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So what about Fourier Domain OCT and Epithelial Thickness Mapping? (October 2015)

Having worked to develop epithelial mapping with its many applications for over 20 years now and given the commonly held (erroneous) belief that OCT would be more accurate for thickness measurement than VHF digital ultrasound simply because of differences in wavelength, I am often asked how OCT compares to VHF digital ultrasound. In my clinical practice, we use both technologies each for its distinct advantages; however, in this inaugural Arcscan Insight blog, I would like to discuss what seem to be relative advantages of VHF digital ultrasound with respect to the earliest detection of keratoconus in refractive surgery screening.

In July 2015 the Journal of Refractive Surgery published a study that we carried out comparing epithelial thickness measurements of very high frequency ultrasound and optical coherence tomography[1] The results showed that the mean OCT measurement of virgin eyes was statistically significantly thinner in 14 of the 17 measured zones than VHF digital ultrasound and in post-laser refractive corneas OCT measurement was statistically significantly thinner in all 17 measured zones.

The difference in measurement between OCT and VHF digital ultrasound was greater on average in post-laser refractive surgery corneas than in virgin corneas. Given the demonstrated high precision of VHF digital ultrasound, the ability for ultrasound to pick up a bigger difference suggests that OCT measurements are not measuring the same thing. The discrepancies are more pronounced in the periphery may be partially explained by the fact that the OCT system scans linearly across the cornea and therefore becomes increasingly non-orthogonal to the corneal surface, while in VHF digital ultrasound arc-scanning the transducer is swept in an arc to maintain orthogonality.

What was especially interesting about this finding is, that OCT measurements include the tear film, whereas VHF digital ultrasound does not due to the scans being performed under immersion. Therefore, we would have expected OCT measurements to be thicker and not thinner than VHF digital ultrasound measurements but the opposite was true, meaning that the actual difference between methods is likely to be 3-4 microns more which translated to OCT measuring the central epithelium approximately 4 microns thinner in virgin corneas and 6 microns thinner in post-laser refractive corneas.

Another critical point regarding the inclusion of tear film in OCT epithelial measurements is the fact that the tear film compensates for small irregularities on the epithelial surface, in the same way that the epithelium compensates for irregularities on the stromal surface; both layers acting as a low-pass filter. It is therefore possible that some detail in the epithelial thickness profile is lost by including a tear film in the actual measurement.

Lastly, one must take into consideration the source of error introduced by the use of different optical and ultrasonic speed constants for the epithelium and the stroma in both technologies.

Combining all these potential error sources still shows a 2.5 micron difference between OCT and VHF digital ultrasound measurements.

Though this implied that both methods are in fairly close agreement for epithelial thickness measurement, we need consider the fact that the main application for epithelial thickness measurement is keratoconus screening – diagnosis or exclusion of keratoconus in cases of equivocal topography, in which differences need to sometimes be detected on a scale of 1-3 microns. Given that the tear film can hide asymmetries on this scale of a few microns, it is possible that OCT may not be able to match the sensitivity VHF digital ultrasound for keratoconus screening.

As a result, both OCT and VHF digital ultrasound corneal scanning are technologies that should be a must for every refractive surgery practice. I firmly believe that the detection of early keratoconus is one of the most challenging tasks for the refractive surgeon and that epithelial thickness mapping will become a minimum standard of care for corneal refractive surgery screening.

[1] Comparison of Corneal Epithelial Thickness Measurement Between Fourier-Domain OCT and Very High-Frequency Digital Ultrasound. Reinstein DZ, Yap TE, Archer TJ, Gobbe M, Silverman RH. J Refract Surg. 2015 Jul;31(7):438-45.

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