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

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Assessing Visual Quality with the Point Spread Function Using the NIDEK OPD-Scan II 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 lens (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] S104 Commercially Sponsored Section journalofrefractivesurgery.com Assessing Visual Quality With the Point Spread Function/Ligabue & Giordano 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 Airy Disk 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 convolution, 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 Ϫ4.25 Ϫ0.25 164. 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 ϩ1.50 Ϫ0.75 The OPD-Scan II measures ocular wavefront, corneal ϫ 15 with 20/30 vision. Slit-lamp examination revealed topography, autorefraction, autokeratometry, and pupil- a cataract in both eyes. Preoperatively, the total and in- Journal of Refractive Surgery Volume 25 January (Suppl) 2009 Commercially Sponsored Section S105 Assessing Visual Quality With the Point Spread Function/Ligabue & Giordano 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.
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