الفهرس 
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Abstract
The diagnosis of advanced keratoconus is not complicated because of the typical biomicroscopic and topographic findings, but the detection of subclinical or forme fruste cases may impose difficulty. It is particularly important to detect the disease among refractive surgery candidates, because keratorefractive procedures may worsen their condition (Correia et al., 2012).
There are currently two major different types of OCTs. Time domain OCT (such as the ASOCT Visante), in which varying the position of the reference mirror produces cross sectional images and Fourierdomain OCT (such as the three dimensional (3D) OCT the slit lampOCT, and the RTVue (Optovue), in which the reference mirror is fixed and Fourier transformation of the spectral interferogram results in the cross sectional images. The Fourier domain system provides faster acquisition times than time domain systems and its higher resolution allows visualization of more details (Lin et al., 2019).
The aim of this study is to determine the accuracy of ASOCT in diagnosing keratoconus and to assess the validity of ASOCT in diagnosing keratoconus
This observational cross-sectional study was carried out on 50 eyes of patients who attending to the out-patient ophthalmic clinic with keratoconus.
In our study regarding the comparison of corneal thickness by OCT and Ultrasound Pachymetry, the mean corneal thickness measurement by OCT and ultrasound pachymetry of right eye of the studied patients was 447.04± 64.78 µm and 440.02± 53.29 µm respectively.
The values showed that OCT values were higher than ultrasound values but not statistically significant (p = 0.114) and these results were confirmed by Bland-Altman analysis. The mean difference was 7.02 µm with 95% limits of agreement from 0.91 µm to 1.14 µm for right eye.
The mean corneal thickness measurement by OCT and ultrasound pachymetry of left eye of the studied patients was 449.10± 53.56 µm and 518.06± 512.76 µm respectively. The values showed that OCT values were lower than ultrasound values but not statistically significant (p = 0.417) and these results were confirmed by Bland-Altman analysis. The mean difference was 68.96 µm with 95% limits of agreement from 0.51 µm to 1.83 µm for left eye.