Introduction
Optical Coherence Tomography (OCT), first introduced in 1996, is an imaging tool that has revolutionized the practice of ophthalmology. It has become the standard of care for daily clinical practice and served a critical role in clinical research to better characterize various retinal diseases.
For this piece, we will focus on the posterior segment. OCT is a cross-sectional imaging tool that is rapid and non-invasive, utilizing reflected light to obtain images of different layers of intraocular tissues. OCT has made diagnosing and managing many conditions more precise, such as diabetic retinopathy, age-related macular degeneration, retinal vascular occlusions, and many more.
Just as with cars, cell phones, or even sports teams, there are many different types of OCTs. With so many options available, which is the right OCT for you?
This article aims to provide general principles to consider when thinking of getting an OCT device. Price can be widely variable depending on which machine you choose. However, we will focus on various OCT machines’ functionality and any unique elements of a given OCT device that may benefit a specific subset of ophthalmologists or optometrists.
Time-domain, spectral-domain and swept-source OCT
To begin, there are three main OCT technologies: Time-domain, Spectral-domain & Swept-source OCT.
The original “time-domain” OCT (TD-OCT) was remarkable for its time. However, relatively low image resolution made it possible to miss various pathologies. “Spectral-domain” OCT (SD-OCT) technology scans up to 100x more A-scans per second as compared to the time-domain OCT, minimizing the risk of missing critical pathology.1 Lastly, “swept-source” OCT (SS-OCT) features the ability to acquire 10x faster image capture speeds than even spectral-domain technology.
Between spectral-domain and swept-source, research has found that choroidal neovascular membranes imaged by swept-source OCT tend to be larger as compared to spectral-domain OCT.2 Swept-source provides imaging of posterior staphylomas in greater detail and does a superior job in visualizing the choroid-sclera interface.3,4
Additionally, swept-source tends to provide better visualization of deeper layers such as the choroid. However, this difference can be mitigated to an extent by enhanced depth imaging (EDI) in spectral-domain devices.
The practical clinical benefits of swept-source versus spectral-domain remain to be strongly validated. Additionally, both technologies have recently developed OCT angiography (OCT-A), although it is not known if this angiography can replace or compare to traditional dye-based angiography. One study compared the clinical performance of four major spectral-domain OCT angiography systems and found that the AngioVue system provided the highest vessel visibility and least amount of motion artifact. At the same time, the Angioplex achieved the shortest acquisition.5
Deciding between OCTs
There is some variability in OCT machines, with many companies and options. OCT scans include Topcon, Spectralis/Heidelberg, RTVue/Optovue, Copernicus/Optopol, Spectral, and many more.
Even within the same company, there can be multiple options. For example, Zeiss itself carries numerous models. The Primus 200 OCT machine represents a smaller-scale machine that does not contain many of the features other Zeiss OCT machines do; however, it comes with a lower cost and smaller size. HD-OCT Cirrus 5000 represents perhaps the opposite, providing a premier device with sophisticated software that can undergo updates that can be transmitted remotely via download (over-the-air). It contains convenience features such as eye-tracking and an incredibly high sample per second rate (nearly 70,000).
The HD-OCT Cirrus 5000 with Angioplex allows a physician or optometrist to go further by allowing imaging of vascular information in addition to the structure. If looking for something in between, one can consider the HD-OCT Cirrus 500. The 500 model is easy to use with straightforward functionality. A new model, the HD-OCT Cirrus 6000 features a scan rate of 100,000 samples per second, superior artifact reduction technology, and a logical way to store data.
Whether it is Zeiss or another company, despite the abundance of OCT machines, most if not all tend to have similar abilities in terms of diagnostic capability. However, key factors to remember are that RNFL thickness values are not interchangeable between the devices, and the devices use different normative data and signal strengths.
Comparison of commonly used OCT Machines
| OCT Machine | Cost | OCT Resolution (AxialxTransverse) | Scan Speed (A-scans/s) |
|---|---|---|---|
| Cirrus HD-OCT 500 6 | $25k to 49k USD | 5x15 μm (in tissue) | 27k to 68k |
| Spectralis SD-OCT7,8 | $>60k USD | 7x14 μm (in tissue) | 40k |
Let's walk through the many variables to consider when deciding on an OCT!
Other variables to consider when choosing an OCT machine
1) New, old, refurbished
If you are starting a new practice, consider a refurbished OCT regardless of the model you choose to get things going. If cost-saving, an older machine may be a better investment while building the funds to buy the newest and latest technology. In terms of saving money, one can even invest in an OCT machine not regulated by the FDA; however, this comes with a higher risk due to lack of standardization.
2) Presence of skilled operators (i.e., dedicated photographers)
The Cirrus HD OCT represents an OCT that is compact and has immense ease of image capture due to the integration of the mouse with the joystick. This machine may not require the presence of a highly skilled operator (dedicated photographers) to capture imaging.
3) Need for ancillary equipment and compatible software
One has to consider integration between software and the presence of imaging services. A commonly cited example is that if one has IMAGEnet available, a device like Topcon 3D OCT 1000 may be a logical choice as the systems have interoperability, making exchanging information and data gathering/presentation more straightforward.
4) Pace of practice
Segmentation quality is essential, as the lower the segmentation quality, the more likely it is that manual adjustment is needed to obtain accurate information. Having to resegment scans can be time-consuming and slow down efficiency.
Conclusions
When purchasing an OCT machine, decide if you want to start with a fully loaded new device or something refurbished and a little older to get things started. Be realistic and remember the nature of your practice, including your tech support, clinical volume, and existing software technologies. Remember, the newest and best does not always translate to practical clinical superiority!
References
- Wojtkowski M et al. Ophthalmic imaging by spectral optical coherence tomography. Am J Ophthalmol. 2004 Sep;138(3):412-9.
- Adhi M, Liu JJ, Qavi AH, et al. Choroidal Analysis in Healthy Eyes using Swept-Source Optical Coherence Tomography Compared to Spectral Domain Optical Coherence Tomography. Am J Ophthalmol. 2014;157(6):1272-81.e1
- Andrew R. Miller, Luiz Roisman, Qinqin Zhang, Fang Zheng, Joao Rafael de Oliveira Dias, Zohar Yehoshua, Karen B. Schaal, William Feuer, Giovanni Gregori, Zhongdi Chu, Chieh-Li Chen, Sophie Kubach, Lin An, Paul F. Stetson, Mary K. Durbin, Ruikang K. Wang, Philip J. Rosenfeld; Comparison Between Spectral-Domain and Swept-Source Optical Coherence Tomography Angiographic Imaging of Choroidal Neovascularization. Invest. Ophthalmol. Vis. Sci. 2017;58(3):1499-1505.
- Lim, L., Cheung, G. & Lee, S. Comparison of spectral domain and swept-source optical coherence tomography in pathological myopia. Eye 28, 488–491 (2014).
- Li XX, Wu W, Zhou H, et al. A quantitative comparison of five optical coherence tomography angiography systems in clinical performance. Int J Ophthalmol. 2018;11(11):1784-1795. Published 2018 Nov 18. doi:10.18240/ijo.2018.11.09
- CIRRUS HD-OCT - Optical Coherence Tomography (OCT) - Glaucoma - Medical Technology | ZEISS United States
- SPECTRALIS OCT - The modular Imaging Platform | Heidelberg Engineering
- Song G, Chu KK, Kim S, et al. First Clinical Application of Low-Cost OCT. Transl Vis Sci Technol. 2019;8(3):61. Published 2019 Jun 28. doi:10.1167/tvst.8.3.61
