With the development of new techniques and equipment, cataract surgery is now considered a therapeutic procedure for extracting an opaque lens as well as a refractive procedure. Although visual acuity remains the gold standard for estimating optical function, the measurement of visual acuity alone is not sufficient to explain the optical condition objectively and precisely.1–3 Ocular aberrations are well-known parameters that can represent optical quality objectively.4 Wavefront analysis has been widely used to isolate the effects of lower-order aberration and higher-order aberration and to measure the contributions of individual aberration on optical quality.5,6 However, wavefront analysis has been limited by poor reproducibility.7,8 In one study using a subjective depth-of-focus analysis in a multivariate model, there was no significant correlation between residual spherical aberration and the objective quality of vision measurements after cataract surgery.9 In addition, the wavefront aberrometer might overestimate the optical quality in eyes with high ocular aberrations or prominent scattered light.10
Optical quality can also be assessed with devices based on the double-pass technique.11–13 The Optical Quality Analysis System (Visiometrics, Terrassa, Spain), a commercially available double-pass device, has been introduced to measure optical quality–associated parameters objectively and precisely in clinical practice.14 Using the double-pass system, many authors evaluated ocular optical performance after a variety of ophthalmological surgical procedures.15–17 The double-pass system also allows the evaluation of the quality and stability of the tear film in detecting mild symptoms of dry eye.18 The double-pass technique is based on recording images from a point source of infrared light after reflection on the retina and a double pass through ocular media.19 The size and shape of this light spot are quantified by measuring the point spread function (PSF).14 The double-pass system provides data on the modulation transfer function (MTF), retinal image quality (MTF cutoff frequency [MTF cutoff], Strehl ratio), intraocular scattering (objective scatter index [OSI]), and objective pseudoaccommodation. In contrast to data obtained using the wavefront aberrometer, the MTF in a double-pass system is directly computed by Fourier transformation from the acquired double-pass retinal image.19 The double-pass system allows reproducible direct objective measurement of the effect of optimal aberrations on optical quality of the human eye.20
In this study, we correlated the MTF cutoff, Strehl ratio, OSI, and objective pseudoaccommodation obtained from the double-pass system with root mean square (RMS) total aberration, RMS higher-order aberration, and spherical aberration obtained from the ray-tracing aberrometer. In addition, we compared parameters of the double-pass system with other well-known parameters of optical quality, such as manifest refraction values, subjective visual acuity, and the contrast sensitivity score.
This study included 40 eyes of 40 patients who underwent cataract surgery with implantation of a monofocal intraocular lens (IOL) (AcrySof IQ, SN60WF; Alcon, Ft. Worth, TX). All patients were treated at Yonsei University College of Medicine, Seoul, Korea. The same surgeon (T.-i.K.) performed all procedures. This study was approved prospectively by the institutional review board of Severance Hospital and conducted according to the Declaration of Helsinki and Good Clinical Practices. All patients signed documents of informed consent for participation in research. The IOL was selected to achieve emmetropia. The AcrySof IQ IOL with a posterior aspheric surface (negative spherical aberration, -0.20) was implanted to decrease the total amount of ocular spherical aberration after cataract surgery.17
Inclusion criteria were age 40 to 80 years and patients who had undergone senile cataract extraction and IOL implantation. Exclusion criteria included previous ocular or intraocular surgery, evidence of trauma on biomicroscopic examination, corneal opacity, fundus abnormalities, glaucoma, uveitis, amblyopia, systemic disease (i.e., diabetes mellitus or evidence of vascular pathology), posterior capsule rupture during surgery, IOL decentration greater than 0.5 mm, or corrected distance visual acuity (CDVA) worse than 0.1 logarithm of the minimum angle of resolution (logMAR) postoperatively. We used a Scheimpflug imaging system (Pentacam; OCULUS Optikgeräte GmbH, Germany) to assess decentration after cataract surgery.21 Any eye with concurrent disease that might influence optical or neural performance was excluded. When a patient underwent operation of both eyes, one eye was randomly selected to avoid correlation effects in statistical analysis.
Postoperative evaluations were performed 3 months after cataract surgery. All patients were examined for uncorrected distance visual acuity (UCDVA), CDVA, manifest refraction values (sphere, cylinder, and spherical equivalent), and slit lamp biomicroscopy. Visual acuity was measured with logMAR UCDVA and CDVA.
Objective evaluation of optical quality was performed using the double-pass system, which allows assessment of retinal image quality at a specific pupil diameter. This double-pass system is based on an unequal pupil configuration.22 The entrance pupil has a fixed diameter of 2 mm. Measurements of optical quality were performed at a 4-mm pupil diameter (the exit pupil) without pharmacologic dilatation. The double-pass system automatically compensated for a patient’s spherical refractive error; cylinder error was not corrected by the machine. We corrected patients’ cylindrical error with a trial lens. A dim light was maintained to assure at least a 4-mm natural pupil diameter. For each parameter, the double-pass device took six measurements and calculated the mean of the measurements.
The first parameter obtained from the double-pass system was the MTF cutoff (cycles per degree [cpd]). The MTF cutoff is the frequency at which the MTF reaches a value of 0.01, the threshold at which the eye can image an object in the retina with a significant 1% contrast. The higher the MTF cutoff value, the better the contrast sensitivity.10 The second parameter was the Strehl ratio, which is the ratio of peak focal intensity in aberrated versus an ideal PSF.5 The Strehl ratio provides general information about the eye’s optical quality. A value of 1 corresponds to a perfect zero aberration optical system. The Strehl ratio was reported to be well correlated with the psychophysical parameters such as contrast sensitivity function and visual acuity for a moderate amount of defocus.23 The third double-pass parameter was the OSI, which quantifies intraocular scattered light. From the image obtained by the double-pass system, the OSI is computed as the ratio between the amount of light in the periphery and in the surroundings of the central peak of the double-pass image. In the particular case of the instrument Optical Quality Analysis System, the central area selected was a circle of a radius of 1 minute of arc, whereas the peripheral zone was a ring set between 12 and 20 minutes of arc.24 The higher the OSI value, the higher the level of intraocular scattering. When interpreting the OSI value as a parameter of the intraocular scattering, evaluation of the contribution of ocular aberrations on the OSI value is required.25 The fourth parameter was the objective pseudoaccommodation, which was calculated using the aberrated PSF, the focus range at which the PSF of the defocus point is better than 50% of the maximum PSF.
Ocular aberrations were measured using the ray-tracing aberrometer (iTrace; Tracey Technologies, Houston, TX) at a pupil size of 4 mm or more in mesopic conditions, without pharmacologic dilatation. For comparison with the double-pass system measurements, data were recalculated at a 4-mm pupil size.26 We measured RMS total aberration, RMS higher-order aberration, spherical aberration, and MTF. The RMS higher-order aberration was analyzed up to the sixth order by expanding the set of Zernike polynomials. The ray-tracing aberrometer uses parallel thin beams in separate and concentric arrays that are projected sequentially onto the eye.27 This method avoids data confusion by both enabling the measurements on a point-by-point basis and providing a robust refractive state.
Contrast sensitivity was measured at five spatial frequencies (1.5, 3, 6, 12, and 18 Hz) using the Optec 6500 vision testing system (Stereo Optical Co., Inc., Chicago, IL). All measurements were obtained under photopic (target luminance value, 85 cd/m2) and mesopic conditions (target luminance value, 3 cd/m2) after correction of refractive errors.
LogMAR acuity values were used for statistical analysis of visual acuity. We analyzed the correlation between optical quality parameters (MTF cutoff, Strehl ratio, OSI, and objective pseudoaccommodation) obtained from the double-pass system and ocular aberrations (RMS total aberration, RMS higher-order aberration, and absolute value of spherical aberration) obtained from the ray-tracing aberrometer. We also analyzed the correlations between the former and other visual quality parameters (manifest refraction values, CDVA, and contrast sensitivity score). To compare the MTF obtained from the double-pass system and ray-tracing aberrometer directly, we manually measured spatial frequency when the MTF reached 10% of its maximum value (MTF10). The correlations were evaluated using the Pearson correlation analysis. A paired t test was used for comparisons of MTF10. Statistical analyses were performed using SAS software (version 9.2; SAS Institute, Inc., Cary, NC). Differences were considered statistically significant for p < 0.05.
Among 40 patients, 22 (55%) were male and 18 (45%) were female. Postoperative data are shown in Table 1. Fig. 1 shows significant correlations between the MTF cutoff and total aberration (r = −0.503, p = 0.003), between the Strehl ratio and total aberration (r = −0.509, p = 0.003; Table 2), between the OSI and total aberration (r = 0.451, p = 0.024), and between the objective pseudoaccommodation and higher-order aberration (r = 0.450, p = 0.031; Table 3). In terms of manifest refraction values and subjective visual acuity, we identified negative correlations between the MTF cutoff and manifest refraction values (r = −0.367, p = 0.036 for sphere; r = -0.462, p = 0.006 for cylinder; r = −0.448, p = 0.008 for spherical equivalent), between the MTF cutoff and CDVA (r = −0.453, p = 0.007), between the Strehl ratio and manifest refraction values (r = −0.366, p = 0.036 for sphere; r = −0.487, p = 0.003 for cylinder; r = −0.471, p = 0.005 for spherical equivalent), and between the Strehl ratio and CDVA (r = −0.354, p = 0.040; Table 2). A positive correlation was found between OSI and CDVA (r = 0.516, p = 0.008; Table 3).
The MTF cutoff showed a positive correlation with the contrast sensitivity score at spatial frequencies of 3, 6, 12, and 18 Hz for photopic conditions (r = 0.539, p = 0.047; r = 0.674, p = 0.008; r = 0.565, p = 0.035; r = 0.534, p = 0.049, respectively) and at spatial frequencies of 3, 6, and 12 Hz for mesopic conditions (r = 0.638, p = 0.019; r = 0.696, p = 0.008; r = 0.655, p = 0.015, respectively; Table 2). The Strehl ratio was positively correlated with the contrast sensitivity score at spatial frequencies of 3 and 6 Hz for photopic conditions (r = 0.544, p = 0.045; r = 0.544, p = 0.044, respectively; Table 2).
The MTF10 obtained from the double-pass system was significantly smaller than that obtained from the ray-tracing aberrometer (Table 4). Pearson correlation analysis showed a significant correlation between both MTF10 values (r = 0.581, p = 0.003; Table 4).
In this study, we analyzed the optical quality of patients who had cataract extraction and monofocal IOL implantation. We measured optical quality and ocular aberrations using the double-pass device and ray-tracing type aberrometer, respectively.28 Our results showed significant negative correlations between retinal image quality parameters (MTF cutoff and Strehl ratio) and total aberration. There were significant positive correlations between the OSI and total aberration and between the objective pseudoaccommodation and higher-order aberration.
The effect of ocular aberration could be summarized as blurring of the retinal image.10,29,30 The blurring of the retinal image reduces the subjective visual acuity, which is directly related to the MTF cutoff value. Our results showed a significant negative correlation between the MTF cutoff and total aberration. On the other hand, intraocular scattering reduces the contrast of the retinal image.30 In this study, total aberration was positively correlated with the OSI. The OSI value of the double-pass system, computed based on the concept that ocular aberrations mainly modify the intensity distribution closer to the peak and the effect of ocular scattering occurs farther from the center, could be used to evaluate ocular scattering objectively.24,31 Because the presence of aberrations could contribute to the outer areas of the double-pass image affecting the OSI value, interpretation of the OSI should include the contribution of ocular aberrations on the OSI.25 Moreover, to avoid the artifact associated with uncorrected refractive errors in the OSI interpretation, we excluded cases with an OSI value of 3 or higher.24 In our study, the Strehl ratio was negatively correlated with total aberration. Ocular aberration causes retinal image degradation, which means nonperfect optical system. Consequently, the Strehl ratio gets closer to zero.
In a study demonstrating the effect of residual ocular spherical aberration on the quality of vision in pseudophakic eyes, Nochez and coauthors5 reported a significant positive correlation between the objective pseudoaccommodation and ocular spherical aberration. However, we found no significant correlation between the objective pseudoaccommodation and spherical aberration. Instead, higher-order aberration was the only value correlated with the objective pseudoaccommodation. The difference between our study and the aforementioned study could be attributed to the methodology for recalculating optical quality parameters and ocular aberrations with a 4-mm pupil. In a study performed by Nochez, ocular aberrations were measured at the 6-mm optical zone and some patients required pharmacologic dilatation. It also should be considered that aberrometer used the Hartmann-Shack method for analysis in that study.
In this study, the values from the double-pass system were measured after correction of refractive error. We found significant correlations between CDVA and retinal image quality parameters (MTF cutoff, Strehl ratio, and OSI). As expected, the Strehl ratio was correlated with visual acuity like that in one study conducted by Villegas et al.,23 which reported that the Strehl ratio is well correlated with visual acuity and contrast sensitivity function. The OSI showed no correlation with refractive error values (defocus and astigmatism), indicating that intraocular scattering is independent from refractive error.24
We also found that the MTF cutoff obtained from the double-pass system demonstrated good correlation with the contrast sensitivity score at all spatial frequencies (except 1.5 Hz) under photopic conditions and with those of moderate spatial frequencies (3, 6, and 12 Hz) under mesopic conditions. In one study analyzing the retinal image quality with the double-pass device, there was a significant positive correlation between the MTF cutoff value and the contrast sensitivity score.10 The MTF value is highest when image contrast is equivalent to contrast in the object. The only significant correlations with Strehl ratio identified in our study were between the Strehl ratio and the contrast sensitivity score at spatial frequencies of 3 and 6 Hz under photopic conditions. We assumed that significant contrast degradation at very low or high spatial frequencies could be the reason why the contrast sensitivity was not correlated with the Strehl ratio at all tested spatial frequencies.
The iTrace ray-tracing aberrometer has the advantage of a larger dynamic range for the measurement of accommodation compared with the Hartmann-Shack principle-based aberrometer.32 In a patient’s eye with a mild to severe amount of scattering, Hartmann-Shack wavefront sensors might overestimate image quality compared with the double-pass system.10 When analyzing the value of MTF10 obtained from both instruments, in this study, the ray-tracing aberrometer showed the larger value of MTF10 compared with the double-pass system. However, there was a significant correlation between the two MTF10 values.
In this study, the effective exit pupil was fixed at 4 mm during measurement with the double-pass system. To examine ocular aberration using the ray-tracing aberrometer, we recalculated our data at a pupil size of 4 mm. Thus, pupil variation did not affect the results of our study. After double pass through the eye and reflection in the retina, monochromatic asymmetric aberrations were cancelled.33 However, it is possible to overcome this limitation by using unequal aperture sizes in the first and second passes.22 In our study, the entrance pupil has a fixed diameter of 2 mm, and the exit pupil has a fixed diameter of 4 mm, which means that monochromatic asymmetric aberrations cannot be cancelled after double pass and reflection.
The double-pass images were recorded using near-infrared light rather than visible light. Albeit patients were comfortable during image acquisition, the magnitude of scatter especially in infrared light were different from that in visible light, rendering measurement of major outcome parameters (MTF, Strehl ratio, and OSI) invalid.34,35 Moreover, there was a large difference in the double-pass measurements between green and infrared light because of a deeper penetration of infrared light in the retina reaching the choroidal layers.36 In our study, we performed repeated measurements of optical quality parameters with the double-pass system to prevent inherent variability in the measurements. Both the tear film dynamics and centration of the system could offer a relative inconsistency of measurements (PSF and MTF) obtained from the double-pass system and the ray-tracing aberrometer. This implies that great caution is needed when interpreting these results.35,37–39
In conclusion, postoperative optical quality parameters obtained from the double-pass system were correlated with the objective measurements (ocular aberrations) and subjective measurements (CDVA and contrast sensitivity score) in pseudophakic eyes.
Department of Ophthalmology
Yonsei University College of Medicine
250 Seongsanno Seodaemun-gu
The first two authors contributed equally to this study and are considered first co-authors.
This work was partially supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST No. 2013R1A1A2058907) and by the Converging Research Center Program through the Ministry of Science, ICT and Future Planning, Korea (2013K000365). No author has a financial or proprietary interest in any material or method mentioned.
Received July 30, 2013; accepted December 17, 2013.
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