Optical coherence tomography (OCT) has revolutionized the diagnostic field of retina in many different ways. Here is a quick primer on how to read an OCT. (This example involves the Stratus Time-Domain model by Carl Zeiss Meditec, Inc., Dublin, Calif.)
- Check the name and date of image: This may seem obvious, but just as we learned in medical school when reading any imaging test like an X-ray, you must confirm that you have the right patient! The date is equally as important as in macular degeneration and diabetic retinopathy, where serial scans are done over multiple visits.
- Center the image: Make sure the OCT image is centered on what you are interested in imaging. Patients with poor fixation (such as disciform scars from advanced macular degeneration) will not fixate well and the image may not be centered on the fovea (or whatever area of interest!). Keep in mind, though, that the infrared image that you are seeing is only an approximation of the patient’s likely position, as the image that is provided is usually taken at the end of the scans and the patient’s eye may have been in a different location during the actual scan.
- Review the OCT image: This is the main reason why you order the OCT. Ensure that the image is in the center and the lines are placed appropriately. These lines are what generate the thickness measurements by the automated software. If they are off (e.g., fibrovascular scarring in advanced macular degeneration), the measurements are not reliable. Next to the image is a small compass that tells the direction of the OCT scan by following the black arrow. (In this case, the OCT moves from supero-nasally to inferio-temporally.) This is very important to orient the image of the OCT.
- Confirm the signal strength: If there is something in the visual axis (i.e. cataract, corneal opacity), it may prevent the OCT from obtaining a good quality image. The signal strength is rated 1 to 10. Usually, you want a signal strength of at least 6. A signal strength of less than 3 creates a poor and unreliable image.
- Review the central subfoveal mean thickness (CSMT): This grid map gives you nine areas with measurements of thickness. The center grid number refers to the central subfoveal mean thickness, which is the circular area (1 mm diameter) centered around the center point. Compared with other measurements (versus center point thickness and total volume), it has a higher reproducibility and stronger correlation. Recent studies have shown that it is the preferred metric for following retinal thickness measurements (i.e diabetic macular edema).
- Double-check the center point thickness (CPT) and its deviation: This box gives both center point thickness and total volume. The CPT is based on the intersection of six radial line scans (compared with 128 macular thickness measurements made for the CSMT). The CPT has a ± indicating the deviation of the measurement. A deviation greater than 10 percent of the CPT implies an unreliable measurement.
It is tempting just to look at the OCT image, but it is very important to understand and look at all these variables to ensure that you have an accurate image that is reliable. These tips are for the Stratus Time Domain machine. Things will change as faster spectral domain machines are becoming available and more research is being done to find which measurement parameter is the best in monitoring retinal thickness.
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About the author: Rahul Khurana, MD, is a retina and uveitis specialist practicing with Northern California Retina Vitreous Associates in the San Francisco Bay area. He also is a clinical instructor in ophthalmology at UCSF Medical Center, where he teaches and staffs UCSF residents and medical students.