Biomedical subjects

Dimitri A Chernyak

Publications and source records attributed to Dimitri A Chernyak.

5 recordsLinked to original sources

PubMed2005

Iris-based cyclotorsional image alignment method for wavefront registration.

In refractive surgery, especially wavefront-guided refractive surgery, correct registration of the treatment to the cornea is of paramount importance. The specificity of the custom ablation formula requires that the ablation be applied to the cornea only when it has been precisely aligned with the mapped area. If, however, the eye has rotated between measurement and ablation, and this cyclotorsion is not compensated for, the rotational misalignment could impair the effectiveness of the refractive surgery. To achieve precise registration, a noninvasive method for torsional rotational alignment of the captured wavefront image to the patient's eyes at surgery has been developed. This method applies a common coordinate system to the wavefront and the eye. Video cameras on the laser and wavefront devices precisely establish the spatial relationship between the optics of the eye and the natural features of the iris, enabling the surgeon to identify and compensate for cyclotorsional eye motion, whatever its cause.

Algorithms↗

PubMed2004

Cyclotorsional eye motion occurring between wavefront measurement and refractive surgery.

PURPOSE: To quantify the cyclorotation occurring between wavefront measurement and laser refractive surgery. SETTING: LaserVue Eye Center Ophthalmic Clinic, Santa Rosa, California, USA. METHODS: The pupil camera of the Visx WaveScan wavefront device was used to obtain images of 51 eyes (26 patients) from 5 to 20 minutes before refractive laser surgery. Additionally, an infrared camera was mounted on the Visx Star S3 ActiveTrak excimer laser system to obtain another image immediately before the laser was fired. After surgery, the 2 sets of images were compared to determine the amount of cyclotorsion between the measurement and surgery. RESULTS: Cyclorotation of individual eyes was as high as 9.5 degrees. The mean was approximately 2.0 degrees for each eye. Binocular excyclotorsion was the predominant trend, affecting 19 of 24 patients. CONCLUSIONS: A low to moderate amount of cyclotorsion was observed in the transition from seated to supine position. Comparison of eye position at the time of measurement to eye position at the time of surgery can be used to adjust the laser ablation algorithm to compensate for this rotational displacement.

Adult↗

PubMed2003

System for the design, manufacture, and testing of custom lenses with known amounts of high-order aberrations.

Now that excimer laser systems can be programmed to correct complex aberrations of the eye on the basis of wave-front measurements, a method is needed to test the accuracy of the system from measurement through treatment. A closed-loop test method was developed to ensure that treatment plans generated by a wavefront measuring system were accurately transferred to and executed by the excimer laser. A surface was analytically defined, and a Shack-Hartmann-based wave-front system was used to formulate a treatment plan, which was downloaded to an excimer laser system. A plastic lens was ablated by the laser and then returned to the wave-front device, where it was measured and compared with the analytically defined wave-front surface. The two surfaces agreed up to 6th-order Zernike terms, validating the accuracy of the system.

Equipment Design↗

PubMedDate not supplied

From wavefront device to laser: an alignment method for complete registration of the ablation to the cornea.

PURPOSE: When customized laser refractive surgery is based on a wavefront measurement, correct registration of the laser ablation to the prescribed treatment area is vital. If the treatment is not ablated exactly where it was measured, the error could negate some benefits of the procedure. Currently, treatments are aligned to the pupil center, which has been shown to shift between measurement and treatment. This article describes an alignment method that uses an image-based coordinate system to compensate for shifts of the pupil center as well as cyclorotation. METHODS: Images of patients' eyes were taken during wavefront measurements and again when patients were in position for surgery. An image-processing method, which matches multiple ocular landmarks in both image sets, was used to align the laser beam to the ablation target. RESULTS: Analyses of the data collected from both instruments suggest that alignment errors caused by movement of the pupil center and cyclorotation could be significantly reduced by using the proposed alignment method. CONCLUSIONS: Because the pupil center undergoes significant movement with changes in lighting conditions, it is unsuitable as the sole alignment point for custom ablations. Several reference points are necessary to compensate for changing pupil size as well as the cyclotorsional eye position changes that occur between wavefront measurement and laser procedure. By providing multiple points for alignment, the registration method described herein increases safety and effectiveness of wavefront-based laser vision correction.

Adult↗

PubMedDate not supplied

Changes in wavefront aberration with pharmaceutical dilating agents.

PURPOSE: Finding the best way to capture the wavefronts of small pupils for refractive surgery has become a more pressing issue as the general population ages. This study explores whether pharmaceutical dilation impacts wavefront measurement and pupil centroid. METHODS: Baseline measurements were performed on 32 eyes using the VISX WaveScan Wavefront system. Pupils were dark adapted. One drop of 0.05% tropicamide was placed in each eye, and wavefront measurements were conducted at 10, 20, and 30 minutes. One drop each of 0.5% tropicamide and 2.5% phenylephrine were administered after the measurements. Wavefronts were captured again 30 minutes after the last eye drop. All patients returned for repeated procedures. Wavefront analysis was performed using the same pupillary area as the smallest capture from the same visit. Pupil center shift was taken into account. RESULTS: Mean patient age was 40 +/- 12 years (range: 20 to 59 years). Mean dark-adapted pupil was 6.40 +/- 1.17 mm. Pupil centers shifted randomly after pharmacological dilation compared to the dark-adapted condition. Pupil centers of 45% of the population shifted by > or = 0.2 mm. Repeatability coefficients were established for the wavefront measurements. After controlling for pupil diameter and pupil center, total high order aberrations root-mean-square (RMS) had changed significantly in 18% of the population. The diluted tropicamide formula, which caused less dilation effect, also induced less high order aberration RMS change. CONCLUSIONS: Pharmaceutical dilation agents cause random shifts of the pupil centroid from the dark-adapted pupil condition and could induce changes in wavefront measurements. Caution is required when resorting to dilation to obtain a wavefront measurement of smaller pupils.

Adult↗