Assessing Visual Quality With the Point Spread Function Using the NIDEK OPD-Scan II

Edoardo A. Ligabue, MD; Cristina Giordano, OD

he measurement of high-contrast visual acuity is ABSTRACT not an adequate representation of the patient’s vi- PURPOSE: To present the use of the point spread sual quality. Visual acuity can be described by the T 1 function (PSF) metric pre- and postoperatively for the following expression : V • A • = 1/, where represents the assessment of visual quality in cataract and refractive minimum angle of resolution. Snellen acuity is a measure of the surgery. ability of the eye to resolve details, and the recorded value pro- vides a metric of the functional integrity of the optical system. METHODS: Case examples of cataract and refractive However, vision is a complex neural process, and the percep- surgery and the effect of accommodation are presented. All PSF measurements were obtained using the NIDEK tion of scotopic symptoms such as halos and glare cannot be OPD-Scan II, and corneal internal and total aberrations described by a single value metric. were simulated using the NIDEK OPD-Station software. Theoretically, a perfect optical system would focus all in- All eyes underwent corneal topography, wavefront aber- coming light rays into one single point that would not cause rometry, autorefraction, keratometry, and pupillometry halos or glare. In the human eye, however, light is distorted, measurements pre- and postoperatively using the OPD- Scan II. The PSF was used to assess visual quality. and this distortion can be measured using wavefront aber- rometry.2 The wave theory of light can be used to explain RESULTS: Four case examples including refractive sur- interference and the diffraction primarily responsible for gery, aspheric multifocal intraocular (IOL) implanta- less-than-perfect optics in the human eye that form the basis tion, toric IOL implantation, and the difference in PSF of the point spread function (PSF).3 For example, when the due to accommodation are presented. crest of one light wave meets the crest of another, the crests combine and undergo constructive interference. Waves that CONCLUSIONS: The PSF in refractive and cataract sur- gery is a clinically valuable metric to evaluate the patient’s combine in this manner are considered in phase. Alternately, visual quality pre- and postoperatively. The OPD-Scan II when the crest of one light wave meets the trough of another, can measure the effect of accommodation and distant the light waves cancel and undergo destructive interference. fi xation providing an assessment of changes in visual These waves are considered out of phase. quality. [J Refract Surg. 2009;25:S104-S109.] Diffraction is defi ned as “any deviation of light rays from a rectilinear path which cannot be interpreted as refl ection or refraction.”2 Diffraction occurs when light waves meet an aper- ture (eg, the pupil in the eye) or an obstacle (eg, the diffractive steps in a multifocal intraocular lens [IOL]). A small aperture causes greater diffraction compared with a large aperture. In the eye, the pupil serves as the aperture, and optics of the eye determine the wavefront aberrations that can be used to simu- late objective visual quality using the PSF.4

From CDI Eye Clinic, Milan, Italy. The authors have no financial interest in the materials presented herein. Correspondence: Edoardo A. Ligabue, MD, CDI Eye Clinic, Corso di Porta Vittoria 5, 20122 Milan, Italy. Tel: 39 248 317260; Fax:39 258 303888; E-mail: [email protected]

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Geometric optics defi nes the image of a point source lometry. All measurements were performed on the same as a point. Physical optics states that light waves from optical axis without moving the instrument or the patient; a point source can interfere. Optically, these effects de- hence, the corneal and whole eye wavefront data can be termine the pattern of the PSF. The PSF for a perfect directly correlated. The OPD-Station software separates optical system is the that is calculated us- corneal (anterior corneal surface) from the internal aber- ing Fraunhofer’s approximation.5 The PSF is the image rations (posterior cornea surface, lenticular, vitreous, and of a point source of light formed on the retina by the retina) to determine the effect of each optical element in eye’s optical system. Every object can be considered the visual system separately. The PSF, early treatment of a combination of a number of point sources of light. diabetic retinopathy study (ETDRS) chart, and MTF can The mathematical process of overlapping all of these be simulated for the total eye or the lenticular or corneal points forming an object is called a , from components of the eye. The OPD-Scan and OPD-Station which the PSF is calculated.6 In a normal eye, the PSF have been described previously.7-9 is generally dependent on pupils because as the pu- pil diameter increases, so do the wavefront aberrations CASE REPORTS that directly affect the PSF. To assess visual quality, the PSF, modulation trans- CASE 1 fer function (MTF), Strehl ratio, and wavefront maps Refractive Surgery Screening and Postoperative can be evaluated. The MTF represents perhaps the Changes. A 30-year-old woman presented for LASIK. most intuitive representation of visual quality. Basi- The right eye is presented as an example. The manifest cally, it is picture of how the optics of an eye distorts refraction was 4.25 0.25 164. The OPD-Station a point source of light. The MTF represents the visual evaluation is presented in Figure 1. The Holladay sum- performance of the eye across the range of spatial fre- mary displays the total, corneal, and internal wavefront quencies used in daily life. Image quality is improved maps of the eye along with the corresponding PSF when the area under the MTF is increased or maxi- measurements. All wavefront maps in Figure 1 present mized. The Strehl ratio is the ratio of the peak height the effect of lower and higher order aberrations of the of the PSF being measured to the peak height of a per- eye. Using the Holladay summary, one can determine fect optical system. The Strehl ratio range is from 0 whether the corneal or internal components (or a com- to 1. In a normal eye, the Strehl ratio does not reach bination) of the eye are detrimental to visual quality. In 1, because diffraction and the optical aberrations limit this case, the total PSF and internal PSF patterns were the visual resolution of the eye. A typical higher order similar, indicating that the majority of refractive error Strehl ratio value for a 5-mm pupil is 0.05. The higher and visual quality was determined by the internal ab- the Strehl ratio, the better the potential image quality. errations of the eye. The wavefront maps show a bull’s The assessment of visual quality is important be- eye pattern consistent with myopia. cause of the introduction of newer IOL and excimer The patient underwent LASIK. The corneal wave- laser technologies that purport to increase visual qual- front map did not show signifi cant aberration 3 months ity compared with conventional models. In addition, postoperatively compared with preoperatively. The a more informed and more demanding patient base postoperative corneal PSF was slightly worse com- makes it necessary to assess the visual quality preop- pared with preoperative corneal PSF, likely due to the eratively for treatment planning. lamellar cut. Postoperatively, the total eye PSF resem- We report the use of these image quality metrics bled a point of light indicating successful reduction in cases of cataract and refractive surgery and assess of refractive error and higher order aberrations. The changes due to accommodation. Clinically, the surgeon Strehl ratio for the total wavefront increased from 0 will be able to understand image perception and visual preoperatively to 0.100 postoperatively, indicating an disturbances that make the patient symptomatic. increase in visual quality.

PATIENTS AND METHODS CASE 2 The OPD-Scan II with OPD-Station software (NIDEK Cataract Screening and Toric IOL Implantation As- Co Ltd, Gamagori, Japan) was used in four cases that in- sessment. A 65-year-old man reporting reduction in vi- cluded toric IOL implantation, preoperative LASIK can- sion along with ghosting at night presented for routine didate assessment, aspheric multifocal IOL implantation, assessment. Only the left eye is presented for illustra- and the effect of accommodation on visual quality. tive purposes. The manifest refraction was 1.50 0.75 The OPD-Scan II measures ocular wavefront, corneal 15 with 20/30 vision. Slit-lamp examination revealed topography, autorefraction, autokeratometry, and pupil- a cataract in both eyes. Preoperatively, the total and in-

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Figure 1. Case 1. A) Pre- and B) postoperative OPD-Station evaluation of a patient who underwent LASIK. Postoperatively, the point spread function (PSF) due to all components (PSF/OPD/Tot) is significantly smaller and less distorted than preoperatively, indicating better uncorrected visual quality. WF/OPD/Tot denotes the higher and lower order aberrations of all components of the eye combined; WF/Corn/Tot denotes the higher and lower order wavefront aberrations of the corneal front surface; and WF/Int/Tot denotes the higher and lower order wavefront aberration of the internal (primarily lenticular) aberrations of the eye. All measurements and simulations were performed for a 4-mm pupil diameter to the eighth Zernike order.

ternal aberration patterns were similar, indicating the CASE 3 internal aberrations were causing the reduction in visu- Changes in Visual Quality Due to Accommodation. al acuity (Fig 2A). The patient underwent SN60T3 24.00 During accommodation, the crystalline lens changes D AcrySof toric IOL implantation (Alcon Laboratories shape so that it is thicker centrally than peripherally. Inc, Ft Worth, Tex). Figure 2B presents the OPD-Station This peripheral fl attening increases negative longitu- evaluation 1 month postoperatively. The wavefront re- dinal .1 This changes the aspheric- fraction shows 0.50 D of astigmatism postoperatively ity and high order aberrations of the crystalline lens, with an excellent wavefront total PSF despite a total which cause a change in the visual quality (Fig 3). For wavefront error 0.800 µm. The discrepancy between example, measurement of a 24-year-old patient look- the wavefront error and PSF highlights the fact that one ing at distance then near induces a myopic shift, a sig- metric or one map cannot suffi ce in clinical assessment nifi cant distortion in the PSF, and a 10-fold decrease in using wavefront aberrometry and that multiple indices the Strehl ratio from 0.022 at distance to 0.002 at near and maps are required for a thorough assessment. The (see Fig 3). pre- and postoperative corneal PSF were somewhat During cataract formation, the amplitude of accom- different, likely because of placement of the incision. modation declines, causing little or no change in the The internal total wavefront was reduced to half the PSF (Fig 4). For example, a patient with an incipient preoperative value after IOL implantation. The internal cataract shows little change in the PSF or Strehl ratio wavefront map showed a band of little to no optical dis- when looking at distance or near. The measurement of tortion (green), indicating that the IOL is positioned on near and distance visual quality can be used to evalu- axis without signifi cant tilt (see Fig 2B). Signifi cant tilt ate the effect of accommodative IOLs. For example, the and IOL decentration lead to coma-like patterns in the PSF of this patient who received an accommodative wavefront maps. IOL shows no difference when looking at distance ver-

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Figure 2. Case 2. A) Pre- and B) postoperative OPD-Station evaluation of a patient who underwent toric IOL implantation. Postoperatively, the point spread function (PSF) due to all components (PSF/OPD/Tot) is significantly smaller and less distorted than preoperatively, indicating better uncorrected visual quality. Postoperatively, the internal PSF also became significantly smaller and less distorted. WF/OPD/Tot denotes the higher and lower order aberrations of all components of the eye combined; WF/Corn/Tot denotes the higher and lower order wavefront aberrations of the corneal front surface; and WF/Int/Tot denotes the higher and lower order wavefront aberration of the internal (primarily lenticular) aberrations of the eye. All measurements and simulations were performed for a 4.8-mm pupil diameter to the eighth Zernike order. sus near, indicating that the IOL remains stable when with the emmetropic eyes. Based on this result, only a presented with a near stimulus (Fig 5). 12% increase in visual performance will be achieved by eliminating the residual refractive error (primarily CASE 4 astigmatism). This small increase did not warrant an Aspheric IOL Implantation. Figure 6 presents OPD- IOL exchange procedure. The patient remained asymp- Scan II evaluation of a patient who underwent implan- tomatic and was satisfi ed with the unaided distance tation of an aspheric AcrySof ReStor IOL (Alcon Labo- and near vision 6 months postoperatively. ratories Inc) in which some residual positive spherical aberration was targeted in the right eye. Postopera- DISCUSSION tively, the ocular spherical aberration was 0.188 µm Visual quality simulations and metrics represent at 5.8 mm of pupil, and corneal spherical aberration an indication of the success of corneal refractive and remained unchanged at 0.368 µm at 5.8 mm of pupil. cataract surgeries. Wavefront aberrometry allows the The PSF showed some glare, likely due to a combina- surgeon to plan the procedure and determine whether tion of the residual astigmatism optics of the IOL. The the origin of visual symptoms is optical. For example, A/B ratio on the MTF graph was 50.3%, indicating the aspheric IOLs could be selected on the basis of corneal visual performance of the patient’s eyes without cor- spherical aberration values provided in the OPD-Sta- rection compared with a pair of emmetropic eyes with tion. The surgeon could select the IOL that leaves the excellent visual acuity and contrast sensitivity. The H/B patient with a slight residual spherical aberration for ratio was 62.4%; this ratio indicates the best correct- depth of focus or completely negate the spherical aber- ed visual performance of the patient’s eyes compared ration depending on the patient’s requirements. As with

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Figure 3. Case 3. A) Point spread function (PSF) at distance and B) PSF due to accom- modation of a 24-year-old emmetropic patient. The change in higher and lower order ocular PSF (PSF/OPD/Tot) is signifi- cant when the patient is presented with a near target. All measurements and simula- tions were performed for a 5.8-mm pupil diameter to the eighth Zernike order.

Figure 4. Case 3. A) Point spread function (PSF) at distance and B) PSF due to accom- modation of a patient with an incipient cata- ract. The change in higher and lower order ocular PSF (PSF/OPD/Tot) is not significantly different when the patient is presented with a near stimulus of 3.00 D. Distance mea- surement and simulation were performed for a 2.9-mm pupil diameter to the sixth Zernike order and near measurement and simulation for a 4-mm pupil diameter to the sixth Zernike order.

Figure 5. Case 3. A) Point spread function (PSF) at distance and B) PSF due to accom- modation for a patient with an accommoda- tive IOL implant. The changes in higher and lower order ocular PSF (PSF/OPD/Tot) do not differ significantly when the patient is presented with a near stimulus of 3.00 D. Distance measurement and simulation were performed for a 5.6-mm pupil diameter to the eighth Zernike order and near measure- ment and simulation for a 5.8-mm pupil diameter to the eighth Zernike order.

LASIK candidates, patients with cataracts demand ex- workup should be similar to that for LASIK for which cellent uncorrected visual acuity with little loss of visu- wavefront aberrometers are routinely used. The ability al quality. Patient needs and the development of premi- to measure visual quality both at near and far will allow um IOLs have changed cataract surgery from functional the surgeon to determine the effects of a multifocal IOL surgery to refractive surgery; hence, the preoperative on near visual quality. In summary, the OPD-Scan II and

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Figure 6. Case 4. Postoperative OPD-Scan II measurement of a patient who received a multifocal aspheric IOL. Axial denotes axial corneal topography (top left); OPD map (top right) is a refractive wavefront map plotting the distribution of refractive power across the pupil due to all wavefront aberrations of the eye. The WF/OPD/Gp map (middle) plots the longitudinal spherical aberration of the eye, the PSF/OPD/Tot plots the PSF of the eye. The MFT/OPD/Tot plots the modulation transfer function of the eye. The corneal and ocular spherical aberration is shown in the red circle. All measurements and simula- tions were performed for a 5.9-mm pupil diameter to the eighth Zernike order.

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