Correction on the distortion of Scheimpflug imaging for dynamic central corneal thickness

Abstract

Abstract. The measurement of central corneal thickness (CCT) is important in ophthalmology. Most studies concerned the value at normal status, while rare ones focused on its dynamic changing. The commercial Corvis ST is the only commercial device currently available to visualize the two-dimensional image of dynamic corneal profiles during an air puff indentation. However, the directly observed CCT involves the Scheimpflug distortion, thus misleading the clinical diagnosis. This study aimed to correct the distortion for better measuring the dynamic CCTs. The optical path was first derived to consider the influence of factors on the use of Covis ST. A correction method was then proposed to estimate the CCT at any time during air puff indentation. Simulation results demonstrated the feasibility of the intuitive-feasible calibration for measuring the stationary CCT and indicated the necessity of correction when air puffed. Experiments on three contact lenses and four human corneas verified the prediction that the CCT would be underestimated when the improper calibration was conducted for air and overestimated when it was conducted on contact lenses made of polymethylmethacrylate. Using the proposed method, the CCT was finally observed to increase by 66 AE 34 μm at highest concavity in 48 normal human corneas.

Determining in vivo elasticity and viscosity with dynamic Scheimpflug imaging analysis in keratoconic and healthy eyes

“…The dynamic change of upper corneal surface was selected to represent the displacement of cornea caused by the air puff force. The size of each image pixel was calibrated and corrected with the value of 18.6 µm .…”

High myopia induced by form deprivation is associated with altered corneal biomechanical properties in chicks

“…By contrast, the air-puff systems (ORA and Corvis) measure in-vivo corneal biomechanical properties and provide multiple useful clinical parameters. Nevertheless, the parameter termed “hysteresis” (provided by ORA) is determined by multiple factors (e.g., corneal thickness, eye size), and Corvis’s Scheimpflug imaging technique may be limited by optical distortions, requiring comprehensive corrections to derive useful corneal biomechanical parameters from the raw data[ 45 , 46 ]. Emerging non-contact/non-invasive devices, including OCE[ 41 ] and Brillouin microscopy[ 42 ], have extended the characterization of corneal biomechanics into mapping the elasticity distribution, but this approach awaits validation of safety and effectiveness prior to its clinical application.…”

Numerical analysis of corneal curvature dynamics based on Corvis tonometer images

“…For curvature calculations in this paper it was assumed that both width and height, related to one pixel, amounts to 16 µm. Li et al used a value of 18.58 µm in their correction procedures representing image length referring to one pixel .…”