IMAGING THE EYE

[caption id='attachment_1302' align='alignright' width='400' caption='Scheimflug (left) and Purkinje (right) images in a pseudophakic eye. Quantitative tools allow measurement of IOL tilt and decentration. Adapted from de Castro et al. JCRS 2007'][/caption]

Recent developments in imaging techniques for the anterior segment are opening up a new era in ophthalmic surgery in which precise customised eye models of individual patients will be available to surgeons, affording more predictable outcomes of cataract and refractive procedures as a result, said Susana Marcos PhD, Instituto de Optica, CSIC, Madrid, Spain.

“Quantitative information on the anterior segment of the eye will help create good customised eye models showing the relationships between the geometry of the eye and the optical aberrations and their clinical significance,†Prof Marcos told the XXIX congress of the ESCRS.

The kind of data that are likely to be most useful in ophthalmic surgery as it relates to vision are the corneal radius and asphericity, anterior and posterior corneal shape, the distances between structures within the eye, anterior and posterior lens shape, lens tilt and decentration and the gradient refractive index distribution of the crystalline lens, she noted

There are already many commercial instruments available which provide some of these kinds of data. However, different instruments give different measurements of the same parameters and the accuracy of the devices in different ranges of applications has yet to be validated. Moreover, there are at present no commercially available instruments that provide measurement of the lens shape, tilt or decentration. Some devices for measuring posterior corneal geometry need yet further validations, she added.

New technology

Prof Marcos noted that she and her associates have developed devices and techniques over the past decade that may help fill some of these gaps and provide information that is not available with currently available machines.

[caption id='attachment_1303' align='aligncenter' width='620' caption='Three-dimensional sOCT image of porcine crystalline lens in vitro (left) and estimated Gradient Index Distribution (right). Adapted from de Castro et al. Optics Express 2010'][/caption]

Among them is a Purkinje imaging system for measuring the dimensions of the crystalline lens and the tilt and decentration of both natural and artificial lenses within the eye. It works by projecting point sources of light on the eye and capturing images from the different reflective interfaces. By using either an equivalent mirror or merit function approach, it is then possible to determine the shape of the crystalline lens from the position of the Purkinje images. In the same way, it also becomes possible to determine the tilt and decentration of the crystalline lens or of an implanted IOL.

Scheimpflug imaging is another technology that can provide information on the anatomy of the cornea and crystalline lens, Prof Marcos noted. However, the geometry of the scanning system, and refraction through ocular media can cause geometric distortion of the images of intraocular structures.

Prof Marcos and her associates have therefore developed algorithms and custom routines that correct the distortion of images obtained with the Pentacam® system (Oculus). The corrected images have yielded more accurate measurements of lens tilt and decentration as well as the radius of curvature of the crystalline lens, she said.

“With Scheimpflug imaging you can derive quantitative information but you have to be sure that the correction algorithms are in place,†she emphasised.

[caption id='attachment_1304' align='alignright' width='400' caption='Three-dimensional OCT reconstruction of the cornea and iris and cross-section in a patient implanted with an intrastromal ring segment, after application of correction of fan and distortion correction algorithms, and image processing tools described by Ortiz et al. Optics Express 2010'][/caption]

In other research, working in collaboration with Michiel Dubbelman and Rob G L van der Heijde in the Netherlands, Prof Marcos and her associates have compared phacometry findings obtained from their Purkinje system with those obtained from the Scheimpflug system in a group of young eyes in a relaxed accommodative state. They found that both instruments gave very similar information and also showed great variability between patients. More recently Prof Marcos’ group has shown similar geometrical changes in the crystalline lenses upon accommodation using quantitative OCT.

GRIN lens model

Prof Marcos noted that OCT surpasses Scheimpflug imaging both in terms of resolution and its ability to provide dynamic images of the eye’s anatomy. That capability in turn enables imaging of the lens as it changes shape during accommodation.

Prof Marcos and her associates have developed a high-resolution spectral domain OCT device, in collaboration with researchers from Copernicus University. By applying fan and optical distortion correction algorithms to the OCT measurements they have been able to obtain accurate quantitative data about the cornea's shape and thickness as well as crystalline lens measurements. In particular, the application of image denoising and segmentation tools, along with the correction of distortions has allowed the team to obtain accurate estimates of corneal elevation from OCT.

One recent achievement of Prof Marcos and her team using their OCT system has been a three-dimensional reconstruction of the crystalline lens gradient index distribution of a porcine and human lens in-vitro. Their findings could have important implications in the understanding of the optical aberrations of the lens, as well as the mechanisms of accommodation and the loss of accommodation with age.

“This relates to structure and optics. We have seen that for the same lens geometry but with a homogenous distribution of refractive index the lens would show a positive spherical aberration. However, in the presence of the measured gradient index the spherical aberration is negative indicating that the gradient index does play a very significant role in the spherical aberration of the eye. It is likely to be playing a role in the decline of accommodative amplitude that occurs with ageing,†she said.

Furthermore, she noted that in more recent research in which they compared human eyes of different ages, their findings indicated that in addition to the changes of shape and elasticity the lens undergoes with age, it also develops a more homogenous refractive index.

“Quantitative information can contribute to an improved understanding of the eye’s accommodative mechanism. In addition, a better understanding of the lens and related intraocular structures could lead to improvements in cataract surgery outcomes and intraocular lens designs,†she concluded.