Optical coherence tomography angiography
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Optical coherence tomography angiography (OCTA) is a noninvasive imaging technique that is being developed to look at the blood vessels in the retina.[1] As of 2018, with further work it is hoped that it will one day be useful to diagnose diabetic retinopathy.[1]
OCTA uses low-coherence interferometry to measure changes in backscattered signal intensity or amplitude in order to differentiate areas of blood flow from areas of static tissue. To correct for patient movement during scanning, bulk tissue changes in the axial direction are eliminated, ensuring that all detected changes are due to red blood cell movement.[2] This form of OCT requires a very high sampling density in order to achieve the resolution needed to detect the tiny capillaries found in the retina. Recent advancements in OCT acquisition speed have made it possible the required sampling density to obtain a high enough resolution for OCTA.[2][3] This has allowed OCTA to become widely used clinically to diagnose a variety of ophthalmological diseases, such as, age related macular degeneration (AMD), diabetic retinopathy, artery and vein occlusions, and glaucoma.[2]
Medical uses
As of 2018, with further work it is hoped that it will one day be useful to diagnose diabetic retinopathy.[1]
In theory, it may also aid in the early diagnosis of arterial occlusion, but no clinical studies have been performed so far.[4]
How it works
OCTA functions by comparing sequential B-scans of the same cross-section of the retina. Many cross-sections are scanned over time to get a 3D view of the retina over time. Low-coherence interferometry is used to perform each B-scan. The device shines an infrared laser onto the retina and measures the interference patterns of the light that is backscattered from the tissue.[5] Areas of change between each sequential B-scan can then be distinguished as sites of blood flow. The B-scans are then added together to get a 3D image of the retina. The resulting image provides structural and functional information of the retinal layers and the vasculature found at each layer, as shown in Figure 1 from Gao et al.[6] Interpretation of the OCT scans and segmentation of the retinal layers rely on software analysis of the collected data. However, once the data processed, OCTA is capable of providing images of the retinal and choroidal vasculature which, as seen in Figure 1, can be split into five layers: the superficial plexus, the deep plexus, the outer retina, the choriocapillaris, and the deeper choroid.
Calculating blood flow
An algorithm developed by Jia et al.,[5] 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.[7] 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.[6] 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.[7]
History
Initial efforts to measure blood flow using OCT utilized the Doppler effect.[8][9] 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.[10]
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.[10][11][12]
By 2012, split spectrum amplitude decorrelation was shown to be effective at increasing SNR and decreasing motion artefacts.[5] 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).[10]
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.[6] 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.[13][14]
OCTA does not need dye, but this method takes a long time to capture an image and 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.[15]
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).[16] For IGCA, the newer method, the excitation wavelength is between 750 and 800 nm while emission occurs above 800 nm.[17]
References
- ^ a b c Gildea, D (2019). "The diagnostic value of optical coherence tomography angiography in diabetic retinopathy: a systematic review". International Ophthalmology. 39 (10): 2413–2433. doi:10.1007/s10792-018-1034-8. PMID 30382465.
- ^ a b c de Carlo, Talisa E; Romano, Andre; Waheed, Nadia K; Duker, Jay S (2015). "A review of optical coherence tomography angiography (OCTA)". International Journal of Retina and Vitreous. 1 (1): 5. doi:10.1186/s40942-015-0005-8. ISSN 2056-9920. PMC 5066513. PMID 27847598.
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: CS1 maint: unflagged free DOI (link) - ^ Drexler, Wolfgang; et al. (2014). "Optical Coherence Tomography Today: Speed, Contrast, and Multimodality". Journal of Biomedical Optics. 19: 071412. doi:10.1117/1.jbo.19.7.071412.
- ^ Kashani, AH; Chen, CL; Gahm, JK; Zheng, F; Richter, GM; Rosenfeld, PJ; Shi, Y; Wang, RK (September 2017). "Optical coherence tomography angiography: A comprehensive review of current methods and clinical applications". Progress in Retinal and Eye Research. 60: 66–100. doi:10.1016/j.preteyeres.2017.07.002. PMC 5600872. PMID 28760677.
- ^ a b c Jia, Y; Tan, O; Tokayer, J; Potsaid, B; Wang, Y; Liu, JJ; Kraus, MF; Subhash, H; Fujimoto, JG; Hornegger, J; Huang, D (2012). "Split-spectrum amplitude-decorrelation angiography with optical coherence tomography". Opt Express. 20: 4710–25. doi:10.1364/OE.20.004710. PMC 3381646. PMID 22418228.
- ^ a b c Gao, Simon S.; Jia, Yali; Zhang, Miao; Su, Johnny P.; Liu, Gangjun; Hwang, Thomas S.; Bailey, Steven T.; Huang, David (2016). "Optical Coherence Tomography Angiography". Investigative Ophthalmology & Visual Science. 57 (9): OCT27–36. doi:10.1167/iovs.15-19043. ISSN 1552-5783. PMC 4968919. PMID 27409483.
- ^ a b Koustenis A, Harris A, Gross J, et al Optical coherence tomography angiography: an overview of the technology and an assessment of applications for clinical research British Journal of Ophthalmology 2017;101:16-20.
- ^ Izatt, J.A.; Kulkami, M.D.; Yazdanfar, S.; Barton, J.K.; Welch, A.J. (1997). "In vivo bidirectional color Doppler flow imaging of picoliter blood volumes using optical coherence tomography". Opt. Lett. 22: 1439–1441. doi:10.1364/ol.22.001439.
- ^ Chen, Z.; Milner, T.E.; Srinivas, S.; Wang, X.; Malekafzali, A.; van Gemert, M.J.C.; Nelson, J.S. (1997). "Noninvasive imaging of in vivo blood flow velocity using optical Doppler tomography". Opt. Lett. 22: 1119–1121. doi:10.1364/ol.22.001119.
- ^ a b c Spaide, R.F.; Fujimoto, J.G.; Waheed, N.K.; Sadda, S.R.; Staurenghi, G. (2017). "Optical coherence tomography angiography". Prog. Retin. Eye Res. 64: 1–55. doi:10.1016/j.preteyeres.2017.11.003.
- ^ Makita, S.; Hong, Y.; Yamanari, M.; Yatagai, T.; Yasuno, Y. (2006). "Optical coherence angiography". Opt. Express. 14: 7821–7840. doi:10.1364/oe.14.007821.
- ^ Fingler, J.; Zawadzki, R.J.; Werner, J.S.; Schwartz, D.; Fraser, S.E. (2009). "Volumetric microvascular imaging of human retina using optical coherence tomography with a novel motion contrast technique". Opt. Express. 17: 22190–22200. doi:10.1364/oe.17.022190. PMC 2791341. PMID 19997465.
- ^ Gass, JDM; Sever, RJ; Sparks, D; Goren, J (1967). "A combined technique of fluorescein fundoscopy and angiography of the eye". Arch Ophthalmol. 78: 455–461. doi:10.1001/archopht.1967.00980030457009.
- ^ Slakter, JS; Yannuzzi, LA; Guyer, DR; Sorenson, JA; Orlock, DA (Jun 1995). "Indocyanine-green angiography". Curr Opin Ophthalmol. 6 (3): 25–32. doi:10.1097/00055735-199506000-00005. PMID 10151085.
- ^ Yannuzzi, LA; Rohrer, MA; Tindel, LJ; et al. (1986). "Fluorescein angiography complication survey". Ophthalmology. 93: 611–7. doi:10.1016/s0161-6420(86)33697-2.
- ^ "Fluorescein Angiography".
- ^ Alander, Jarmo T.; Kaartinen, Ilkka; Laakso, Aki; et al. (2012). "A Review of Indocyanine Green Fluorescent Imaging in Surgery". International Journal of Biomedical Imaging. 2012: 1–26. doi:10.1155/2012/940585. PMC 3346977. PMID 22577366.