Emerging medical technologies such as optical coherence tomography (OCT) have become the standard of care in the diagnosis as well as the management and follow-up of many ocular diseases, including glaucoma and various retinal pathologies such as age-related macular degeneration. As a technician, it is of vital importance that you be able to quickly and accurately capture the data. With a good understanding of basic ocular anatomy, familiarity with OCT and its functions, and a recognition of the barriers to obtaining quality data, you should be able to collect a wealth of reliable data with ease.

TECHNOLOGICAL OVERVIEW

Using a principle called interferometry, the OCT device is able to obtain a virtual histological section with a resolution of around 5 μm. Similar to ultrasound but using light energy instead of sound energy and producing a higher-resolution image, an OCT scan is relatively quick and noninvasive. Spectral domain OCT applies a newer imaging modality and offers significant benefits over time domain OCT, including faster acquisition times and higher-resolution images. An advantage of OCT is the high reproducibility of scans. Using reference points to compare previous scans, the instrument is able to show change in ocular pathology over time with a high degree of accuracy.

UNDERSTANDING POTENTIAL OCT APPLICATIONS

Having a conceptual understanding of the ocular anatomy is crucial to obtaining quality OCT readings. The three main structures of the eye that will be imaged are the optic nerve, the macula, and the anterior chamber angle. Other areas of the eye may be imaged at the doctor's request (eg, a lesion in the retinal periphery). Knowing these anatomical landmarks as well as their relationship to one another will help you confidently obtain quality images.

Retinal Disease
The macula is an important anatomical landmark in the eye that is located between the temporal superior and inferior vascular arcades and is roughly 5.5 mm in diameter. It typically appears as a darkened region and may have a yellowish hue secondary to the accumulation of xanthophyll pigments such as lutein and zeaxanthin. The fovea, located in the center of the macula, is responsible for finely detailed central vision. In a healthy patient, the fovea will present as a shallow depression in the center of the macula and will be easily imaged using OCT. Damage to this part of the eye tends to be highly symptomatic and can compromise the patient's central vision. Quality repeatable OCT scans are essential for following a multitude of conditions such as age-related macular degeneration and diabetic macular edema.

The patient will typically fixate on a central target, while you optimize and capture the data. Most OCT devices will assign a color code for various degrees of tissue reflectivity, thereby helping to delineate the various retinal layers. Subtle changes, however, are often best appreciated using the gray scale. In addition to high resolution of the retinal layers, newer versions of OCT using spectral domain technology are able to visualize the vitreous— in particular, the vitreal retinal interface. The vitreous is the jelly-like substance that occupies the largest portion of the globe. It has attachments in several places throughout the eye, with two of the attachments being at the optic nerve and macula. As the vitreous composition changes with age, traction can develop on retinal structures such as the macula, leading to conditions like vitreoretinal traction. OCT has allowed clinicians not only to visualize these conditions but also to understand their etiology and devise appropriate treatment plans.

Glaucoma
Another important structure that is commonly analyzed with OCT is the optic nerve, which comprises a collection of hundreds of thousands of visual fibers coursing together to carry visual signals to the brain. Because one of the most common diseases of the optic nerve is glaucoma, patients diagnosed with or suspected to have the disease will often undergo OCT of the optic nerve.

The optic nerve is an easily identifiable anatomical landmark. It generally appears as a round disc approximately 1.75 mm in size that is pink to orange in color with distinct margins. In the center of the optic nerve is a depression called the cup, which progressively enlarges in glaucoma. The retinal vasculature can easily be seen radiating from the center of the optic nerve.

For scans of the optic nerve, patients will typically look at a target slightly displaced nasally of center. You will then center the scan on the optic nerve and capture the image. Spectral domain OCT is able to capture a wealth of data concerning the optic nerve, including disc size and area as well as cup size, and it can calculate the thickness of the retinal nerve fiber layer (RNFL). Disease conditions of the optic nerve often affect the RNFL, making this a value of particular importance. In glaucoma, the RNFL thins as the disease progresses. Using a normative database, most OCT devices are able to calculate whether the RNFL of a particular patient falls within normal limits. Thus, OCT has become a valuable test for the early diagnosis and monitoring of glaucoma. Recent advances in OCT technology enhancing optic nerve topography and the assessment of peripapillary RNFL thickness and the actual number of ganglion cells have made possible not only the accurate detection of glaucoma but also the accurate detection of its progression.

Corneal and Anterior Chamber Angle Pathology
One of the more recent modalities in OCT has been the imaging of the anterior chamber anatomy as well as corneal imaging. The iridocorneal angle is where the majority of aqueous fluid produced by the ciliary body drains. It is essential for the clinician to have a thorough understanding of a patient's angle anatomy when diagnosing various disease states, notably glaucoma. Using a technique called gonioscopy, the clinician is able to evaluate the structures of the angle and determine whether it is wide open, narrow, or closed. Some angles may be difficult to grade, however, because of an absence of pigment, an overabundance of pigment, or hazy media. OCT imaging may be able to provide another view of the angle anatomy, thus allowing the clinician to confirm the diagnosis. OCT can also image and document various corneal conditions such as the depth of an opacity or confirm that a corneal graft is in adequate condition.

STEPS TO OBTAINING AN OCT SCAN

First, position the patient comfortably facing the OCT device. Then, enter his or her information into the database and select the appropriate scan. Place the patient's head comfortably in the headrest and make the necessary height adjustments.

Instruct the patient to blink a few times and have him or her focus on the fixation point. Ask the patient to hold his or her eye as still as possible and to try not to blink.

Obtain the scan and check it for reliability and artifact. If the scan is adequate, save the image and provide it to the clinician.

IMAGE ACQUISITION: CONSIDERATIONS

The patient must be able to fixate steadily on a target for the duration of the scan. Excessive movement will degrade the image.

The patient's eyes must remain open. Blinking will produce unwanted artifacts.

Uncooperative patients or those with significant media opacity will present more difficulty in terms of obtaining quality images.

Patients with dry eye disease can be given artificial tears to optimize the images.

While usually unnecessary, a very mild dilating agent can be used if needed.