ANTERIOR SEGMENT IMAGING Anterior segment biometry with 2 ...

ARTICLE

MINISECTION: ANTERIOR SEGMENT IMAGING Anterior segment biometry with 2 imaging technologies: Very-high-frequency ultrasound scanning versus optical coherence tomography David P. Pin˜ero, MSc, Ana Beleń Plaza, MSc, Jorge L. Alio´, MD, PhD

PURPOSE: To determine the interchangeability of 2 anterior segment imaging systems: a very-highfrequency (VHF) ultrasound scanning system (Artemis 2, Ultralink LLC) and an optical coherence tomography (OCT) system (Visante, Zeiss). SETTING: Vissum Instituto Oftalmologico de Alicante, Alicante, Spain. METHODS: This study comprised 20 eyes without pathology or previous surgery. The anterior chamber depth (ACD), central corneal thickness (CCT), angle-to-angle distance (ATA), and the iridocorneal angle size (IAS) at the 0-degree and 180-degree positions were measured with 2 imaging techniques: VHF ultrasound scanning and OCT. Analysis of agreement and interchangeability was performed by the Bland and Altman method. In addition, each measurement was performed 3 times consecutively to determine intrasession repeatability by means of the coefficient of variation (CV) and the intraclass correlation coefficient (ICC). RESULTS: No statistically significant differences were found between imaging techniques in ACD, CCT, or ATA (P>.40). The ranges of agreement were 0.20 mm, 16.11 mm, and 0.80 mm for ACD, CCT, and ATA, respectively. Regarding IAS, no statistically significant differences were found in the nasal (P Z .78) or temporal (P Z .63) measurements between devices. However, the range of agreement for nasal (14.3 degrees) and temporal (14.90 degrees) values was relevant, indicating the 2 techniques cannot be used interchangeably for IAS measurement. Excellent intrasession repeatability scores were obtained (CV and ICC). CONCLUSION: The Artemis 2 and the Visante OCT systems provide equivalent and repeatable measurements of the ACD, CCT, and ATA and can be used interchangeably for these purposes. J Cataract Refract Surg 2008; 34:95–102 Q 2008 ASCRS and ESCRS

Biometric analyses of the anterior segment of the eye have become increasingly important in the decision making of any anterior segment surgical protocol, especially in the preoperative and postoperative evaluation of intraocular lenses (pIOLs). Knowing the accurate anatomical dimensions of the anterior chamber allows the clinician to select the appropriate-sized IOL to avoid complications.1 In addition, anterior segment analysis is a valuable tool for a better understanding of the outcomes of cataract surgery (IOL position, capsule opacification)2 and keratorefractive surgery (corneal transparency, configuration Q 2008 ASCRS and ESCRS Published by Elsevier Inc.

of the flap, corneal thickness profiles)3,4 as well as anterior segment pathology (tumors, uveitis, aniridia).5–7 These uses have generated great expectations for the field of anterior segment imaging. Developments in digital processing, computation, and optical devices have created a significant increase in the interest in anterior segment imaging techniques.8 As a result, several devices with different physical bases have been developed. These can be classified into 3 categories: Scheimpflug method–based systems,9 veryhigh-frequency (VHF) ultrasound scanning devices,10 and optical coherence tomography (OCT) systems.11 0886-3350/08/$dsee front matter doi:10.1016/j.jcrs.2007.08.033

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ASI: ANTERIOR SEGMENT BIOMETRY WITH VHF ULTRASOUND VERSUS OCT

With some instruments, corneal contact is necessary for taking measurements or immersion of the eye in saline solution is required to provide a medium for the transmission of the waves generated by the transductor; other systems are noninvasive. In addition, microscopic evaluation of the cornea with a confocal or specular microscope has been introduced into clinical practice. The aim of this study was to analyze the similarities and evaluate the interchangeability of 2 anterior segment imaging devices: a VHF ultrasound scanning system (Artemis 2, Ultralink LLC) and an OCT system (Visante, Carl Zeiss Meditec AG). PATIENTS AND METHODS Patient Cohort This study comprised 20 eyes of 20 patients. The eyes were chosen randomly (random sampling). A random number sequence (dichotomic sequence 0 and 1) was created using computer software. Based on the sequence, patients from the refractive surgery consultation at Vissum Instituto Oftalmologico de Alicante were included in the study (1) or not included in the study (0). The only exclusion criterion was active ocular pathology or previous ocular surgery. After being informed of their inclusion in the study, all patients signed an informed consent document in accordance with the Helsinki Declaration. In all cases, a complete ophthalmologic examination was performed to screen patients. In addition, anterior segment analysis was performed with 2 instruments: the Artemis 2 VHF ultrasound system and the Visante OCT system. The same experienced independent examiner (DPP) performed all measurements. To corroborate the accuracy of the procedure, each measurement was performed 3 times consecutively. Next, the intrasession repeatability was analyzed.

Very-High-Frequency Anterior Segment Ultrasound Analysis The Artemis 2 is a VHF ultrasound eye scanner that uses a probe of 50 MHz. The device is connected to a computer

Accepted for publication August 27, 2007. From the Vissum/Instituto Oftalmolo´gico de Alicante (Pin˜ero, Plaza, Alio´), and the Division of Ophthalmology (Alio), Universidad Miguel Hernańdez, Alicante, Spain.

Figure 1. Scan profile obtained with the Artemis 2 VHF ultrasound system. The image shows the profile of the anterior segment.

with software that analyzes the acquired data. The U.S. Food and Drug Administration approved its use in 2000. Thousands of ultrasound samples of the anterior segment are captured during an arc motion of the probe and then digitized. After all the information is processed, anatomical profiles of the anterior segment are provided. Different distances and dimensions can be measured with accuracy12 over these profiles with a digital calibrated caliper (Figure 1). In addition, high-resolution images of the corneal structure can be obtained by placing the probe closer to the eye. The images are useful for analyzing the thickness distribution of each corneal layer.4 The patient must be properly positioned to obtain good images of the anterior segment. In addition, the patient must lean over the support mechanism and continuously look at a blinking fixation light to ensure accurate centration of the scans. Then, the interface eye-transducer is filled with saline solution and used as the acoustic coupling medium between the eye and transducer. In this study, 1 horizontal scan was obtained and then analyzed. Using the digital tools (calipers and angular sizers) for measuring provided by the Artemis 2 software, 5 anatomical parameters of the anterior segment were measured: anterior chamber depth (ACD), central corneal thickness (CCT), angle-to-angle distance (ATA), and iridocorneal angle size (IAS) at the 0-degree (nasal) and 180-degree (temporal) positions.

Optical Coherence Tomography Anterior Segment Analysis The Visante OCT system uses an infrared light of a 1310 nm wavelength to obtain, by interferometry, scans of different structures of the anterior segment.11 The system is connected to a computer with software that provides different options of image capturing and measurement.

No author has a financial or proprietary interest in any material or method mentioned. Supported in part by the Spanish Ministry of Health, Red Tema´tica de Investigacioń en Oftalmologıá, Subproyecto de Cirugıá Refractiva y Calidad Visual (C03/13), and Spanish Generalitat Valenciana (ref: Grupos05/036) Grants and Support for Scientific Research and Technological Development in the Comunidad Valenciana for the year 2005. Corresponding author: Jorge L. Alio´, MD, PhD, Avenida de Denia s/n, Edificio Vissum, 03016 Alicante, Spain. E-mail: jlalio@vissum. com.

Figure 2. Scan profile obtained with Vivante OCT. The image shows an anterior segment profile with some anatomical measurements, the ATA distance, and ACD.

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Table 1. Results for each anatomical parameter by imaging technique. Mean G SD Technique

ACD (mm)

ATA (mm)

CCT (mm)

Nasal IAS (Degrees)

Temporal IAS (Degrees)

Artemis 2 VHF US Visante OCT P value

3.07 G 0.40 3.16 G 0.41 .45

12.23 G 0.59 12.14 G 0.52 .69

527.78 G 22.54 528.00 G 20.93 .90

37.33 G 7.88 36.51 G 10.39 .78

37.40 G 8.25 36.13 G 8.60 .63

ACD Z anterior chamber depth; ATA Z angle-to-angle distance; CCT Z central corneal thickness; IAS Z iridocorneal angle size; OCT Z optical coherence tomography; VHF US Z very-high-frequency ultrasound

First, the patient’s head is well positioned on the chin and forehead rests and the patient is asked to fixate on the central light. An image appears on the screen, and its focus is changed until the profile of the anterior segment is obtained. The noise, saturation, and polarization of each scan can be modified to obtain a more detailed image. After an image with good focus and centration is acquired, different structures can be measured (Figure 2) using digital tools such as calibrated calipers or angular sizers. The same 5 anterior segment parameters were measured as with the Artemis 2 system.

Statistical Analysis Statistical analysis of data was performed using the SPSS for Windows (version 11.0, SPSS, Inc.). The normality of all anatomical data distributions was confirmed by the Kolmogorov-Smirnov test, and parametric statistics were always applied. The unpaired Student t test was used for comparing the instruments in each anatomical parameter. In addition, the Pearson coefficient was calculated to assess the correlation between anatomical parameters measured with each imaging technique. All tests were 2 tailed, and a P value less than 0.05 was considered statistically significant. The Bland-Altman analysis13 was used for checking the interchangeability of both imaging techniques for measuring the 5 anatomical parameters. In our study, the objective was to know if ultrasound and optical coherence tomography (OCT) scanners can be used interchangeably for the characterize of the main anatomical parameters of the anterior segment. This is a graphical method for assessing if there is an agreement between two clinical procedures.13 Specifically, Bland-Altman plots show the differences between the methods plotted against the mean of the 2 methods. The limits of agreement (LoA) are defined as the mean G 1.96 SD of the differences.13 If the limits are clinically relevant, the 2 methods cannot be used interchangeably. Intrasession repeatability of the measurement method was analyzed using the within-subject coefficient of variation (CV) (reliability) and the intraclass correlation coefficient (ICC). The CV is defined as the ratio of the standard deviation of the repeated measurements to the mean. The lower this ratio, the higher the repeatability. Nonparametric statistics were used to compare the CV corresponding to each technique (Mann-Whitney U test) because the normality of these data distributions could not be assumed. The ICC is an analysis of variance–based correlation that measures the relative homogeneity within groups (between the repeated measurements) in ratio to the total variation. This parameter is normally used to assess test–retest reliability. The number of patients to include in this study was decided based on the results of sample-size calculations.

Twenty eyes provided a statistical power higher than 85% for all comparison tests. This power reached a value higher than 95% for the hypothesis tests corresponding to the ACD and CCT.

RESULTS The mean age of the patients was 32.82 years G 7.91 (SD) (range 23 to 51 years). Comparison of Measurements Table 1 shows the measurements for each anatomical parameter by both imaging techniques. The ACD measured with the Visante OCT is slightly higher than that obtained with the Artemis 2 VHF ultrasound system, but the differences were not statistically significant (P Z .45, bilateral unpaired Student t test; 95% confidence interval [CI], 0.16 to 0.36). Regarding the ATA diameter, the Artemis 2 provided a slightly higher mean value, but difference between the devices did not reach statistical significance (P Z .69, bilateral unpaired Student t test; 95% CI, 0.41 to 0.27). The mean pachymetry values were similar, with no statistically significant difference between the 2 devices (P Z .90, bilateral unpaired Student t test; 95% CI, 12.44 to 14.14). The angle measurements (Table 1) in the nasal and temporal locations were also similar, with no statistically significant differences between the 2 devices (nasal: P Z .78, bilateral unpaired Student t test; 95% CI, 6.62 to 4.97; temporal: P Z .63, bilateral unpaired Student t test; 95% CI, 6.53 to 4.00). A statistically significant correlation was found between devices in all anatomical parameters (P!.02 Table 2. Pearson correlation coefficient for each anatomical parameter analyzed. Technique

ACD ATA CCT Nasal IAS Temporal IAS

Pearson coefficient (r) P value

0.97

0.73

0.93

0.65

0.53

.01

.01

.01

.02

.02

ACD Z anterior chamber depth; ATA Z angle-to-angle distance; CCT Z central corneal thickness; IAS Z iridocorneal angle size


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Figure 3. Bland-Altman plot showing the differences in ACD between Artemis 2 and Visante OCT plotted against the mean value of both. The upper and the lower lines represent the LoA calculated as mean G1.96 SD.

Figure 5. Bland-Altman plot showing the differences in CCT between Artemis 2 and Visante OCT plotted against the mean value of both. The upper and the lower lines represent the LoA calculated as mean G1.96 SD.

in all cases). Table 2 shows the Pearson correlation coefficient for each parameter. The Pearson coefficient was higher than 0.5 for all parameters.

Interchangeability Analysis Figures 3 to 7 show the Bland and Altman plots. The range of agreement, defined as 1.96 SD, was 0.20 mm

Figure 4. Bland-Altman plot showing the differences in ATA diameter between Artemis 2 and Visante OCT plotted against the mean value of both. The upper and the lower lines represent the LoA calculated as mean G1.96 SD.

Figure 6. Bland-Altman plot showing the differences in nasal IAS between Artemis 2 and Visante OCT plotted against the mean value of both. The upper and the lower lines represent the LoA calculated as mean G1.96 SD.


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lower than 4%. Statistically significant differences between devices were found in the CV corresponding to all anatomical parameters. Specifically, the Artemis 2 showed less variability in ATA and temporal IAS measurements, and the Visante showed less variability in ACD, CCT, and nasal IAS measurements. The angle measurements were less repeatable than the other anatomical parameters. Table 4 shows the ICCs for each imaging technique. Excellent intrasession scores were obtained in all cases, which confirmed the consistency of the data obtained.

Figure 7. Bland-Altman plot showing the differences in temporal IAS between Artemis 2 and Visante OCT plotted against the mean value of both. The upper and the lower lines represent the LoA calculated as mean G1.96 SD.

for the ACD measurements, which indicates a high level of agreement between the devices in this parameter. For the ATA and CCT measurements, the ranges of agreement were 0.80 mm and 16.11 mm, respectively. Considering the properties of both magnitudes (ATA and CCT), the LoA were acceptable. For the nasal and temporal IAS, the ranges of agreement were 14.13 degrees and 14.90 degrees, respectively. These values were relevant and indicated that the variability was high enough. Therefore, the LoA were large and clinically relevant (nasal IAS limits: 12.85 degrees and 15.41 degrees; temporal IAS limits: 12.98 degrees and 16.82 degrees). Intrasession Repeatability Table 3 shows the CVs for each imaging technique. For all anatomical parameters and each imaging device, the CV of the repeated measurements was

DISCUSSION Ultrasonic systems allow visualization of anterior segment structures, even in the presence of optical opacities. Different applications have been described for this ophthalmic technology; these include intraocular tumors analysis,6,7 determination of the position of a pIOL,14,15 planning for refractive surgery retreatment,16 microkeratome cut analysis in LASIK,17 study of intraocular pathology,5–7,18 and analysis of some posterior segment structures.19 As resolution normally improves with frequency, VHF waves are used for most anterior segment imaging, providing an axial resolution of less than 40 mm.12 A main advantage of this technology is its accuracy and repeatability.20 The problems arise from the measuring procedure. Most ultrasound imaging devices require physical contact between the cornea and the probe, which can be uncomfortable for some patients. The Artemis 2 VHF ultrasound device used in our study does not require cornea–probe contact; rather, an interface eye transducer of saline is used as the acoustic coupling medium between the cornea and probe. However, the position of the head is uncomfortable for patients and the procedure requires an experienced examiner. On the other hand, interferometry has been used over the past few years as a basis for anterior segment imaging. This technology is called anterior segment OCT (AS-OCT). The systems based on this technology have the following advantages: The measuring procedure is fast and easy for the examiner and patient, and no corneal touch or specific interphase is needed for

Table 3. Results of intrasession repeatability for each imaging technique and for each anatomical parameter evaluated by the CV. Mean Coefficient of Variation (%) G SD Technique Artemis 2 VHF US Visante OCT P value

ACD

ATA

CCT

Nasal IAS

Temporal IAS

0.49 G 0.24 0.38 G 0.41 .01

0.20 G 0.15 0.35 G 0.21 .01

0.42 G 0.62 0.00 G 0.00 !.01

3.33 G 1.53 2.36 G 2.17 .01

3.51 G 1.80 3.96 G 9.69 !.01

ACD Z anterior chamber depth; ATA Z angle-to-angle distance; CCT Z central corneal thickness; IAS Z iridocorneal angle size; OCT Z optical coherence tomography; VHF US Z very-high-frequency ultrasound


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Table 4. Results of intrasession repeatability for each imaging technique and for each anatomical parameter evaluated by the ICC. Intraclass Correlation Coefficient (95% CI) Technique

ACD

ATA

CCT

Nasal IAS

Temporal IAS

Artemis 2 VHF US 0.9995 (0.9989-0.9998) 0.9992 (0.9982-0.996) 0.9893 (0.9775-0.9954) 0.9906 (0.9801-0.9960) 0.9897 (0.9782-0.9956) Visante OCT 0.9995 (0.9991-0.998) 0.9968 (0.9934-0.9986) 1.000 (1.000-1.000) 0.9971 (0.9942-0.9987) 0.9328 (0.8635-0.9700) ACD Z anterior chamber depth; ATA Z angle-to-angle distance; CCT Z central corneal thickness; CI Z 95% confidence interval; IAS Z iridocorneal angle size; OCT Z optical coherence tomography; VHF US Z very-high-frequency ultrasound

measuring. They provide a more comfortable procedure of measurement for the patient. All anterior segment structures in front of the iris and posterior segment structures visible through the pupillary aperture can be analyzed with this technology. Several applications have been described for these devices including analysis of corneal pathology,21 corneal refractive surgery changes,22 the position of pIOLs,23 the configuration of the iridocorneal angle area, and changes in the central part of the lens during accommodation.24 In addition, AS-OCT provides a resolution similar to that of ultrasound devices (axial resolution approximately 20 mm) and excellent repeatability.25 However, AS-OCT has an important limitation; that is, it cannot detect structures behind the iris. Therefore, it is not possible to estimate the sulcus-to-sulcus distance, a crucial measurement for accurate determination of the size of a posterior pIOL in an eye. In addition, the device cannot analyze any object behind any opaque structure. There are few published comparisons of ultrasound and OCT systems.26,27 Dada et al.27 obtained similar ACD, CCT, and nasal and temporal IAS values with the Visante OCT device and ultrasound biomicroscopy (Paradigm) but did not perform an analysis of the interchangeability of the devices. In our study, we analyzed the interchangeability of those anatomical parameters as well as ATA measurements between the Visante OCT with Artemis 2 VHF ultrasound systems. No statistically significant differences were found in ACD measurements, although the Visante OCT had a slight tendency to give larger values. This agrees with the results obtained by Dada et al.27 The range of agreement between devices in this parameter was very low (0.20 mm) and not clinically relevant. This means the 2 instruments can be used interchangeably to measure ACD. The intrasession repeatability for this parameter was excellent. Therefore, both devices can be used for preoperative screening of patients for phakic refractive surgery. The ACD is a crucial parameter for this kind of surgery because it indicates whether a pIOL can be placed without risk to the endothelium or risk for angle closure.

The mean CCT values were very similar and well correlated for both devices, with no statistically significant differences between them. The range of agreement between the imaging systems was not clinically relevant; this indicates that the interchangeability of systems is possible for the measurement of the CCT. The intrasession repeatability was excellent for both

Figure 8. Measurement of the nasal IAS with the Visante OCT. A and B show 2 different measurements of the same angle. The differences in IAS depending on the tangential point considered in the iris plane can be seen.



instruments, although the Visante OCT showed less variability in the results. Regarding angle measurements, no statistically significant differences were found between imaging techniques. The ranges of agreement between techniques for nasal and temporal IAS were clinically relevant because differences up to 14 degrees between the devices are possible. In addition, intrasession repeatability for nasal and temporal IAS measurements was good, but the coefficients of variation obtained were, in general, higher. There is one explanation for the higher level of variability in the angle measurements; that is, pupil size can vary, which changes the angle structures and thus generates new angle values. The luminance of the fixation light or test is not the same between the 2 instruments; in addition, the accommodation status can change during the measurement. Finally, the manner of measuring angles is subjective. With both instruments, you must draw 2 tangential lines to the structures that form the angle, and this process can vary from one measurement to another depending on where you locate the tangential point (Figure 8). This is an important source of variability. Other parameters that do not depend on the examiner’s selection are necessary for defining the angle structure. To our knowledge, this is the first study to compare ATA distance measurements between ultrasound and OCT systems. Furthermore, the few studies that report data on ATA diameter28–30 found a mean value very similar to ours. The ATA measurements obtained with both devices were well correlated with no statistically significant differences between them. The range of agreement between devices was acceptable; thus, the 2 devices can be used interchangeably. However, there was a slight tendency toward larger ATA values with the ultrasound device. Finally, the intrasession repeatability was excellent for both devices. In conclusion, the Artemis 2 VHF ultrasound system and Visante OCT system can be used interchangeably for the measurement of the ACD, CCT, and ATA. For the angle measurements with both devices, it is necessary to define new objective parameters to avoid the bias induced by the subjective delimitation of the angle structure. For measuring parameters behind the iris, sulcus-to-sulcus distance, or ciliary body size, only ultrasonic methods such as the Artemis 2 VHF ultrasound system can be used because anterior OCT systems cannot detect these structures due to the masking effect of the pigment layers of the anterior uvea. REFERENCES 1. Alio JL. Advances in phakic intraocular lenses: indications, efficacy, safety, and new designs. Curr Opin Ophthalmol 2004; 15:350–357

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First author: David P. Pin˜ero, MSc Vissum/Instituto Oftalmolo´gico de Alicante and Division of Ophthalmology, Universidad Miguel Hernańdez, Alicante, Spain