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At first glance, both OCT examination and fundus photography (FP) have great potential for effective screening of retinal abnormalities. However, OCT imaging provides an improved diagnosis of many diseases, and the role of FP as the gold standard is losing popularity. In this post, we will look at the critical limitations of fundus photography and explore why OCT examination is gaining credibility among ophthalmologists and optometrists worldwide.

In the industrialized world, human eye diseases develop quickly and progress rapidly, and many of them can lead to blindness. Ophthalmologists and optometrists are well aware that many of these diseases, such as age-related macular degeneration (AMD), diabetic retinopathy, or glaucoma, occur in the retina. That is why the ability to depict the retina, observe its blood circulation and analyze the resulting images is crucial. 

Various retinal imaging techniques can improve patient care by increasing the quality of screening and monitoring. Modern technologies, such as fundus photography and optical coherence tomography imaging, have a positive effect on the daily practice of ophthalmologists and optometrists, facilitate early diagnosis and allow better management of eye disorders. Currently, special attention is paid to these two methods and their ability to provide a comprehensive description of the morphology and function of the retina. We will discuss both methods to highlight their pros and cons.

Fundus photography: benefits and limitations

Being widely available, the fundus imaging system is vital for visualization of retinal and optic nerve conditions. Fundus photography is easy to use and cost-effective, contributing to its rapid spread over the past few years. However, this method also has a few disadvantages which make it less effective than OCT examination. Let’s take a closer look at the benefits and limitations of fundus imaging systems.

The benefits of the fundus photo

Fundus photography is a quick and simple non-invasive technique that allows eye care specialists to visualize the retina and provide the accurate diagnosis. FP shows the landmarks of the eye. In addition, fundus photo provides an early and accurate diagnosis, which is highly important for timely treatment and improved therapy. 

Fundus photography helps ophthalmologists and optometrists not only identify retinal abnormalities and pathologies but also to monitor the progression of eye diseases. In this way, any eye care specialist can develop an effective treatment plan for different people with different eye types.

The limitations of the fundus photo

Despite all the benefits of the fundus photo, this technology also has some disadvantages. FP allows eye care specialists to examine the retina by looking at it from above. They may see an uneven retinal surface or curvature. However, FP does not allow observing the microscopic changes inside the retina which correspond to early stages of the disease. It, therefore, can be obtained with OCT image interpretation.

oct examination

A key disadvantage of fundus photography compared to optical coherence tomography imaging is its lower resolution. Thus, the pathology size detected in the fundus photography is larger. The FP is unable to detect the invisible pathologies on different retinal layers, which usually present at the stage when the patient does not even have any complaints. In fact, the fundus imaging system sees what the human eye can see. With this technology, an ophthalmologist or optometrist detects only pathologies that are visible to human eyes.

Main principles of OCT examination

OCT examination has revolutionized retinal research, allowing doctors to review the pathophysiology of many diseases. But what is the main difference between OCT and fundus photography? FP is the process of photographing the back of the eye using a specialized camera consisting of a microscope attached to a camera with a flash. In contrast, optical coherence tomography imaging estimates the depth at which a particular backscatter occurred by measuring its flight time

The reflection of light allows determining exactly from what retinal layer the signal is coming. As we know that it takes more time for the light to return from deeper layers. The physical principle of OCT examination is similar to ultrasound. The only difference is that the OCT does not use acoustic waves but near-infrared optical wavelength radiation.

oct examination

Modern OCT examination allows doctors to get images with a reasonably high resolution, ranging from 1 to 10 μm. In fact, optical coherence tomography is also called an optical retinal biopsy. The architecture of the retinal structure in the images is very close to the histological structure of the retina. Histologically, the retina consists of 10 layers, but OCT technology allows anyone to assess the retina itself and the structures surrounding it. The modern classification has 18 zones (layers), which can be estimated and described using this technology.

How OCT examination boosts your working process 

Modern equipment allows patients to undergo both OCT and fundus photography quite comfortably – without dilation of the pupil and through a non-contact method of research. But optical coherence tomography imaging has many advantages that make this method the most progressive, leaving all competitors behind. 

OCT imaging system is a highly informative method of retinal examination, and because of its resolution, it is called histology or microscopy. With this technology, ophthalmologists see what could only be seen under a microscope without OCT.

Advantages of oct examination

Usually, thinking of the benefits of OCT, eye care specialists  talk about three key points:

  • High scanning speed
  • Non-invasiveness
  • Contactless

However, experienced ophthalmologists and optometrists know these are not the only advantages. Let’s discuss how OCT image interpretation helps examine the layers of the retina and determine the causes of eye diseases.

Determining pathologies at early stages

Many diseases at the early stages are almost invisible to even an experienced optometrist or ophthalmologist. Most retinal abnormalities progress with age and develop slowly and gradually, so diagnosing them is pretty difficult. However, modern OCT image interpretation allows physicians to detect the warning signs of the disease, classify hundreds of pathologies, and re-monitor images to track the progression of pathologies.

Moreover, OCT image interpretation helps ophthalmologists understand the pathophysiology of retinal diseases, for example, how macular holes arose. This discovery showed doctors that they often misdiagnosed fluid location in the retina. Modern OCT examination help determine the location of abnormal new blood vessels, which is especially important when working with patients suffering from wet AMD.

oct examination

Measuring thickness

OCT imaging allows eye care specialists to measure the retina’s thickness and the magnitude of the pathological process in μm. It is advantageous for the diseases that cause fluid accumulation, such as retinal vein occlusion (RVO) and diabetic macular edema (DME).

oct examination

Fundus photography does not provide such an opportunity because the supervision of the dynamics is unavailable in FP. Because OCT imaging allows the retina to be examined in layers, any eye care specialist can detect changes in the structure of the eye that will never be able to be tracked by the FP. 

In addition, creating a map of the total thickness of the retina or its layers is crucial for monitoring patients with glaucoma, for example. The retinal nerve fiber thickness in such patients becomes thinner as the disease progresses so it is vital to monitor it.

Determining the severity of eye disease

Well-made retinal images allow to determine the severity and stage of the disease, compare images after examination with documented results, and track disease progression. Moreover, obtaining clear images of the retina helps different eye care specialists who monitor the same patient to choose the most accurate diagnosis.

Providing high patient tolerance

Needless to say that patient cooperation is highly important while performing any type of diagnosis. If a patient moves during the procedure, the quality of the image may deteriorate significantly. However, with modern optical coherence tomography principles, the acquisition time is shorter which results in fewer motion-related artifacts. 

OCT uses completely safe laser light, avoiding all the side effects or risks. Moreover, with its scanning speed, the process becomes comfortable and effortless both for the ophthalmologist/optometrist and the patient.

Disadvantages of OCT examination

Despite the high-quality information provided with optical coherence tomography imaging, the technology also has a few limitations. As OCT uses light waves, some images can contain media opacities. Thus, the OCT scan can be limited by staging a hemorrhage in the vitreous body, a dense cataract, or clouding of the cornea.

Current use of OCT examinations

Although standard fundus imaging is widely used, more and more eye care specialists are switching to modern OCT systems that provide more detailed information about various retinal abnormalities.

Today, the commercially available and clinical standard of choice for most specialists is SD-OCT (spectral-domain OCT) systems, which provide volumetric images of the human retina with a lateral resolution of better than 20 μm. Current SD-OCT devices use retinal images to re-trace the same image area during several subsequent examinations to monitor treatment progress.

The ophthalmological practice also uses SS-OCT (swept-source OCT) systems, which provide access to a large number of parameters of the eye, which is important for measurements through dense cataracts. SS-OCT supports high image speed and a large scanning depth range compared to SD-OCT. However, the cost of SS-OCT devices is much higher than their counterparts, so these systems have not yet gained widespread clinical implementation. Assuming that the cost of lasers will decrease, it is likely that SS-OCT will eventually also replace SD-OCT in most daily clinical practice.

In general, the modern OCT devices available today, whether SS-OCT or SD-OCT, are multimodal, which means that ophthalmologists can quickly and easily acquire an incredible amount of information. In addition to image acquisition, modern OCT imagin systems are equipped with special software. It collects retinal images and compares the results to regulatory databases. This allows doctors to make better patient treatment decisions.

The future of retinal imaging with OCT examinations

Fundus photo and OCT are pretty difficult to compare because these are completely different technologies. These diagnostic methods carry different information and can sometimes even complement each other. After many years of using the fundus imaging system, this method has been perfected, the quality of cameras has increased, and it has become possible to take pictures without dilating the pupil. 

For example, FP is a great method for revealing vascular diseases of the eye. However, in most cases, the resolution of OCT is much higher than the resolution of fundus photography. FP will never be able to track invisible changes in the retina structure that OCT can track.

oct examination

OCT image interpretation makes it possible to examine 18 zones of the retina, which allows ophthalmologists and optometrists to investigate pathologies in the early stages and detect foci of diseases up to 20 μm. That is why both young specialists and experienced professionals should choose OCT to examine the patient’s retina.

The future of OCT examination is definitely connected to technologies. 

For instance, mobile apps for ophthalmologists, such as Altris Education OCT, help eye care specialists learn OCT image interpretation on millions of labeled scans.

Altris AI web platform supports ophthalmologists and optometrists in decision-making: the system detects 54 pathologies and 49 pathological signs on OCT  providing eye care specialists with a higher level of confidence in diagnostics. 

The combination of the knowledge of eye care specialists powered by AI technologies will result in higher diagnostic standards for the industry and better patient outcomes. Imagine how many diseases can be prevented if detected at early stages!

Article autor:
Maria Znamenska

Ph.D. Ophthalmology