Limits of Higher Order Correction based on Spot Size, Ablation Depth, and Tracker Responsiveness

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2 Limits of Higher Order Correction based on Spot Size, Ablation Depth, and Tracker Responsiveness Michael Bueeler a,b, Michael Mrochen a,b, Theo Seiler b a Swiss Federal Institute of Technology Zurich, Institute of Biomedical Engineering, Switzerland b IROC Insitute for Refractive and Oculoplastic Surgery, Zurich, Switzerland

3 Purpose Introduction Investigation Effect of incomplete eye movement compensation due to eye-tracker/laser-system latency on the correction of higher-order wavefront aberrations. Questions to be answered - What laser spot parameters (spot size, ablation depth per pulse) provide the best stability of the correction in the presence of eye movements / artefacts? - Tolerable latencies for the correction of higher-order aberrations?

4 Methods - Numerical simulations of scanning spot corrections (6 mm OZ) - Measured eye movements - Variation of the following parameters: Laser spot size D (Gaussian beam) (250, 500, 750, 1000 microns) Laser ablation depth per pulse t (0.25, 0.50, 0.75, 1.00 microns) D t Eye tracker latency (0, 4, 32, 96 ms and no eye-tracking) Type of corrected aberration

5 Simulation process Calculation of laser shot list ablation profile spot diameter ablation depth per pulse Scanning spot ablation from anterior corneal surface Assessment of surface quality (surface variance) eye movement artefacts tracker latency measured eye movement data Smoothing of corneal surface (low-pass filtering) Assessment of image quality (PSF, Strehl)

6 Eye movements & Latency horizontal eye movements [microns] Eye movements measured at 250 Hz No compensation Std = 100 microns time [sec] eye movement artefacts [microns] Compensation with 96 ms latency Std = 61 microns time [sec] eye movement artefacts [microns] 300 Compensation with 16 ms latency Std = 20 microns time [sec]

7 Some qualitative results

8 Results Example: 4D myopic correction (Spot 500 / 0.5 microns) Compensation with 0 ms latency Compensation with 96 ms latency

9 Example: primary spherical aberration (Spot 250 / 1.0 microns) Ideal ablation profile Ablated w 0 ms latency Ablated w 96 ms latency

10 Influence of the spot parameters D = 250 mm d c = 0.25 mm D = 250 mm d c = 1.0 mm D = 1000 mm d c = 0.25 mm D = 1000 mm d c = 1.0 mm

11 Surface quality

12 Variance of profile difference

13 Surface variance ratio post- / pre-op as function of latency Surface variance post-op to pre-op Spot diameter: 250 microns 1000 microns Primary coma (Z 3-1 VSIA = 0.6 microns, 6 mm pupil) Eye-tracker latency [ms] Ablation depth per pulse [microns]: / / / /0.25

14 Surface variance post-op to pre-op Secondary coma (Z 5-1 VSIA = microns, 6 mm pupil) Spot diameter: 250 microns 1000 microns Eye-tracker latency [ms] Ablation depth per pulse [microns]: / / / /0.25

15 Image quality

16 PSF cross sections after correction of a primary spherical aberration Spot diameter: 1000 microns Central ablation depth : 1.0 microns Spot diameter: 250 microns Central ablation depth : 1.0 microns

17 Strehl ratio post- / pre-op as function of latency postop / Strehl preop Strehl intensity value ratio Strehl Spot diameter: 250 microns 1000 microns Eye tracker latency [ms] Primary coma (Z 3-1 VSIA = 0.6 microns, 6 mm pupil) Ablation depth per pulse [microns]: 1000/ / / /1.00

18 Latency thresholds derived from the Strehl intensity data when intending to reduce the spot diameter from 1000 to 250 microns at a constant ablation depth of 0.25 or 1.0 microns. Primary coma Primary spherical Secondary coma Secondary spherical Constant ablation depth [microns] Latency threshold from Strehl ratio [ms]

19 Conclusions - Best approximation of the profile in case of perfect movement compensation: Small spot size / small ablation depth per pulse - Most stable ablation in the presence of movement artefacts: Large spot size / small ablation depth per pulse - Asymptotical approach to the case no eye tracking with increasing latency -> Strehl ratio at ~100 ms latency similar to no eye tracking value - Latency thresholds for changing the spot diameter from 1000 to 250 microns: ª 5 ms for the correction of large aberrations < 5 ms for small aberrations

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