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Oct angiogram
1. OCT Angiogram
Dr Md Afzal Mahfuzullah
MCPS.FCPS.
Fellowship in Vitreo Retina
Assistant Professor
Department of Ophthalmology
Bangabandhu Sheikh Mujib Medical University
2. OCT A
OCT-Angiography is a new, non-invasive diagnostic method through
which the vascular structures of the retina and choroid may be
visualized in three dimensions without the need for contrast agent
injection
3. Through acquisition software and more advanced hardware,
OCTAngiography enables imaging of the retinal vascular flow.
4. Principle
• OCT-Angiography is based on the principle of diffractive particle
movement detection, such as red blood cells,
• on sequential OCT B-scans performed repeatedly at the same retina
location, therefore showing the presence of blood vessels.
5. Principle Cont
• The method is based on differences between the B-scans to generate a
movement-related contrast.
• Specially a contrast related to erythrocyte movement in the vascular
system.
6. Principle Cont
• To generate the image of the retinal microvascularization, each B-scan
of the examination pattern is consecutively repeated several times.
• The contrast comparisons on consecutive B-scans at the same location
reveal some areas with a contrast change over time and some areas
with a constant contrast.
7. Principle Cont
• The temporal change in contrast in a specific location is attributed to
the movement of erythrocytes, which therefore indicates the location
of the vessels.
8. Scan architecture
• As every OCTA obtained is essentially a cube scan, it is a three-
dimensional (3D) assessment of the retinal vasculature unlike
traditional fluorescein or ICG angiography, which is two dimensional.
• Evaluate the scans from the inner retinal surface right down to the
choroid in a continuous manner.
9. Scan architecture,cont
• OCTA machines have taken the cube and split it into slabs to reflect a
known anatomic layer of the retinal vasculature, referred to as
autoseg-mentation.
10. Software slabs
• The AngioVuesoftware on the Optovue OCTA splits the volume cube
up into the following four slabs:
• 1. Inner retinal slab extends from 3 μm below the internal limiting
membrane to 15 μm below the inner plexiform layer. This incorporates
the known anatomic location of the superficial retinal vascular plexus,
which is generally what we see on traditional FA .
11. Software slabs
• Middle retinal slab extends from 15 μm below the inner plexiform
layer to 70 μm below the inner plexiform layer and incorporates the
known location of the deep retinal capillary plexus.
• This plexus is poorly seen on traditional FA and beautifully seen on
OCTA.
12. Software slabs
• Outer retinal slab extends from 70 μm below the inner plexiform layer
to 30 μm below the retinal pigment epithelium (RPE) reference line.
• This region anatomically corresponds to a part of the retina within
which there is NEVER any vasculature in a normal individual.
• This slab can be very useful to identify type 2 (subretinal)
neovascular membranes.
13. Software slabs
• Choriocapillaris extends from 30 μm below the RPE reference to 60
μm below the RPE reference.
• It incorporates the choriocapillaris and allows detection of early type
1 (sub-RPE) choroidal neovascular membranes.
14. Software slabs
• FOR the evaluation of some CNVMs, manual manipulation of the
boundaries of the slab to be visualized is best to truly view the extent
and nature of the CNVM complex.
15. Optic nerve head evaluation
• Optic nerve head and peripapillary retina.
• These scans feature an autosegmentation (four zones)
• 1. The optic nerve head.
• 2. Above the optic nerve head or vitreous, which can be used to
evaluate for the presence of new vessels of the disc or a vascularized
hyaloid artery.
16. Optic nerve head evaluation,Cont
• 3. Radial peripapillary capillaries, which can be evaluated for ischemia
and its potential role in glaucoma. These capillaries are seen in
exquisite detail on OCTA, while poorly viewed on traditional
fluorescein and ICG angiography.
• 4. Choroid/lamina cribrosa.
17. Terminologies
• Amplitude/Magnitude/Intensity Variance -These refer to methods
which detect motion or flow by looking for change in the OCT signal
over time as measured by the variance or decorrelation of signal
amplitude, intensity, or their log transforms.
• Phase Variance -Phase variance uses changes in the phase of the OCT
signal as the means of detecting flow.
•
18. Terminologies,cont
• Split-spectrum Amplitude-decorrelation Angiography (SSADA) is an
efficient algorithm which improves the signal-to-noise ratio of flow
detection by maximizing the extraction of flow information from
speckle variation.
• This is achieved by splitting the OCT spectrum, which increases the
number of usable image frames and reduces noise from axial bulk
motion.
19. Analysis in OCTA
• Segmentation:
• OCT angiography produces volumetric flow information.
• To allow for rapid identification and interpretation of pathological
vascular features, segmentation of key anatomic layers is required .
• En Face Projection: En face projection produces two-dimensional
(2D) views of segmented tissue layers
• Slabs and Slices : These refer to the tissue volume used for en face
projection.
• Slabs refer to thick tissue sections such as the inner retina or outer
retina, whereas slices refer to thin sections of a few microns used to
examine fine details.
20. Signal analysis
• Nonvascular Flow Signal
• In OCT angiography, background bulk tissue motion can generally
be subtracted because it is associated with a uniform decorrelation.
• However, the decorrelation signal in some very highly
backscattering structures can still rise above the background in some
instances.
• These include the RPE, hard exudates, regions of pigment
accumulation, thrombosed aneurysms, and retinal hemorrhages.
21. Signal analysis
• Flow Projection Artifact
• Shadowgraphic flow projection artifacts are the result of fluctuating
shadows cast by flowing blood in a superficial vascular bed that cause
variation of the OCT signal in deeper layers.
22. Signal analysis,Cont
• Flow Index and Vessel Density
• 1. The flow index is calculated as the average decorrelation value
(which is correlated with flow velocity) in the selected region.
• 2. The vessel density is calculated as the percentage area occupied by
vessels in the selected region.
• Avascular area is a significant area (larger than the normal gap
between capillaries) devoid of flow signal on an en face angiogram.
23. Signal analysis,Cont
• Nonperfusion (Capillary Dropout) Area
• Nonperfusion area refers to an avascular area that should
normally be vascular.
• For example, on an OCT angiogram of the macula, any retinal
avascular area outside of the FAZ considered retinal nonperfusion
(capillary dropout) area.
24. Signal analysis,Cont
• Neovascularization Area
• Neovascularization area is the sum of pixel areas in a pathologic
neovascular net identified on an en face OCT angiogram.
• In proliferative diabetic retinopathy, the area is of vessel growth
above the ILM.
• In age-related macular degeneration, the area is of
neovascularization in the outer retina
25. FFA / ICGA vs OCTA
• Scans can be acquired in a few seconds and does not require
intravenous injection
• Fluorescein or ICG angiography requires multiple image frames
taken over several minutes and can cause nausea, vomiting and rarely
anaphylaxis.
• The fast and noninvasive nature of OCT angiography also means
that follow-up scans can be conducted more frequently
26. FFA / ICGA vs OCTA,Cont
• Dye leakage in fluorescein angiography is the hallmark of important
vascular abnormalities such as neovascularization and
microaneurysms.
• OCT angiography does not employ a dye and cannot evaluate
leakage.
• OCT angiography detects vascular abnormalities by other methods
based on depth and vascular pattern
27. Additional advantage in OCTA
• Dye leakage and staining do not occur in OCT angiography, the
boundaries, and therefore areas, of capillary dropout and
neovascularization can be more precisely measured.
• Conventional angiography is two- dimensional, which makes it
difficult to distinguish vascular abnormalities within different layers.
• The 3D nature of OCT angiography allows for separate evaluation
of abnormalities in the retinal and choroidal circulations.
28. OCTA in Clinical Use
• ARMD
• CNVM
• Diabetic Retinopathy
• Arterial and Venous occlusive disease
• CSCR
• Mac Tel 2
• Glaucoma
• Anterior Segment ( Cornea, Conjunctiva, Iris )
30. OCTA for Anterior Segment Vasculature
• The ocular surface and iris vasculature are not easily accessible, and
fluorescein angiography is rarely performed for such evaluations and cannot
be easily repeated.
• OCTA can be repeated over time as often as needed.
• Incredible potential for following disease evolution and monitoring
treatment efficacy.
• Corneal neovascularization is a potentially severe complication in various
corneal diseases and a high-risk factor for corneal rejection following
keratoplasty.
31. The conjunctiva assessment
• The tumor development
• Bleb formation after glaucoma surgery
• Early detection of iris neovascularization is also a major goal when
monitoring ischemic diseases of the retina
32. Technique and difficulty
• To obtain a scan of the anterior segment in the AngioVue OCTA system
,the anterior segment optical adaptor lens (L-CAM) is used.
• A specific anterior module (angiocornea) is used to perform anterior
segment scans.
• Anterior segment OCTA does not tolerate any eye movement of the
patient because even micromovements create transverse artifacts on the
final images.
• Scans cannot be performed when patients are unable to fixate, or have
continuous eye or eyelid movements such as nystagmus or symptoms
causing abnormal blinking rate or blepharospasm.
33. OCTA for Conjunctival Vessel Assessment:
• Application in Glaucoma Surgery
• OCTA is also helpful for documenting the vascular patterns in
conjunctival inflammation or wound healing
• Monitoring bleb formation and evaluating proper functioning of the
filtering bleb.
• Investigative tool to study the conjunctival and episcleral vasculature
changes after trabeculectomy
• Postoperative: the vasculature alterations include much higher vascular
density, dilated and tortuous vessels, and vascular anastomoses.
34. Iris Vessels evaluation
• OCTA of the iris appears to be able to demonstrate vessels difficult to
photograph or to clinically observe by slit-lamp examination
• The iris angiograms show radial iris vessel patterns in normal
lightcolored Eyes.
• In darker iris, the pigment pro- duces shadowing and artifacts that
obscure the vasculature .
35. Bleb evaluation in post trab
• Post-mitomycin C ischemic blebs
• OCTA shows avascular zones.
• Avascular spaces between dense vascular networks may reflect the
presence of aqueous humor and are therefore indicating proper wound
healing and bleb formation.
• Absence of free vessel intervals and increased vessel density may reflect
inflammatory states and early stages of bleb scarring and loss of
functionality
36. Total Retinal Blood Flow
• Fourier-domain OCT utilizes optical phase information to precisely
measure Doppler velocity.
• It measures the axial flow velocity, which is the velocity component in
the direction of the OCT probe beam.
• It provides quantitative measurement of high flow velocities in the retinal
vessels of the optic disc, and is done by scanning multiple con- centric
circles around the optic disc.
37. Total Retinal Blood Flow,Cont
• This technique has been used to investigate TRBF in several ocular
diseases, including DR, retinal vein occlusions, uveitis, and glaucoma.
• In eyes with vein occlusions, the TRBF was reduced in the eye with the
vascular occlusion, when compared to both the fellow eye and the normal
age-matched eyes.
38.
39. Conclusion
• OCTA has rapidly expanded as an imaging modality that has been used to
qualitatively and quantitatively describe changes in retinal and choroidal
vasculature associated pathology.
• It also has the potential to enhance our understanding of the disease
mechanism, since microvascular changes can be correlated to structural
features.
• Currently, OCTA is widely used in the clinical setting to guide treatment
or diagnosis decisions.
