Anterior segment imaging Flashcards
What are 6 forms of anterior segment imaging?
- Keratometry
- Corneal topography
- Scanning slit videokeratography
- Scheimpflug imaging
- In vivo confocal microscopy
- Anterior segment optical coherence tomography (OCT)
What is keratometry a measurement of?
the anterior surface curvature of the central cornea
Waht is the approximate diameter of the central cornea?
~3mm
What pathological feature does keratometry tell us about?
assesses axis and magnitude of astigmatism
How is keratometry usually expressed?
two corneal curvature values, 90 degrees apart (i.e. max and min ‘k’ values)
What are the 2 types of manual keratometers?
- von Helmholtz (Bausch and Lomb)
- Javal Schiotz
What are 3 examples of devices that can perform automated keratometry?
- autorefractors
- corneal topographers
- IOLMaster - provides IOL calculations for cataract surgery
What are 3 indications for performing keratometry?
- CL fitting
- ocular biometry
- assessment of changes in corneal curvature post-surgery
What does keratometry provide no information about?
points central or peripheral to the points measured
What is the difference between the von Helmholtz and Javal Schiotz keratometers?
von Helmholtz: fixed object size, variable image size
Javal Schiotz: variable object size, fixed image size
What is corneal topography?
mapping of curvature across the entire corneal surface
What type of systems is corneal topography traditionally performed using?
Placido-based systems - analysing multiple concentric rings of light reflected off the anterior corneal surface, thus indirectly measuring corneal curvature (‘videokeratoscopy’)
What 2 types of technology are now used to perform corneal topography rather than Placido-based systems?
- Scanning slit technology (Orbscan, Bausch and Lomb)
- Scheimplug imaging (Pentacam, Oculus)
What are 3 additional measurements that direct measurements of corneal curvature provide (with scanning slit technology or Scheimplug imaging)?
- evaluation of posterior corneal surface
- corneal thickness maps
- greater coverage of the peripheral cornea
What are 4 indications for performing corneal topography?
- assess corneal curvature (post operative changes and prior to laser refractive procedures)
- detection of macro-irregularities such as astigmatism, keratoconus, pellucid marginal degeneration
- assessment of CL fit and monitoring of warpage
- measurement of corneal thickness
What are 3 examples of causes of macro-irregularities of the cornea?
- astigmatism
- keratoconus
- pellucid marginal degeneration
How is corneal curvature expressed from corneal topography?
as radii of curvature in millimetres, or in keratometric dioptres; colour scale used representing the range of values
What are 2 ways that curvature maps can be constructed for corneal topography?
- comparing data with themselves (relative or normalised scales)
- comparing data to set ranges (absolute scale)
What are 5 groups into which normal corneas are usually classified?
- round
- oval
- symmetric bow tie
- asymmetric bow tie
- irregular
What is the type of astigmatism present int he average adult cornea?
steeper in vertical meridian compared with horizontal - with-the-rule astigmatism
What is scaning slit videokeratography?
high-resolution video camera is used to capture reflections from multiple slits of light projected through the cornea at 45 degrees. software calculates corneal thickness and posterior corneal surface by direct triangulation
What is an example of a type of machine which performs scanning slit videokeratography?
- Orbscan (Bausch and Lomb)
What is an example of a type of machine which performs scanning slit videokeratography?
- Orbscan (Bausch and Lomb)
How does Orbscan scanning slit videokeratography software calculate elevation?
the points per half-slit from both the anterior and posterior surfaces
then indirectly calculates the corneal thickness
What is a limitation/caution with use of scanning slit videokeratogrpahy?
inability to detect interfaces e.g. post-LASIK flap
What is Scheimpflug imaging?
corneal topography with high resolution of the entire cornea and ability to measure corneas with severe irregularities
What are 4 indications for Scheimpflug imaging?
- High resolution needed to view entire cornea and measure cornea with severe irregularities
- Corneal wavefront analysis for measurement of higher-order aberrations
- Measurement of corneal thickness from limbus to limbus
- Measurement of AC depth and angle estimation
How does Scheimpflug imaging differ from conventional techniques?
the object plane, lens plane, image plane are not parallel to each other but intersect in a common straight line; this allows generation of optical sections with a wide depth of focus
Within what time period can a 3D model of the anterior segment be generated from 50 slit images + evaluating 500 measured points from each image, usng the non-contact rotating Scheimpflug camera?
2 seconds
What is the most commonly used Scheimplug imaging device?
the Pentacam (oculus)
What is In vivo confocal microscopy?
non-invasive technique that images the cornea and conjunctiva in vivo
What are 3 indications for in vivo confocal microscopy?
- diagnosis of infectious keratitis (e.g. fungal elements and Acanthamoeba cysts)
- detection of corneal pathology, including dystrophies, degenerations, deposits, and infections (e.g. fungi elements and Acanthamobea)
- Post-surgical analysis (e.g. refractive surgery, collagen cross-linking, filtering blebs post-trabeculectomy)
What are 4 types of corneal pathology that in vivo confocal microscopy can detect?
- dystrophies
- degenerations
- deposits
- infections e.g. fungi and acanthamoeba
What are 4 types of surgery after which in vivo confocal microscopy may be used for pos-surgical analysis?
- refractive surgery
- collagen cross-linking
- filtering blebs post-trabeculectomy
What is the principle of IVCM?
pinhole apertures are used to focus point source of light on tissue and collect light waves reflected specifically from this point - apertures are conjugate to the focal plane (=confocal)
How can many points be simultaneously examined with IVCM?
array of apertures used to examine many points at once
How is IVCM used to create 2D and 3D appreciation of cornea?
array of apertures is scanned rapidly across the field to create 2D corneal image. scans down through conrnea creating optical sections, allow visualisation of microstructure at various depths
What device is the most commonly used laser scanning confocal microscope?
HRT Rostock Cornea Module (Heidelberg)
What are 6 indications for anterior segment optical coherence tomography (OCT)?
- direct visualisation of AC angle - glaucoma assessment
- measurement of relative corneal epithelial tickness + depth of anterior stromal scars
- post op assessment of lamellar corneal graft positioning and thickness
- assessment of post-LASIK flap thickness
- measurement of AC depth prior to insertion of phakic IOLs
- imaging of glaucoma filtering blebs
What are 2 types of glaucoma in which anterior segment OCT is particularly useful for AC angle assessment?
- angle-closure glaucoma
- plateau iris configuration
What are the basic principles of anterior segment OCT?
OCT uses interferometry to provide cross-sectional images of cornea and anterior segment
How does anterior segment OCT compare with posterior segment OCT?
longer wavelength light sources are used, commonly 1310nm vs 800nm
What type of OCT devices are commonly used for anterior segment OCT?
time domain OCT devices e.g. Visante (Zeiss)
What type of OCT devices provides greatly increased image acquisition speed for better coverage of the corneal surface and improved resolution, and what are 3 examples?
Spectral domain (SD) OCT devices e.g. RTVue, Optovue, Spectralis
What are 2 examples of slit-lamp adapted OCT devices?
SL SCAN-1 (Topcon) + SL-OCT (Heidelberg)
