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Originally published In Press as doi:10.1167/iovs.07-1216 on September 4, 2008
 (Investigative Ophthalmology and Visual Science. 2009;50:290-294.)
© 2009 by The Association for Research in Vision and Ophthalmology, Inc.
doi:10.1167/iovs.07-1216
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Comparison of Lens Thickness Measurements Using the Anterior Segment Optical Coherence Tomography and A-scan Ultrasonography

Yangfa Zeng, Yizhi Liu, Xing Liu, Chuying Chen, Yuanling Xia, Miaoling Lu, and Mingguang He

From the State Key Laboratory of Ophthalmology, Zhongshan Ophthalmic Center, Sun Yat-sen University, Guangzhou, People’s Republic of China.


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
PURPOSE. To assess lens thickness (LT) measurements with anterior segment optical coherence tomography (AS-OCT) in comparison with A-scan ultrasonography (A-scan US).

METHODS. Enrolled were 66 eyes of 48 phakic elderly volunteers aged ≥50 years and 56 eyes of 56 young participants aged 18 to 40 years. LT was measured with the internal manual caliper tools in AS-OCT. The A-scan US measurements were based on an average of 10 consecutive automatic measurements. Reproducibility was assessed by three measurements each with AS-OCT and A-scan US independently obtained by two observers who were masked to one another’s results.

RESULTS. The failure rates of AS-OCT and A-scan US were 9.1% and 7.6%, respectively, in elderly subjects, but no failure was observed in young subjects. The LT values measured by AS-OCT were significantly greater than A-scan US; the paired difference was 0.135 mm in elderly and 0.101 mm in young subjects (P < 0.001). These differences did not correlate with the nuclear cataract grades (r = 0.078, P = 0.558). Intraobserver agreement on AS-OCT (95% limits of agreement [LoA]: –0.049 to +0.045 mm; ICC: 0.999) was better than A-scan US (95%LoA: –0.194 to +0.218 mm; ICC: 0.974). The 95% LoA of interobserver agreement using AS-OCT and A-scan were –0.084 to +0.073 and –0.278 to +0.239 mm, respectively, and the ICCs were 0.996 and 0.960, respectively.

CONCLUSIONS. AS-OCT can be used to measure lens thickness in most eyes with clear or opacified lenses. It appears to be an alternative means of measuring lens thickness, particularly when a noncontact method is needed.

The accurate measurement of lens thickness (LT) is of paramount importance for biometric studies of myopia and primary angle-closure glaucoma (PACG).1 2 3 Thickening and anterior positioning of the lens is recognized as a major anatomic predisposing factor for the development of angle closure.2 4 5 A-scan ultrasonography (A-scan US) and Scheimpflug photography are the techniques commonly used for the LT measurement in vivo. Although US technique is considered to be a reference standard for the in vivo biometric measurement, it is time consuming and often involves corneal contact; thus, it is less patient friendly and could increase the chance of infection.6 In contrast to A-scan US, Scheimpflug photography is a noncontact and faster procedure, but it requires pupil dilation for LT measurement, which may change the anterior chamber configuration and make it less suitable for biometric studies.7

Anterior segment optical coherence tomography (AS-OCT) is a fast, noncontact method for imaging the anterior chamber. It allows complete visualization of the anterior segment with a high axial resolution of ~18 µm and horizontal resolution of ~60 µm. The use of an infrared light source in the AS-OCT may keep the pupil size unaltered, thereby presumably giving a more accurate LT value. The AS-OCT technique has been reported for the quantitative measurements of corneal thickness, anterior chamber depth, anterior chamber width and anterior chamber angle.6 8 Studies on anterior segment biometry have suggested a high reproducibility of the measurements of anterior chamber depth and anterior chamber width.6 9 One study on accommodation used this technique for lens imaging.10 Several studies have reported a comparison between AS-OCT and ultrasound, but these were mainly on the anterior chamber parameters rather than LT measurement.11 12 To our knowledge, no published study has reported LT measurement using AS-OCT technique. Therefore, we attempted to assess the accuracy of AS-OCT, in comparison with ultrasound biometry, on the measurement of LT in elderly subjects with lens opacity. This comparison was further assessed in young subjects with clear lenses and substantial accommodation. The interobserver and intraobserver reproducibilities were also assessed in elderly subjects.


   Methods
Top
 Abstract
 Methods
Results
 Discussion
 References
 
Participants
Phakic volunteers age 50 years or older were consecutively recruited from the cataract clinic of Zhongshan Ophthalmic Center in Guangzhou. The study was approved by the ethics committee of Zhongshan Ophthalmic Center and was performed in accordance with the tenets of the Declaration of Helsinki. Written informed consent was obtained from all participants. Lens nucleus grading was assessed by using slit lamp according to Emery classification.13 Exclusion criteria were anterior segment disease, visual acuity < 0.05, spherical correction > 3.0 D, cylindric power > 2.0 D, history of eye injury, previous intraocular surgery, corneal opacity, or any type of glaucoma.

Another group of young people aged 18 to 40 years who attended excimer laser clinic were also enrolled before laser treatment. The subjects were enrolled as long as the spherical equivalent was ≥ –1.00 D. LT was measured using both A-scan ultrasound and AS-OCT. Reproducibility assessment was not attempted in these subjects.

Instruments
LT was measured with an AS-OCT (Viscante; Carl Zeiss Meditec, Dublin, CA) as well as an A-scan US (CineScan A/B, Quantel Medical, Bozeman, MT). The measurements, performed independently by two experienced operators, were made with AS-OCT first, followed immediately by A-scan US. The operators were masked to one another’s results. The measurements were performed without pupil dilation in the same room in standard illumination conditions.

The AS-OCT images were acquired in a nonaccommodated state. The subject’s refractive correction was used to adjust the internal fixation target for the patient’s distance correction by using the system’s internal program.14 During imaging, the subjects were asked to fixate on the target. AS-OCT measurements of LT were centered on the pupil and performed at the horizontal meridian (nasal–temporal angles at 0–180°) using the anterior segment single-scan tools. The default fixation angle position in the AS-OCT is aligned along the visual axis. If the scans were noticeably off from the horizontal, the fixation angle was adjusted to align the image along the geometric axis.14 The image was considered optimally aligned when both the reflections of anterior and posterior poles of the crystalline lens were visible and on the same line perpendicular to the lens surface. Two ophthalmologists independently captured the lens images and measured the LT by using the device’s manual caliper tools (see Fig. 2-2 ). The LT was measured as the distance between the anterior and posterior poles of the crystalline lens (see Fig. 2-3 ). If the crystalline lens anterior pole or posterior pole could not be seen, the LT was considered unmeasurable.


Figure 1
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  		FIGURE 1. Lens thickness measurement by A-scan ultrasound. With A-scan ultrasound, it was difficult to differentiate the peaks of the lens capsule from cortical opacity. In this recording, the peaks of lens posterior capsule was not defined correctly by A-scan US.

 

Figure 2
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  		FIGURE 2. Lens thickness measurement by AS-OCT. (1) Lens with grade 5 nuclear opacity. (2) AS-OCT could not penetrate dense cortical cataracts. (3) LT measurement with the manual calipers. (4) A clear lens.

 
After AS-OCT measurements, the A-scan US was used to measure LT after topical anesthesia with 0.4% oxybuprocaine (Benoxil; Santen Pharmaceuticals, Osaka, Japan) was instilled in the eyes of supine subjects. Two ophthalmic technicians independently performed the measurements. The A-scan US measurement was made with a handheld 10-MHz probe with sound velocity setting of 1641 m/s for the lens. The patients were asked to look at a blinking light in the center of the ultrasound probe. The A-scan probe was gently placed on the central cornea, with the pupil used as a reference. The device automatically made 10 consecutive measurements of axial length, anterior chamber depth (ACD), LT, and vitreous chamber depth. Care was taken to ensure that the measurement was on-axis and without indentation. The readings were obtained when a satisfactory scan image was achieved. A good image was defined as one with well-defined echoes corresponding to the cornea, the anterior and posterior poles of the crystalline lens, and the posterior wall of the eye. The LT measurements were based on the average of 10 consecutive measurements. The measurement series was repeated if the SE of the 10 programmed consecutive measurements was >0.12 mm. If three consecutive measurements were not able to achieve SE ≤ 0.12 mm, the LT was considered unmeasurable.

The reproducibility assessment was based on the definitions adopted by the British Standards Institution.15 16 In the present study, we investigated both intraobserver and interobserver reproducibility of AS-OCT and A-scan US. Intraobserver reproducibility of AS-OCT was investigated by acquiring image of lens on two separate occasions by the same operator (YZ). The images were analyzed later by another grader twice with an interval of 1 week. To investigate interobserver reproducibility of AS-OCT, two operators (YZ and YX) independently obtained one scan in each subject during the same scanning session. For the A-scan US measurement, intraobserver reproducibility of the A-scan US was investigated by measuring LT on two separate occasions with an interval of a few minutes by the same operator (CC). To investigate interobserver reproducibility of A-scan US, two operators (CC and ML) each obtained a measurement in every subject during the same session.


   Results
Top
 Abstract
 Methods
 Results
Discussion
 References
 
In this study, we enrolled 48 (66 eyes) consecutive elderly patients: 22 men and 26 women; the mean age was 65.4 ± 10.7 years (range, 50–88). The median nuclear cataract grade (Emery Classification)13 was 3 (range, 1–5; 25th percentile, 2; 75th percentile, 3). Table 1 shows the failure rate of the two instruments. Six eyes were not measurable for LT with AS-OCT: two participants were unable to fixate on the system’s internal fixation target, three could not produce a crystalline lens posterior pole due to dense and opaque cortical cataracts, one lens could not be completely captured, possibly due to the LT being greater than 6 mm. A-scan US scans were not successful in five eyes. Failure was defined as SE > 0.12 mm in three consecutive scan sessions: One patient (one eye) was unable to cooperate, four eyes had dense cortical or nuclear opacity that resulted in additional peaks or dense posterior cortical opacity, which hampered the accurate detection of the lens capsule (Fig. 1) . Of interest, for the three LT measurements that were unsuccessful with AS-OCT, A-scan was also unsuccessful; however, for another three OCT measurements that failed due to poor fixation and thick lens, we were able to obtain successful A-scan measurements.


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  		TABLE 1. Failure Rates for AS-OCT and US A-scan LT Measurement in Elderly Subjects

 
In 56 young subjects (age, 27.7 ± 5.8 years; range, 18–40), the spherical equivalent was –4.84 ± 0.29 D based on cycloplegic refraction. AS-OCT and A-scan US measurements were measurable in all the young subjects. In young subjects, the AS-OCT imaging may not provide a clear boundary of the posterior capsule, but the posterior pole reflection is clearly displayed in all images (Fig. 2-4) .

Table 2 shows the LT measurements of AS-OCT and A-scan US by all examiners in elderly subjects. The interobserver variations were not significant for AS-OCT (paired t-test, P = 0.301) and ultrasound (P = 0.264). There was no significant difference between the first and the second measurement for the same observer in both AS-OCT (paired t-test, P = 0.515) and A-scan US measures (P = 0.385).


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  		TABLE 2. Distribution of LT Measured by Various Examiners Using AS-OCT and A-scan US

 
The mean difference between AS-OCT and A-scan US (1st measurement by 1st examiner using AS-OCT versus 1st measurement by 1st examiner using A-scan US) was 0.135 mm (SD, 0.150 mm; 95% confidence interval [CI], 0.095–0.174 mm; paired t-test, P < 0.001) in the elderly subjects and 0.101 mm (SD, 0.111 mm; 95% CI, 0.072–0.131mm; paired t-test, P < 0.001) in young subjects. AS-OCT gave greater measures than A-scan US, and these differences tended to be consistent across the measurement ranges. In elderly people, the Pearson correlation coefficient (r) between AS-OCT and A-scan US was 0.946 (P < 0.001; Fig. 3 ). These differences in LT measurements with AS-OCT and A-scan US did not correlate with the nuclear cataract grades (r = 0.078, P = 0.558).


Figure 3
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  		FIGURE 3. Scatterplot of lens thickness data collected by AS-OCT and A-scan US. Correlation coefficient = 0.946.

 
Table 3 shows the 95% limits of agreement (LoA) and intraclass correlation coefficient (ICC) of LT measurements by AS-OCT and A-scan US in both intraobserver and interobserver agreement. Although two measurements correlated highly with each other (ICC range, 0.996–0.999 for AS-OCT and 0.960–0.974 for A-scan), the 95% LoA suggested that agreement tends to be better in AS-OCT, and intraobserver agreement in general is superior to interobserver agreement.


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  		TABLE 3. Intra- and Interobserver Reproducibility of LT Measurements by AS-OCT and A-scan US

 

   Discussion
Top
 Abstract
 Methods
 Results
 Discussion
References
 
In this study, we measured the LT of cataractous eyes with AS-OCT and A-scan US, both instruments were able to measure LT in 58 of 66 cataractous eyes. As a whole, the failure rate of AS-OCT (9.1%) was slightly higher than that of A-scan US (7.6%). In young subjects with clear lenses, all LT measurements are successful, which suggests that AS-OCT is able to quantify the LT in eyes with clear lens and most of those with lens opacity. We also found that dense cortical lens opacity may compromise the visibility of the posterior capsule, but nuclear and posterior subcapsular won’t affect the visibility. In a clear lens, the posterior capsule is not fully visible, but the posterior pole of the lens is clearly justified by the reflection (Fig. 2-4) . In previous studies, the opacity of the cornea and lens was suggested as the cause of the failure of optical LT measurement.17 18 Sacu et al.17 reported the failure rate of ACMaster (Carl Zeiss Meditec) LT measurements to be 3 of 11 in cataractous eyes, and Goel et al.18 reported similar measurement difficulties with the IOLMaster (Carl Zeiss Meditec) in eyes with dense cataract. AS-OCT imaging requires that the subject fixate on an internal target. One patient could not cooperate and fixate on the internal target. On the other hand, the depth of the scan was only 6 mm. Therefore, if an LT was more than 6 mm, the AS-OCT could not capture the posterior poles of the crystalline lens in a single image. In LT measurements with A-scan US, we found that the cortical lens opacity also compromised the accuracy of measurements. It could be difficult to differentiate the peaks of lens posterior capsule and posterior cortical opacity (Fig. 1) . In four of five eyes with failed US measurement, three consecutive measurements were unable to achieve SE ≤ 0.12 mm due to significant cortical lens opacity.

In our study, the AS-OCT tended to give greater LT values (mean difference, 0.135 mm in the elderly and 0.101 mm in young subjects) than did the A-scan US, although both measurements correlated highly with each other and the difference was small. These two measures are designed based on different physical principles: A-scan ultrasound estimates the distances according to the speed of sound in ocular media and the ultrasonic echoes from the interfaces of ocular structure, whereas AS-OCT quantifies the distance based on a linear scan, using infrared light and the principles of low-coherence interferometry. The disparities between AS-OCT and ultrasonic measurements in ocular tissues are different: AS-OCT tends to underestimate central corneal thickness (CCT),19 20 but to overestimate the ACD in comparison to ultrasound.6 The difference in ACD may be explained by indentation of the ultrasound probe, but this should not be the case with LT measurements, given that the indentation will not change the echoes of lens capsules. The disparities in the corneal and LT measurements are mainly attributable to the difference in physical principles of the measurements, optical refractive index, and ultrasonic velocity of the tissues and therefore the results in the cornea and lens are not directly comparable.

Second, accommodation status in AS-OCT and A-scan US may also account for the differences on LT. AS-OCT is measured with minimal or no accommodation, whereas A-scan US inevitably introduces accommodation and therefore may yield greater LTs when subjects have to fixate on the blinking light on the probe.14 However, the fact that the AS-OCT gives greater values than US may suggest that this difference could have been even greater. In fact, we observe that the difference (AS-OCT versus A-scan) becomes smaller in young subjects. This suggests that the accommodation makes the lens slightly thicker in A-scan US measurement. But given that the difference is very small, the influence of accommodation may be negligible in clinical evaluation.

Drexler et al.21 further suggested that the LT measurement was less affected by greater degrees of nuclear cataract with the optical method than with US. Dubbelman et al.22 found that effects of age-dependent changes of velocity and refractive index on LT measurement were comparable in the Scheimpflug and US methods. However, we did not identify such associations between the measurements and nuclear lens opacity grades.

Greater reproducibility of AS-OCT measurement has been suggested for the ACD and anterior chamber width in comparison with the IOLMaster (Carl Zeiss Meditec) and US.6 23 In our study, the intraobserver and interobserver reproducibility of AS-OCT were better than those of A-scan US. It has been suggested that the accuracy of ACD measurements by A-scan US was affected by operator experience, differences in probe tip handling, off-axis measurement, and pupil diameter.6 14 AS-OCT allows a visualization of the anterior and posterior poles of the lens using the reflection flare and therefore the axis of measurement may be more consistent among different measurements, whereas this is not possible in A-scan US.

In conclusion, our study demonstrated a good correlation between AS-OCT and A-scan US in LT measurements. AS-OCT tends to systematically give greater values, but the differences are small and clinically less important. AS-OCT is able to measure LT in most cases and may be subject to less measurement variation than US. This newly available anterior segment technique offers a new noncontact method for the quantification of LT that may help research on myopia and angle closure.


   Footnotes
 
Submitted for publication September 17, 2007; revised March 3 and July 20, 2008; accepted October 24, 2008.

Disclosure: Y. Zeng, None, Y. Liu, None; X. Liu, None; C. Chen, None; Y. Xia, None; M. Lu, None; M. He, 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.

Corresponding author: Yizhi Liu, Zhongshan Ophthalmic Center, Guangzhou 510060, People’s Republic of China; yzliu62{at}yahoo.com.


   References
Top
 Abstract
 Methods
 Results
 Discussion
 References
 

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