Basic Interpretation of OCT angiography
Basic Interpretation of OCT angiography
Introduction:
OCTA is a non-invasive, non-dye imaging system that has
ability to recognize both structural and blood flow information within retina.
Concept of OCTA is that it employs motion contrast imaging
resolution volumetric blood flow information which helps in generating angiographic images
.
Use of SSADA( Split spectrum amplitude decorrelation
angiography) algorithm helps to compare decorrelation signal between segmental
OCT B scan taken at precisely same cross section in order to construct a map of
blood flow.
As retina has capillaries in various layers allows retinal
segmentation into specific layers.
4 slabs which has been arbitrarily divided :
1. Superficial retinal capillary plexus: capillary
network in ganglion cell layer. Between ILM to inner plexiform layer
2. Deep retinal capillary plexus: capillary network
between inner plexiform and outerplexiform layer
3. Outer retina : Between outerplexiform layer to retinal
pigmen epithelium
It should be avascular, but flow projection artifacts from the inner
retina can be seen.
4. Choriocapillaries
denotes network of choroicapillaries below RPE and bruchs membrane
Normal OCT Angio: :
1.
Superficial
retinal capillary plexus
Well
arranged network of superficial capillaries with normal retinal circulation and
circular Foveal avascular zone
2.
Deep
retinal capillary plexus
|
Foveal avascular zone
|
|
Fine network of capillaries
|
|
Projection artifact
|
Around the
avascular area (FAZ), the capillaries form continuous peri-foveal arcades with
regular meshes. Note normal FAZ is slightly larger in deep capillary plexus
than superficial capillary plexus
3.
Outer
retinal layer
The outer
retina slab shows flow projection artifacts cast by flowing blood in the inner
retinal vessels onto the RPE.
4.
Choriocapillaries
Dense homogenous
layer of choriocapillaries . Foveal avascular zone canno be seen in
choriocapillaries and outer retinal layer. .
Interpretation of OCT angio can be done through looking these
following points:
1. Superficial retinal capillary plexus
-
Density of capillaries:
Areas
of capillary dropout appears as hyporeflective compared to surrounding areas
EX:
Vein occlusion, Diabetes, PFT
|
Areas of capillary dropout
|
-
Vessel architecture
Microaneuryms : dilated hyperintense
outpouching seen in vein occlusion and diabetes
Telengiectasia: irregular
hyperintense dilatation and tortuosity of capillaries
-
FAZ
-
normal FAZ in superficial capillary plexus is smaller
than in deep capillary plexus.
Enlargement
of foveal avascular zone seen in ishemic macular edema and vein occlusion
-
New vessels; neovascularization can be seen as
irregular tuft of capillaries.
2. Deep
capillary retinal plexus:
-
Density of capillaries:
Capillary
dropout can be een as hypointense areas . Usually in various conditions like vein occlusion
and PFT capillary dropouts are more in deep capillary retinal plexus than superficial
capillary plexus.
Ex:
DM, Vein occlusion
-
Telengiectasia: these abnormal hyperintense telangiectasia
noted will be more compared to superficial capillary plexus.
Ex
PFT
-
FAZ: enlargement of foveal avascular zone
-
Cytic spaces:
can be seen in deep capillary plexus in
patients wih CME
3. Outer
retinal layer
-
Normally this layer should be avascular . but usually
artifacts from super and deep capillary retinal plexus will project into outer retinal
layer
-
In RAP abnormal vessels can be seen
-
In type 2 CNVM:
when vessels reach subretinal these will project into subretinal layer.
-
4. Choriocapillaries
-
At this layer we seen very fine homogenous vascular layer of choriocapillaries.
-
Loss of choriocapillaries
In
patients with inflammatory chorioretinopathies, loss of choriocapillaries can
be seen.
drusen
-
CNVM: well defined vessels which are fine in early
CNVM but then large vessels will be seen which in larger and mature
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