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
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?
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
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)
Eye movements & Latency horizontal eye movements [microns] 300 200 100 0-100 -200 Eye movements measured at 250 Hz No compensation Std = 100 microns -300 0 5 10 15 20 25 30 time [sec] eye movement artefacts [microns] 300 200 100 0-100 -200 Compensation with 96 ms latency Std = 61 microns -300 0 5 10 15 20 25 30 time [sec] eye movement artefacts [microns] 300 Compensation with 16 ms latency Std = 20 microns 200 100 0-100 -200-300 0 5 10 15 20 25 30 time [sec]
Some qualitative results
Results Example: 4D myopic correction (Spot 500 / 0.5 microns) Compensation with 0 ms latency Compensation with 96 ms latency
Example: primary spherical aberration (Spot 250 / 1.0 microns) Ideal ablation profile Ablated w 0 ms latency Ablated w 96 ms latency
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
Surface quality
Variance of profile difference
Surface variance ratio post- / pre-op as function of latency Surface variance post-op to pre-op 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Spot diameter: 250 microns 1000 microns Primary coma (Z 3-1 VSIA = 0.6 microns, 6 mm pupil) 0.0 0 20 40 60 80 100 Eye-tracker latency [ms] Ablation depth per pulse [microns]: 1.00 0.25 1.00 0.25 250/1.00 250/0.25 1000/1.00 1000/0.25
Surface variance post-op to pre-op 7 6 5 4 3 2 1 Secondary coma (Z 5-1 VSIA = 0.075 microns, 6 mm pupil) Spot diameter: 250 microns 1000 microns 0 0 20 40 60 80 100 Eye-tracker latency [ms] Ablation depth per pulse [microns]: 1.00 1.00 0.25 0.25 250/1.00 1000/1.00 250/0.25 1000/0.25
Image quality
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
Strehl ratio post- / pre-op as function of latency postop / Strehl preop Strehl intensity value ratio Strehl 18 16 14 12 10 8 6 4 Spot diameter: 250 microns 1000 microns 0.25 0.25 1.00 2 1.00 0 0 20 40 60 80 100 Eye tracker latency [ms] Primary coma (Z 3-1 VSIA = 0.6 microns, 6 mm pupil) Ablation depth per pulse [microns]: 1000/0.25 250/0.25 1000/1.00 250/1.00
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] 0.25 1.00 0.25 1.00 0.25 1.00 0.25 1.00 Latency threshold from Strehl ratio [ms] 3 6 3 5 8 5 0.5 0.5
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