Day 7(4): Visualizing the Ocular Fundus Flashcards

Question

What layers are present in the fovea?

Answer 1

ABSENT: innermost layers: NFL, GCL, IPL, INL PRESENT: (LIGHT) Henle's Layer/OPL: high conc. of CONES (DARK) ONL (LIGHT) ELM (DARK) Myoid Zone: inner segment (LIGHT) Ellipsoid Zone: junction of inner & outer segment (DARK) Outer Segment (LIGHT) Interdigitation Zone: junction of outer segment and RPE (DARK + LIGHT) Retinal Pigment Epithelium

Answer 2

- Imaging technique that uses low-coherence light to capture micron-resolution, 2D and 3D images from within an optical media - Multiple A scans fired in rapid succession - Relies on optical differences of tissues - Tomography: generating a 2D image of a section through a 3D object using a penetrating wave - Based on low-coherence interferometry using: 1. Michelson interferometer 2. Near-infrared light: wavelength ~ 800 nm - Long wavelength light: allows penetration into the scattering medium - Analogue: B-Mode Ultrasound + But uses light instead of sound + Light travels faster permitting > 100X greater resolution How does it work? 1. Light source is directed towards a beam splitter which splits beam into two. 2. One beam directed into a reference mirror while the other is directed into the retina. 3. Reflections of both light waves are received by an interferometer which superimposes the two to form an interference pattern. 4. Interference pattern is analyzed by a detector and forms a 2D or 3D image of the retina.

Answer 3

Ultrasound - sound is slower - image is blurred similar to a photo of a moving subject or with slow shutter speeds - based on reflection of sound waves (10^7 Hz) - (+) echo delay - needs direct contact for propagation - poor resolution: 150 um OCT - light travels faster - faster shutter speed - based on reflection of low-coherence light (near infrared: 800 nm ~ 10^14 Hz) - direct contact is NOT necessary - high resolution: < 10 um

Answer 4

- Technique which uses the interference of superimposed waves to extract information - Light from a single source is split into two beams that travel in different optical paths, which are then combined again to produce interference - The resulting interference fringes give information about the difference in optical path lengths Michelson interferometer - Usually used in OCT - 1st interferometer arm: focused onto the tissue sample - 2nd interferometer arm: bounced off a reference mirror - Reflected light from the tissue sample is combined with reflected light from the reference mirror - Low coherence light: interferometric signal observed only at a limited depth - Records one thin optical slice of the sample at a time - By performing multiple scans, moving the reference mirror between each scan, an entire three-dimensional image of the tissue can be reconstructed

Answer 5

1. Time Domain OCT: lowest resolution 2. Spectral Domain OCT 3. Swept Source OCT: highest resolution similar to a histologic section even at the fovea

Answer 6

- Light source: broad band of light from a super luminescent diode - Reference mirror: moves back and forth - Image: 10 um (grainy) - Disadvantage: + Mirror movement and speed determines image quality and resolution (TIME-domain) + Speed of movement is limited thus image is poorer in quality

Answer 7

- Light source: broad band of light from a super luminescent diode (similar to TD-OCT) - Reference mirror: FIXED - Utilizes the Fourier analysis: converts a signal from its original domain (often time) to a representation in the frequency domain - Frequency domain: analysis of signals with respect to frequency, rather than time. - Image: 5-7 um (better resolution than TD-OCT) - Added components: 1. Diffraction Grating Detector: - receives interference pattern from the interferometer - separates the light wave into its component wavelengths 2. Spectrophotometer or Spectrum Analyzer - receives light from the DGD - analyzes spectrum of light and forms a spectral interferogram - processed interferogram subjected to fast Fourier transform to form the final image

Answer 8

- Light source: Swept-Source Laser + Type of laser in which the output wavelength is adjustable over a wide range of wavelengths + Starts with a particular wavelength of light and "sweeps" across the spectrum of light with a range of wavelengths + Capable to producing narrow wavelength or coherent light - Reference mirror: FIXED (similar to SD-OCT) - Image: < 5 um (best resolution similar to a histologic section) - Deeper penetration: imaging even the choroid and sclera - Added components: 1. Balanced Detector - receives interference pattern from the interferometer - diffraction grating unnecessary because wavelengths of light has already been separated at the source 2. Spectrophotometer or Spectrum Analyzer - similar to a SD-OCT - receives light from the BD - analyzes spectrum of light and forms a spectral interferogram - processed interferogram subjected to fast Fourier transform to form the final image

Answer 9

TD-OCT Light source: 820 nm Detector: Single Axial resolution: 10 um Scanning speed: 400 A-scans per second (slow) Scanning depth: 2 mm Retinal thickness: ILM to IS/OS (Ellipsoid Zone) Speed of data acquisition: Slower than eye movement SD-OCT Light source: 840 nm (broader bandwidth) Detector: Spectrophotometer + FFT Axial resolution: 5-7 um Scanning speed: 25,600 A scans per second (18,000 - 40,000) Scanning depth: 2 mm Retinal thickness: ILM to RPE Speed of data acquisition: Faster than eye movement SS-OCT Light source: 1050 nm (range of wavelengths using a swept laser) Detector: Spectrophotometer + FFT Axial resolution: < 5 um (best resolution) Scanning speed: 100,000 A scans per second Scanning depth: 2.6 mm (deeper penetration) Retinal thickness: ILM to choroid, sclera, LC Speed of data acquisition: Faster than eye movement

Answer 10

- Principle of REFLECTIVITY: proportion of incident light directly backscattered by tissues HIGH: reflects back ALL light - Color: White, Red - E.g.: NFL, RPE, silicone oil-retina interface, dense scar tissue MODERATELY HIGH: reflects back MAJORITY of light - Color: Orange, Yellow - E.g.: Plexiform layers (IPL, OPL) and photoreceptor layer (yellow to green), scar tissue, choroidal neovascularization, hemorrhages MODERATE: reflected and scattered light in equal proportion - Color: Green - E.g.: Plexiform layers (IPL, OPL) and photoreceptor layer (yellow to green), vitreous bands MODERATELY LOW: absorbs MAJORITY of light - Color: Blue - E.g.: Nuclear layers (GCL, INL, ONL), choroid (green to blue), vitreous debris LOW: absorbs or transmits ALL light - Color: Violet, Black - E.g.: Vitreous gel, Aqueous humor, Sclera, silicone oil, cysts

Answer 11

- Imaging of the microvasculature of the retina and choroid - Uses Fourier domain (SD or SS) OCT: high imaging speed enables measurement of blood flow - Uses the reflective properties of the surface of moving RBCs accurately outline vessels - Able to distinguish slow flow in small blood vessels from biological motion in extravascular tissue - Examples: 1. Split-Spectrum Amplitude Decorrelation Angiography (SSADA) 2. Phase Variance OCT (pvOCT)

Answer 12

1. Scan Information 2. En face image: image of the surface of the fundus 3. Topographic map with reflectivity scale: color-coded topography of the retinal surface 4. En face with topographic overlay: combines 2 and 3 5. Retinal thickness map - Normal: GREEN; within 90% of normal population (5 - 95%) - Thickened: + PINK: upper 1% (> 99%); extremely thickened + PALE YELLOW: upper 4% (95 - 99%); moderately thickened - Thinned: + BRIGHT YELLOW: lower 4% (1 - 5%); moderately thinned + RED: lower 1% (< 1%); extremely thinned 6. B-scan Images: combination forms a 3D image - Horizontal + imaged at the 180 degree meridian + see NASAL and TEMPORAL retina - Vertical: + imaged at the 90 degree meridian + see SUPERIOR and INFERIOR retina 7. Segmentation Layers: to assess extent or depth of lesions

Answer 13

1. Assess centration of the scan on the fovea. - presence of foveal depression - absence of INNER retina layers in the fovea: NFL, GCL, IPL, INL - tapering of retinal layers approaching the fovea + appearance: hourglass or bowtie + NEVER lost even in pathologic conditions 2. Assess retinal topography and thickness 3. Evaluate integrity of layers in the B-scan image 4. Describe macular and retinal pathologies if present - Parameters: location, size, depth of involvement -E.g. Cysts, Elevations, Depression

Answer 14

1. Age-Related Macular Degeneration - intraretinal cysts: cystic changes in the macula (hyporeflective) - neovascularization: thickening of choroid (hyperreflective) - drusen: irregularities in the RPE surface (hyperreflective) 2. Macular Edema - intrarenal exudates: fluid collection in between the layers of the NSR in the macular/foveal area - intraretinal cysts 3. Central Serous Retinopathy - fluid collection between NSR and RPE in the macular/foveal area - detachment of NSR from RPE 4. Vitreo-Macular Traction - traction of the vitreous on the fovea causing detachment of NSR layers from the RPE - end-point: macular hole 5. Macular Hole - full-thickness: complete absence of all layers; only RPE is present - lamellar/partial-thickness: partial defect with some layers present 6. Retinal Detachment - detachment of NSR from RPE in the peripheral retina - Rhegmatogenous: due to retinal tear from traction in the vitreous base - Tractional: due to fibrovascular proliferation and scar formation - Exudative: due to fluid collection in the subretinal space, (-) tear 7. Pigment Epithelial Detachment - detachment of RPE from the choroid 8. Epiretinal Membrane - thick hyperreflective layer on the surface of the NSR - vs ILM: nondescript or a very thin hyperreflective line

Day 7(4): Visualizing the Ocular Fundus Flashcards

(38 cards)