Overview of Corneal Imaging
Corneal imaging is a tool that eye doctors use to look at the shape and curvature of the clear front surface of your eye, known as the cornea. There are two main types of this imaging – corneal topography and corneal tomography.
Corneal topography focuses on the front surface of the cornea. It shows this information in a color-coded map, sort of like a weather map for your eye. Corneal tomography, on the other hand, also measures how thick the cornea is. This lets the doctor also see the back surface of the cornea.
These imaging studies are really important in eye care. They help doctors track how an eye disease is evolving, measure if treatments are working over time, and decide who might be a good candidate for certain eye surgeries. This article aims to explain the basics of corneal topography and tomography, when they are used, and how to understand the results they provide.
Why do People Need Corneal Imaging
Corneal imaging is a type of eye exam that doctors use for various reasons:
* To check the features of the cornea (the clear, front surface of your eye). These features include its elevation (how high it sits), shape, curvature (how curved it is), and thickness – all of which can be understood through a process called topography.
* To look at the back surface of your cornea and the front chamber of your eye, using a technique called optical coherence tomography.
* To help make decisions about medical treatments before and after eye surgery.
* To watch for changes in an existing corneal disease to see if it’s getting better or worse.
In simple words, the corneal imaging gives eye doctors a detailed look at various parts of your eye to better understand your eye health and decide on the best treatment if needed.
When a Person Should Avoid Corneal Imaging
Corneal topography and tomography are eye tests that don’t involve any physical contact, so anyone can have these tests done. However, care must be taken if you’ve used artificial tears just before the test. This is because the artificial tear layer could be misinterpreted as the natural shape of your cornea, which might lead to inaccurate results.
Equipment used for Corneal Imaging
The Placido Disc Topography is a method used to measure the foremost surface of your eye’s cornea. This technique was developed in the 1880s and it works by directing rings of alternating dark and light circles, also known as mires, onto the corneal surface. The reflection of these circles is then recorded. Depending on how the circles reflect, eye doctors can get a visual understanding of the shape of your cornea. Today, advanced mathematical equations help to turn this visual data into more practical, numerical data. However, this method does have some limitations; it cannot provide information about the back of the cornea, and it only allows for a 60% view of the corneal surface. Despite these limitations, many eye doctors still use Placido Disc devices because they give important information about the corneal surface.
There are also methods, like Slit-Scanning Elevation Topography, that can create images of both the front and back surfaces of the cornea. It does this by shining a narrow beam of light onto the cornea, which triggers two reflections – one from the front surface and the other from the back. These reflections form a triangle that can be interpreted using mathematical equations to recreate the cornea’s surfaces. Additionally, since the two reflections have the same reference point, this method can also provide measurements of the cornea’s thickness.
A limitation of the Slit-Scanning method is that measurements towards the edge of the cornea can be less accurate. Scheimpflug Imaging is a method that was designed to overcome that problem. This method takes into account the fact that the cornea, light source, and image aren’t always aligned, improving the image resolution. This is particularly useful for cases where the cornea is severely distorted.
The Anterior-Segment Optical Coherence Tomography (AS-OCT) method produces cross-sectional images of the front part of the eye – similar to an ultrasound, but instead of using sound waves, it uses infrared light. This method involves splitting a beam of light, with one beam going to a reference mirror at a known distance, and the second one going to the cornea. The way the light scatters when it meets different structures in the eye is then recorded by the OCT device. The AS-OCT method is a standard technique when precise measurements of the cornea are required.
How is Corneal Imaging performed
Eye doctors use different types of maps and imaging to examine the front part of your eye, known as the cornea. These tools allow them to see the shape, steepness, and height differences on the surface of the cornea.
The most commonly used map is the axial map. It shows the distance from a certain point on the cornea to a reference plane in the lens. It gives a 2D view of the cornea, using warm colors like red, orange, and yellow to indicate steeper (or more curved) areas and cool colors like blue and green for flatter areas. For example, with a condition called keratoconus where the cornea has a cone-like bulge, the steepest, or apex, would be red surrounded by gradually cooler colors.
The instantaneous map, on the other hand, shows the slope of the cornea at any given point and can give a better overall sense of shape. The colors used are the same as in axial maps.
The elevation map compares the cornea to a best-fit sphere (a perfect round shape) to show differences in height from point to point. Warmer colors indicate areas above the sphere, cooler colors below, and green shows areas aligned with it.
Pachymetry maps show corneal thickness – warm colors for thinner areas and cool colors for thicker regions. The refractive power map displays the cornea’s ability to bend light to correct vision.
Doctors also use specific imaging methods like Pentacam, Galilei, and AS-OCT. These tools help identify conditions like keratoconus and allow viewing of the anterior segment from different orientations. But keep in mind, they require patients to maintain a fixed gaze, and eye movements could affect their accuracy. Also, each imaging device serves a specific purpose and cannot be used interchangeably. For example, imaging devices are essential to accurately view the cornea’s posterior (back) surface before deciding on surgical procedures, whereas ultrasound is the gold standard for measuring central corneal thickness.
In short, depending on the health and condition of your cornea, your doctor will decide which method of imaging to use. Each has its strengths and weaknesses but together they provide a complete profile of your cornea’s health.
Possible Complications of Corneal Imaging
Corneal topography and tomography are procedures that involve taking images of the eye’s cornea. They are non-invasive, meaning they don’t involve any surgery or physical penetration into the body. These procedures are quite safe and do not typically have major complications or side effects.
What Else Should I Know About Corneal Imaging?
Doctors use a tool called AS-OCT to see if you’re a good candidate for vision-correcting surgery such as LASIK or PRK (photorefractive keratectomy). Before being approved for the surgery, you should be at least 18 years old, have a stable vision for a year or more, and have no eye or cornea diseases. The thickness of your cornea is also considered very important.
In such surgeries, the shape of the cornea (the clear, front surface of your eye) is changed by removing a small bit of it. This can make the cornea weaker and contribute to a condition called ‘corneal ectasia’, where it bulges in a cone shape. One of the common reasons to not proceed with these surgeries is keratoconus, which is a disease causing similar cornea thinning and bulging.
If your cornea is too thin (< 480 μm), doing LASIK or PRK is not advisable. New research also shows that if more than 40% of your corneal tissue is changed during surgery, it might increase your chances of developing keratoconus. After the surgery, doctors use tools to examine your cornea and determine if any further procedures will be beneficial.
Keratoconus is one of the most common causes of cornea bulging. It can be detected and monitored with modern imaging techniques. When keratoconus is still at the early or ‘subclinical’ stage, it’s challenging to diagnose because your eyes might still appear normal. However, using these imaging techniques doctors can track changes, assess the risk, and monitor the effectiveness of any treatment to slow down its progression.
Treatment options can range from using contact lenses to surgical procedures, including the implantation of ring segments in the cornea or ‘cross-linking’ to strengthen it. The Pentacam system, a high-tech imaging device, can show your doctor many details about your cornea and help detect keratoconus as well as other conditions.