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

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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.

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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.

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-         CNVM: well defined vessels which are fine in early CNVM but then large vessels will be seen which in larger and mature