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New Retinal Technologies Joseph Sowka, OD, FAAO, Diplomate Optical Coherence Tomography (OCT) • Noninvasive, noncontact, transpupillary imaging technology • Analogous to ultrasound B-wave imaging or radar except light is used instead of acoustic or radio waves • Can image retinal structures in vivo with a resolution of 10 to 17µ • Cross-sectional images of the retina are produced using the optical backscattering of light in a fashion analogous to B- scan ultrasonography • The anatomic layers within the retina can be differentiated and retinal thickness can be measured. Principles of OCT • Uses a super luminescent diode as a light source • Coupled to fiber optic delivery system • Same fiber optic delivery is used to return the signal to detector • Signal sent to “demodulator” • The analogue image converted to digital -> then is sent computer • Uses low coherence (white light) interferometry to perform high resolution imaging • Low coherence light allows for a propagation speed nearly one million times faster then sound • Allows for ultra-fast echo delay time resolution (tens of femtoseconds) OCT vs Ultrasound OCT • Uses low coherent light instead of sound waves • Does not require direct contact on cornea • Light allows higher spatial resolution • 10-20X higher resolution vs. echo • Light allows for better imaging of fine structures • Speed allows for more precision • Only able to use on tissues optically accessible • Dense cataract, VH Ultrasound • Uses sound waves • Requires direct contact on the cornea of the imaging device • Resolution depends on frequency or wavelength of the sound waves (10 MHz) • Sound waves utilize time delay • Sound waves are million times slower then light • Ideal for opaque media 1 • Allows for tissue differentiation Principles of OCT • OCT computes tomographic images based on the amount of incident light which is reflected for a given tissue (or indices =n) • OCT is particularly sensitive to light that has only one back-scattering event • Light returns without more scattering or absorbing events • This property makes the OCT highly precise Retinal Examination by OCT • Similar to using 78 D lens • Mounted to Slit lamp delivery system • OCT beam is directed to eye via fiber optic sampling arm using 2 galvanometer-driven mirrors • Allows for scanning the beam of light across the retina – image time 2.5 sec • Allows for direct visualization of the fundus • Via visible light or IR Videoscopy • • • Provides Cross-sectional images of retinal structures • Allows clinical correlation • Better anatomic perspective • Supplements other diagnostic testing Provided a better understanding of vitreo-macular interactions • Related diseases Instrumental in understanding new diseases Clinical Pearl: The most common program used for retinal evaluation by the OCT is the Fast Macula program. Optos Panoramic200 • Panoramic200 • Red and green wavelength lasers • 30,000 rpm spinning polygon • Virtual scanning point just behind the pupil in the anterior lens • Since the DR is in the sensory retina, the green laser is best at detecting it and it is even better on the gray scale representation. Red laser image may show choroidal changes in possible choroidopathy. 2
