Wednesday, September 02, 2009

Reading an OCT 101

By Rahul Khurana, MD

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, CA, the most common OCT machine in the world).

Figure 1. Sample OCT - click on image for larger view

1. Name and Date of Image: This may seem obvious but always check the name of the patient and the date the image was taken. 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.

2. Centration: 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!).

3. 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 (i.e. fibrovascular scarring in advanced macular degeneration), the measurements are not reliable. Next to the image is a small compass which 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.

4. Signal Strength: If there is something in the visual axis (i.e. cataract, corneal opacity), it prevents the OCT from obtaining a good quality image. The signal strength is rated 1-10. Usually, you would like a signal strength of at least 5. A signal strength less than 3 creates a poor and unreliable image.

5. 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 is the preferred metric for following retinal thickness measurements (i.e. diabetic macular edema).

6. Center Point Thickness (CPT): 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. If this deviation is greater than 10% of the CPT, that 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 which is currently the most common model used in the world. 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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