Download Assessing Visual Quality With the Point Spread Function Using the

Assessing Visual Quality With the
Point Spread Function Using the
NIDEK OPD-Scan II
Edoardo A. Ligabue, MD; Cristina Giordano, OD
ABSTRACT
PURPOSE: To present the use of the point spread
function (PSF) metric pre- and postoperatively for the
assessment of visual quality in cataract and refractive
surgery.
METHODS: Case examples of cataract and refractive
surgery and the effect of accommodation are presented.
All PSF measurements were obtained using the NIDEK
OPD-Scan II, and corneal internal and total aberrations
were simulated using the NIDEK OPD-Station software.
All eyes underwent corneal topography, wavefront aberrometry, autorefraction, keratometry, and pupillometry
measurements pre- and postoperatively using the OPDScan II. The PSF was used to assess visual quality.
RESULTS: Four case examples including refractive surgery, aspheric multifocal intraocular lens (IOL) implantation, toric IOL implantation, and the difference in PSF
due to accommodation are presented.
CONCLUSIONS: The PSF in refractive and cataract surgery is a clinically valuable metric to evaluate the patient’s
visual quality pre- and postoperatively. The OPD-Scan II
can measure the effect of accommodation and distant
fixation providing an assessment of changes in visual
quality. [J Refract Surg. 2009;25:S104-S109.]
T
he measurement of high-contrast visual acuity is
not an adequate representation of the patient’s visual quality. Visual acuity can be described by the
following expression1: V • A • = 1/, where represents the
minimum angle of resolution. Snellen acuity is a measure of the
ability of the eye to resolve details, and the recorded value provides a metric of the functional integrity of the optical system.
However, vision is a complex neural process, and the perception of scotopic symptoms such as halos and glare cannot be
described by a single value metric.
Theoretically, a perfect optical system would focus all incoming light rays into one single point that would not cause
halos or glare. In the human eye, however, light is distorted,
and this distortion can be measured using wavefront aberrometry.2 The wave theory of light can be used to explain
interference and the diffraction primarily responsible for
less-than-perfect optics in the human eye that form the basis
of the point spread function (PSF).3 For example, when the
crest of one light wave meets the crest of another, the crests
combine and undergo constructive interference. Waves that
combine in this manner are considered in phase. Alternately,
when the crest of one light wave meets the trough of another,
the light waves cancel and undergo destructive interference.
These waves are considered out of phase.
Diffraction is defined as “any deviation of light rays from a
rectilinear path which cannot be interpreted as reflection or
refraction.”2 Diffraction occurs when light waves meet an aperture (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 simulate 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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journalofrefractivesurgery.com
Assessing Visual Quality With the Point Spread Function/Ligabue & Giordano
Geometric optics defines the image of a point source
as a point. Physical optics states that light waves from
a point source can interfere. Optically, these effects determine the pattern of the PSF. The PSF for a perfect
optical system is the Airy Disk that is calculated using Fraunhofer’s approximation.5 The PSF is the image
of a point source of light formed on the retina by the
eye’s optical system. Every object can be considered
a combination of a number of point sources of light.
The mathematical process of overlapping all of these
points forming an object is called a convolution, from
which the PSF is calculated.6 In a normal eye, the PSF
is generally dependent on pupils because as the pupil diameter increases, so do the wavefront aberrations
that directly affect the PSF.
To assess visual quality, the PSF, modulation transfer function (MTF), Strehl ratio, and wavefront maps
can be evaluated. The MTF represents perhaps the
most intuitive representation of visual quality. Basically, it is picture of how the optics of an eye distorts
a point source of light. The MTF represents the visual
performance of the eye across the range of spatial frequencies used in daily life. Image quality is improved
when the area under the MTF is increased or maximized. The Strehl ratio is the ratio of the peak height
of the PSF being measured to the peak height of a perfect optical system. The Strehl ratio range is from 0
to 1. In a normal eye, the Strehl ratio does not reach
1, because diffraction and the optical aberrations limit
the visual resolution of the eye. A typical higher order
Strehl ratio value for a 5-mm pupil is 0.05. The higher
the Strehl ratio, the better the potential image quality.
The assessment of visual quality is important because of the introduction of newer IOL and excimer
laser technologies that purport to increase visual quality compared with conventional models. In addition,
a more informed and more demanding patient base
makes it necessary to assess the visual quality preoperatively for treatment planning.
We report the use of these image quality metrics
in cases of cataract and refractive surgery and assess
changes due to accommodation. Clinically, the surgeon
will be able to understand image perception and visual
disturbances that make the patient symptomatic.
CASE 1
Refractive Surgery Screening and Postoperative
Changes. A 30-year-old woman presented for LASIK.
The right eye is presented as an example. The manifest
refraction was 4.25 0.25 164. The OPD-Station
evaluation is presented in Figure 1. The Holladay summary displays the total, corneal, and internal wavefront
maps of the eye along with the corresponding PSF
measurements. All wavefront maps in Figure 1 present
the effect of lower and higher order aberrations of the
eye. Using the Holladay summary, one can determine
whether the corneal or internal components (or a combination) of the eye are detrimental to visual quality. In
this case, the total PSF and internal PSF patterns were
similar, indicating that the majority of refractive error
and visual quality was determined by the internal aberrations of the eye. The wavefront maps show a bull’s
eye pattern consistent with myopia.
The patient underwent LASIK. The corneal wavefront map did not show significant aberration 3 months
postoperatively compared with preoperatively. The
postoperative corneal PSF was slightly worse compared with preoperative corneal PSF, likely due to the
lamellar cut. Postoperatively, the total eye PSF resembled a point of light indicating successful reduction
of refractive error and higher order aberrations. The
Strehl ratio for the total wavefront increased from 0
preoperatively to 0.100 postoperatively, indicating an
increase in visual quality.
PATIENTS AND METHODS
The OPD-Scan II with OPD-Station software (NIDEK
Co Ltd, Gamagori, Japan) was used in four cases that included toric IOL implantation, preoperative LASIK candidate assessment, aspheric multifocal IOL implantation,
and the effect of accommodation on visual quality.
The OPD-Scan II measures ocular wavefront, corneal
topography, autorefraction, autokeratometry, and pupil-
CASE 2
Cataract Screening and Toric IOL Implantation Assessment. A 65-year-old man reporting reduction in vision along with ghosting at night presented for routine
assessment. Only the left eye is presented for illustrative purposes. The manifest refraction was 1.50 0.75
15 with 20/30 vision. Slit-lamp examination revealed
a cataract in both eyes. Preoperatively, the total and in-
Journal of Refractive Surgery Volume 25 January (Suppl) 2009
lometry. All measurements were performed on the same
optical axis without moving the instrument or the patient;
hence, the corneal and whole eye wavefront data can be
directly correlated. The OPD-Station software separates
corneal (anterior corneal surface) from the internal aberrations (posterior cornea surface, lenticular, vitreous, and
retina) to determine the effect of each optical element in
the visual system separately. The PSF, early treatment of
diabetic retinopathy study (ETDRS) chart, and MTF can
be simulated for the total eye or the lenticular or corneal
components of the eye. The OPD-Scan and OPD-Station
have been described previously.7-9
CASE REPORTS
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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
internal aberrations were causing the reduction in visual acuity (Fig 2A). The patient underwent SN60T3 24.00
D AcrySof toric IOL implantation (Alcon Laboratories
Inc, Ft Worth, Tex). Figure 2B presents the OPD-Station
evaluation 1 month postoperatively. The wavefront refraction shows 0.50 D of astigmatism postoperatively
with an excellent wavefront total PSF despite a total
wavefront error 0.800 µm. The discrepancy between
the wavefront error and PSF highlights the fact that one
metric or one map cannot suffice in clinical assessment
using wavefront aberrometry and that multiple indices
and maps are required for a thorough assessment. The
pre- and postoperative corneal PSF were somewhat
different, likely because of placement of the incision.
The internal total wavefront was reduced to half the
preoperative value after IOL implantation. The internal
wavefront map showed a band of little to no optical distortion (green), indicating that the IOL is positioned on
axis without significant tilt (see Fig 2B). Significant tilt
and IOL decentration lead to coma-like patterns in the
wavefront maps.
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CASE 3
Changes in Visual Quality Due to Accommodation.
During accommodation, the crystalline lens changes
shape so that it is thicker centrally than peripherally.
This peripheral flattening increases negative longitudinal spherical aberration.1 This changes the asphericity and high order aberrations of the crystalline lens,
which cause a change in the visual quality (Fig 3). For
example, measurement of a 24-year-old patient looking at distance then near induces a myopic shift, a significant distortion in the PSF, and a 10-fold decrease in
the Strehl ratio from 0.022 at distance to 0.002 at near
(see Fig 3).
During cataract formation, the amplitude of accommodation declines, causing little or no change in the
PSF (Fig 4). For example, a patient with an incipient
cataract shows little change in the PSF or Strehl ratio
when looking at distance or near. The measurement of
near and distance visual quality can be used to evaluate the effect of accommodative IOLs. For example, the
PSF of this patient who received an accommodative
IOL shows no difference when looking at distance verjournalofrefractivesurgery.com
Assessing Visual Quality With the Point Spread Function/Ligabue & Giordano
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
presented with a near stimulus (Fig 5).
CASE 4
Aspheric IOL Implantation. Figure 6 presents OPDScan II evaluation of a patient who underwent implantation of an aspheric AcrySof ReStor IOL (Alcon Laboratories Inc) in which some residual positive spherical
aberration was targeted in the right eye. Postoperatively, the ocular spherical aberration was 0.188 µm
at 5.8 mm of pupil, and corneal spherical aberration
remained unchanged at 0.368 µm at 5.8 mm of pupil.
The PSF showed some glare, likely due to a combination of the residual astigmatism optics of the IOL. The
A/B ratio on the MTF graph was 50.3%, indicating the
visual performance of the patient’s eyes without correction compared with a pair of emmetropic eyes with
excellent visual acuity and contrast sensitivity. The H/B
ratio was 62.4%; this ratio indicates the best corrected visual performance of the patient’s eyes compared
Journal of Refractive Surgery Volume 25 January (Suppl) 2009
with the emmetropic eyes. Based on this result, only a
12% increase in visual performance will be achieved
by eliminating the residual refractive error (primarily
astigmatism). This small increase did not warrant an
IOL exchange procedure. The patient remained asymptomatic and was satisfied with the unaided distance
and near vision 6 months postoperatively.
DISCUSSION
Visual quality simulations and metrics represent
an indication of the success of corneal refractive and
cataract surgeries. Wavefront aberrometry allows the
surgeon to plan the procedure and determine whether
the origin of visual symptoms is optical. For example,
aspheric IOLs could be selected on the basis of corneal
spherical aberration values provided in the OPD-Station. The surgeon could select the IOL that leaves the
patient with a slight residual spherical aberration for
depth of focus or completely negate the spherical aberration depending on the patient’s requirements. As with
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Assessing Visual Quality With the Point Spread Function/Ligabue & Giordano
Figure 3. Case 3. A) Point spread function
(PSF) at distance and B) PSF due to accommodation of a 24-year-old emmetropic
patient. The change in higher and lower
order ocular PSF (PSF/OPD/Tot) is significant when the patient is presented with a
near target. All measurements and simulations 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 accommodation of a patient with an incipient cataract. 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 measurement 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 accommodation for a patient with an accommodative 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 measurement and simulation for a 5.8-mm pupil
diameter to the eighth Zernike order.
LASIK candidates, patients with cataracts demand excellent uncorrected visual acuity with little loss of visual quality. Patient needs and the development of premium IOLs have changed cataract surgery from functional
surgery to refractive surgery; hence, the preoperative
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workup should be similar to that for LASIK for which
wavefront aberrometers are routinely used. The ability
to measure visual quality both at near and far will allow
the surgeon to determine the effects of a multifocal IOL
on near visual quality. In summary, the OPD-Scan II and
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Assessing Visual Quality With the Point Spread Function/Ligabue & Giordano
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 simulations were performed for a 5.9-mm pupil
diameter to the eighth Zernike order.
OPD-Station are important instruments for the pre- and
postoperative assessment of point spread function in a
variety of patients.
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