Diagnostics

Building Valuable Skills in Ophthalmic Imaging

Knowing a few tricks of the trade can make you a valuable asset to the practice

By Timothy J. Bennett, CRA, OCT-C, FOPS, Harrisburg, Pa

Ophthalmic imaging has long played an important role as an adjunct to ophthalmic examination. With the recent advent of new technology and treatment options for many ocular conditions, imaging has become a critical component of the comprehensive ophthalmic examination in some practice settings. Fundus photography, optical coherence tomography and fundus autofluorescence results are frequently used as a foundation for treatment plans in glaucoma, diabetic retinopathy and age-related macular degeneration (AMD).

As utilization continues to grow, ophthalmic medical personnel with imaging skills are increasingly in demand. Learning these skills can expand your role as a valued member of your practice’s eye-care team. Here are a few basic principles and tips to help you build your ophthalmic imaging skills.

Ignore the Numbers

There are two situations in imaging where reliance on numbers can be detrimental to image quality. Optical coherence tomography (OCT) instruments generate “quality” numbers that assess the signal strength of the scans. They are useful in a general sense, but it’s more important to look at the quality of the OCT scan rather than the quality number generated by the instrument. When evaluating OCT results for quality ask these questions:

■ Is the scan centered on the area of interest or pathology?

■ Does it have good edge-to-edge saturation?

■ Is it free from artifacts?

These are subjective judgments, but ultimately, they are more relevant to capturing good diagnostic images than signal strength numbers. Often, the “best” scan may have a low signal strength number.

Correctly adjusting the eyepiece reticle is essential for consistent well-focused fundus photos. The imager needs to relax their accommodation at distance to avoid accommodative shift during photography. A commonly taught technique involves adjusting the crosshairs three successive times, noting the diopter setting each time, and using the average of these numbers. Unfortunately, this technique actually promotes accommodation to near while recording the numbers. The best strategy is to ignore the diopter numbers altogether, relax your accommodation and pay constant attention to the crosshairs and the image of the retina. As long as the crosshairs and the aerial image of the fundus both appear sharp at capture, the focus will be correct.

Focusing the Fundus Camera

Assuming the eyepiece is set correctly, another factor that can affect focus is viewing-lamp brightness. The brighter the view, the easier it is to focus accurately. Unfortunately, viewing levels that promote good focus can be uncomfortable for the patient, adversely affecting cooperation. Briefly turning the lamp up to adjust focus and then dropping it down to a more tolerable level is a simple technique that can help both photographer and patient.

If the camera is equipped with a green filter, it can be used as a focusing aid. Because the sensitivity of the eye peaks in the yellow-green portion of the spectrum, a green filter improves overall contrast and visibility of the fundus at comfortable brightness levels. After focusing with green light, the filter can be removed from the light path just before exposure. Using these basic techniques and carefully monitoring focus will allow us to get maximum resolution and diagnostic information.

To Blink or not to Blink?

Because OCT and fundus photography are non-contact techniques, image quality is dependent on a regular ocular surface and clear ocular media. These tests are often performed after a patient has undergone an extensive screening workup that includes IOP measurement, and application of topical anesthetic and dilating solutions. Patients may also undergo gonioscopy or macular contact lens examination prior to imaging. A disrupted tear film is an unintended side effect of these procedures and can adversely affect imaging quality.

It may seem counter-intuitive, but encouraging patients to blink frequently helps improve cooperation and image quality for both fundus photographs and OCT images. In our clinic, patients are often surprised that we encourage them to blink, having had procedures done in other clinics where they were sternly cautioned against blinking. Frequent blinking helps refresh the tear film and makes the patient feel more at ease. You’ll soon learn to recognize a patient’s blinking rhythm and you can time your image capture just as the lid is retracting after a blink. Gently encouraging the patient to “hold your gaze for just a moment” often works when you need just a moment longer to capture a good image. When frequent blinking doesn’t work, application of artificial tears can also make a difference in patients with dry eyes or compromised tear film.

Don’t Overdo It

Overexposure, oversaturation and over-sharpening can all have an adverse effect on digital fundus photographs. Even a slight amount of overexposure can be detrimental to image quality.Decreasing the flash setting will help retain detail in the optic nerve or other bright structures and prevent the unnatural look associated with overexposure.

The factory default settings for saturation and sharpness are often set a little too high for accurate color rendition of fundus images. Oversaturation can occur when the camera’s contrast or gamma settings are too high.If you suspect this is the case, you may be able to adjust the software settings to reduce saturation and contrast.

All digital images have some level of image sharpening applied to them, but over-sharpening can easily be avoided.If your images appear too grainy or noisy, consult with your vendor to see if the default system settings can be adjusted to reduce the baseline amount of sharpening. OP

Mr. Bennett is an ophthalmic photographer in the Penn State University Department of Ophthalmology at Milton S. Hershey Medical Center. He is a nationally recognized author, lecturer and educator in the field of ophthalmic photography and has served as president of the Ophthalmic Photographers’ Society.