Optical coherence tomography angiography
Optical coherence tomography angiography (OCTA)[1][2][3][4] is a non-invasive imaging technique based on optical coherence tomography (OCT) developed to visualize vascular networks in the human retina,[4][5][6][7][8][9] choroid,[10][11][12][13] skin[14] and various animal models.[15][16][17] OCTA may make use of speckle variance optical coherence tomography.
OCTA uses motion contrast between cross-sectional OCT scans (B-frames) to differentiate blood flow from static tissue, enabling imaging of vascular anatomy.
Medical uses
While conventional dye-based angiography is still the common gold standard, OCTA has been evaluated and used across many diseases.[4][5][25] OCTA was first introduced in clinical eyecare 2014.[26]
OCTA has applications in several diseases, including leading causes of blindness such as glaucoma[24] and age-related macular degeneration.[27] In diabetic retinopathy (DR), OCTA was shown to resolve previously established markers of severe disease (i.e., vitreous proliferation). Moreover, OCTA was shown to provide a plethora of additional biomarkers including subclinical loss of vessel density.[28][29][30][31] Thus, OCTA may offer in future the potential to monitor the progression of DR at an earlier, pre-clinical state. Similarly, OCTA was shown to provide more refined information compared to dye-based angiography in other vascular occlusive diseases such as central (or branch) retinal vein occlusion.[32][33]
How it works
OCTA detects moving particles (
Calculating blood flow
An algorithm developed by Jia et al.[1] is used to determine blood flow in the retina. The split-spectrum amplitude decorrelation angiography (SSADA) algorithm calculates the decorrelation in the reflected light that is detected by the OCT device.
The blood vessels are where the most decorrelation occurs allowing them to be visualized, while static tissue has low decorrelation values.[39] The equation takes into account fluctuations of the received signal amplitude or intensity over time. Greater fluctuations receive a greater decorrelation value and indicate more movement.
A significant challenge when trying to image the eye is patient movement and saccadic movement of the eye. Movement introduces a lot of noise into the signal making tiny vessels impossible to distinguish. One approach to decreasing the influence of movement on signal detection is to shorten the scanning time. A short scan time prevents too much patient movement during signal acquisition. With the development of Fourier-domain OCT, spectral-domain OCT, and swept source signal acquisition time was greatly improved making OCTA possible.[40] OCTA scan time is now around three seconds, however, saccadic eye movement still causes a low signal-to-noise ratio. This is where SSADA proves to be very advantageous as it is able to greatly improve SNR by averaging the decorrelation across the number of B-scans, making the microvasculature of the retina visible.[39]
History
Initial efforts to measure blood flow using OCT utilized the Doppler effect.[41][42] By comparing the phase of successive A-mode scans, the velocity of blood flow can be determined via the Doppler equation. This was deemed Optical Doppler Tomography; the development of spectral domain OCT (SD-OCT) and swept-source OCT (SS-OCT) greatly improved scan times since this phase information was readily accessible. Still, Doppler techniques were fundamentally limited by bulk eye motion artefacts, especially as longer scan times became important for increasing sensitivity.[43] In the mid-2000s systems began compensating for bulk eye motion, which significantly reduced motion artefacts. Systems also began to measure the variance and power of the Doppler phase between successive A-mode and B-mode scans; later it was shown that successive B-mode scans must be corrected for motion and the phase variance data must be thresholded to remove bulk eye motion distortion.[43][2][44][45]
By 2012, split spectrum amplitude decorrelation was shown to be effective at increasing SNR and decreasing motion artefacts.[37] Commercial OCT-A devices also emerged around this time, beginning with the OptoVue AngioVue in 2014 (SD-OCT) and the Topcon Atlantis/Triton soon after (SS-OCT).[43]
Other angiography techniques
The most common angiographic techniques were fluorescein (FA) or indocyanine green angiography (ICGA), which both involve the use of an injectable dye. Intravenous dye injection is time-consuming and can have adverse side effects. Furthermore, the edges of the capillaries can become blurred due to dye leakage and imaging of the retina can only be 2D when using this method.[40] With OCTA, dye injection is not needed making the imaging process faster and more comfortable while at the same time improving the quality of the image.
The current gold standards of angiography, fluorescein angiography (FA) and indocyanine green angiography (ICGA), both require dye to be injected.[46][47]
OCTA does not need dye but is susceptible to motion artefacts. The dyes used in FA and ICGA can cause nausea, vomiting, and general discomfort, and only have an effective lifetime on the order of a few minutes.[48]
From a physics perspective, both dye-based methods utilize the phenomenon of fluorescence. For FA, this corresponds to an excitation wavelength of blue (around 470 nm) and an emission wavelength near yellow (520 nm).[49] For IGCA, the newer method, the excitation wavelength is between 750 and 800 nm while emission occurs above 800 nm.[50]
References
- ^ PMID 22418228.
- ^ PMID 19529151.
- ^ PMID 19532651.
- ^ PMID 25897021.
- ^ PMID 27409483.
- PMID 32712135.
- S2CID 26880837.
- PMID 25317632.
- PMID 30382465.
- PMID 31528658.
- S2CID 208215569.
- S2CID 29133966.
- PMID 32413411.
- PMID 29185292.
- PMID 19838301.
- PMID 31394180.
- PMID 27699105.
- ^ PMID 27847598.
- PMID 25079820.
- PMID 28186181.
- PMID 24679442.
- PMID 26308529.
- PMID 26795548.
- ^ PMID 24629312.
- PMID 28760677.
- PMID 29229445.
- PMID24679442.
- S2CID 49213827.
- PMID 31360604.
- PMID 27472076.
- PMID 26338819.
- S2CID 22808467.
- PMID 34003897.
- PMID 21559130.
- PMID 19498514.
- PMID 19997465.
- ^ S2CID 13838091.
- S2CID 231192745.
- ^ S2CID 11456379.
- ^ PMID 27409483.
- PMID 18188263.
- S2CID 30205866.
- ^ PMID 29229445.
- PMID 19997465.
- PMID 19532651.
- PMID 6046840.
- S2CID 43888613.
- PMID 3523356.
- ^ "Fluorescein Angiography". American Academy of Ophthalmolog. Archived from the original on 27 May 2016.
- PMID 22577366.