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K. Azartash, J. T. Kwan, J. R. Paugh, A. L. Nguyen, J. V. Jester, E. Gratton; Preliminary Tear Film Thickness Data in Humans Measured With a Novel Technique. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4151.
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The purpose of this work was to gather normative and repeatability data in normals and dry eye subjects using a new, non-invasive method to measure the thickness of pre-corneal tear film.
Human subjects were screened for dry eye using a battery of clinical tests and classified as dry or normal. Tear film thickness of the inferior paracentral corneal area was measured with spatial correlation analysis. The technique is termed Fluctuation Analysis by Spatial Image Correlation (FASIC), wherein a series of images are obtained using laser illumination and a cMOS camera. The spatial correlation image of laser speckle is calculated for every frame. A sinusoidal background due to interference caused by the tear film appears in this spatial correlation image together with other features. We developed a mathematical model to obtain the thickness of the tear film from this sinusoidal background.
Mean tear thickness values (± SD) were 3.08 µm (0.11) and 2.41 µm (0.12) on the initial visit for normals (n = 11) and dry eye subjects (n = 10), respectively, and were statistically significantly different (p < 0.001, 2-sample t-test). Mean tear thickness values (± SD) were 3.09 µm (0.12) and 2.41 µm (0.12) on the second visit for normals (n = 9) and dry eye subjects (n = 6), respectively, and were also statistically significantly different (p < 0.001, 2-sample t-test). Repeatability was good between visit 1 and 2 (all subjects combined, intraclass correlation coefficient = 0.987).
The FASIC technique for imaging the ocular surface appears to provide tear thickness values in agreement with other non-invasive methods. Moreover, the technique can differentiate between normal and dry eye patient types and appears to be a highly reliable measure. No corneal changes have been observed after FASIC was performed on subjects.
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