Optical Coherence Tomography in ENT - With Special Reference to the Larynx

1. Introduction

• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, p. 584
• Cummings Otolaryngology Head and Neck Surgery, p. 2287

2. Physical Principle

• Wavelength: 800-1300 nm NIR light (shorter wavelengths give better resolution; longer wavelengths penetrate deeper into scattering tissue)
• Axial resolution: Determined by the coherence length of the light source - typically 1-15 µm in tissue
• Lateral resolution: Determined by the focal spot size of the beam optics
• Depth of penetration: 1-3 mm, limited by optical scattering within biological tissue (most turbid tissues allow only ~1-2 mm in practice)

3. Mechanism of Image Formation

3.1 The Michelson Interferometer Setup

1. Sample arm: Light is directed into the tissue via a probe or endoscopic catheter. At each depth within the tissue, small amounts of light are backscattered due to variation in optical refractive index between tissue components (cell membranes, organelles, collagen fibres, basement membrane, vessels).
2. Reference arm: Light travels to a mirror at a precisely known distance and returns to the beam-splitter.

3.2 From A-Scan to 2D Image

• A-scan: One axial reflectivity profile at a single lateral point
• B-scan: Multiple A-scans acquired as the beam is swept laterally, producing a 2D cross-sectional image (equivalent to a vertical histological section)
• C-scan / en face: Lateral scanning at a fixed depth, producing a face-on image like a confocal microscopy view

3.3 Time-Domain vs Fourier-Domain OCT

4. OCT in the Larynx - Specific Applications

4.1 Normal Laryngeal Anatomy on OCT

1. Stratified squamous epithelium (SS): Relatively uniform, moderately backscattering layer at the surface
2. Basement membrane (BM): A thin, highly reflective bright band separating epithelium from lamina propria
3. Lamina propria (LP): Deeper, heterogeneous zone of connective tissue

"Conventional OCT studies of healthy laryngeal tissues have demonstrated that OCT can detect spatial changes in the thickness and transparency of the epithelium, the content of the connective tissues including the presence of glands and vessels, as well as pronounced transitions in connective tissue type and the architecture of the basement membrane."

4.2 Detection of Basement Membrane Integrity

• In benign lesions (polyps, cysts, Reinke's oedema, papilloma), the BM remains intact and identifiable as a bright reflective band
• In carcinoma in situ (CIS), the BM is intact but the epithelial architecture is disordered
• In microinvasive SCC, the BM is disrupted - irregular, discontinuous, or completely absent - with tumour nests penetrating into the submucosa

4.3 Vocal Fold Vibration and Dynamic OCT

4.4 Pediatric Laryngology

• Develop computerized airway models for predicting neonatal subglottic stenosis following prolonged intubation
• Localize the site of upper airway obstruction in children with sleep-disordered breathing
• Image pediatric vocal fold paralysis intraoperatively to assess mucosal integrity and extracellular matrix changes

4.5 Subglottic / Tracheal Applications

4.6 Other ENT Applications

5. Clinical Indications in Laryngology

1. Evaluation of superficial and subtle laryngeal lesions - lesions suspicious for early cancer in which standard imaging and white-light endoscopy are indeterminate
2. Differentiation of benign from microinvasive or early malignant lesions - particularly T1 glottic tumours, leukoplakia, and erythroplakia
3. Assessment of basement membrane integrity in lesions suspicious for CIS or microinvasive SCC
4. Grading of precancerous lesions (dysplasia) in conjunction with microlaryngoscopy
5. Guiding targeted biopsy - identifying the most representative site for tissue sampling, reducing sampling error
6. Surgical margin assessment following transoral laser microsurgery or endoscopic resection of laryngeal tumours
7. Monitoring disease progression or response to treatment (radiotherapy, photodynamic therapy) without tissue removal
8. Intraoperative real-time guidance during phonomicrosurgery under surgical microscopy
9. Assessment of vocal fold scar - PS-OCT for collagen architecture and lamina propria layering
10. Pediatric subglottis - post-intubation stenosis assessment and airway modelling

6. Contraindications

1. Bulky or exophytic laryngeal lesions - the BM cannot be consistently identified because thick hypercellular tissue increases backscattering and limits light penetration; biopsy on clinical criteria alone is more appropriate in this situation
2. Heavily keratinized surfaces - keratin scatters light intensely and may obscure deeper structures
3. Active haemorrhage at the imaging site - blood absorbs and scatters NIR light, degrading image quality
4. Severely obstructed airway - delivering a probe to the larynx in acute airway compromise is contraindicated; airway security takes priority
5. Uncooperative or paediatric patient (for office-based awake OCT) - patient motion causes significant imaging artifact; general anaesthesia or microlaryngoscopy setting may be required
6. Limitation to superficial imaging - OCT cannot evaluate deep cervical lymph nodes, cartilage invasion, or subglottic extension beyond 1-3 mm; CT/MRI remain essential for advanced staging

7. Delivery Methods

• Microlaryngoscopy-integrated OCT: Probe passed through the operating laryngoscope under general anaesthesia with the surgical microscope; allows hands-free OCT simultaneously with microscopic visualization. Good probe stability, minimal motion artifact.
• Office-based awake transnasal OCT: A flexible fiberoptic probe is introduced through the nasal passage to the larynx in an awake patient. Burns et al. described the largest published series of this technique, demonstrating feasibility for non-anaesthetic outpatient evaluation. Challenges include patient gag reflex, motion artifact, and short focal length requiring careful probe positioning.
• Handheld probe: Investigational; being explored for intraoperative use similar to a nerve stimulator.
• Surgical microscope-coupled OCT: Allows real-time imaging during phonomicrosurgery.

8. Advantages

9. Disadvantages and Limitations

10. OCT Variants Relevant to ENT

1. Polarization-sensitive OCT (PS-OCT): Exploits birefringence of organized collagen. Relevant for vocal fold scar, cartilage assessment, and distinguishing fibrotic from normal lamina propria.
2. Fourier-domain OCT (FD-OCT) / Swept-source OCT (SS-OCT): Faster acquisition, less motion artifact, greater depth; preferred for in-office and intraoperative laryngeal work.
3. Long-range OCT (LR-OCT): Extended depth of field; used for airway cross-sectional modelling in subglottic stenosis and paediatric airway.
4. Optical frequency domain imaging (OFDI): High-speed variant; demonstrated for parathyroid identification in thyroid/parathyroid surgery.
5. Micro-OCT (µOCT): Ultra-high resolution system offering ~1 µm axial resolution; still largely experimental but has been demonstrated for parathyroid tissue identification.
6. Dynamic/4D OCT and optical Doppler tomography (ODT): Allows imaging of vocal fold vibration mechanics and mucosal wave motion in real time.
7. Spectroscopic OCT: Encodes spectral information within the OCT signal to provide molecular contrast beyond pure structural imaging.

11. Current Evidence and Status

• Good histological correlation for normal and pathological laryngeal tissue has been demonstrated across multiple groups (Armstrong et al., 2006 [PMID 16826043]; Burns et al., 2012 [PMID 22913932])
• OCT in conjunction with microlaryngoscopy significantly increases sensitivity for distinguishing benign from malignant lesions and grading precancerous lesions compared to microlaryngoscopy alone
• Office-based awake transnasal OCT of the larynx has been demonstrated as feasible, with the potential to diagnose laryngeal lesions, guide targeted biopsy, and monitor disease noninvasively
• OCT is most useful for subtle superficial lesions - particularly potential T1 tumours, CIS, and early dysplastic disease
• Large-scale randomized studies to justify replacement of histological biopsy are lacking, and OCT currently serves as an adjunct rather than a replacement for tissue diagnosis

"Whilst these studies are predominantly non-randomized pilot or proof of concept studies they illustrate potential clinical applications for widespread clinical practice... we currently lack large-scale studies to justify replacement of histological confirmation." (p. 584)

12. Summary Table

13. Conclusion

• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol 1, pp. 584-586; Vol 2, p. 962
• Cummings Otolaryngology Head and Neck Surgery, pp. 2287-2292
• Armstrong WB et al. Optical coherence tomography of laryngeal cancer. Laryngoscope 2006. [PMID 16826043]
• Burns JA. Optical coherence tomography: imaging the larynx. Curr Opin Otolaryngol Head Neck Surg 2012. [PMID 22913932]
• Tibbetts KM, Tan M. Role of Advanced Laryngeal Imaging in Glottic Cancer. Otolaryngol Clin North Am 2015. [PMID 26096138]
• Benboujja F, Hartnick C. Quantitative evaluation of the human vocal fold ECM using OCT. Sci Rep 2021. [PMID 33510352]