Repeatability and Reproducibility of Pachymetric Mapping with Visante Anterior Segment–Optical Coherence Tomography

Abstract

purpose. To determine the repeatability and reproducibility of central and peripheral corneal pachymetry mapping with anterior segment–optical coherence tomography (AS-OCT).

methods. An observational cross-sectional study involving two groups: 27 healthy eyes and 20 eyes with keratoconus. Each subject underwent scanning sessions with AS-OCT to determine intraobserver repeatability, interobserver reproducibility, and additionally for healthy eyes, intersession reproducibility for different regions of the cornea up to a 10-mm diameter. Main outcome measures were reproducibility and repeatability coefficients, intraclass correlation coefficients, and coefficients of variation of the average central (0–2 mm), pericentral (2–5 mm), transitional (5–7 mm), and peripheral (7–10 mm) corneal thicknesses generated by the Visante AS-OCT (Carl Zeiss Meditec, Inc., Dublin, CA) pachymetric mapping protocol.

results. The coefficients of repeatability were less than 2% in healthy subjects and less than 3% in patients with keratoconus. The reproducibility coefficients were less than 2% and 4% in healthy subjects and patients with keratoconus, respectively. There was no significant difference between scans obtained by different observers or during different visits. The intraclass correlation coefficients were greater than 0.99 and 0.97 in healthy subjects and patients with keratoconus, respectively.

conclusions. With the pachymetric mapping protocol of Visante AS-OCT, these results suggest that central and peripheral corneal thickness measurements in healthy subjects and in eyes with keratoconus are repeatable and reproducible.

The measurement of corneal thickness has various important applications in monitoring corneal edema and ectatic dystrophies such as keratoconus, measuring intraocular pressure, and calculating risk of progression to glaucoma in patients with ocular hypertension. ^{1 2} It is also essential to the selection of patients for and the planning of refractive surgical procedures such as LASIK and phototherapeutic keratectomy (PTK). 

Traditional methods of corneal thickness measurement employ spot pachymetry techniques, such as ultrasound pachymeters, which are reliable, easy to use, and inexpensive. ³ However, they require corneal contact, and their reliability is highly dependent on accurate probe alignment by the operator. Moreover, the points of reflection of ultrasound within the cornea are ill defined. ⁴ Pachymetric mapping systems, such as with slit scanning systems or Scheimpflug systems, permit quantification of corneal thickness over a wider area, allow easy and rapid visualization of abnormal thickness patterns such as keratoconus, and allow better preoperative planning of refractive procedures that do not just involve the center of the cornea. Slit scanning corneal topography/pachymetry (Orbscan II; Bausch & Lomb, Inc., Rochester, NY) is the current reference pachymetric mapping system for many surgeons performing refractive procedures. It generates corneal thickness maps by using a ray-tracing algorithm based on optical principles of reflection of light, but its limited resolution places constraints on identification of corneal surface reflections in the presence of corneal opacities. 

Optical coherence tomography (OCT) is a non–contact-imaging technique based on principles of low-coherence interferometry. The commercially available retinal OCT scanner (Carl Zeiss Meditec, Inc., Dublin, CA) can be used to obtain pachymetry measurements automatically or manually from a single linear cross-sectional image through a selected location. ⁵ Anterior segment (AS)-OCT employs a longer wavelength light source that allows higher resolution imaging and better delineation of the anterior and posterior surfaces of the cornea with the computer tracing algorithm. ⁶ Its high-speed scanning system enables the generation of pachymetric maps, in addition to linear cross-sectional images. To our knowledge, no study on the reproducibility of peripheral corneal thickness measurements with this algorithm has been published. The purpose of this study was to determine repeatability and reproducibility of pachymetry mapping over a 10-mm diameter in healthy eyes and in keratoconus eyes, by using the AS-OCT pachymetry mapping protocol. 

Subjects and Methods

The local ethics committee approved this observational cross-sectional study, which adhered to the tenets of the Declaration of Helsinki. Twenty-seven healthy eyes from 27 subjects (12 men and 15 women; mean age, 44.2 years; range, 18–71) and 20 eyes from 10 subjects (7 men and 3 women; mean age 30.8 years; range 20–42), with diagnoses of keratoconus were consecutively recruited into the study over a 4-month period at Hong Kong Eye Hospital. Inclusion criteria for healthy subjects included best corrected visual acuity better than 20/25, normal corneal appearance on slit lamp, and no history of prior ocular surgery or trauma. Keratoconus was diagnosed in an eye if there was scissoring reflex on retinoscopy and central or paracentral steepening of the cornea on computerized topography with at least one of the following slit lamp findings of keratoconus: central or paracentral thinning, anterior bulging or conicity, hemosiderin deposition (Fleischer’s ring), stromal striae (Vogt’s striae), Descemet’s breaks, apical scars, and subepithelial fibrosis. We included a spectrum of keratoconic eyes of different severity in this study with keratometry readings as follows: early, ≤51 D (nine eyes); moderate, between 51 and 56 D (five eyes); and severe ≥56 D (six eyes). All subjects underwent visual acuity testing with refraction, slit lamp and fundal examination. All subjects gave informed consent after explanation of the purpose of the study and before investigations were performed. Best corrected visual acuity in the healthy group ranged from 20/20 to 20/25; and in the group with keratoconus, from 20/20 to 20/60. All subjects were able to fixate well at the internal fixation target used. 

The scanner used was the Food and Drug Administration–approved Visante AS-OCT (Carl Zeiss Meditec). The performance of the anterior segment OCT prototype (Carl Zeiss Meditec, Inc.) has been described previously. ⁷ The Visante AS-OCT uses a 1310-nm superluminescent diode source, and operates up to a speed of 4000 axial scans per second. The “pachymetry map” protocol, consisting of 10-mm radial lines in eight equally spaced meridians centered on the corneal vertex reflection, was used. Each radial line is composed of 128 A-scans, and the entire map contains 1024 A-scans acquired in 0.5 seconds. The software automatically delineated anterior and posterior corneal boundaries in the cross-sectional images along the eight meridians. Pachymetry maps were generated over a 10-mm diameter circle, and thickness values were provided for four sectors: a central zone (0–2 mm) and pericentral (2–5 mm), transitional (5–7 mm), and peripheral (7–10 mm) zones which were further subdivided into eight sectors in between the eight radial lines. Corneal thicknesses for each sector were generated by interpolation, and presented as maximum, minimum, and average thicknesses (Fig. 1) . All measurements were taken between 10 AM and 4 PM (to minimize any effect of diurnal variation on corneal thickness). 

Subjects underwent scanning sessions with the pachymetry mapping protocol as follows: two scans were performed by operator 1 (SM) and a further scan by operator 2 (GL) at the first session, to determine intraobserver and interobserver reproducibility. Intersession reproducibility was assessed by an additional scan performed by operator 1 a week later in healthy subjects. In eyes with keratoconus, intervisit reproducibility was not assessed, as observed differences may have been the result of variations in corneal thickness. Patients were instructed to look at an internal fixation target during scanning. Rough alignment was achieved by centering the cornea on the real-time video image, and fine alignment was adjusted using a horizontally directed scan. Cross-sectional scans were displayed continuously on the integrated video monitor at a rate of up to eight frames per second. The operator adjusted the system to position the vertex at the center of the AS-OCT image and to maximize the vertex reflection. Patients were instructed to keep their eyes wide open during scanning, and when necessary, the lids were gently held apart (with care not to exert pressure on the globe) to ensure that the lids did not block the 10-mm diameter corneal mapping. Acceptable scans were selected as soon as they appeared. Images were judged to be of adequate quality based on the following criteria: good demarcation of the anterior and posterior corneal boundaries and absence of artifacts owing to motion or eyelid margins. Subjects were repositioned between OCT scans. For paired intraobserver measurements, not more than 3 minutes elapsed between the first and second measurements. For paired interobserver measurements, not more than 10 minutes elapsed between the first and second measurements. 

Statistical Analysis

Mean and SD of average corneal thickness measurements in each of the four regions (central, pericentral, transitional, and peripheral) in healthy eyes and eyes with keratoconus were calculated for each observer during each visit. Repeatability and reproducibility coefficients and intraclass correlation coefficients (ICCs) were calculated for the average corneal thicknesses in each of the four regions, as summarized in the pachymetry map printouts. As recommended by Bland and Altman and proposed by the British Standard Institution, the coefficient of repeatability was defined as 2 SDs of the differences between pairs of measurements in the same subjects obtained during the same visit by the same observer divided by the average of the means of each pair of readings. ^{8 9 10} The coefficient of reproducibility was defined as 2 SDs of the difference between measurements obtained during repetition of the test under different conditions (different sessions or different observers), divided by the average of the means of each pair of readings. For intraobserver, interobserver, and intersession measurements, coefficients of variation were obtained by taking the within-subject SD divided by the within-subject mean (pooled for the 27 healthy eyes, and similarly pooled for the 20 keratoconic eyes). In addition, intraobserver, interobserver, and intersession reproducibility were estimated using the intraclass correlation coefficient (ICC) determined on the basis of analysis of variance for mixed models for each situation as proposed by Bartko and Carpenter. ¹¹ Values close to 1 indicate high reproducibility of the method under study. This calculation was performed for each corneal zone. Statistical significance was defined as P < 0.05. Statistical analyses were performed with a commercially available statistical software package (SPSS for Windows, ver. 11.0; SPSS Inc, Chicago, IL). 

Results

For the 27 healthy eyes and 20 keratoconic eyes, the mean and SDs of corneal thickness measurements in the central region (0–2 mm) the and pericentral (2–5 mm), transitional (5–7 mm), and peripheral (7–10 mm) regions obtained at each session are presented in Table 1 . The coefficients of variation, and coefficients of repeatability and reproducibility (in micrometers) for each corneal region are shown in Table 2and should be interpreted with respect to the mean corneal thickness measurements (Table 1)and the axial resolution of the scanning protocol used. The ICCs for each region as estimates of intraobserver, interobserver, and intersession reproducibility are also presented in Table 2 . There was no significant association between the within-subject SD and mean intraobserver, interobserver, or intrasession corneal thickness measurements. The Wilcoxon matched-pairs test showed no significant systematic differences between measurements obtained by different observers or during different visits in healthy or keratoconus eyes. There were no missing or excluded data. 

Discussion

In our study, Visante AS-OCT (Carl Zeiss Meditec, Inc.) enabled high-resolution qualitative and quantitative imaging of the cornea. It could be of particular use in the monitoring of patients with corneal disease, such as abnormal thickening or thinning, and to assess the efficacy of therapeutic intervention in longitudinal clinical trials. The clinical utility of any instrument depends strongly on the reproducibility of its measurements. The purpose of this study was therefore to characterize the repeatability and reproducibility of central and peripheral corneal thickness measurements with the pachymetry mapping protocol of Visante AS-OCT. 

In this study, corneal thickness measurements obtained with the pachymetry mapping protocol of Visante AS-OCT were shown to be repeatable and reproducible. Measurements made for healthy subjects showed that the coefficient of repeatability ranged from 1.1% to 1.6%, and coefficients of reproducibility ranged from 1.1% to 1.8% in central and peripheral regions of the cornea. In the periphery, mean reproducibility coefficients were less than 13 μm (i.e., the difference between any two measurements for the same subject is expected to be less than 13 μm for 95% of all pairs of measurements). A change in peripheral corneal thickness greater than 13 μm would therefore be more likely to represent actual change rather than measurement error. For keratoconic eyes, the coefficient of repeatability was less than 2.1% in all regions, and the coefficient of reproducibility was less than 3.3% in all regions. In eyes with keratoconus, mean reproducibility coefficients were less than 17 μm in all regions. The ICCs were particularly high for all regions in both groups, confirming good reliability of measurements with the mapping protocol. Intrasession coefficients of variation of less than 1% and 2% in healthy subjects and patients with keratoconus, respectively, also demonstrated the high level of intrasession reproducibility. Similarly, Li et al. ⁷ also reported good reproducibility of approximately 2 μm over a 7-mm diameter with the anterior segment OCT prototype (Carl Zeiss Meditec, Inc.) in healthy eyes undergoing LASIK. 

Repeatability and reproducibility of measurements are dependent on such factors as variations in corneal thickness along neighboring points, consistency of positioning over the same points during scanning, and number of sampling points for each region. As expected, measurements in the central and paracentral regions of the cornea in keratoconic eyes showed greater variability, compared with similar regional measurements in normal eyes. This finding was especially true in eyes in which significant variations in corneal contour due to corneal thinning were present (Fig. 1) . Minimal changes in scan positioning in these areas resulted in significantly different corneal thickness measurements. However, our results showed that repeatability and reproducibility remained good in all regions of the cornea. Measurement variability in eyes with keratoconus might be expected to decrease if more eyes were at a milder stage of the disease and to increase if more eyes had severe disease. 

The consistency of positioning over the same points during different scans also affects the reliability of measurements. AS-OCT enables continuous monitoring of the subject’s eye during scanning, and the vertical flare from the vertex further assists in confirming proper centration. For the pachymetry mapping protocol, each scan combining the eight radial line scans requires a slightly longer acquisition time (0.5 second) than each line scan obtained separately using the single scan protocol (0.125 second). This could result in difficulty in positioning the scans over the same location each time, owing to eye motion or unstable fixation. However, this effect is likely to be minimal as acquisition with the mapping protocol is still rapid. On the other hand, scan positioning errors with separate single scan alignments, especially in locations where the contour of the cornea varies, such as in areas of thinning in keratoconic eyes, could result in higher variability of measurements. Acquiring all the tomograms with a single alignment may reduce such scan positioning errors. However, there are 128 A-scans per line in the pachymetry mapping protocol, compared with 256 scans per line in the single line scan protocol. The number of points measured also decreases from the center to the periphery of the cornea, where the radial lines are more spaced, which may account for the greater variability of measurements in the peripheral cornea. Visante AS-OCT can also generate difference maps allowing for rapid visualization of the spatial distribution of differences and areas of measurement discrepancies (Fig. 2) . Intersession reproducibility might further be improved by software for cross-correlation and coregistration of pairs of pachymetry maps before calculation of such difference maps, so as to minimize errors resulting from inconsistent positioning in different sessions. 

In addition, software can affect the reliability of measurements with AS-OCT. Visante generates cross-sectional images as gray-scale or false-color representation of differential back-scattering contrast between various tissues. Anterior and posterior corneal boundaries are identified in the computer algorithm by signal peaks at the air–tear film interface and cornea–aqueous interface on each A-scan. Automated scan processing allows correction for image distortion due to refractive index transition at the air–cornea interface. From these dewarped images, corneal thickness is measured as the distance between the anterior and posterior corneal surfaces along lines perpendicular to the anterior corneal surface at the point of measurement, to generate thickness profiles on each of the eight meridional cross-sections. The vertical flare on cross-sectional images, produced when strong reflection from the anterior vertex of the cornea saturates the dynamic range of the OCT system, results in slightly broadened anterior and posterior signal peaks which might blur the corneal boundaries identified by the algorithm and contribute to measurement errors. Nevertheless, we found that these computer algorithm-identified boundaries generally had good agreement with boundaries identified manually with calipers and that its effect is likely to be minimal. 

High repeatability of central corneal thickness (CCT) measurements has been demonstrated with the current gold standard spot pachymetry technique, ultrasound pachymetry. Marsich and Bullimore ¹² reported 95% limits of agreement (LOA) of −22 to +24 μm in healthy eyes. Although spot pachymetry is inexpensive and easy to perform, thicknesses obtained are only spot measurements highly dependent on the operator’s probe placement. Other pachymetric mapping systems such as Orbscan (Bausch & Lomb) have also been shown to have good reproducibility of CCT measurements, with 95% LOA of −10 to +17 μm, but poorer peripheral reproducibility, with 95% LOA of −39 to +43 μm (i.e., a range of approximately 80 μm), which is most likely the result of a greater number of incomplete or erroneous slit images acquired from the peripheral cornea using slit-scanning technology. ¹³  

Our results suggest that AS-OCT has better central and peripheral repeatability than Orbscan, the current reference pachymetric mapping system. One of the reasons for this may be the much faster acquisition time with Visante AS-OCT (0.5 seconds), compared with Orbscan II (2.1 seconds), which can help reduce errors due to unstable fixation. Orbscan reconstructs corneal height profiles from optical sections obtained from slit-scanning using ray-tracing algorithms. With Orbscan II, scattering from corneal haze and stromal interfaces tends to interfere with the identification of corneal surface reflections, and corneal thickness may be underestimated. Orbscan II may also be unable to construct maps when there are significant irregularities in the corneal surface. In contrast, AS-OCT offers higher resolution, cross-sectional tomographic imaging of the cornea by measuring backscattered light. There is clearer delineation of anterior and posterior corneal boundaries, free of interference from stromal opacities, as the longer wavelength light scatters less in opaque tissues, allowing for deeper penetration. This consistency of pachymetric mapping in eyes with corneal opacities has recently been demonstrated by Khurana et al. ¹⁴  

Our study showed the reliability of central and peripheral pachymetric mapping with Visante AS-OCT. The validity of measurements by an instrument is the other important issue; we have previously shown that AS-OCT underestimates central corneal thickness compared with ultrasound pachymetry. ¹⁵ Further study of the agreement of AS-OCT measurements in different regions of the cornea, including the periphery, with spot pachymetry or other pachymetric mapping systems is needed for a full evaluation of the utility of measurements with this new imaging device. 

Clinicians should also note that with time-domain OCT systems such as Visante AS-OCT, scanning speed still does not allow for “true corneal mapping,” since the cornea is scanned in eight meridians only, and thicknesses in each sector are derived by interpolation of points sampled along these meridians (which become more spaced toward the periphery), so that small areas of localized thickness variations between the sampled lines may not be reflected in the map. This limitation may be overcome with newer spectral domain OCT systems, ¹⁶ and optical frequency domain imaging (OFDI) systems ¹⁷ which, although not yet commercially available, have increased speed and sensitivity, can generate images of better quality and are able to provide more representative pachymetric mapping in a shorter time interval. 

In conclusion, the pachymetry mapping protocol of Visante AS-OCT offers clinicians a rapid and easy method to obtain corneal thickness maps with a high degree of repeatability and reproducibility. These findings are particularly useful, as they indicate that corneal thickness changes of greater than 2% in any region of the cornea up to a diameter of 10 mm in healthy eyes are likely to be caused by actual changes in thickness rather than by measurement errors of the mapping system. Our results suggest that the pachymetry mapping protocol is also a reliable method for monitoring patients with corneal thinning, which may be useful for designing staging systems or assessing efficacy of therapeutic interventions. 

 Submitted for publication May 19, 2007; revised July 18, 2007; accepted October 15, 2007.

 Disclosure: S. Mohamed, None; G.K.Y. Lee, None; S.K. Rao, None; A.L. Wong, None; A.C.K. Cheng, None; E.Y.M. Li, None; S.C.C. Chi, None; D.S.C. Lam, None

 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.

 

Figure 1.

 

Pachymetric mapping in a patient with keratoconus. The AS-OCT pachymetry maps generated by different scans performed during the same session by the same operator. Images on the right represent one of the eight cross-sectional images (orientation, 45°), with corneal boundaries identified by the algorithm (white lines) used to generate the corresponding maps. Vertical flares from the vertex of the cornea are seen in both images. Average, maximum, and minimum corneal thicknesses for the four corneal zones (central, pericentral, transitional, and peripheral) are shown at the bottom right of the pachymetry maps. A small displacement of the tomogram resulted in a significant variation of the average CCT measurement, from 385 (A) to 402 (B) μm, mostly because of the large variation in corneal contour due to thinning at this location.

Table 1.

 

Average Corneal Thickness Measurements

A. Healthy Eyes
	Zone (mm)	R1	R2	R3	R4	Δ (R1 − R2)	Δ (R1 − R3)	Δ (R1 − R4)
	0–2	543.4 ± 34.2	543.1 ± 34.0	542.0 ± 34.6	543.4 ± 35.7	0.3 (−0.65–1.24)	1.33 (−0.22–2.88)	0.07 (−1.36–1.51)
	2–5	570.9 ± 34.8	570.9 ± 35.2	571.1 ± 36.1	572.2 ± 36.2	0.07 (−1.16–1.31)	−0.26 (−2.23–1.71)	−1.37 (−3.05–0.30)
	5–7	615.5 ± 37.5	616.2 ± 37.6	615.6 ± 38.5	616.5 ± 39.1	0.63 (−0.96–2.22)	−0.07 (−2.30–2.16)	−1.00 (−2.37–0.37)
	7–10	677.2 ± 40.3	676.1 ± 39.7	676.3 ± 41.6	673.9 ± 40.5	1.11 (−1.10–3.32)	0.89 (−1.56–3.34)	3.30 (1.22–5.37)
	n	27 eyes	27 eyes	27 eyes	27 eyes	27 pairs	27 pairs	27 pairs

B. Eyes with Keratoconus
	Zone (mm)	R1	R2	R4	Δ (R1 − R2)	Δ (R1 − R4)
	0–2	467.3 ± 49.3	468.6 ± 49.0	466.1 ± 50.3	−1.35 (−3.06–0.36)	0.55 (−1.04–2.14)
	2–5	515.2 ± 41.7	514.6 ± 40.7	512.7 ± 41.9	−0.43 (−2.51–1.66)	2.45 (−0.49–5.39)
	5–7	577.6 ± 42.6	575.3 ± 39.9	573.4 ± 41.1	2.30 (−5.11–0.51)	4.20 (−0.60–7.80)
	7–10	646.1 ± 39.3	643.2 ± 37.9	640.0 ± 38.2	2.85 (−0.26–5.96)	6.05 (2.00–10.10)
	n	20 eyes	20 eyes	20 eyes	20 pairs	20 pairs

 Data are expressed at the mean ± SD. R1 represents measurements by observer 1; R2 represents repeat measurements by observer 1 at the same session. R3 represents repeat measurements by observer 1 at a second session (1 week later); R4 represents measurements by observer 2 at the first session. Δ represents difference. Data in parentheses represent 95% CI.

Table 2.

 

Intraclass Correlations and Coefficients of Repeatability and Reproducibility

A. Healthy Eyes
	Zone (mm)	ICC (R1, R2)	ICC (R1, R3)	ICC (R1, R4)	CoR (R1, R2)	CVw	CoR (R1, R3)	CVw	CoR (R1, R4)	CVw
	0–2	0.998 (0.995–0.999)	0.993 (0.986–0.997)	0.995 (0.988–0.998)	4.63 (0.86%) (3.69–6.40)	0.3% (0.2–0.4)	7.68 (1.41%) (6.04–10.53)	0.5% (0.4–0.7)	7.09 (1.31%) (5.60–9.73)	0.5% (0.4–0.6)
	2–5	0.996 (0.991–0.998)	0.990 (0.978–0.996)	0.993 (0.984–0.997)	6.12 (1.07%) (4.82–8.40)	0.4% (0.3–0.5)	9.78 (1.71%) (7.71–13.39)	0.6% (0.5–0.8)	8.29 (1.45%) (6.54–11.36)	0.5% (0.4–0.7)
	5–7	0.994 (0.987–0.997)	0.989 (0.976–0.995)	0.996 (0.991–0.998)	7.87 (1.28%) (6.21–10.81)	0.5% (0.4–0.6)	11.06 (1.80%) (8.70–15.13)	0.6% (0.5–0.9)	6.79 (1.10%) (5.35–9.31)	0.4% (0.3–0.5)
	7–10	0.990 (0.978–0.996)	0.989 (0.975–0.995)	0.992 (0.981–0.996)	10.94 (1.62%) (8.62–15.00)	0.6% (0.5–0.8)	12.10 (1.79%) (9.54–16.60)	0.7% (0.5–0.9)	10.25 (1.52%) (8.09–14.08)	0.5% (0.4–0.7)
	Pairs (n)	27	27	27	27	27	27	27	27	27

B. Eyes with Keratoconus
	Zone (mm)	ICC (R1, R2)	ICC (R1, R4)	CoR (R1, R2)	CVw	CoR (R1, R4)	CVw
	0–2	0.997 (0.993–0.999)	0.988 (0.969–0.995)	7.17 (1.53%) (5.46–10.48)	0.6% (0.4–0.8)	15.24 (3.22%) (11.61–22.29)	1.2% (0.9–1.8)
	2–5	0.997 (0.991–0.999)	0.989 (0.971–0.996)	6.56 (1.28%) (5.07–9.73)	0.5% (0.3–0.7)	12.30 (2.39%) (9.37–17.96)	0.9% (0.7–1.3)
	5–7	0.989 (0.973–0.996)	0.983 (0.957–0.993)	11.74 (2.04%) (8.95–17.18)	0.7% (0.6–1.1)	15.07 (2.62%) (11.45–22.00)	0.9% (0.7–1.4)
	7–10	0.985 (0.962–0.994)	0.975 (0.937–0.990)	13.02 (2.02%) (9.90–19.01)	0.8% (0.6–1.1)	16.95 (2.64%) (12.89–24.78)	1.0% (0.7–1.4)
	Pairs (n)	20	20	20	20	20	20

 Data in parentheses represent 95% CI. Data in the top half of the “CoR” columns are represented by x (y). This means that the difference between two measurements for the same subject is expected to be less than x micrometers for 95% of pairs of measurements; y is the value of the repeatability coefficient. R1 represents measurements by observer 1; R2 represents repeat measurements by observer 1 at the same session; R3 represents repeat measurements by observer 1 at a second session (1 week later); R4 represents measurements by observer 2 at the first session. CoR (R1, R2) coefficient of repeatability; CoR (R1, R3), CoR (R1, R4), coefficients of reproducibility; CV_w, coefficient of variation.

Figure 2.

 

Difference maps generated by AS-OCT software illustrating the spatial distribution of interobserver differences in pachymetry measurements in a normal (A) and a keratoconic (B) eye. In the normal eyes, differences in measurements increased toward the periphery. In keratoconic eyes, differences increased near areas of central or paracentral thinning. Similar difference maps could be generated to enable rapid assessment of intersession changes in pachymetry measurements in longitudinal studies.