After working through this chapter, you should be able to:
Explain what interference is and how it can be used to measure distances
Explain what OCT stands for
Explain (in simple terms) how time-domain OCT works
Explain (in simple terms) how Fourier-domain OCT works
Explain (in simple terms) how swept-source OCT works
Describe clinical applications of OCT
This chapter will focus on the exciting world of optical coherence tomography (OCT) imaging, which is currently considered the ‘gold standard’ of imaging the eye. However, OCT relies on the wave theory of light, so we need to start with some light revision (as always, pun intended).
Some light revision
In chapter 10 we discussed that light can be thought of as a wave , and that multiple waves can superimpose on top of one another (by arriving at a point at the same time) and combine to produce a resultant wave . This resultant wave may have a smaller or larger amplitude (brightness or intensity) depending on how the waves interact. Remember, as well, that if the waves are coherent (meaning they possess the same wavelength and frequency), then they will produce either constructive or destructive interference . The type (and extent) of the interference will depend on whether the waves arrive in-phase (path difference a whole multiple of the wavelength) to produce constructive interference (an increase in amplitude) or if they arrive out-of-phase (path difference half a wavelength out) to produce destructive interference (a decrease in amplitude). Fig. 17.1 shows a reminder of this.
In chapter 10 we also discussed that the path difference travelled by two beams of light can help us measure distances. This requires two identical waves (which can be from a single light source split into two) to travel separate distances before meeting again at a detector which can quantify the resultant amplitude and determine the relative path difference. This has been discussed before in the context of the Michelson interferometer , which we will review in the next section.
The simplest interferometer to start with is the Michelson interferometer, which utilises light from a coherent light source and splits it into two ( Fig. 17.2 ). One of the paths of light travels to a moveable mirror a known distance away, whilst the other path of light travels to something we want to measure the distance of. In the case of the Michelson interferometer, the second path of light travels to a fixed mirror.
As discussed in chapter 10 , the idea of this setup is that by moving the moveable mirror set amounts, the interference pattern at the detector will cycle through constructive interference (increase in amplitude) and destructive interference (decrease in amplitude). For example, if we move the mirror by a distance that equates to 0.25 of the wavelength of the light, then the path difference will equate to 0.5 of the wavelength (as the light will experience +0.25 approaching the mirror and +0.25 after reflecting, totalling +0.5). This means that, providing we know the wavelength of the light, by measuring the interference at the detector we can determine the distance the light has travelled.
This is, in relatively simple terms, how OCT works.
What is OCT?
OCT is a method of structurally imaging the individual layers of the eye. Typically this is used to image the retinal layers at the back of the eye, but it can also be used to image the anterior eye as well. If we use the example of imaging the retinal layers, OCT utilises the principles of interferometry to measure how light reflects from each of the individual layers in order to determine the distance they are away from the detector ( Fig. 17.3 ). This distance information can then be transformed into a black-and-white image of the layers, which can help to monitor health and disease.
Interferometry and OCT
As discussed, OCT systems utilise these principles of interferometry in order to image the layers of the eye, but there are several different types of OCT system.
Fibre-based time-domain OCT (TD-OCT)
The first type of OCT is called fibre-based time-domain OCT (or TD-OCT for short). This method of OCT utilises a moveable mirror (just like in the Michelson interferometer) which means it measures the interference patterns over time as the mirror is moved (hence why it’s called a time-domain). They usually use a low-coherence, near-infrared light source which is produced using a superluminescent diode . Fig. 17.4 shows roughly how the system works (you’ll notice it’s very similar to that of the Michelson interferometer), but with a lens system for helping to image side to side (laterally) across the back of the eye. One of the paths of light is incident upon the moveable mirror ( reference beam ), whilst the other is incident upon the eye ( measurement beam ), and the measurement beam is reflected, or backscattered, from the back of the eye with different delay times which are dependent on the optical properties of the tissue and the distance away from the light source. The software within the system can then interpret the interference fringe (or reflectance ) profiles as they pass through the detector in order to determine the depth of the tissue and produce the nice black-and-white image of conventional OCT (as shown in Fig. 17.4 ). Importantly, because these images reveal cross-sectional views of the retina, they are imaging the z-plane (as opposed to the x-y, transverse plane; Fig. 17.5 ). However, of all the types of OCT, this system is slow and of the lowest resolution (lowest-quality images).