Transferring wavefront measurements to ablation profiles Michael Mrochen PhD Swiss Federal Institut of Technology, Zurich IROC Zurich
corneal ablation Calculation laser spot positions Centration Calculation ablation profile Eye Tracking Wavefront sensing Performance Laser Centration during measurement photoablation Patient Eye Biological response wound healing
corneal ablation Calculation laser spot positions Centration Calculation ablation profile Eye Tracking Wavefront sensing Predictability of the refractive outcome Performance Laser Centration during measurement photoablation Patient Eye Biological response wound healing
corneal ablation Calculation laser spot positions Centration Calculation ablation profile Wavefront sensing Centration during measurement Accommodation Tear Film Refractive errors Pupil Size Opacity of media Age Eye Tracking Performance Laser photoablation Patient Eye Biological response wound healing
Patients expectations Is it save and predictablil? Correction for far or near distance monovision / presbyopia high expectations on visual performance Cost / service ratio
corneal ablation
corneal ablation
Preview!!!! See also: Mirko Jankov (Poster session) Can dry eye influence the wavefront measurement Takashi Fujikado (Sunday) Wavefront sensing and the tear film
corneal ablation Calculation laser spot positions Centration Calculation ablation profile Eye Tracking Wavefront sensing Fixation Target line of sight pupil size Performance Laser Centration during measurement photoablation Patient Eye Biological response wound healing
corneal ablation Calculation laser spot positions Centration Calculation ablation profile Wavefront sensing Centration during measurement Optical Setup System calibration Wavelength Zernike calculation Dynamic range of sensor Eye Tracking Performance Laser photoablation Patient Eye Biological response wound healing
Chromatic aberrations Φ = 1D F 486 nm d 588 nm C 656 nm Wavefront sensors usually work in the near infrared wave length > 750 nm
Preview!!!! See also: Larry Thibos (Hot Topics) Does Chromatic Aberration Hinder or Help?
Wavefront sensing ~ 96 spots over a 7 mm pupil ~ 1100 spots over a 7 mm pupil
corneal ablation Calculation laser spot positions Centration Calculation ablation profile Eye Tracking Wavefront sensing Centration during measurement Optical eye model K-Readings Topography of the cornea Biometric data of the eye Performance Laser photoablation Patient Eye Biological response wound healing
Ablation profiles Ablation profile Subjective Refraction Pupil size corneal curvature age female / mail patien expectations y 2 2 1 ( r) = R1 ( r r0 ) y 2 2 2 ( r) = R2 ( r r0 ) Optical zone radius r 0 2 ( r0 ), 0 r 0 ( n 1) D r a( r) = r 2 D = D before D after = ( n 1) 1 R 1 1 R 2
Ablation profiles. the corneal topography information specifying corneal shape has very little effect on the desired ablation depth for an optimal refraction. Stanley Klein, J Opt. Soc. Am A (1999)
Ablation profiles Wavefront guided treatments. the first surface of the cornea and internal optics partially compensate for each other's aberrations and produce an improved retinal image. it shows the limitation of corneal topography as a guide for new refractive procedures and provides a strong endorsement of the value of ocular wave-front sensing for those applications.. Pablo Artal, J. Vis. (2001)
Ablation profiles wavefront aberrations of the internal structures Corneal wavefront aberrations Total wavefront aberrations
Ablation profiles Converting wavefronts into corrections 1st - order approximation! Wavefront inversion 2 µm 2 µm 2 µm 2 µm Pupil diameter 6 mm Pupil diameter 6 mm Ablations profile shift vertical axis [mm] 10 8 6 4 2 2 4 6 8 10 horizontal axis [mm] 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8.0 6.0 4.0 2.0 0 ablation depth [µm] 12 µm 6 µm Pupil diameter 6 mm Ablations profil conversion 6 µm W ( x, y) a( x, y) = n = 1.337 n 1 6 µm Pupil diameter 6 mm
Ablation profiles Total wavefront Wavefront of higher orders Pre-OP Post-OP 6 months vertical axis [mm] vertical axis [mm] Ablation profile used! 10 8 6 4 2 10 8 6 4 2 2 4 6 8 10 horizontal axis [mm] 2 4 6 8 10 horizontal axis [mm] 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8.0 6.0 4.0 2.0 0 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8.0 6.0 4.0 2.0 0 ablation depth [µm] classical ablation profile ablation depth [µm]
Ablation profiles a ray that is less bent when it enters the eye is expected to intersect the lens much further away form the axis. the spherical aberration is expected to be much higher in the operated eye. Fabrice Manns, SPIE Ophthalmic Technologies XI (2001)
Ablation profiles f 1
Ablation profiles f f 1 2 2 f
Ablation profiles because of individual interactions of the aberrations in the ocular components, a combination of corneal and total aberration measurements is critical to understand the individual outcomes, and by extension, to designing customized ablation algorithms. Susana Marcos, IOVS (2001)
Preview!!!! See also: Susana Marcos (Sunday) From theoretical laser ablation profile design ro real outcomes: implications for optimized corneal refractive correction Steve Burns (Hot topics) What s better than a perfect optical correction?
corneal ablation Calculation laser spot positions Centration Calculation ablation profile Eye Tracking Wavefront sensing Centration during measurement Overlapping of spots assumed ablation depth per pulse corneal shape factors thermal heating Performance Laser photoablation Patient Eye Biological response wound healing
Ablation depth [µm] Principles of of laser-tissue interaction 1.4 1.2 1.0 0.8 0.6 0.4 0.2 Tissue removal Threshold process Ablation threshold @ 193 nm 50-60 mj/cm 2 0.0 10 100 1000 Radiant exposure [mj/cm²) Planned ablation profile Ablation threshold ~ 50 mj/cm 2 Central ablation depth of a single laser spot ~ 0.5 microns Ablation diameter of a single spot 0.5-1.0 mm Ablation diameter of a single spot 0.5-1.0 mm
corneal ablation Treatment zone Optical zone Spot overlapping Laser pulses
corneal ablation Small laser beam Pulse ablation profile Large laser beam Pulse ablation profile Achieved profile Attempted profile on the cornea Achieved profile
Example Coma-like Aberration Ablation depth 0.5µm 0.25µm 0.125µm C7 = 0.5µm Pulse diameter 1.5mm 1.0mm 0.5mm
Example Example 6th 6th order order astigmatism astigmatism 1.5mm Pulse diameter 1.0mm C23 = 0.25µm 0.5mm 0.5µm Ablation depth 0.25µm 0.125µm
Treatment time!! Reducing the spot diameter by a factor of 2 results in an increase of the treatment time by a factor of 4. treatment time ~ ( spot diameter 2 )
corneal ablation Calculation laser spot positions Centration Calculation ablation profile Eye Tracking Wavefront sensing Centration during measurement Fixation Target line of sight pupil size rough corneal surface Performance Laser photoablation Patient Eye Biological response wound healing
Centration Difficulty 2 Centration: A task with 6 degrees of freedom Horizontal shifts Vertical shifts Rotation around longitudinal axis (cyclotorsion) Rotation around horizontal axis Rotation around vertical axis Z - distance The coordinate systems used in the measurement (M) and the treatment (T) have to coincide exactly!
Centration 2 Types of centration errors: Systematic centration errors causing constant decentration Random (dynamic) centration errors causing the ablation to be smeared
Centration Systematic centration errors avoid with precise alignment techniques Random (dynamic) centration errors avoid with active eye tracking
Centration Required Accuracy Treat 95% of normal eyes to: Diffraction limit 10 th percentile of rms of normal eyes Same image quality Torsional 3 mm 3 deg 6 deg 29 deg alignment 7 mm 1 deg 4 deg 21 deg Lateral 3 mm 0.21 mm 0.41 mm 0.85 mm centration 7 mm 0.07 mm 0.22 mm 0.62 mm
corneal ablation Calculation laser spot positions Centration Calculation ablation profile Eye Tracking Wavefront sensing Centration during measurement Sapling rate / Latency Resolution Pupil Size Paraxial errors of entrance pupil Performance Laser photoablation Patient Eye Biological response wound healing
Eye - tracking The eye moves during treatment
Eye - tracking How does latency cause positioning errors? Latency Image Acquisition Image Transfer Image Processing Position Control Ablation image Eye Tracker x/y Scanner Device beamsplitter α/β Camer a Scanner mirror ablation beam Laser infrared illumination Eye
Eye - tracking Image Acquisition Image Transfer Image Processing Position Control Ablation Eye Tracker Scanner Device Camera Laser
Eye - tracking Image Acquisition Image Transfer Image Processing Position Control Ablation image Eye Tracker Scanner Device Camera Laser
Eye - tracking Image Acquisition Image Transfer Image Processing Position Control Ablation image Eye Tracker x/y Scanner Device Camera Laser
Eye - tracking Image Acquisition Image Transfer Image Processing Position Control Ablation image Eye Tracker x/y Scanner Device α/β Camera Laser
Eye - tracking Image Acquisition Image Transfer Image Processing Position Control Ablation image Eye Tracker x/y Scanner Device α/β Camera Laser
Eye - tracking Eye motion Positioning Error Latency Eye Motion during Latency => Positioning Error
Eye - tracking Vertical deviation [µm] 400 Vertical deviation [µm] 400 0 ms latency 4 ms latency 200 200 0 0-200 -200-400 -400-200 0 200 400 Horizontal deviation [µm] Vertical deviation [µm] 400 200 8 ms latency -400-400 -200 0 200 400 Horizontal deviation [µm] Vertical deviation [µm] 400 200 No tracking Positioning errors increase with increasing latency 0 0-200 -200-400 -400-200 0 200 400 Horizontal deviation [µm] SMI -400-400 -200 0 200 400 Horizontal deviation [µm]
Preview!!!! More detailed information on the assumptions, stability, and outcomes of different scanning - spot laser parameters such as ablation depth, spot diameter, and eye-tracking latency treatments are presented tomorrow by Michael Bueeler
corneal ablation Calculation laser spot positions Centration Calculation ablation profile Eye Tracking Wavefront sensing Centration during measurement Energy stability Beam profile Scanning technology Wavelength Performance Laser photoablation Patient Eye Biological response wound healing
Excimer laser corneal ablation Beam profiling / shaping Beam delivery Imaging / focusing optics
corneal ablation
corneal ablation Calculation laser spot positions Centration Calculation ablation profile Eye Tracking Wavefront sensing Centration during measurement Tissue absorption Laser pulse duration Tissue properties Biomechanical properties of tissue Performance Laser photoablation Patient Eye Biological response wound healing
Principles of of laser-tissue interaction Dissoziation and vaporisation Vaporisation Ft h F0 Dissoziation da b l - α z ~e Tissue z
Principles of of laser-tissue interaction Radiant exposure (fluence) Absorption Breaking of molecular bonds Increase of temperature Breaking of hydrogen bonds Dissociation and vaporization Tissue removal Stress waves Ablation plume dynamics
Ablation depth [µm] Principles of of laser-tissue interaction 1.4 1.2 1.0 0.8 0.6 0.4 0.2 Tissue removal Threshold process Ablation threshold @ 193 nm 50-60 mj/cm 2 0.0 10 100 1000 Radiant exposure [mj/cm²)
Spot cross-section The illumination problem
The illumination problem 1.00 Fluence losses z α 0.95 Ae ff y kor(r) 0.90 0.85 0.80 0.75 F = 150mJ/cm²; R = 6.5 mm F = 150mJ/cm²; R = 7.0 mm F = 150mJ/cm²; R = 7.8 mm F = 150mJ/cm²; R = 8.3 mm r x 0.70 0 1 2 3 4 5 radius r[mm]
corneal ablation Calculation laser spot positions Centration Calculation ablation profile Eye Tracking Wavefront sensing Centration during measurement Epithelium smoothing Flap / Hinge Biomechanical changes DLK Performance Laser photoablation Patient Eye Biological response wound healing
Biomechanical effect IOP
Biomechanical effect IOP
Biomechanical effect IOP
Biomechanical effect myopic shift! IOP
Biomechanical effect Do we have a method for stiffening the cornea?
Biomechanical effect Cross-linking by UV - light and riboflavin is able to increase Young s module of the cornea by a factor of 5
SUMMARY Transfering wavefronts onto the cornea includes complex physical, optical, and biological assumtions that are not fully understood or studied
corneal ablation Calculation laser spot positions Centration Calculation ablation profile Eye Tracking Wavefront sensing Further research is required to increase Performance Laser Centration during measurement photoablation Patient Eye Biological response wound healing
corneal ablation Calculation laser spot positions Centration Calculation ablation profile Eye Tracking Wavefront sensing Centration during measurement the predictability of the refractive outcomes Performance Laser photoablation Patient Eye Biological response wound healing