assessment and laser treatment monitoring
• Possible future role as substitute for excisional biopsy ("optical biopsy")

• Imaging of the tympanic membrane - detecting effusion, assessing viscosity of middle ear fluid
• Useful in diagnosing tympanic cavity effusion (otitis media with effusion)
• High-resolution imaging of middle ear structures
• OCT-A (Angiography variant) can monitor microcirculation in otitis and cochlear ischemia
• Feasibility studies of OCT applied to the inner ear are ongoing

• FD-OCT and LR-OCT used to create computerized airway models for predicting neonatal subglottic stenosis following intubation
• Localizing site of upper airway obstruction in children with sleep-disordered breathing

• Evaluation of oral cavity and pharyngeal mucosa - differentiating benign from malignant lesions
• Tracheal/bronchial cartilage evaluation - detecting congenital absence of cartilaginous tracheal rings
• Potential role in image-guided surgery and robotic systems

• Feasibility studies of OCT applied to nasal tissue are underway
• Potential for assessing mucosal changes in rhinosinusitis by monitoring microcirculation

Limitations

• Imaging depth limited to 2-3 mm (cannot image deep structures)
• Cannot replace histopathology currently (insufficient large-scale trials)
• Interpretation requires expertise
• Not yet widely available in routine ENT practice
• Large-scale randomized studies are lacking to justify replacement of conventional histological biopsy

Future Directions

• Spectroscopic OCT
• OCT combined with image guidance and robotic systems
• Intraoperative OCT
• OCT-Angiography (OCTA) for microvascular assessment in ENT inflammation and cochlear ischemia

Key Points Summary

• OCT = optical analogue of ultrasound; uses interferometry
• Resolution 1-15 μm; depth 2-3 mm
• "Optical biopsy" - subepithelial imaging without tissue removal
• Most studied ENT application: laryngeal imaging with microlaryngoscopy
• PS-OCT specifically useful for vocal fold and collagen assessment
• No radiation, non-invasive, real-time, endoscope-compatible
• Substantial promise but remains largely investigational in ENT

CONE BEAM COMPUTED TOMOGRAPHY (CBCT) IN ENT

Introduction

• Grainger & Allison's Diagnostic Radiology; Scott-Brown's Otorhinolaryngology Head & Neck Surgery Vol. 2

Principle / Technical Aspects

• Utilizes a divergent, pyramidal (cone-shaped) x-ray beam - unlike conventional CT's thin fan beam
• The system rotates around the patient in a single 180-360 degree arc, acquiring multiple sequential planar projections (300+ captures)
• Detected by either:
  • An image intensifying tube coupled to a CCD sensor, or
  • A flat-panel detector (amorphous silicon)
• Base images (similar in appearance to cephalometric radiographs) are integrated and displayed as a 3D volumetric dataset
• The dataset is made up of voxels (3D pixels - isotropic, with all three dimensions equal), unlike conventional CT which has anisotropic voxels
• Images reconstructed by computer algorithm; can be viewed in any plane (axial, coronal, sagittal) and as 3D reconstructions

• Small FOV: 5 cm (localized area, e.g., single tooth/joint)
• Large FOV: up to 13-22 cm (full facial skeleton, skull base)
• FOV should be tailored to the diagnostic task to minimize radiation

Advantages of CBCT over Conventional CT

Applications in ENT

• Imaging the external auditory canal (EAC) - exostoses, stenosis, atresia
• Middle ear and mastoid - ossicular chain assessment, chronic otitis media, pre-surgical mapping
• Inner ear - cochlear anatomy for cochlear implant pre-operative planning
• Facial nerve canal - anatomical delineation
• Detecting anatomical variants important for surgical planning:
  • Low-lying tegmen tympani
  • Anteriorly placed / dehiscent sigmoid sinus
  • High-riding jugular bulb projecting into hypotympanum
• HRCT including CBCT is the most common technique for assessing the middle ear cleft
• Cholesteatoma assessment (though MRI better differentiates soft tissue types)

• Multiplanar (coronal, sagittal, axial) views of paranasal sinuses provide sufficient detail for diagnosis and identification
• Pre-operative planning for functional endoscopic sinus surgery (FESS)
• Identification of sinonasal pathology, mucosal thickening, polyps (limited soft tissue resolution)
• Low-radiation "point-of-care" imaging in ENT outpatient settings

• Assessment of the close relationship between maxillary sinus floor and root apices - superior to plain films
• Pre-surgical implant planning and orthodontic treatment planning
• Evaluation of jaw pathology, cysts, tumours (bony component)
• Assessment of temporomandibular joint
• Tissue spaces around the jaws - odontogenic infection spreading to deep neck spaces

• Nasal bone fractures
• Septal assessment
• Orbital wall involvement in trauma

Limitations of CBCT

1. Poor soft tissue contrast - cannot differentiate fluid from cholesteatoma, or distinguish tissue types
2. Beam hardening artefacts - dark bands across images caused by dense structures (restored dentition, metallic implants) - limits use in dental caries diagnosis
3. Higher radiation dose than conventional dental radiography (though lower than conventional CT)
4. Large FOV scans include skull base and spine - may require co-reporting by a radiologist
5. Streak artefacts degrade image quality
6. Not suitable for assessing intracranial soft tissues

Radiation Dose Reduction Strategies

• Limit FOV to area of diagnostic interest only (in width and height)
• 50% tube current reduction can be applied for orthodontic / pre-surgical implant planning without significant loss of diagnostic quality
• Selecting 180-degree arc (vs. 360) for lower dose; however, at slight cost to image quality

CBCT vs. CT: Key Differences

Key Points Summary

• CBCT uses cone-shaped x-ray beam in a single 180-360° arc rotation
• Provides isotropic voxels and 3D reconstruction without geometric distortion
• Lower radiation dose than conventional CT; higher than plain radiography
• Superior bone detail; poor soft tissue resolution
• ENT uses: temporal bone (pre-op cochlear implant, EAC, middle ear), paranasal sinuses (FESS planning), skull base, maxillofacial trauma
• Specific advantage in paediatric ENT - radiation reduction is clinically significant
• Cannot differentiate soft tissue pathology (e.g., cholesteatoma vs. fluid) - MRI preferred for that

Sources

• Scott-Brown's Otorhinolaryngology Head & Neck Surgery (Vols 1 & 2)
• Grainger & Allison's Diagnostic Radiology
• Brogdon's Forensic Radiology
• Textbook of Family Medicine (9e)
• ENT Today - Emerging Use of OCT in Otolaryngology
• Almanac of Clinical Medicine - OCT in Otolaryngology (Shakhova et al.)