The Triton OCT has a normative reference database whereby thickness maps are compared and significant deviations from normal are automatically identified. A version of the Triton (Triton plus) is also capable of performing Fluorescein Angiography (FA), red-free imaging, and fundus autoflourescence (FAF). Most recently, the Triton also includes the capability to perform OCT-Angiography, which creates a 3D map of the microvasculature. It is a useful and indispensable tool to aid the clinician in the detection and management of many ocular pathologies. The Triton OCT is not approved for sale in the US yet.

The Triton is part of the third generation of OCTs. The first-generation OCT was developed more than 20 years ago at Massachusetts Institute of Technology by Jim Fujimoto, David Huang, Michael Hee, and others [1]. This OCT was slow, had poor resolution, and operated on a time domain principle. That is, it utilized a moving reference mirror in the interferometer, which limited the speed. The light source was not very broad-band (±25 nm) and so the depth resolution was limited (10–20 μm). Despite these limitations, the time-domain OCT was a commercial success and it became the standard of care for retina and glaucoma. More recently, a second-generation OCT was developed, which utilized Fourier Domain (also referred to as Spectral Domain) methodology. It is faster (20–70 kHz) and has an improved depth resolution (5–8 μm) primarily because it utilized a stationary reference mirror and had a broader-band light source (±50 nm). The third-generation OCT, swept source OCT (SS-OCT), includes several major advances in technology. First, SS-OCT utilizes a swept source technology in combination with a light source with a longer wavelength. Swept source utilizes a narrow band wavelength laser and is swept across a broad range of wavelengths. This eliminates the need for a spectrometer, which allows for much faster scanning speeds (100 kHz). The longer wavelength light source (centered on 1050 nm) provides greater penetration due to the nature of longer wavelengths (less scatter, better penetration). This subsequently allows imaging and quantitative evaluation of the choroid for the first time [2–11] as well as better penetration through cataracts and other media opacities. SS-OCT also has a shallower drop in sensitivity with depth, which means there is high sensitivity throughout the entire image from top to bottom compared to time domain or Fourier domain OCT (Fig. 2.2). This allows imaging of the vitreous [12–16] while maintaining good visibility of the choroid in the same scan. Another advantage of the longer wavelength is that it is less visible to the human eye, as it is centered at 1050 nm with a range of ±50 nm. This is advantageous because the patient typically does not see the scanning light and so they are not distracted and are less likely to move the eye to follow the scan beam. Over the years, OCT technology has continued to advance and this third-generation OCT, SS-OCT, is the latest iteration and offers many advantages over the older versions.

The Triton OCT also allows the user to visualize the retina and optic nerve using an en face approach. In en face mode the user can view a 3D cube of data from a top-down, or en face perspective. This allows for detailed surface evaluation. The software allows the user to view not only the top surface, but at any plane down into the deepest layers of the retina including the choroid and lamina cribrosa. The user also has the option to select the depth of the layer to be visualized, which further enhances any desired structure (Fig. 2.3).

In addition to the en face viewing capability, the Triton also offers the opportunity to better visualize the vitreous using the Enhanced Vitreous Visualization (EVV) mode. This proprietary method improves the signal-to-noise ratio at all levels in the image, which greatly enhances the visualization of structures, especially in the vitreous (Fig. 2.4).

Another helpful software feature, known as SMARTTrack™, is the ability to perform real-time tracking for line scanning and OCT-Angiography. This feature utilizes the live fundus image to lock on and track eye movements (through the use of landmarks such as blood vessels and the optic disc). It updates in real time such that once activated, the scan will move to compensate for eye movements, staying in the same place on the retina. A related software feature, called fundus guided acquisition™ (FGA), allows the user to identify and indicate a precise location on the fundus image where the OCT scan will be taken. The tracking feature keeps the scan locked onto this location during scanning, and follow-up features allow the user to take the scan again at a later time in the same location.

There are numerous advantages of SS-OCT over SD-OCT. Traditional spectral domain OCT (SD-OCT) utilizes a shorter wavelength (850 nm) and subsequently image quality can be negatively affected by media opacities such as nuclear sclerosis (cataract), hemorrhage, intravitreal gas, and oil. The use of a 1050 nm wavelength with SS-OCT, in contrast, offers the unique advantage of increased depth of penetration through a variety of media opacities.

A novel application of SS-OCT is the integration of OCT angiography (OCT-A). OCT-A is a novel and non-invasive imaging approach to visualizing the human ocular microvascular network. This method provides vascular information, in fine detail, of all retinal layers beyond what can be seen with conventional fluorescein angiography (FA), but without the use of a dye injection (a full 3D microvasculature map can be generated in 3–4 s). This is achieved by scanning the same location in a repeated fashion and then detecting intensity differences over time. These intensity changes over time can be attributed to motion (i.e., bloodflow within the vasculature). The method utilized in the Triton system is known as OCTARA™, which stands for OCTA Ratio Analysis. This name describes the basic process Topcon uses to detect the retinal and choroidal microvascular pattern (it is a ratio analysis of the intensity changes over time). The algorithm represents a relative measurement of OCT signal amplitude change that optimizes angiographic visualization over both the retina and choroid and also enhances the minimum detectable signal.

Similar to the en face software, the vasculature can be visualized at any depth using the modifiable segmentation lines. Four key retinal vascular layers are presented by default: (1) the Superficial Capillary Plexus (SCP), which is segmented from the internal limiting membrane (ILM) to approximately the inner plexiform layer/inner nuclear layer border (IPL/INL); (2) the Deep Capillary Plexus (DCP), which is segmented from approximately the IPL/INL border down 70 μm; (3) the Outer retina, which is segmented from 70 μm below the IPL/INL border to Bruch’s Membrane (BM); and (4) the Choriocapillaris, which is segmented from BM down 10 μm (Fig. 2.5).

Figure 2.6 shows an example of the OCT Angiography image in the superficial layers of a normal eye and an eye with BRVO. Scan areas can be selected from 3 × 3 mm², 4.5 × 4.5 mm², and 6 × 6 mm². Several inherent advantages of the Triton swept source methodology also help with the OCT Angiography imaging method, including the faster speed and better depth penetration compared to spectral-domain OCT. These have been found to facilitate better detection of choroidal neovascular membranes (CNVMs) [17].

The Triton software also comes with a wide array of scan types and clinical reports. One important new scan is a wide-field (12 × 9 mm ) protocol with 256 b-scans of high resolution (512 a-scans) It provides thickness maps over a large area (over 30 × 40°) and high resolution B scans in the same report (Fig. 2.7). There are also detailed 3D macula reports and glaucoma reports, including the new Hood Report.

The Hood Report is a new one-page report specialized for glaucoma assessment developed by Don Hood, professor, Columbia University, NY (Fig. 2.8). This report has several key features that make it especially helpful for guiding clinical decisions related to glaucoma detection and management. The layout of the report is designed to guide the clinician through several key aspects of the OCT results to improve evaluation. On the top left of the report is a large B scan image of the peripapillary RNFL surrounding the optic disc at a distance of 3.45 mm. Below this image is the peripapillary RNFL thickness profile (black curve) from this B scan superimposed on the normative limits (color regions) of the reference database. Unlike the typical RNFL thickness profile, the temporal side of the RNFL profile is in the center of the image. That is, the RNFL circumpapillary B scan (and thickness profile map) begins at the nasal side of the optic disc, then works its way superiorly, temporally, inferiorly, and then back to nasal again (NSTIN). In the past, most imaging devices start this display on the temporal side and move superiorly, nasally, inferiorly, then back temporally (TSNIT). There is an important advantage of the NSTIN view [18]. In particular, the clinician can easily relate the changes in the RNFL thickness to changes in the visual field. In fact, the horizontal lines with arrows indicate the portion of the NSTIN plot associated with the central ±8° and the central ±15° of the visual field. These are the regions of the macula and perimacula most often affected by glaucomatous damage [19].