Biomedical subjects

Rob G L van der Heijde

Publications and source records attributed to Rob G L van der Heijde.

5 recordsLinked to original sources

PubMed2006

Spherical aberration of the anterior and posterior surfaces of the human cornea.

A ray-tracing procedure was applied to corrected Scheimpflug photography measurements to determine the spherical aberration of the anterior and posterior surfaces of the cornea. It was found that the total spherical aberration of the cornea increases slightly with age. The spherical aberration of the posterior corneal surface is negative at a young age and becomes positive at an older age. To make an accurate description of the spherical aberration for the whole eye, the posterior surface must also be measured.

Adolescent↗

PubMed2005

Dynamic mechanical properties of human lenses.

The purpose of this study was to determine the shear compliance of human crystalline lenses as a function of age and frequency. Dynamic mechanical analysis was performed on 39 human lenses, ranging in age from 18 to 90 years, within the frequency range of 0.001-30 Hz. The lenses were stored at -70 degrees C before being measured. The influence of freezing on the mechanical properties was determined using pairs of porcine lenses, with one lens measured directly after enucleation and the other after freezing. The measurement method had a repeatability standard deviation of 4 and 6% for the storage and loss compliance, respectively. The reproducibility standard deviation was 31 and 33% for the storage and loss compliance respectively. On average, freezing increased the storage compliance by 8% and increased the loss compliance by 32%, both depending slightly on age and frequency. The human lenses exhibited a distinct viscoelastic behavior. The storage and loss compliance depended strongly on age and decreased a factor 1000 over a lifetime. Dynamic mechanical analysis has proven to be a successful technique for characterizing the mechanical properties of the human crystalline lens. The shear compliance decreases exponentially with age.

Adolescent↗

PubMed2005

Comment on "Scheimpflug and high-resolution magnetic resonance imaging of the anterior segment: a comparative study".

In a recent paper, Koretz et al. [J. Opt. Soc. Am. A 21, 346 (2004)] conclude that the results of the "corrected Scheimpflug technique" to obtain the shape of the human lens differ significantly from their magnetic resonance imaging (MRI) data. We demonstrate that the conclusions are based on incorrect statistical methods. A straightforward statistical comparison shows that there is no significant difference between the corrected Scheimpflug results and the MRI data.

Adult↗

PubMed2004

Corneal surface reconstruction algorithm that uses Zernike polynomial representation.

We developed an algorithm that directly determines Zernike coefficients for the corneal anterior surface derived from the reflection image of a stimulus with pseudorandom encoding. This algorithm does not need to include calculation of corneal height maps. The numerical performance of the algorithm is good. It has the potential of determining corneal shape with submicrometer accuracy in obtaining Zernike coefficients. When applied to real eye measurements the accuracy of the procedure will be limited by the topographer that is used.

Algorithms↗

PubMed2002

Radius and asphericity of the posterior corneal surface determined by corrected Scheimpflug photography.

PURPOSE: To obtain the shape of the posterior corneal surface in a healthy population, using Scheimpflug photography corrected for distortion due to the geometry of the Scheimpflug imaging system and the refraction of the anterior corneal surface. METHODS: The posterior corneas of 83 subjects, ranging in age from 16 to 62 years, were measured in the vertical meridian using corrected Scheimpflug photography. The aspherical shape of the anterior Corneal surface was also determined in conjunction with the correction of Scheimpflug images. RESULTS: The average radius of the anterior corneal surface was 7.87 +/- 0.27 mm (SD), while the average radius of the posterior corneal surface was 6.40 +/- 0.28 mm (SD). The ratio between the posterior and the anterior radius of curvature was 0.81 +/- 0.02. The asphericity of the anterior and the posterior corneal surfaces was 0.82 +/- 0.18 and 0.62 +/- 0.27, respectively. The asphericity of the posterior corneal surface decreased significantly with age. The posterior/anterior asphericity ratio is also dependent on age and was 0.98 +/- 0.17 at 16 years of age and 0.53 +/- 0.30 at 62 years of age. CONCLUSION: Corrected Scheimpflug photography is an appropriate technique for measuring the radius and asphericity of the posterior corneal surface. The asphericity of the posterior corneal surface changes with age.

Adolescent↗