imaging the posterior pole Flashcards
what are the methods of imaging the posterior pole?
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)
what does the OCT allow you to do?
. image the retinal layers as a cross section
how does the OCT work?
. 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
what are the different ways to measure reflected light?
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
- spectral domain OCT(SD-OCT) measures multiple wavelengths of reflected light across a spectrum
- 40,000 A-scans per second
- higher quality image
how to relate retinal histology to an OCT image ?
. 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
what are the retinal layers?
. 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
what is the structure of the choroid ?
. choroid does not have the same regular structure as the retina
. the choroid is highly fenestrated , has many blood vessels
how to know what is a left eye from the OCT image?
. 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
what is a C-scan?
. collection of B-scans that been put together
what quantitative data does the OCT allow us to take?
. the OCT also allows us some quantitative data
. takes measurements of the retinal thickness- this allows us to make comparisons with normative data
what is the nerve fibre layer assessment ?
. OCT can give us an assessment of the nerve fibre layer
how is the anterior chamber imaging carried out in an OCT?
. 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
what are the OCT at city?
- spectralis OCT, heidelberg
- with FAF
- no fundus camera - Topcon OCT
- no FAF
- with fundus camera
where to begin with OCT image analysis?
- look at overall profile e.g.foveal dip
- look for separation of the retinal layers
- consider which layer of the retina is affected
what is confocal scanning laser ophthalmoscope (cSLO)?
. 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
what are the lasers used with cSLO?
- 830nm infrared
- see through ‘haze/bleeding’
- identification of retinal oedema/pigmentary changes - 795nm diode laser
- indocyanine green angiography (ICG)- useful for viewing choroid
- looks at choroidal structure - choroidal neovacularisation (CNV) - 514 nm argon with 500nm barrier filter
- imaging of superficial retinal layers
- green reflectance image at 514nm (red-free) - 488nm diode laser
- fundus autofluorescene and FA ‘retina’
- excites lipfuscin
- looks for ‘health’ of RPE
what is fundus autofluorescence (FAF)?
- 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
what does hyper-autofluorescence mean?
hyper-autofluorescence is a sign of increased lipofuscin accumulation
- may indicate degnerative changes or oxidative injury
what does hypo-autofluroscence indicate?
- dark areas or hypo-autofluorescence indicate missing or dead RPE cells
what is fluorescein angiography ?
. 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
what are the phases of FA?
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
what is fluorescein angiography used for ?
- 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
how is FA used in diabetic retinopathy ?
. identify extent of ischaemia
. location of microaneurysms
. presence of neovascularization
. extent of macular oedema
how is FA used in AMD?
. identify the presence of location and characteristic feature of choroidal neovascularization
how to interpret FA?
- we look for areas of hypofluorescence
this can indicate
- blockage
- haemmorrage - sub-, intra or pre-retinal
- inflammatory cells, melanin, exudates - impaired vascular filing
- we look for areas of hyperfluorescence- this can be due to - leakage
e. g. NVM - transition defect
e. g. AMD
what are side effects of FA?
- 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
what is indocyanine green?
- 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
what is the use of ultrasound B-scan?
uses . high media opacities . retinal detachment . tumour size . intra-ocular foreign body
