OpticalCoherenceTomography-OCT.pdf

OPTICAL COHERENCE TO M O G R A P H Y MODULE: 6LMS0096 DR CHRISTIAN FRENCH  LEARNING OBJECTIVES Understand how the theory of physical phenomena involving light and acoustic waves, interferometry and the doppler effect can be exploited to image ti...

OPTICAL COHERENCE TO M O G R A P H Y MODULE: 6LMS0096 DR CHRISTIAN FRENCH  LEARNING OBJECTIVES Understand how the theory of physical phenomena involving light and acoustic waves, interferometry and the doppler effect can be exploited to image tissues in medicine. Be able to explain the various principles involved in imaging ocular tissues; including fundus photography and ultrasonography. Appreciate the relationship between resolution and penetration of an imaging modality; the associated advantages and disadvantages of each and relate this to ocular anatomy. Be able to compare and contrast the various types of OCT; Time and Spectral Domain and Swept Source; anterior and posterior scans and OCT-A(ngiography). Attempt to interpret an OCT report, identifying key details and considerations. Consider some examples of pathologies which can be observed with OCT, and some case studies from clinical practice; appreciating how OCT can compliment the diagnostic process. L I G H T P E N E T R AT I O N OF THE EYE  L I G H T P E N E T R AT I O N OF THE EYE Chromatic aberration takes place within the eye: Exploited during refraction: duochrome Can also be exploited when imaging the retina en face* Colour fundus photography: Blue: superficial retina Green: mid-retina / vasculature Red: deep retina / choroid * en face = face on; used in OCT literature L I G H T P E N E T R AT I O N OF THE EYE Superficial retina Absorbed by deeper layers (RPE) ∴ dark background Transparent structures: 450nm RNFL ILM Retinal folds / cysts Epiretinal membrane Easily affected by scatter (∴ corneal / lenticular clarity vital) L I G H T P E N E T R AT I O N OF THE EYE Retinal vasculature Less RPE absorption Easier distinction of vessel types due to concentration 540nm of oxy-haemoglobin in arteries and veins Basis of retinal oximetry L I G H T P E N E T R AT I O N OF THE EYE Deep retina / choroid Most retinal structures: varying shades of red ∴ other features ‘filtered 615nm out’ Choroidal vasculature visible ONH: featureless Uses: Pigmentary disturbances Neavus, melanoma  L I G H T P E N E T R AT I O N OF THE EYE Filtering performed with photographic negatives. Same principle used with digital photography: Colour channel extraction (RGB) BUT removing 2/3 of photographic data, ∴ ↓ resolution (↑ contrast)  L I G H T P E N E T R AT I O N OF THE EYE Filtering performed with photographic negatives. Same principle used with digital photography: Colour channel extraction (RGB) BUT removing 2/3 of photographic data, ∴ ↓ resolution (↑ contrast) Green channel Monochromatic isolated filter  U LT R A S O N O G R A P H Y A high energy acoustic wave is transmitted Tissue border through tissue(s), partially: Acoustic wave Probe  U LT R A S O N O G R A P H Y A high energy acoustic wave is transmitted Tissue border through tissue(s), partially: Acoustic wave Absorbed by tissue (∴ signal progressively weakens with continuing tissue penetration) Probe  U LT R A S O N O G R A P H Y A high energy acoustic wave is transmitted through tissue(s), partially: Absorbed by tissue (∴ signal progressively weakens with continuing tissue penetration) Pass through the tissue and continue onwards  U LT R A S O N O G R A P H Y A high energy acoustic wave is transmitted through tissue(s), partially: Absorbed by tissue (∴ signal progressively weakens with continuing tissue penetration) Pass through the tissue and continue onwards Reflect back from the tissue and return to the source  U LT R A S O N O G R A P H Y A high energy acoustic wave is transmitted through tissue(s), partially: Absorbed by tissue (∴ signal progressively weakens with continuing tissue penetration) Pass through the tissue and continue onwards Reflect back from the tissue and return to the source Echoes received by transducer are converted into a signal and displayed on the instrument Dense tissue = absorbent but strong echoed reflected  U LT R A S O N O G R A P H Y & O C T Ultrasound: high amplitude acoustic wave (20kHz) Requires contact solution Velocity (air): ~340m/s Velocity (water): ~1,480m/s OCT: light source Velocity unaffected by medium But requires optically clear medium ∴ ultrasonography still used in ophthalmology Exploits interference (rather than echoes) Soft-tissue modalities: cannot penetrate bone Real-time imaging (near-instantaneous processing) Non-ionising energy (unlike x-rays / MRI)  INTERFERENCE When energy (light or sound) is transmitted, it undergoes both absorption and reflection to varying degrees. When energy waves meet, their interaction is governed by their phase (interference): In phase = constructive (additive) Peak-to-peak Out of phase = destructive (subtractive) Peak-to-trough  INTERFERENCE When energy (light or sound) is transmitted, it undergoes both absorption and reflection to varying degrees. When energy waves meet, their interaction is governed by their phase (interference): In phase = constructive (additive) Peak-to-peak Out of phase = destructive (subtractive) Peak-to-trough  INTERFERENCE Optical applications: Ophthalmic lenses: Anti-reflection vs. mirror coatings AR thickness = ¼λ (out-of-phase by ½λ ∴ maximal deletion: destructive interference) Mirror thickness = ½λ (out-of-phase by 1λ ∴ maximal addition: constructive interference) Diagnostic imaging: Optical coherence tomography  INTERFERENCE Optical applications: Ophthalmic lenses: Anti-reflection vs. mirror coatings AR thickness = ¼λ (out-of-phase by ½λ ∴ maximal deletion: destructive interference) Mirror thickness = ½λ (out-of-phase by 1λ ∴ maximal addition: constructive interference) Diagnostic imaging: Optical coherence tomography Interference  TISSUE P E N E T R AT I O N When a single wave is transmitted through tissue(s), it will partially: Pass through the tissue and continue onwards Reflect back from the tissue and return to the source Tissue properties determine amount of absorption, transmission and reflectance (and subsequent interference of transmitted and reflected beams). ∴ can distinguish different tissue types (and properties)  TISSUE P E N E T R AT I O N When a single wave is transmitted through tissue(s), it will partially: Pass through the tissue and continue onwards Reflect back from the tissue and return to the source Tissue properties determine amount of absorption, transmission and reflectance (and subsequent interference of transmitted and reflected beams). ∴ can distinguish different tissue types (and properties)  A AND B SCANS Single point = A-scan Multiple points = B-scan Peaks identify tissue boundaries Intensity of peak relates to reflectance Single-dimensional data  A AND B SCANS Single point = A-scan Multiple points = B-scan Peaks identify tissue boundaries Intensity of peak relates to reflectance Two-dimensional data  A AND B SCANS Single point = A-scan Multiple points = B-scan Peaks identify tissue boundaries Intensity of peak relates to reflectance Two-dimensional data  A AND B SCANS Single point = A-scan Multiple points = B-scan Peaks identify tissue boundaries Intensity of peak relates to reflectance Two-dimensional data  A AND B SCANS Single point = A-scan Multiple points = B-scan Peaks identify tissue boundaries Intensity of peak relates to reflectance Two-dimensional data  A AND B SCANS Single point = A-scan Multiple points = B-scan Peaks identify tissue boundaries Intensity of peak relates to reflectance Two-dimensional data 512 A-scans  A AND B SCANS Multiple B-scans can be merged together into a 3-dimensional ‘C-scan’. The OCT cube is specified by the number of A- and B-scans: e.g. 512x128. This is important for defining the resolution of an OCT (and the sales pitch: “65,536 scans!”) 128 b-scans 512 A-scans  IMAGE RESOLUTION There is a trade-off between tissue penetration depth and image resolution  IMAGE RESOLUTION There is a trade-off between tissue penetration depth and image resolution Additional factor: motive for imaging Ocular biometry only requires distances ∴ less-complex instrumentation sufficient  IMAGE RESOLUTION There is a trade-off between tissue penetration depth and image resolution Additional factor: motive for imaging Ocular biometry only requires distances ∴ less-complex instrumentation sufficient Evaluation of macular health requires high resolution (≤10μm)  OPTICAL COHERENCE TO M O G R A P H Y  OCT Developed for ophthalmology in 1991 Original method: Time-domain (TD-OCT) Based on Michelson Interferometer principle More recently: Fourier-domain Spectral-domain (SD-OCT) Swept-source (SS-OCT) Technological advancements have improved both spatial resolution and tissue penetration depths (including choroidal vasculature).  OCT All OCTs use light split into two paths; a measurement arm and reference arm. Michelson interferometer OCT plots are the product of interference patterns between these two arms. Only paths of similar lengths (although can be out-of-phase) will interfere (thus early OCTs required moveable reference mirrors) – TD-OCT More recently (Fourier-domain): Spectrometer: SD-OCT Rapidly tuneable laser: SS-OCT  TIME-DOMAIN OCT A monochromatic light source transmits light towards a beam splitter. Moveable Mirror Mono- chromatic light source Sample Beam Splitter Photo Detector  TIME-DOMAIN OCT A monochromatic light source transmits light towards a beam splitter. Moveable One beam continues through to the sample Mirror (ocular tissue) where it is reflected back towards the beam splitter by the optical tissues. Mono- chromatic light source Sample Beam Splitter Photo Detector  TIME-DOMAIN OCT A monochromatic light source transmits light towards a beam splitter. Moveable One beam continues through to the sample Mirror (ocular tissue) where it is reflected back towards the beam splitter by the optical tissues. Mono- chromatic light source Sample Beam Splitter Photo Detector  TIME-DOMAIN OCT A monochromatic light source transmits light towards a beam splitter. Moveable One beam continues through to the sample Mirror (ocular tissue) where it is reflected back towards the beam splitter by the optical tissues. The split beam is also directed towards a Mono- chromatic moveable mirror which rapidly oscillates back light source Sample Beam Splitter and forth to alter the beam path length. This allows for evaluating different phase Photo shifts in interference. Detector  TIME-DOMAIN OCT A monochromatic light source transmits light towards a beam splitter. Moveable One beam continues through to the sample Mirror (ocular tissue) where it is reflected back towards the beam splitter by the optical tissues. The split beam is also directed towards a Mono- chromatic moveable mirror which rapidly oscillates back light source Sample Beam Splitter and forth to alter the beam path length. This allows for evaluating different phase Photo shifts in interference. Detector  TIME-DOMAIN OCT A monochromatic light source transmits light towards a beam splitter. Moveable One beam continues through to the sample Mirror (ocular tissue) where it is reflected back towards the beam splitter by the optical tissues. The split beam is also directed towards a Mono- chromatic moveable mirror which rapidly oscillates back light source Sample Beam Splitter and forth to alter the beam path length. This allows for evaluating different phase Photo shifts in interference. Detector Both beams return to the beam splitter and pass through to a photo-detector array.  TIME-DOMAIN OCT A monochromatic light source transmits light towards a beam splitter. Moveable Resolution limited One beam continues through to the sample Mirror by speed of mirror (ocular tissue) where it is reflected back towards the beam splitter by the optical tissues. The split beam is also directed towards a Mono- chromatic moveable mirror which rapidly oscillates back light source Sample Beam Splitter and forth to alter the beam path length. This allows for evaluating different phase Photo shifts in interference. Detector Both beams return to the beam splitter and pass through to a photo-detector array.  FOURIER-DOMAIN OCT To improve resolution, a range of wavelengths needs to be used simultaneously. This replaces Reference Mirror the need to have a moving reference mirror. (λ rather than path-length) This can be achieved by: A broadband light source passing through Broadband Source a spectroscope: Spectral domain Sample Beam Splitter A tuneable laser sweeping rapidly through a range of wavelengths: Swept source Photo Detector Light rays arriving at the photodetector array are amalgamated by Fourier transformation for analysis.  OCT T Y P E S S U M M A RY Axial Scanning Transverse Tissue Type Light source Variable (depth) speed resolution range resolution Vitreo- Superluminescent Moveable 400 A-scans / retinal Time Domain 10µm 20µm diode (810nm) mirror second interface to RPE Posterior Broadband 27,000-70,000 Spectral cortical superluminescent Spectroscope A-scans / 5-7µm 14-20µm Domain vitreous to diode (840nm) second sclera Posterior 100,000 – Tuneable laser cortical Swept Source Tunable laser 400,000 scans / 5µm 20µm (1050nm) vitreous to second sclera  OCT IMAGE COLOURING Data produced by OCT is not a real image. Fundus photography still the only modality to capture a real true-colour image of the ocular structures. OCT data has false colouring applied in two styles: False colour ↑ reflectance = warmer colours Monochromatic / B&W ↑ reflectance = ↑ white Same data ∴ no difference? (Preference for monochromatic)  OCT SCAN TYPES Predominantly disc (ONH)- and macular-centred scans Currently no established protocol in the UK ∴ practitioner judgement 3D OCT cube Standard approach; 512x128 etc. Widefield OCT scan including significant retinal landmarks (macula and ONH) Raster Series of lines, rapid scan Good for poor fixation Single line scan  R E V I E W : R E T I N A L A N ATO M Y 1. Retinal Pigment Epithelium 2. Photoreceptor Layer 3. Outer Limiting Membrane 4. Outer Nuclear Layer 5. Outer Plexiform Layer 6. Inner Nuclear Layer 7. Inner Plexiform Layer 8. Ganglion Cell Layer 9. Nerve Fibre Layer 10. Inner Limiting Membrane  R E V I E W : R E T I N A L A N ATO M Y Layer Initials Location 1 Retinal Pigment Epithelium RPE RPE cells 2 Photoreceptor Layer PL Cell bodies of rods and cones 3 Outer Limiting Membrane OLM Zonulae adherents: photoreceptors and Müller cells 4 Outer Nuclear Layer ONL Nuclei of: rods & cones 5 Outer Plexiform Layer OPL Synapses of: rods & cones, bipolar, horizontal cells 6 Inner Nuclear Layer INL Nuclei of: bipolar, horizontal, amacrine, Müller cells 7 Inner Plexiform Layer IPL Synapses of: bipolar, amacrine, ganglion cells 8 Ganglion Cell Layer GCL Nuclei of: ganglion cells 9 Nerve Fibre Layer (R)NFL Ganglion cell axons 10 Inner Limiting Membrane ILM Müller cell terminations and basement membrane  R E V I E W : R E T I N A L A N ATO M Y We can’t yet observe individual cells like a microscope slide, but some orientation of the retinal layers is required for documenting location of lesions, abnormalities etc., and also being aware of what cells are being affected (link to Sx?)  O C T R E T I N A L A N ATO M Y Software has internal edge-detection algorithms to determine boundaries within retina. As this isn’t a ‘real’ image, this process is not always perfect. Also used when defining RNFL thickness in ONH-centred scans (“glaucoma scan”) Useful for progression analysis (RNFL, lesion size etc.) Correct fixation and alignment is vital!  LINKING STRUCTURE AND APPEARANCE Hyper-reflective = strong signal (bright/warm) Hypo-reflective = weak signal (dim/cool) Dense = no data beyond tissue (absorbent) Clear = no data (transmissive)  LINKING STRUCTURE AND APPEARANCE Hyper-reflective = strong signal (bright/warm) Hypo-reflective = weak signal (dim/cool) Dense = no data beyond tissue (absorbent) Clear = no data (transmissive)  LINKING STRUCTURE AND APPEARANCE Hyper-reflective = strong signal (bright/warm) Hypo-reflective = weak signal (dim/cool) Dense = no data beyond tissue (absorbent) Clear = no data (transmissive)  LINKING STRUCTURE AND APPEARANCE Hyper-reflective = strong signal (bright/warm) Hypo-reflective = weak signal (dim/cool) Dense = no data beyond tissue (absorbent) Clear = no data (transmissive)  LINKING STRUCTURE AND APPEARANCE Hyper-reflective = strong signal (bright/warm) Hypo-reflective = weak signal (dim/cool) Dense = no data beyond tissue (absorbent) Clear = no data (transmissive)  OCT A NGIOGRAPHY Newer OCT technology allows greater tissue Mid-phase fluorescein angiograms (BRVO) penetration depths ∴ imaging of choroidal vasculature now possible (OCT angiography / OCT-A) (Historically performed with fluorescein angiography (FA) / indocyanine green (ICG)) Utilises Doppler effect Static retinal tissue vs. mobile blood cells Algorithm removes background to highlight areas of flow (i.e. blood vessels) Identified dual layer choroidal vasculature OCT-A of same patient that wasn’t seen with traditional FA.  OCT A NGIOGRAPHY Newer OCT technology allows greater tissue penetration depths ∴ imaging of choroidal vasculature now possible (OCT angiography / OCT-A) (Historically performed with fluorescein angiography (FA) / indocyanine green (ICG)) Utilises Doppler effect Static retinal tissue vs. mobile blood cells Algorithm removes background to highlight areas of flow (i.e. blood vessels) Identified dual layer choroidal vasculature that wasn’t seen with traditional FA.  ANTERIOR OCT It is also possible to perform anterior segment OCT (separate / same instrument). Structures to image: Cornea Anterior Chamber Angle Crystalline Lens  ANTERIOR OCT It is also possible to perform anterior segment OCT (separate / same instrument). Structures to image: Cornea Measure corneal thickness Digital thickness callipers CCT measurement for laser surgery? Assess fit of RGP? Monitor keratoconic? (If no topographer) Central reflex artefact = optimal alignment Anterior Chamber Angle Crystalline Lens  ANTERIOR OCT It is also possible to perform anterior segment OCT (separate / same instrument). Structures to image: Cornea Measure corneal thickness Digital thickness callipers CCT measurement for laser surgery? Assess fit of RGP? Monitor keratoconic? (If no topographer) Central reflex artefact = optimal alignment Anterior Chamber Angle Crystalline Lens  ANTERIOR OCT It is also possible to perform anterior segment OCT (separate / same instrument). Structures to image: Cornea Anterior Chamber Angle Visualise structures in angle Alternative to gonioscopy Not replacing yet…! Monitor narrow angles Pre-dilation check in suspect patients? Crystalline Lens  ANTERIOR OCT It is also possible to perform anterior segment OCT (separate / same instrument). Structures to image: Cornea Anterior Chamber Angle Crystalline Lens Alternative method for ocular biometry? SS-OCT Limited by properties of iris  OCT R E P O RT S O C T G L A U C O M A R E P O RT Patient details (DOB etc.) Eye (OD, OS, OU) Scan type Quality index Significance map (grid-breakdown) Thickness heat map (thick = warm) Fundus photograph O C T G L A U C O M A R E P O RT Patient details (DOB etc.) Eye (OD, OS, OU) Scan type Quality index Significance map (grid-breakdown) Thickness heat map (thick = warm) Fundus photograph O C T G L A U C O M A R E P O RT Patient details (DOB etc.) Eye (OD, OS, OU) Scan type Quality index Significance map (grid-breakdown) Thickness heat map (thick = warm) Fundus photograph O C T G L A U C O M A R E P O RT Patient details (DOB etc.) Eye (OD, OS, OU) Scan type Location, size, scan details Quality index Significance map (grid-breakdown) Thickness heat map (thick = warm) Fundus photograph O C T G L A U C O M A R E P O RT Patient details (DOB etc.) Eye (OD, OS, OU) Scan type Quality index Based on strength of returning signal Colour coded Reduced with media opacity etc. Significance map (grid-breakdown) Thickness heat map (thick = warm) Fundus photograph O C T G L A U C O M A R E P O RT Patient details (DOB etc.) Eye (OD, OS, OU) Scan type Quality index Significance map (grid-breakdown) Significant RNFL thickness difference from age-matched individuals Thickness heat map (thick = warm) Fundus photograph O C T G L A U C O M A R E P O RT Patient details (DOB etc.) Eye (OD, OS, OU) Scan type Quality index Significance map (grid-breakdown) Thickness heat map (thick = warm) Tissue thickness Visual marker for areas of thinning/thickening Fundus photograph O C T G L A U C O M A R E P O RT Patient details (DOB etc.) Eye (OD, OS, OU) Scan type Quality index Significance map (grid-breakdown) Thickness heat map (thick = warm) Tissue thickness Visual marker for areas of thinning/thickening Fundus photograph Reference image O C T G L A U C O M A R E P O RT RNFL thickness defined by machine (edge-detection). Circular region around ONH flattened out into linear “scan”. Comparison with age-matched individuals. RNFL asymmetry index and averages. Sectoral breakdown to highlight regions of RNFL loss O C T G L A U C O M A R E P O RT RNFL thickness defined by machine (edge-detection). Circular region around ONH flattened out into linear “scan”. Comparison with age-matched individuals. RNFL asymmetry index and averages. Sectoral breakdown to highlight regions of RNFL loss O C T G L A U C O M A R E P O RT RNFL thickness defined by machine (edge-detection). Circular region around ONH flattened out into linear “scan”. Comparison with age-matched individuals. RNFL asymmetry index and averages. Sectoral breakdown to highlight regions of RNFL loss O C T G L A U C O M A R E P O RT RNFL thickness defined by machine (edge-detection). Circular region around ONH flattened out into linear “scan”. Comparison with age-matched individuals. RNFL asymmetry index and averages. Sectoral breakdown to highlight regions of RNFL loss O C T G L A U C O M A R E P O RT RNFL thickness defined by machine (edge-detection). Circular region around ONH flattened out into linear “scan”. Comparison with age-matched individuals. RNFL asymmetry index and averages. Sectoral breakdown to highlight regions of RNFL loss. O C T G L A U C O M A R E P O RT RNFL thickness defined by machine (edge-detection). Circular region around ONH flattened out into linear “scan”. Comparison with age-matched individuals. RNFL asymmetry index and averages. Sectoral breakdown to highlight regions of RNFL loss. O C T G L A U C O M A R E P O RT ONH measurements Various C:D ratios given NB still record subjective based on ophthalmoscopy! O C T G L A U C O M A R E P O RT O C T G L A U C O M A R E P O RT  OCT PAT H O L O G I E S & CASE STUDIES  M A C U L A R D E G E N E R AT I O N Geographic atrophy of the macular region Dense, reflective drusen deposits Exposure of underlying choroidal tissue ∴ less impedance of OCT beam and greater tissue penetration  M A C U L A R D E G E N E R AT I O N Geographic atrophy of the macular region Dense, reflective drusen deposits Exposure of underlying choroidal tissue ∴ less impedance of OCT beam and greater tissue penetration  MACULAR OEDEMA Unilateral Fluid-filled cyst within retina Density of fluid within cyst evident from scan U N U S U A L V E S S E L O R I E N TAT I O N M O N I TO R I N G P R O G R E S S I O N CASE STUDY #1  C A S E H I S TO RY Px: ♂, 48 years old Reason for visit: Routine examination Presenting Sx: Due for exam but also felt ‘wanted eyes checking’. Not aware of any significant change to vision (although glasses are a few years old) GH: DM (T-II) – insulin, gliclazide (HbA1c 4.2); U/a thyroid (takes liothyronine), OSA FHx: Brother – glaucoma, DM (T-II) OcHx: DR screening (3/12 ago, all clear) Rx: R: +2.50 / -1.25 x 87 (6/5) +1.75 Add (N5 @ 37cm) L: +2.75 / -1.00 x 110 (6/5) +1.75 Add (N5 @ 37cm) (NCT) IOPs: R: 16, 17, 18mmHg L: 18, 16, 19mmHg Visual field plot (Humphrey FDT) OPHTHALMOSCOPY OPHTHALMOSCOPY O C T R E P O RT O C T R E P O RT O C T R E P O RT What does OCT tell us about right eye RNFL thickness? Is it the same conclusion we have come to? MANAGEMENT  MANAGEMENT Referral: Urgent HES: Seen by ophthalmologist Idiopathic papilloedema Suspect cause: DM-II No treatment given Review with optometrist in 1/12 ? ? 1 MONTH REVIEW 1 MONTH REVIEW CASE STUDY #2  C A S E H I S TO RY Px: ♀, 67 years old Reason for visit: 6/52 post-operative review following R/E cataract extraction Presenting Sx: Initially happy with result, but recently feels it has worsened and is like it was before. She is aware of a dull ache in the eye. She has completed her course of drops and been discharged by HES. Rx: R: ∞ / -0.50 x 165 (6/36-2) +2.50 Add (N24 @ 37cm) L: -1.50 / -0.75 x 20 (6/7.5)+2.50 Add (N5 @ 37cm) Previous Rx: R: ∞ / -0.50 x 165 (6/36-2) +2.50 Add (N24 @ 37cm) L: -1.50 / -0.75 x 20 (6/7.5)+2.50 Add (N5 @ 37cm) “UNEXPLAINED” REDUCED VISION “UNEXPLAINED” REDUCED VISION “UNEXPLAINED” REDUCED VISION MANAGEMENT  MANAGEMENT Referral: Urgent HES: Seen by ophthalmologist Pseudophakic Cystoid Macular Oedema (CMO) / Irvine-Gass Syndrome Topical NSAIDs (No standardised treatment regime) Review in 1/12 3 M O N T H S L AT E R  S U M M A RY The theory of physical phenomena involving light and acoustic waves, interferometry and the doppler effect can be exploited to image tissues in medicine. The various principles involved in imaging ocular tissues; including fundus photography and ultrasonography. The relationship between resolution and penetration of an imaging modality along with the associated advantages and disadvantages of each, relating this to ocular anatomy. Considered the various types of OCT; Time and Spectral Domain and Swept Source; anterior and posterior scans and OCT-A(ngiography). Introduced interpretation of an OCT report, identifying key details and considerations. Considered some examples of pathologies which can be observed with OCT, and some case studies from clinical practice; appreciating how OCT can compliment the diagnostic process.