What is OCT Angiography?
Optical coherence tomography (OCT) has become the standard imaging technique for the diagnosis and management of ocular diseases. It provides three-dimensional (3D) imaging with micrometer-scale depth resolution of the anterior segment, retina, and optic nerve head. Traditional OCT provides structural images which enhance the clinician’s ability to detect and monitor fluid exudation linked to vascular diseases. However, structural OCT is only sensitive to backscattered light intensity. Therefore it is not capable of detecting capillary dropout or neovascularization associated with some of the major causes of blindness.
In the past, these features have been assessed clinically with fluorescein (FA) or indocyanine green (ICG) angiography. Leakage of the dye on the angiogram was used to identify vascular abnormalities. However both FA and ICG techniques require intravenous dye injection, which can cause side effects such as nausea, and rarely, anaphylaxis. Furthermore, the cycle of dye injection and photography through the phases of dye transit is time consuming, requiring tens of minutes, and can only be done in all phases for one eye per injection. Furthermore, capillary contrast is highly variable and often obscure by dye leakage, requiring highly subjective interpretation by expert clinicians. Due to all these limitations, Fluorescein and ICG angiography are only used for critical diagnosis in sight-threatening complications of age related macular degeneration (AMD), diabetic retinopathy (DR) and other retinal vascular diseases. Because OCT angiography is noninvasive, it can be used for more routine screening examinations for early diagnosis of vascular abnormalities, and for routine follow-up examinations to evaluate the efficacy of treatments such as intravitreal injection of anti-angiogenic agents.
Use high speed Fourier domain OCT (FD-OCT)
Spectral Domain or Swept Source
Software upgrade of existing commerical ophthalmic OCT
Combined amplitude/phase based variance
No dye injection
Measures flow, not leakage
Measures Doppler shift or variations in speckle patterns in consecutive OCT cross sections taken at the same location.
What is SSADA?
Split-spectrum amplitude-decorrelation angiography, or SSADA, is an algorithm that detects motion in blood vessel by the variation in reflected OCT signal amplitude between consecutive cross-section scans. The novelty of SSADA lies in how the OCT signal is processed to enhance flow detection and reject axial bulk motion noise. Specifically, the algorithm splits the OCT image into narrow spectral bands, thereby increasing the number of cross-sectional image frames that could be used to compute variation (decorrelation) in the speckle pattern caused by the motion of blood cells. Each spectral band provides different speckle contrast from the interference of blood cells and surrounding structures, and therefore independent information on speckle variance due to blood flow. Averaging the decorrelation values from multiple spectral bands enhances the flow signal. Furthermore, the narrow spectral bands impose a lower axial resolution that is less susceptible to bulk tissue motion due axial eye pulsation. A 4 fold improvement in the signal-to-noise ratio of flow detection has been demonstrate using SSADA with 11-fold spectral split.*
Cross-sectional OCT angiograms combine color-coded flow information superimposed on gray-scale structural information. Therefore, both blood flow and retinal structural information are presented together. This is useful for clinical evaluation on the depth of abnormalities such retinal or choroidal neovascularization.
Since OCT angiography generates a data cube, segmentation and en face presentation of vascular perfusion at various layers of the retina can summarize the flow information at relevant anatomic layers or slabs. These images can be more easily interpreted by clinicians and aid in their ability to recognize abnormalities in vascular patterns.
SSADA is capable of flow detection both at the optic nerve head or the macula and quantifies the data as both flow index and vessel density. The flow index is the average decorrelation value and contains information on capillary flow velocity. Decorrelation values increases with flow velocity until a saturation point is reached in larger blood vessels. This means flow index contains information on vessel flow in addition to vessel density. Vessel density is defined as the percentage area occupied by flow pixels. These parameters have been used to study pathology in AMD, glaucoma and diabetic retinopathy.
Split-Spectrum Amplitude Decorrelation Angiography (SSADA)
*Gao, S.S., Liu, G., Huang, D., Jia, Y., (2015) Optimization of the split-spectrum amplitude-decorrelation angiography algorithm on a spectral optical coherence tomography system, Opt Letters 40, 2305-2308
Neovascular Age-Related Macular Degeneration
Top row shows an example of healthy eye and bottom row a glaucomatous eye. In comparison to the healthy eye (B), en face OCT angiogram of glaucomatous eye (F) shows reduced density of the peripapillary microvasculature network. Patches of nonperfusion in glaucoma correlated well with the locations of retinal nerve fiber layer thickness maps deficits (G) and visual field loss (H).
The type I CNV is identified by OCT angiography (C), but it is not well defined by fundus photography (A) or fluorescein angiography (B). The black square outlines the areas shown on the angiograms. The CNV area is shown on the en face color-coded OCT agngiogram (C). The dashed yellow line shows the location of the OCT cross section (D). Analysis of the yellow highlighed CNV flow reveals the CNV is predominantly under the retinal pigment epithelium.
This figure quantifies inner retinal blood flow in nonproliferative diabetic retinopathy with macular edema. Fluorescein angiography (Left) shows defined foveal avascular zone (FAZ) as the area inside white dashed line, parafoveal region between white and blue dashed lines, and perifoveal zone between blue and green dashed lines. Enlargement of the FAZ is shown extending into parafoveal region (Middle). Scattered areas of macular nonperfusion are colored blue and presented on the 6x6 mm OCT angiogram (Right).
OCT angiography produces 3D data that must be segmented into different slabs before it can be evaluated by clinicians. First, computer segmentation of OCT images provides a reference plane or surface. Appropriate tissue slabs are then defined based on these reference planes. The useful reference planes include the inner limiting membrane (ILM), outer boundary of the inner plexiform layer (IPL), outer boundary of the outer plexiform layer (OPL), and Bruch’s membrane (BM). In scans of healthy eyes, automated algorithms perform well in identifying these reference planes. However, in cases where the retina is deformed, manual correction or adjustments of slab boundaries may be required.
Multiple approaches for OCT angiography have been developed. These include amplitude-based, phased-based, or combined amplitude/phase variance-based methods. Furthermore, new software algorithms have been developed which allow existing OCT hardware to perform OCT angiography. These methods use either the Doppler shift or variations in speckle pattern caused by moving red blood cells to detect both transverse and axial flow. These methods have become practical now because the high speed of Fourier-domain OCT allows multiple cross-sectional images to be obtained at the same location in very quick succession to detect relative motion in voxels contain blood flow. Both varieties of Fourier-domain OCT: spectral (a.k.a. spectral-domain or spectrometer-based) or swept-source, could be used. Three-dimensional volumetric OCT angiography can be obtained in seconds. Subsequent processing allows the visualization of vascular networks down to the capillary level in both the retinal and choroidal circulations. Because of the high axial resolution of OCT, the retinal circulation could be further subdivided into the superficial and deep plexi as well as abnormal neovascularization into the vitreous space, and the choroidal circulation could be subdivided into the choriocapillaris, the deeper choroid, and abnormal neovascularization above the Bruch’s membrane and into the retina.