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 Table of Contents  
ORIGINAL ARTICLE
Year : 2014  |  Volume : 4  |  Issue : 4  |  Page : 163-169

Central corneal thickness measurement by Fourier domain optical coherence tomography, ocular response analyzer and ultrasound pachymetry


1 Department of Ophthalmology; Department of Community Health Development, Far Eastern Memorial Hospital, Ban-Chiao District; Department of Ophthalmology, National University Hospital, Taipei, Taiwan
2 Lo’s Eye Clinic, Changhua City, Changhua County, Taiwan
3 Department of Ophthalmology, National University Hospital, Taipei, Taiwan

Date of Web Publication1-Oct-2014

Correspondence Address:
Shu-Wen Chang
Department of Ophthalmology, Far Eastern Memorial Hospital, Number 21, Section 2, Nanya South Road, Banciao District, New Taipei City 220
Taiwan
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Source of Support: None, Conflict of Interest: None


DOI: 10.1016/j.tjo.2014.08.001

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  Abstract 


Purpose: To assess the repeatability, reproducibility, and agreement of central corneal thickness (CCT) measured by non-contact Fourier domain optical coherence tomography (FD-OCT; OptoVue) with the other two contact devices, ocular response analyzer (ORA; Reichert Ophthalmic Instruments) and Ultrasound Pachymetry (USP; DGH Technologies).
Methods: This observational cross-sectional study measured CCT sequentially using FD-OCT, ORA and USP. The first 16 volunteers (32 eyes) received three measurements by two independent examiners in a single session to determine intra-observer repeatability and inter-observer reproducibility. An additional 27 volunteers (54 eyes) received one measurement by the same examiner. The measurements of all 86 eyes were analyzed for the difference, correlation, and agreement among the three devices.
Results: FD-OCT measured the thinnest while USP measured the thickest CCT (548.6 ± 28.3 μm, 556.9 ± 28.8 μm, and 560.0 ± 28.8 μm by FD-OCT, ORA, and USP, respectively, p < 0.001). The mean differences (lower/upper limit of agreement) for CCT measurements were 8.4 ± 7.6 |μm (-6.5/23.2) between ORA and FD-OCT, 11.4 ± 7.3 |μm (-2.8/25.7) between USP and FD-OCT, and 3.1 ± 5.1 |μm (-6.9/13.1) between ORA and USP. The intra-class correlation coefficients were above 0.98 for all tested groups. FD-OCT had the lowest intra-examiner variability (coefficient of repeatability of 0.64%) and lowest inter-examiner variability (coefficient of reproducibility of 1.16%).
Conclusion: FD-OCT, ORA, and USP demonstrated good inter-observer reproducibility and intra-observer repeatability. The three measurements were highly correlated; however, systematic differences between the three tested devices did exist. FD-OCT was a reliable and examiner-independent method in CCT measurement.

Keywords: corneal thickness, Fourier domain optical coherence tomography, ocular response analyzer, repeatability and reproducibility, ultrasound pachymetry


How to cite this article:
Chang SW, Su PF, Lo AY, Huang JY. Central corneal thickness measurement by Fourier domain optical coherence tomography, ocular response analyzer and ultrasound pachymetry. Taiwan J Ophthalmol 2014;4:163-9

How to cite this URL:
Chang SW, Su PF, Lo AY, Huang JY. Central corneal thickness measurement by Fourier domain optical coherence tomography, ocular response analyzer and ultrasound pachymetry. Taiwan J Ophthalmol [serial online] 2014 [cited 2019 Nov 20];4:163-9. Available from: http://www.e-tjo.org/text.asp?2014/4/4/163/204133




  1. Introduction Top


Accurate central corneal thickness measurement (CCT) has clinically significant implications in glaucoma diagnosis and follow up.[1],[2],[3],[4],[5],[6],[7],[8] The accuracy of corneal thickness measurement is also important in the evaluation ofendothelial safety ofnewlyemergent surgery modality,[9] dry eye therapy effect,[10] disease progression,[11] and refractive surgery evaluation.[12] Measurement of corneal thickness per se or monitoring its temporal alteration is considered as an overall functional evaluation of the corneal endothelium before and/or following intraocular surgery[11],[13] and penetrating keratoplasty,[14],[15] as well as in cases of prolonged contact lens wear[16] when in vivo confocal microscopy and specular microscopy are not possible.

Factors such as tear film thickness, topical eye drops used before examination including anesthetics and fluorescein, duration of contact lens wearing, diurnal variation, and pre-existing corneal pathologies or previous surgeries may influence corneal thickness measurement.[17],[18] A corneal thickness measurement instrument less influenced by the tear film thickness that could be examined without topical anesthetics would thus provide more valuable and consistent information. There are many modalities available for corneal thickness measurement. Conventional ultrasonic pachymetry (USP) with a 10-MHz probe has been the gold standard with the advantages of ease of use, portability, low cost, and wide availability. Despite its high degree of inter-observer reproducibility and intra-observer repeatability, this technique is still operator dependent. Misalignment, corneal indentation, and variations in placing the probe all influence the final measurement. Furthermore, the requirement for cornea–probe contact and the resultant increased patient discomfort, risk for epithelial erosion, and transmission of infection have led to the development of several non-contact methods using various optical principles such as Scheimpflug imaging (Pentacam; Galilei Dual Scheimpflug Analyzer),[19],[20],[21] optical low-coherence reflectometry pachymeter,[22] slit-scan pachymetry (Orbscan),[20] and optical coherence tomography (Visante AS-OCT, RTvue-100 OCT).[21],[23],[24] Pentacam and Orbscan are clinically practical methods in corneal thickness measurement for refractive surgery. By contrast, AS-OCT measures the corneal thickness from linear cross-sectional images. It underestimates corneal thickness compared with USP, Pentacam, and Orbscan in unoperated eyes, although there is good repeatability and reproducibility among these instruments.[23],[25] However, these results cannot be used interchangeably due to different design methodologies in previous studies.

Fourier Domain optical coherence tomography (FD-OCT; RTVue-100/CA, OptoVue, Fremont, CA, USA) is a newly emerged noncontact optical device capable of illustrating both retinal and corneal thickness and pathologies. For anterior segment measurement in FD-OCT, the information in an entire A-scan is acquired by a charge-coupled device (CCD) camera simultaneously. The A-scan acquisition rate is limited by the CCD camera frame transfer rate and the computer calculation time to perform the Fourier transform of the CCD-acquired raw data into A-scan information. It takes 26,000 A-scans per second, with a frame rate of 256–4096 A-scans per frame. The ocular response analyzer (ORA; Reichert Ophthalmic Instruments, Depew, NY, USA) is a new instrument designed to measure intraocular pressure (IOP) and the corneal-compensated IOP measurements obtained by the ORA are proposed to be independent of the corneal biomechanical properties. It is equipped with 20-MHz ultrasonic pachymetry (range 200–999 μm, accuracy ± 5 μm, display resolution ± 1 μm) for corneal thickness measurement. Both FD-OCT and ORA have gained more popularity in recent years. However, FD-OCTand ORA are usually owned by retinal and cornea/glaucoma specialists, respectively. Comparison of corneal thickness measurement by the two devices would provide interchangeable information to most ophthalmologists for their daily practice.

In the present study, FD-OCT with low magnification cornea lens adapters (CAM-L), ORA and 10-MHz ultrasound pachymetry (USP; DGH-500 Pachette, DGH Technologies, Exton, PA, USA) were compared for their repeatability, reproducibility, and agreement of CCT between methods.


  2. Methods Top


2.1. Patients

This prospective study followed the tenets of the Declaration of Helsinki, and the protocol was reviewed and approved by the Institutional Board of our hospital. Informed consent was obtained from the patients before inclusion. Forty-three healthy young volunteers (17 male and 26 female) were randomly selected for the study from patients who visited the outpatient clinic.

2.2. Measurements

All measurements were taken between 10 AM and 4 PM (at least 2 hours after awaking), when corneal thickness is considered stable. Corneal thickness measurements were conducted in the sequential order of FD-OCT, ORA, and USP. Room illumination was set at 233–236 lux (TES-1339; TES Electrical Electronic Corp.,

Taipei, Taiwan). Patients who had a history of previous ocular surgery, ocular abnormalities other than cataract or refractive error, or were unable to cooperate in the examination were excluded. Contact lens wearers were asked to cease lens wearing for 1 week prior to data collection. Informed consent was obtained from all participants.

The FD-OCT RTVue-100/CA is a special version of the RTVue system that includes two cornea lens adapters, that is CAM-L (low-magnification cornea lens adapter) and CAM-S (high-magnification cornea lens adapter), for imaging the cornea and anterior chamber. Both lenses can be used to measure corneal flap or stromal thickness but only CAM-L can provide a corneal thickness map. We thus selected CAM-L for this study. In pachymetry map mode, the instrument has a scanning range of 8 mm × 6 mm and scanning depth of 2 mm. In this defined area, a total of 8 x 1024 scans were performed in 0.32 seconds (operator’s manual). For examination, the patients were positioned with a headrest and external illuminations [two short goose neck cables with 735 nm light-emitting diode (LED)] were used for pupil illumination. To allow more precise alignment, the examiner observed a real-time image of the patient’s eye on the video monitor. The cross-hair indicating the center of area of interest was centered on the pupil center. As soon as the image was perfectly aligned, the patients were asked to keep their eyes open during image capture. At the end of measurement, FD-OCT displayed a value of CCT that was an average of the central 2 mm of the cornea. This was different from ORA and USP, which showed the thickness of cornea at the point of contact. Each FD-OCT measurement was completed within 1 minute.

Five minutes after FD-OCT measurement, the cornea was anesthetized with topical 0.5% proparacaine hydrochloride (Alcaine, Alcon, Belgium) and CCT was measured with the ORA. For corneal thickness measurement by ORA, the ultrasound probe was placed manually as perpendicular as possible to the cornea at the pupil center, while the patient was instructed to fixate on a distant target. After contact with the cornea, the device automatically took several hundred measurements of corneal thickness (operator’s manual). After measuring, three values were displayed: (1) mean corneal thickness; (2) thinnest corneal thickness; and (3) standard deviation (SD) of measurement. Measurement was repeated if the SD was >1.0. Each ORA measurement took 1–2 minutes in cooperative patients and >5 minutes in uncooperative patients.

Five minutes after ORA measurement, five consecutive measurements of the CCT were made using 10-MHz UPS in a manner similar to ORA. Every five measurements by USP took about 2 minutes. The lowest and highest values were excluded. The mean of three measurements was calculated for further analysis.

Volunteers underwent measurement sessions with the following protocol. For a total of 43 volunteers (86 eyes), the first 16 (32 eyes) were examined three times with each instrument. Two measurements were performed by Examiner 1 (PFS) with a further measurement by Examiner 2 (AYL) in a single session to determine intra-observer repeatability and inter-observer reproducibility of each device. The examiners completed their examination on one instrument before measuring the volunteer with a different device. The patients were asked to take their faces away from the chinrest between the measurements. The remaining 27 volunteers (54 eyes) were examined with each instrument by Examiner 1 only. The first measurement performed on all 43 volunteers (86 eyes) by

Examiner 1 was used to calculate the difference, correlation and agreement between the three devices.

2.3. Statistical analysis

Inclusion of 86 eyes has 83% power to detect the difference with a significance level of 0.017 in CCT measurement by three devices. Besides, 32 eyes to detect the repeatability and reproducibility also had 81% and 80% power, respectively. CCT measurements using the three methods were compared using repeated-measures analysis of variance as a within-patient factor. Within-patients, pair-wise comparisons were performed using Bonferroni adjustment for multiple comparisons and three comparisons were made. A p value ≤0.017 was considered statistically significant. Agreement among instruments was evaluated using the method described by Bland and Altman.[27] Ninety-five percent limits of agreement were defined as the mean ± 1.96 SD. The intra-class correlation coefficient (ICC) is a measure of correlation, consistency or conformity for a data set of multiple groups. A score of 1.0 means perfect agreement, while 0.99–0.81 represent almost perfect agreement. The coefficient of repeatability was developed following the standard proposed by Bland and Altman. It was defined as 2 SDs of the differences between pairs of measurements in the same participants measured by the same observer, divided by the average of the means of each pair of readings. The coefficient of reproducibility was defined as 2 SDs of the difference between measurements obtained during repetition of the test with different observers, divided by the average of the means of each pair of readings.[24] For both coefficients, smaller values meant better consistency. Statistical analysis was performed using SPSS version 13.0 (SPSS Inc., Chicago, IL, USA) and MedCalc version 9.3.7.0 (MedCalc Software, Mariakerke, Belgium).


  3. Results Top


The mean age of the 43 patients was 35.1 ± 15.8 years (range: 9–74 years). The mean refraction was −3.16 ± 3.63 diopters (range: from −11.75 D to +3.50 D) and −0.70 ± 0.60 (range: from −2.75 D to 0 D) for sphere and astigmatism refraction, respectively.

3.1. Agreement of FD-OCT, ORA, and USP in CCT measurement

There was good correlation among the three measurements (between FD-OCT and ORA correlation coefficient r = 0.98, p < 0.001; between ORA and USP r = 0.99, p < 0.001; and between FD-OCT and USP r = 0.98, p < 0.001). To test for clustering and asymmetry of the sample population, kurtosis and skewness were calculated for FD-OCT, ORA, and USP [Table 1]. All results were within the range of −1 and +1, indicating normal distribution of the sample population. The mean CCT and 95% confidence interval measured by FD-OCT, ORA, and USP are summarized in [Table 1]. FD-OCT measured the thinnest, while USP measured the thickest central corneal size (p < 0.001). The mean differences in CCT measurements among the three devices and 95% limits of agreement are shown in [Figure 1]. Although Bland–Altman plots showed good agreement among the three methods, significant fixed biases existed among the three devices (p < 0.001, repeated-measures analysis of variance). Both FD-OCT and ORA (p < 0.001) underestimated CCT relative to USP (p < 0.001 for both pairs). FD-OCT also measured significantly thinner CCT than ORA did (p < 0.001).
Table 1: Central corneal thickness (CCT) measurements by Fourier domain optical coherence tomography (FD-OCT), ocular response analyzer (ORA), and ultrasound pachymetry (USP).

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Figure 1: Bland–Altman plots comparing central corneal thickness measurements using the three modalities. (A) Comparison between ocular response analyzer and Fourier domain optical coherence tomography (p < 0.001). (B) Comparison between ultrasound pachymetry and Fourier domain optical coherence tomography (p < 0.001). (C) Comparison between ultrasound pachymetry and ocular response analyzer (p < 0.001).

Click here to view


3.2. Intra-examiner repeatability and inter-examiner reproducibility of FD-OCT, ORA, and USP in CCT measurement

Although all three devices showed good agreement, the intra-examiner repeatability coefficient was smallest for FD-OCT and largest for ORA [Table 2]. These intra-examiner differences in ICC and repeatability for the three devices showed most clustered and reliable results using FD-OCT and most scattered measurement using ORA [Figure 2].
Table 2: Intra-examiner repeatability and inter-examiner reproducibility of FD-OCT, ORA and USP in measuring CCT.a

Click here to view
Figure 2: BlandeAltman plots comparing intra-examiner (A–C) and inter-examiner (D–F) differences in central corneal thickness measurements using Fourier domain optical coherence tomography (A and D), ocular response analyzer (B and E), and ultrasound pachymetry (C and F).

Click here to view


Inter-examiner ICC was largest for FD-OCT and smallest for USP. However, the inter-examiner reproducibility coefficient was smallest for FD-OCT and largest for ORA [Table 2]. Analysis of inter-examiner reproducibility revealed a lower ICC, higher coefficient of repeatability, and wider 95% limit of agreement than for intra-examiner differences by all three devices. Bland–Altman plots illustrated the greater inter-examiner difference over intra-examiner difference in measuring each individual CCT by all three methods [Figure 2].


  4. Discussion Top


While modern developments emphasize the importance of corneal thickness measurement and provide additional non-contact instruments, surgical criteria for corneal refractive surgery and IOP adjustment are based on USP measurement. Systematic evaluation of newly emerging instruments helps provide essential information on the interchangeability of measurements. Refractive surgeons and clinicians should be aware of their systematic bias when using these devices. Our results showed good agreement of CCT measured by FD-OCT, ORA, and UPS. However, these instruments showed a systematic difference: FD-OCT tended to underestimate corneal thickness relative to ORA and USP, while ORA showed a slight underestimation when compared to USP.

Previous studies with various OCT devices showed underestimations[25] (range: 6.4–49.4 μm) of CCT with respect to USP. Our result with FD-OCT agreed with these reports, showing an underestimation of 11.4 μm compared to USP measurement. There are several possible reasons for this discrepancy. First, OCT has better measurement centration and perpendicularity than UPS. UPS is operator dependent. While skilled examiners can minimize the effect of probe misalignment, this innate error cannot be abolished and may bias the ultrasound toward thicker measurements. Second, differences in design methodology between the OCT and USP could account for underestimation. OCT devices have a high axial resolution that allows corneal boundaries to be clearly defined by distinct signal peaks. Despite the fact that FD-OCT uses an 840-nm superluminescent diode source that allows less delineation of the anterior and posterior corneal boundaries than the 1310-nm source used by the Visante AS-OCT (Carl Zeiss Meditec, Inc., Dublin, CA, USA), both OCT devices still provide more than sufficient detail in their corneal imaging.[24] By contrast, the posterior reflection point in UPS may be located between Descemet’s membrane and the anterior chamber. The exact location of the signal peak, however, remains unknown. This ambiguity of signal peak location may contribute to at least part of the greater variation in USP measurement.

Alteration in corneal hydration due to use of topical 0.5% proparacaine hydrochloride prior to ORA and USP measurements should also be considered as a potential confounding factor. Two or more drops of topical anesthetics has been demonstrated to increase measured corneal thickness.[26] However, most researchers conclude that one drop of topical anesthetics does not affect corneal thickness measurement over a 10-minute examination period.[26] Our examinations used only one drop of 0.5% propar-acaine and were completed within 10 minutes, therefore, 0.5% proparacaine-related corneal hydration could be excluded. Topical proparacaine can potentially increase the measurement of corneal thickness by increasing tear film thickness. In theory, OCT measurement includes the tear film, whereas ultrasound contact probe displaces the tear film. Therefore, applying one drop of proparacaine after FD-OCT but before ORA and USP should not contribute to the difference in corneal thickness measured in our study.

The present study also assessed intra-examiner repeatability and inter-examiner reproducibility. FD-OCT had lower intra-examiner variability and inter-examiner variability than ORA and USP had. Compared to other OCT devices, FD-OCT showed equal if not better repeatability and reproducibility. Mohamed et al[24] have reported the ICC/repeatability coefficient for the Visante OCT to be 0.998/0.86% and 0.995/1.31% for intra- and inter-observer, respectively. Our FD-OCT measurements showed similar ICC values (0.998 and 0.993 for intra- and inter-observer, respectively) but a slight lower repeatability coefficient (0.64% and 1.16% for intra- and inter-observer, respectively).

Good repeatability and reproducibility of measurements depend on short acquisition time, consistent positioning over the same points during scanning, and corneal thickness variation along neighboring points. First, rapid acquisition time is essential to minimize both patient and machine motion artifacts during a scanning session. With an acquisition speed of 0.32 seconds, FD-OCT not only has a decisive advantage over conventional pachymetry, but also has a slight edge over the Visante OCT which requires 0.5 seconds per acquisition. ORA usually requires 2–5 seconds to measure corneal thickness. This limitation might contribute to its lower repeatability and reproducibility coefficient compared to USP and Visante AS-OCT.[24] Second, simple and consistent positioning of scanning over the same points during different scans also increased reliability of FD-OCT measurements. FD-OCT provides a clear fixation target and enables continuous monitoring of the participant’s eye to allow proper centering during a scanning session. The FD-OCT operator’s manual recommends aligning the aiming circle (inner circle, 4 mm diameter and outer circle, 6 mm diameter) with respect to the center of the pupil. We would recommend keeping a constantly lit room during examination. Corneal thickness is measured over the pupil center, thus, a constantly lit room allows the pupils to remain constricted during examinations, which eliminates the problem of pupil shift and facilitates examiners to locate precisely the true pupil center. By contrast, ORA and USP rely on the operator’s consistency in positioning over the same points during measurement, which inevitably is more prone to intra- and inter-observer variations. Third, FD-OCT uses an average 2-mm central zone as its measuring value, which helps eliminate any focal irregularities of the cornea that might be picked up by USP and maintains a good intra- and inter-examiner consistency.

Both ORA and USP utilize contact-based ultrasound methods, which are examiner dependent. Correct centering, perpendicular alignment, and corneal indentation of the probe can all affect the repeatability and reproducibility of measurement. This explains why the inter-examiner repeatability coefficient was higher than the intra-examiner repeatability coefficient for both ORA and USP. Besides calibration differences, we believe the rapid, automatic repeated measurement function may have perpetuated this difference. Unlike conventional single shot measurement with USP in which the examiner realigns the probe after each measurement, automatic measurement of ORA is potentially more susceptible to misalignment and to recording wrong corneal thickness due to incorrect positioning over the center. In addition, continuous measurement has the propensity for examiners to over-indent progressively the cornea during a measuring session, which would result in a lower corneal thickness measurement compared to that with USP. However, the intra-examiner repeatability and inter-examiner reproducibility in CCT measurement were good for ORA. ORA measures other parameters such as corneal hysteresis, corneal compensated IOP, and Goldmann-correlated IOP, therefore, it might provide more information for clinical practice than USP does, especially for glaucoma specialists and corneal/refractive surgeons.

Our study had some limitations. First, ORA and USP measurements were conducted back to back within 10 minutes. This could have increased the likelihood of epithelial trauma, albeit negligibly, which may have potentially affected the accuracy of USP measurements. Second, the volunteers participating in this study were all normal healthy candidates. More studies are needed to see if the systematic biases identified with normal individuals in this study stands in post-Laser-Assisted in situ Keratomileusis (LASIK), kera-toconus and corneal opacity populations. Third, choosing pupil center (FD-OCT) over the maximal reflection point (Visante OCT) as the reference point seemed to have no effect on repeatability and reproducibility in measuring normal corneas. However, the effect in special cases of advanced keratoconus, pellucid marginal degeneration, and keratoglobus remains to be verified. Progressive changing of cornea curvature in these patients may make it more difficult to find vertical reflection, making it a poor selection for reference point. Fourth, clinicians should be aware that FD-OCT, like time-domain OCT (Visante OCT), has not yet achieved true corneal mapping. Despite faster scanning speeds and more scanning points over time, FD-OCT still scans in only eight meridians and derives the thicknesses in each sector by interpolating points sampled along these meridians. Therefore, small areas of localized thickness variation between the sampled lines may not be reflected in the 24 map.[24]

Evaluating changes in corneal thickness among different days emphasizes the temporal variations of corneal thickness in each individual. This provides important information when evaluating outcomes of corneal surgery or corneal diseases. The major goal of this study was to examine the inter-observer and intra-observer reproducibility and repeatability of three devices, thus, we did not study the reproducibility among different days.

In conclusion, FD-OCT, ORA, and USP measurements were highly correlated. FD-OCT demonstrated the lowest inter and intra-observer variability, although it underestimated corneal thickness compared with ORA and USP. It also took less time and was easier to operate in corneal thickness measurement. However, significant discrepancies among instruments do exist and results from different instruments should be interpreted with necessary adjustment.

Conflicts of interest: All authors declare no conflicts of interest. None of the authors have any financial or proprietary interests in the products mentioned in this report.



 
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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2]


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  In this article
Abstract
1. Introduction
2. Methods
3. Results
4. Discussion
References
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