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0V2022 Introduction to eye diseases > imaging the posterior pole > Flashcards

imaging the posterior pole Flashcards

1
Q

what are the methods of imaging the posterior pole?

A 
1- optical coherence tomography(OCT)
2- confocal scanning laser ophthalmoscopy (cSLO)
. fundus autoflurescence
. fluroscein angiography (FA)
. indocyanine green (ICG)
3- ultrasound (A-or B-scan)
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2
Q

what does the OCT allow you to do?

A

. image the retinal layers as a cross section

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3
Q

how does the OCT work?

A

. based on non-invasive imaging technology
. based on low-coherence interferometry - where by we use 830nm light beam (infra-red) directed at a target area
. magnitude and relative location of the reflected light used to produce images
. image based on optical properties of the microstructure of the tissue
. each imaged point generates an axial A-scan
- the more A-scans, the higher the resolution
. the scanning beam moves across the tissue and a cross section image is created = tomogram = B-scan
. a 3-D cube (c-scan) is a collection of tightly packed B-scans

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4
Q

what are the different ways to measure reflected light?

A

1 . time domain OCT(TD-OCT) uses a moving mirror to measure time taken for light to be reflected

  • 400 A-scans per second
  • grainy image
  1. spectral domain OCT(SD-OCT) measures multiple wavelengths of reflected light across a spectrum
    - 40,000 A-scans per second
    - higher quality image
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5
Q

how to relate retinal histology to an OCT image ?

A

. each retinal layer has a different reflectance for the 830nm diode laser and so it is possible to discriminate the retinal layer and location of any pathology

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6
Q

what are the retinal layers?

A 
. nerve fibre layer
. ganglion cell layer
. inner plexiform layer
. inner nuclear layer
. outer plexiform layer
. outer nuclear layer
. inner segments 
. outer segments
. pigment epithelium 
. choroid
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7
Q

what is the structure of the choroid ?

A

. choroid does not have the same regular structure as the retina
. the choroid is highly fenestrated , has many blood vessels

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8
Q

how to know what is a left eye from the OCT image?

A

. if the nerve fibre layer is thickest on the left side, means optic disc is on the left
. this means that it’s left eye

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9
Q

what is a C-scan?

A

. collection of B-scans that been put together

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10
Q

what quantitative data does the OCT allow us to take?

A

. the OCT also allows us some quantitative data

. takes measurements of the retinal thickness- this allows us to make comparisons with normative data

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11
Q

what is the nerve fibre layer assessment ?

A

. OCT can give us an assessment of the nerve fibre layer

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12
Q

how is the anterior chamber imaging carried out in an OCT?

A

. using a separate detachment, we can image the anterior chamber using the OCT
. this allows us to image the scleral spur and look at drainage angle

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13
Q

what are the OCT at city?

A
  1. spectralis OCT, heidelberg
    - with FAF
    - no fundus camera
  2. Topcon OCT
    - no FAF
    - with fundus camera
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14
Q

where to begin with OCT image analysis?

A
  1. look at overall profile e.g.foveal dip
  2. look for separation of the retinal layers
  3. consider which layer of the retina is affected
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15
Q

what is confocal scanning laser ophthalmoscope (cSLO)?

A

. ophthalmic imaging technology
. laser light instead of bright white light to illuminate retina
. light shone on object and reflected light captured through pinhole
- allows light reflected from focal plane
- blocks scattered light
- focused, high contrast images

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16
Q

what are the lasers used with cSLO?

A
  1. 830nm infrared
    - see through ‘haze/bleeding’
    - identification of retinal oedema/pigmentary changes
  2. 795nm diode laser
    - indocyanine green angiography (ICG)- useful for viewing choroid
    - looks at choroidal structure - choroidal neovacularisation (CNV)
  3. 514 nm argon with 500nm barrier filter
    - imaging of superficial retinal layers
    - green reflectance image at 514nm (red-free)
  4. 488nm diode laser
    - fundus autofluorescene and FA ‘retina’
    - excites lipfuscin
    - looks for ‘health’ of RPE
17
Q

what is fundus autofluorescence (FAF)?

A
  • used to assess RPE health
  • lipofuscin is a byproduct of cell function
  • accumulates in RPE with age/disease
  • eventually leads to cell death
  • blue light at 488nm causes lipofuscin to autofluoresce
  • black and white image is created
  • dark spots=optic nerve head, blood vessels and macula as blue light absorbed by macular pigment
18
Q

what does hyper-autofluorescence mean?

A

hyper-autofluorescence is a sign of increased lipofuscin accumulation
- may indicate degnerative changes or oxidative injury

19
Q

what does hypo-autofluroscence indicate?

A
  • dark areas or hypo-autofluorescence indicate missing or dead RPE cells
20
Q

what is fluorescein angiography ?

A

. fluorescein sodium dye injected as a bolus into a peripheral vein

  • dye metabolized by kidneys and eliminated via urine within 24 to 36 hrs
  • fluorescein excites at 490nm and emits at 530 nm
  • specialized fundus camera or cSLO to capture rapid-sequence photographs of retina
  • argon laser (488nm) to excite with barrier filter to allow only 530nm through
  • photographs or video images are taken as dye passes through blood vessels in eye
21
Q

what are the phases of FA?

A

1- choroidal
. 10 sec after post injection

2- early arteriovenous
. 1 sec after choroid

3- venous
. 20-25 secs

4- late phase
. no fluorescein after 10 minutes

22
Q

what is fluorescein angiography used for ?

A
  • used to demonstrate abnormalities within neurosensory retina, RPE, sclera, choroid and optic nerve
  • used for clinical diagnosis of retinal or choroidal vascular diseases
    . diabetic retinopathy, age-related macular degenration, hypertensive retinopathy and vascular occlusions
  • used to
    . determine extent of damage
    . develop a treatment plan
    . to monitor the results of treatment
23
Q

how is FA used in diabetic retinopathy ?

A

. identify extent of ischaemia
. location of microaneurysms
. presence of neovascularization
. extent of macular oedema

24
Q

how is FA used in AMD?

A

. identify the presence of location and characteristic feature of choroidal neovascularization

25
Q

how to interpret FA?

A
  • we look for areas of hypofluorescence

this can indicate

  1. blockage
    - haemmorrage - sub-, intra or pre-retinal
    - inflammatory cells, melanin, exudates
  2. impaired vascular filing
    - we look for areas of hyperfluorescence- this can be due to
  3. leakage
    e. g. NVM
  4. transition defect
    e. g. AMD
26
Q

what are side effects of FA?

A
  • occurs in 5-10% px
  • skin/urine discolourations
  • reaction at injection site
  • nausea- very common
  • syncope - fainting due to stress
  • anaphylactic shock (1:2000)
  • death - 1:220,000
27
Q

what is indocyanine green?

A
  • imaging of choroid
  • 790 nm IR laser with 835nm barrier filter
  • ICG is larger molecule so stays in choroid
  • used to diagnose certain types of AMD
  • preferred if using laser photocoagulation to identify leaky vessels
  • safer than FA
  • nausea less frequent
  • ICG contains iodine - which may be contra-indicated in px with allergic or liver disease
28
Q

what is the use of ultrasound B-scan?

A 
uses
. high media opacities
. retinal detachment 
. tumour size
. intra-ocular foreign body

0V2022 Introduction to eye diseases (22 decks)

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