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1 November 2007 Accuracy of cornea and lens biometry using anterior segment optical coherence tomography
Mark C. M. Dunne, Leon N. Davies, James Wolffsohn
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Abstract
We assess the accuracy of the Visante anterior segment optical coherence tomographer (AS-OCT) and present improved formulas for measurement of surface curvature and axial separation. Measurements are made in physical model eyes. Accuracy is compared for measurements of corneal thickness (d1) and anterior chamber depth (d2) using-built-in AS-OCT software versus the improved scheme. The improved scheme enables measurements of lens thickness (d3) and surface curvature, in the form of conic sections specified by vertex radii and conic constants. These parameters are converted to surface coordinates for error analysis. The built-in AS-OCT software typically overestimates [mean±standard deviation(SD)]d1 by +62±4 μm and d2 by +4±88μm. The improved scheme reduces d1(-0.4±4 μm) and d2 (0±49 μm) errors while also reducing d3 errors from +218±90 (uncorrected) to +14±123 μm (corrected). Surface x coordinate errors gradually increase toward the periphery. Considering the central 6-mm zone of each surface, the x coordinate errors for anterior and posterior corneal surfaces reached +3±10 and 0±23 μm, respectively, with the improved scheme. Those of the anterior and posterior lens surfaces reached +2±22 and +11±71 μm, respectively. Our improved scheme reduced AS-OCT errors and could, therefore, enhance pre- and postoperative assessments of keratorefractive or cataract surgery, including measurement of accommodating intraocular lenses.

1.

Introduction

The anterior segment optical coherence tomographer (AS-OCT) (Visante™, Carl Zeiss Meditec, Dublin, California) has been used for anterior chamber depth measurements1, 2, 3 and phakic intraocular lens implant evaluations.4 Accurate measurements of anterior segment ocular dimensions are important for cataract surgery and keratorefractive surgery and to determine the risk of angle closure glaucoma.1 AS-OCT has built-in software to obtain corneal thickness and anterior chamber depth.4 However, we are unaware of any previous reports on (1) the accuracy of AS-OCT measurements using a model eye or (2) the measurement of cornea and lens surface curvature using this instrument. The purpose of our study was to carry out these assessments and present a new scheme for measuring axial distances and surface curvature.

2.

Methods

The physical model eye, in which the cornea and intraocular lens were varied, was described by Kirschkamp 5 and has been used for evaluation of phakometry6 and Hartmann-Shack aberrometry.7 The five variations studied included a range of corneal radii (anterior: 6.8to8.7mm ; posterior: 6.3to8.1mm ), lens radii (anterior: 7.6to13.6mm ; posterior: 8.3to4.5mm ), anterior chamber depths (1.9to4.1mm) , and lens thicknesses (3.5to4.5mm) . All model corneal thicknesses were 0.8mm . Model corneas and lenses were made from PMMA (refractive index 1.493) and were separated by water (refractive index 1.333).

For each model eye, three horizontal AS-OCT images, captured in the anterior segment single-image capture mode, were analyzed. The first image included corneal thickness, anterior chamber depth, and corneal surface curvature corrected using built-in AS-OCT software. The Visante software (version 1.0.12.1896) detects and fits the corneal surfaces through an edge-detection algorithm. This fit was manually optimized using the built-in AS-OCT software, particularly in the peripheral cornea where the fit was often poor. The second image included these parameters without correction. The third image involved repositioning the instrument head to enable measurement of uncorrected lens thickness and surface curvatures. The Visante software again attempts to detect the corneal surfaces, but as they are not present, distorts the images based on the first and second surface profiles detected from the anterior plane of the image. Therefore the surface correction was removed prior to analysis.

The AS-OCT does not currently measure surface curvature. A Liberty BASIC 4.0 (Shoptalk Systems, Framingham, Massachusetts) program was written to measure coordinates of corneal and lens surfaces to subpixel precision. A pixel-to-millimeter conversion factor [ 1pixel=22±0.1μm , mean±standarddeviation(SD) ] was determined by measuring the diameter of the recess in which model corneas were held. Twenty x and y coordinates were determined for each surface using a cursor. The x coordinate (xoriginal) and gray-scale value (G) of the pixel on which the cursor was centered were retrieved. Subpixel analysis involved sampling the gray-scale profile of a vertical line of pixels centered on the cursor. These pixels were assigned a location value (L) ranging from 5 ( 5pixels above the cursor) to +5 ( 5pixels below). The peak of the gray-scale profile was calculated and used to adjust x [Eq. 1]. Note that Eq. 1 works only for the edges of model corneas and lenses that appeared as a narrow band.

Eq. 1

xadjusted=xoriginal+(LG)G.
Conic sections were derived from surface coordinates using Eq. 2 in which r0 is the vertex radius, and p is the conic constant that determines whether the surface becomes flatter (p<1) or steeper (p> 1) in the periphery.8 A second-order polynomial fitted to y2 plotted as a function of x yielded coefficients for x (=2r0) and x2 (=p) :

Eq. 2

y2=2r0xpx2.
Our comparison of measured to known model eye dimensions involved three steps. In step 1, calculations were carried out to add optical distortion, due to refraction at the corneal and lenticular surfaces, to the known model eye dimensions. This step ensured that instrument distortion correction factors were transferable to any human eye. Having completed this step, any remaining discrepancy between known and measured eye dimensions was assumed to be due to instrument distortion. In step 2, averaged instrument distortion correction factors were derived for each eye component. Step 3 involved carrying out calculations (effectively the reverse of those carried out in step 1) to correct optical distortion.

Optical distortion was added to known model eye dimensions, in step 1, using well-known paraxial ray tracing formulas. These calculations made use of refractive indices corrected for OCT light wavelength4 (1.310μm) . Refractive indices were corrected using Eq. 3 in which nλ is the refractive index at wavelength λ , and n is the refractive index for white light.9

Eq. 3

nλ=n+0.05120.1455λ+0.0961λ2.
Averaged instrument correction factors, derived in step 2, for axial distances were calculated by dividing optically distorted axial distances, calculated in step 1, by the corresponding distances measured in uncorrected AS-OCT images. For surface curvature, optically distorted surface x coordinates were divided by uncorrected x coordinates, at fixed values of y . Model eye dimensions dictated the fixed y values used for corneas (from 0.5to5.0mm in 0.5mm steps) and lenses (from 0.5to3.0mm in 0.5mm steps). Surface x coordinates were calculated using

Eq. 4

x=r0(r02py2)0.5p.
The instrument distortion correction factor for the anterior corneal surface was determined using three steel ball bearings. The diameters of these were measured with Precision Gold electronic digital callipers (Maplin Electronics, South Yorkshire, England) and converted to radii of curvature; 7.49±0.005 , 7.94±0.003 , and 9.52±0.004mm ( mean±95% confidence interval). Table 1 shows the correction factors [Eqs. (5) to (11)] that were derived as described above and which are transferable to any human eye.

Table 1

AS-OCT instrument distortion correction factors.

ComponentCorrection FactorEquation Number
Surface curvature
 anterior cornea r1xI=r1xU(1.55820.0037y+0.0025y2) (5)
 posterior cornea r2xI=r2xU(1.54540.0012y+0.0020y2) (6)
 anterior lens r3xI=r3xU(1.8412+0.0501y0.0519y2) (7)
 posterior lens r4xI=r4xU(1.5373+0.0036y0.0035y2) (8)
Axial distances
 corneal thickness d1I=d1U(0.6350) (9)
 anterior chamber d2I=d2U(0.90020.0848d2U+0.0314d2U2) (10)
 lens thickness d3I=d3U(28.18413.634d3U+1.7017d3U2) (11)
These were derived from measurements on each model eye but are transferable to any human eye. Equations (5) to (8) convert surface x coordinates measured from uncorrected AS-OCT images (r1−4xU) , at any value of y , to values corrected for instrument distortion (r1−4xI) . Equations (9) to (11) convert axial distances measured from uncorrected AS-OCT images (d1−3U) to values corrected for instrument distortion (d1−3I)

Before applying paraxial formulas to correct optical distortion in step 3, it was necessary to convert the surface x coordinates corrected for instrument distortion (r14xI) into local sagittal radii of curvature (r14sI) . This was achieved by fitting second-order polynomial curves to the square of the fixed y coordinates (y2) plotted as a function of the surface x coordinates (r14xI) to calculate vertex radii (r140I) and conic constants (p14I) . The values r10I and p1I represented the fully corrected anterior corneal surface vertex radius (r10) and conic constant (p1) as this surface was viewed in air. Equation 12 was then used to calculate sagittal radii of curvature (r23sI) for each fixed y coordinate.10

Eq. 12

rs=[r02+(1p)y2]0.5.

Table 2 shows the paraxial formulae [Eqs. (13 to (45)] used to correct optical distortion, which are transferable to any human eye. These calculations were repeated for sagittal radii of curvature at each fixed y coordinate.

Table 2

Paraxial ray tracing formulas [Eqs. (13) to (45)] used to correct sagittal radii of curvature (r2−4sI) and axial distances (d1−3I) for optical distortion for each fixed y coordinate.

EquationEquation Number
Surface vertex powers (based on vertex radii r10I , r20I , and r30I )
F10I=[1000(n21)]r10I (13)
F20I=[1000(n3n2)]r20I (14)
F30I=[1000(n4n3)]r30I (15)
Ray trace for optically corrected corneal thickness (d1)
L1=1000d1I (16)
L1=L1+F10I (17)
d1=(1000n2)L1 (18)
Ray trace for optically corrected posterior corneal sagittal radius of curvature (r2s)
L1=1000(d1I+r2sI) (19)
L1=L1+F10I (20)
r2s=[(1000n2)L1]d1 (21)
Ray trace for optically corrected anterior chamber depth (d2)
L1=1000(d1I+d2I) (22)
L1=L1+F10I (23)
L2=L1{1[d1(1000n2)]L1} (24)
L2=L2+F20I (25)
d2=(1000n3)L2 (26)
Ray trace for optically corrected anterior lens sagittal radius of curvature (r3s)
L1=1000(d1I+d2I+r3sI) (27)
L1=L1+F10I (28)
L2=L1{1[d1(1000n2)]L1} (29)
L2=L2+F20I (30)
r3s=[(1000n3)L2]d2 (31)
Ray trace for optically corrected lens thickness (d3)
L1=1000(d1I+d2I+d3I) (32)
L1=L1+F10I (33)
L2=L1{1[d1(1000n2)]L1} (34)
L2=L2+F20I (35)
L3=L2{1[d2(1000n3)]L2} (36)
L3=L3+F30I (37)
d3=(1000n4)L3 (38)
Ray trace for optically corrected posterior lens sagittal radius of curvature (r4s)
L1=1000(d1I+d2I+d3I+r4sI) (39)
L1=L1+F10I (40)
L2=L1{1[d1(1000n2)]L1} (41)
L2=L2+F20I (42)
L3=L2{1[d2(1000n3)]L2} (43)
L3=L3+F30I (44)
r4s=[(1000n4)L3]d3 (45)
Refractive indices ( n2 to n4 ) were corrected for AS-OCT light wavelength using Eq. 3 (see text). These equations are transferable to any human eye.

Finally, a scheme described by Douthwaite11 was adopted to derive optically corrected vertex radii (r240) and conic constants (p24) from the optically corrected sagittal radii (r24s) calculated for each fixed y coordinate, as described in Table 2. This involved plotting rs2 (on the y axis) against y2 (on the x axis) to produce a straight line for which, according to Eq. 46, the intercept is ro2 and the slope is (1p) .

Eq. 46

rs2=r02+(1p)y2.

3.

Results

Figure 1 shows the profiles and axial separations and Table 3 the errors of uncorrected and corrected measurements made in all five model eyes. Uncorrected AS-OCT measurements overestimate cornea and lens thickness while underestimating anterior chamber depth and surface curvature. Correction with built-in AS-OCT software substantially reduced these errors, but greater accuracy was achieved with the new scheme.

Fig. 1

Profiles and axial separations of actual surfaces (solid lines) compared to uncorrected measured surfaces (dashed lines) and those corrected using built in AS-OCT software (dashed lines) and the equations shown in Tables 1, 2 (circles). Each profile represents the average of the five model eyes tested.

064023_1_044706jbo1.jpg

Table 3

Errors ( mean±SD , μm ) for ocular components measured in five model eyes using AS-OCT with and without correction using in-built software and the new scheme.

ComponentUncorrectedCorrected
AS-OCT SoftwareNew Scheme
Corneal thickness +60±0 +62±4 0.4±4
Anterior chamber depth 96±111 +4±88 0±49
Lens thickness +218±90 +14±123
Anterior corneal surface 221±33 68±15 3±10
Posterior corneal surface 264±69 88±22 0±23
Anterior lens surface 264±70 +2±22
Posterior lens surface 246±213 +11±71
Coordinate errors are shown for corneal and lens surfaces. As surface x coordinate errors gradually increased toward the periphery, these are only shown for y coordinate values of 3mm . This represents the maximum errors expected when sampling surfaces over a zone of 6mm diameter, i.e., similar to corneal ablation zones in keratorefractive surgery or optic zone diameters of intraocular lenses. Positive values represent overestimated axial distances or surface curvature.

4.

Discussion

AS-OCT is a noncontact procedure capable of providing a comprehensive evaluation of anterior segment biometry. While there have been several reports on the use of model eyes to assess biometric accuracy, that of Dubbelman 12 using corrected Scheimpflug images is of particular relevance as it also covered the entire anterior segment with a noncontact technique. Their typical levels of accuracy were similar to ours for corneal thickness (1 compared to 0.4μm ) and lens thickness (44 compared to 14μm ). After conversion of their surface radius errors to x coordinate errors at y=3mm , their errors for the anterior cornea (2 compared to 3μm ), posterior cornea (1 compared to 0μm ), anterior lens (2 compared to 2μm ), and posterior lens (4 compared to 11μm ) were also comparable with ours. Interestingly, anterior corneal surface errors of both imaging techniques fall within clinically acceptable limits13 of 0.02mm (corresponding to x coordinate errors of 2μm at y=3mm ).

Kashiwagi 14 used a model eye to assess the accuracy of their scanning peripheral anterior chamber depth analyzer and found errors of up to 1.8%. This equates to errors of up to 70μm and is, again, comparable with those of our study ( 98μm based on two SDs from the mean).

Barry 6 used different variations of the model eye described in our study to assess the accuracy of a Purkinje imaging method and found that anterior and posterior lens surface radii could be measured to an accuracy of 0.02to0.10mm and 0.10to0.55mm , respectively; equivalent to x coordinate errors (at y=3mm ) of 1 to 5 and 16to96μm , respectively. These errors are a little higher than those of Scheimpflug and AS-OCT imaging techniques. A more detailed comparison between Scheimpflug and Purkinje imaging methods has been described elsewhere.15

Paraxial ray tracing equations were used to demonstrate that these produce results to acceptable levels of accuracy. Exact ray-tracing formulas may further improve accuracy but at the loss of convenience; clinicians can program spread sheets with the paraxial equations shown in Table 2 rather than having to purchase specialist software for exact ray tracing.

We were concerned that instrument distortion correction factors for corneal thickness might not be transferable to human eyes as all model eyes had a fixed corneal thickness of 0.80mm . To test this, we made further measurements on five contact lenses with the same anterior surface radii (7.8mm) and refractive indices (1.493) but with central thicknesses of between 0.3 and 0.7mm in 0.1mm steps. Corneal thickness instrument correction factors and errors for these did not differ from those shown in Tables 1, 3.

Although the formulas shown in Table 1 adequately correct instrument distortion, exploration of the possible sources of this distortion was beyond the scope of this study. Such exploration would also require commercially sensitive information about the instrument’s design.

In conclusion, the improved computing scheme reduces the errors of the built-in AS-OCT software and enables measurement of surface curvature in the form of vertex radii and conic constants. Further, the equations of our new scheme are made available for readers to use in their own clinics and laboratories. The accuracy of this technique is comparable to that of other noncontact techniques. Surface curvatures can be measured to within clinically acceptable limits over zones of at least 6mm diameter, i.e., similar to optic zone diameters of intraocular lenses and corneal ablation zones in keratorefractive surgery.16

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©(2007) Society of Photo-Optical Instrumentation Engineers (SPIE)
Mark C. M. Dunne, Leon N. Davies, and James Wolffsohn "Accuracy of cornea and lens biometry using anterior segment optical coherence tomography," Journal of Biomedical Optics 12(6), 064023 (1 November 2007). https://doi.org/10.1117/1.2821844
Published: 1 November 2007
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