Ophthalmic technology: Where is it taking us?

With the huge leaps in technology, that occur on a yearly, if not monthly basis, it is not possible to be able to describe all the latest advances in the ophthalmic world.
However, the intention in this article is to describe a small sample of some significant developments in the area of ophthalmic imaging in recent years. Specifically, this feature will describe:

Wavefront aberrometry (Zywave, Bausch and Lomb Surgical)
Imaging of the optic nerve head and retina (Heidelberg Retina Tomograph, Optical Coherence Tomography (OCT) and GDx Nerve Fibre Analyser)
Anterior segment and crystalline lens imaging (Scheimpflug photography).

Wavefront aberrometry
Wavefront aberrometry, for measuring the complete refractive status of an optical system, has also recently been introduced more widely into the clinical practice.
In optometry and ophthalmology it is routine to measure the spherical and cylindrical components of refractive errors and subsequently correct these by spectacles, contact lenses, intraocular lenses or refractive surgery. However, these sphero-cylindrical corrections account only for the lower order aberrations of the eye and do not address the higher order optical aberrations.
Ocular monochromatic aberrations result from optical inconsistencies within the media and local irregularities of the optical surfaces of the eye.
These vary from individual to individual,1 but the extent of these optical aberrations is seen as the most significant variable affecting the resolving power of the healthy eye.
A wavefront aberration is most easily appreciated by studying light after reflection from a point surface on the retina.2 Light passing through the pupil can be considered as a bundle of rays, or as a virtual surface perpendicular to these rays. This surface is the wavefront of light reflected out of the eye (Figure 1).
In this figure, 'P' is the point source from which light is emerging, and the wavefront is the virtual surface perpendicular to this emergent bundle of rays.
The shape of this virtual surface is called the 'wavefront aberration function' of the eye and in an optically perfect, emmetropic, eye would be a circular plane wave of similar size to the pupil (Figure 2).
The aberrations of the eye, or the so-called 'wavefront error', are measured by comparing the shape of the wavefront emanating from the eye to this conceptualised ideal wavefront: namely, the distance between the reflected wavefront and the ideal reference wavefront.

Measuring wavefront aberrations
A number of different techniques have been developed to describe the eye's optical aberrations in a comprehensive manner.3
In ophthalmic clinical practice, the most widely used of these is the objective aberrometer, based upon the Scheiner-Hartmann-Shack principle. Astronomers have taken advantage of the properties of this aberrometer to measure the optical aberrations of telescopes and the atmosphere for years.
The Hartmann-Shack aberrometer was designed to measure the aberrations of the eye at multiple locations, simultaneously, within the pupil area.4
The aberrometer uses a Hartmann screen (Scheiner disc with multiple apertures) in which each aperture contains a tiny lens, thereby constituting an array of lenslets. These lenslets capture a portion of the emerging wavefront and project it onto a CCD camera chip. The displacement of the projected image, vertically and horizontally, represents the distortion of light caused by the optical system's aberrations.

Analysing wavefront aberrations
The eye produces a wavefront, which is a complex 3-D 'surface'. This surface can be described by an infinite number of mathematical expressions called Zernike polynomials. Spherical and cylindrical refractive errors are quantified by the first two 'orders' of Zernike polynomials, and are of a parabolic and saddle-shape, respectively. The orders of aberrations are listed in Table 1.
Figure 3 shows three-dimensional representations of spherical refractive error (Figure 3A), cylindrical refractive error (Figure 3B) and trefoil (Figure 3C).
Similar to the maps produced by corneal topography, qualitative analysis of a wavefront can be displayed as a colour-coded map. The warmer colours represent the advancing portions of the wavefront and the cooler colours represent the trailing parts. An example of a wavefront from an eye that exhibits predominantly 'trefoil', assessed using the Bausch & Lomb Zywave aberrometer, is shown in Figure 4.

Clinical applications of wavefront aberrometry
Measuring wavefront aberrations using the Hartmann-Shack aberrometer has a number of applications, such as displaying the aberrations of light produced by abnormalities of the tear film, the cornea, refractive surgery and cataract.5
The most widely recognised clinical application of measuring wavefront aberration is in corneal refractive surgery, such as RK, PRK and Lasik.
However, like spectacles and contact lenses, these techniques correct only the lower-order aberrations of the eye. Until recently, these techniques have not corrected, and in some cases have actually increased, higher order aberrations.6 Such under or uncorrected higher-order aberrations are believed to contribute to subjective symptoms of glare, halos and visual difficulties in low lighting conditions.
A new technique of laser refractive surgery has, therefore, been developed in an attempt to correct both lower and higher order aberrations of the eye. Pre-operative assessment includes both corneal topography measurement, for example, Orbscan II and evaluation of the ocular aberrations using an objective aberrometer, such as Zywave.
The data from these two analyses are mathematically combined to allow an individualised excimer laser ablation to be performed.

Posterior segment imaging
In recent years it has been established that the gold standard for evaluating progression in glaucoma is observation of changes in the optic disc, since early detection of changes in the neural rim may precede visual field deterioration. Until recently, the main means of observing the changes was stereo disc photography (Figure 5) but this visualisation tends to be qualitative and rather subjective, and it relies upon clear media and a dilated pupil for good imaging. They also require a not insignificant amount of time for interpretation.
In recent years, however, a number of computerised techniques have become available for quantifying optic disc changes.7,8 The Heidelberg Retina Tomograph, the Optical Coherence Tomographer and the GDx Nerve Fibre Analyser are among these. They each work in a different way, and have advantages and disadvantages, which will be outlined below.

The Heidelberg Retina Tomograph
The Heidelberg Retina Tomograph is a confocal laser scanning system (670nm) for the acquisition and analysis of 3-D images of the posterior segment of the eye. The data created, serve to quantitatively describe the retinal topography and progressive topographic change. The main application of the HRT is the assessment of the glaucomatous optic nerve head.
A 3-D image acquired with the HRT is made up of a series of 32 optical sections at different locations of the focal plane, as shown in Figure 6. From this layered 3-D image, a topography image is computed which consists of more than 65,000 local measurements of the retinal surface height (Figure 7a). The topography image is colour coded, with dark colours representing elevated structures and light colours representing depressed structures (Figure 7b).
In the case of optic nerve head analysis, after the definition of the disk margin, the HRT software computes a set of stereometric parameters that quantitatively describe the shape of the optic nerve. The results of the topographic description are used to classify an optic nerve head, sector by sector, as being normal (green tick) or outside normal (red cross) limits.
More than a decade of research with the Heidelberg Retina Tomograph and similar instruments showed that quantification of the optic nerve head topography provides an important tool for glaucoma detection and follow-up.
Therefore, the HRT II, a more user-friendly clinical instrument was developed, specifically for topographic optic nerve head analysis in an almost completely automated system.
After image acquisition, the examination results, (classification as shown in Figure 8a and Figure 8b, and progression analysis as shown in Figure 9), are presented, and the total examination time is only a few minutes.
The only manual step required in the analysis process, is the definition of the optic disc margin, after which, the automatic analysis continues with the computation of the stereometric parameters, the classification of the eye, a comparison to previous exams (if applicable), and the presentation of the results.
Recent research is showing new and exciting uses for the HRT II, when additional software modules are added, in the examination of the macular area, and in studies of retinal disease.

GDx Nerve Fibre Analyser
The GDx is a scanning laser polarimeter, which assesses the peripapillary retinal nerve fibre layer.
It takes advantage of the polarising nature of the retinal nerve fibre layer (Figure 10) by using a polarised diode light source (780nm) to allow visualisation of the profile of the layer (Figure 11).
The results obtained with the GDx are illustrated below for a patient with a normal optic nerve head (Figure 12), and for a patient with optic disc asymmetry in the right eye.
The data, which is collected in a novel, if indirect method, is well presented with the GDx, and the instrument itself is simple to operate and does not require the patient's pupil to be dilated.
Like the HRT, the software has been designed to provide information regarding progression of glaucoma. One potentially significant disadvantage however, has to be interference with the results caused by the structures anterior to the retina, since the cornea and the lens also affect the polarisation of the light, and by varying degrees between subjects. This has been balanced by specialised software modification in recent models.

Optical Coherence Tomography
This commercial instrument utilises yet another principle. It measures the echo time-delay of light, back-scattered from different layers in the retina, using low-coherence interferometry.
The instrument allows the retinal nerve fibre layer to be observed in cross-section as shown in the image in Figure 14. The Optical Coherence Tomography has the advantage of being capable of analysing the retina as well as the optic nerve head and therefore has many uses, such as allowing detailed examination of patients with macular holes, with cystoid macular oedema, or with diabetic maculopathy, for example.

Anterior segment imaging (Scheimpflug photography)
An imaging technique for the anterior segment is the Nidek EAS-1000 Anterior Eye Segment Analysis System, which has been in use over the past decade, primarily in research as a non-invasive tool for in vivo examination of the crystalline lens.9
It provides a unique slit image of the anterior segment, a retroillumination image of the lens, and it performs analysis of different parameters of these images. The slit-image is acquired using the Scheimpflug principle (Figure 15) and can be captured, at any angle, by rotation of the system. If 60 of these images are taken at 3¡ intervals over the whole of the lens, it is possible to reconstruct a 3-D image of the lens.10

The Scheimpflug principle
The Scheimpflug camera (camera based on the Scheimpflug principle) forms an image of the anterior segment of the back scattered light from the full thickness of the lens. The exact mechanism is illustrated in Figure 15.
The slit beam plane and the image plane meet at one point S and are bisected by an objective plane containing the camera lens (that is: q1 = q2). A non-coherent light source (S) illuminates the lens and forms a slit image in the image plane by the objective lens.
If the planes are arranged as illustrated in Figure 15, the image will contain the backscattered light from the cornea and the full thickness of the lens.
Analysis of the slit images, as shown in Figure 16 for a normal anterior segment, includes linear and multilinear biometric analysis, (performed as shown in Figure 17 by the Nidek EAS-1000), including the radius of curvature of the anterior corneal surface, pachymetry, anterior chamber depth, anterior chamber angle and lens thickness.
Further analysis enables assessment of the lens and includes densitometric analysis of different points within the lens, average density between two points of the lens and average density of an integral area of the lens. This is of interest for analysing lenticular opacities as seen in Figure 18.
Further analysis of lens opacities can be performed using the retroillumination images. A histogram displaying the areas of opacification to areas of transparence can be displayed as well as a colour-coded map illustrating the differences in densities. Analysis of intraocular lenses includes the degree of tilt and decentration of intraocular lens implants, computed by taking two images perpendicular to each other and comparing the optical axis of the eye to the optical axis of the lens implant.
As with much of the innovative instrumentation described in this article, the cost of the Nidek EAS-1000 Anterior Eye Segment Analysis System may be considered prohibitive in terms of regular use in ophthalmic clinics.
However, the potential for exciting clinical research, which may ultimately dictate the future management of cataract patients can clearly be seen.


References
1 Collins MJ, Wildsoet CF, Atchison DA. Monochromatic aberrations and myopia. Vision Res, 1995; 35(9):1157-63.
2 Thibos LN. Wavefront data reporting and terminology. J Refract Surg, 2001; 17; S578-583.
3 Howland HC. The history and methods of ophthalmic wavefront sensing. J Refract Surg (2000) 16; S552-3.
4 Thibos LN. Principles of Hartmann-Shack aberrometry. J Refract Surg, 2000; 16; S563-565.
5 Thibos LN, Hong X. Clinical applications of the Hartmann-Shack aberrometer. Optom & Vis Sci, 1999; 76:817-825.
6 Applegate RA, Howland HC, Sharp RP, Cottingham AJ, Yee RW. Corneal aberrations, visual performance and refractive keratectomy. J Refract Surg, 1998; 14;397-407.
7 Zangwill LM, Bowd C, Berry CC, Williams J, Blumenthal EZ, Sanchez-Galeana CA, Vasile C, Weinreb RN. Discriminating between normal and glaucomatous eyes using the Heidelberg Retina Tomograph, GDx Nerve Fiber Analyzer, and Optical Coherence Tomograph. Arch Ophthalmol, 2001; 119(7):985-993.
8 Bowd C, Zangwill LM, Blumenthal EZ, Vasile C, Boehm AG, Gokhale PA, Mohammadi K, Amini P, Sankary TM, Weinreb RN. Imagin of the optic disc and retinal nerve fiber layer; the effects of age, optic disc area, refractive error and gender. J Opt Soc Am, 2002; 19 (1):197-207.
9 Sasaki K, Sakamoto Y, Shibata T et al. The Multi-Purpose Camera: A New Anterior Eye Segment Analysis System. Ophthal Res, 1990; 22(supp 1):3-8.
10 Masters BR. Three-dimensional microscopic tomographic imaging of the cataract in a human lens in vivo. Optics Express, 1998; 3(9):332-338.


Acknowledgements
The author would like to acknowledge Professor Charles McGhee, Associate Professor Helen Danesh-Meyer and Dr Nisha Sachdev of the Department of Ophthalmology, University of Aukland, for advice. This article is based on a presentation at the OEL Roadshow last autumn.


Dr Jennifer P Craig is a senior lecturer in ophthalmology at the University of Auckland, New ZealandWith the huge leaps in technology, that occur on a yearly, if not monthly basis, it is not possible to be able to describe all the latest advances in the ophthalmic world.
However, the intention in this article is to describe a small sample of some significant developments in the area of ophthalmic imaging in recent years. Specifically, this feature will describe:

Wavefront aberrometry (Zywave, Bausch and Lomb Surgical)
Imaging of the optic nerve head and retina (Heidelberg Retina Tomograph, Optical Coherence Tomography (OCT) and GDx Nerve Fibre Analyser)
Anterior segment and crystalline lens imaging (Scheimpflug photography).

Wavefront aberrometry
Wavefront aberrometry, for measuring the complete refractive status of an optical system, has also recently been introduced more widely into the clinical practice.
In optometry and ophthalmology it is routine to measure the spherical and cylindrical components of refractive errors and subsequently correct these by spectacles, contact lenses, intraocular lenses or refractive surgery. However, these sphero-cylindrical corrections account only for the lower order aberrations of the eye and do not address the higher order optical aberrations.
Ocular monochromatic aberrations result from optical inconsistencies within the media and local irregularities of the optical surfaces of the eye.
These vary from individual to individual,1 but the extent of these optical aberrations is seen as the most significant variable affecting the resolving power of the healthy eye.
A wavefront aberration is most easily appreciated by studying light after reflection from a point surface on the retina.2 Light passing through the pupil can be considered as a bundle of rays, or as a virtual surface perpendicular to these rays. This surface is the wavefront of light reflected out of the eye (Figure 1).
In this figure, 'P' is the point source from which light is emerging, and the wavefront is the virtual surface perpendicular to this emergent bundle of rays.
The shape of this virtual surface is called the 'wavefront aberration function' of the eye and in an optically perfect, emmetropic, eye would be a circular plane wave of similar size to the pupil (Figure 2).
The aberrations of the eye, or the so-called 'wavefront error', are measured by comparing the shape of the wavefront emanating from the eye to this conceptualised ideal wavefront: namely, the distance between the reflected wavefront and the ideal reference wavefront.

Measuring wavefront aberrations
A number of different techniques have been developed to describe the eye's optical aberrations in a comprehensive manner.3
In ophthalmic clinical practice, the most widely used of these is the objective aberrometer, based upon the Scheiner-Hartmann-Shack principle. Astronomers have taken advantage of the properties of this aberrometer to measure the optical aberrations of telescopes and the atmosphere for years.
The Hartmann-Shack aberrometer was designed to measure the aberrations of the eye at multiple locations, simultaneously, within the pupil area.4
The aberrometer uses a Hartmann screen (Scheiner disc with multiple apertures) in which each aperture contains a tiny lens, thereby constituting an array of lenslets. These lenslets capture a portion of the emerging wavefront and project it onto a CCD camera chip. The displacement of the projected image, vertically and horizontally, represents the distortion of light caused by the optical system's aberrations.

Analysing wavefront aberrations
The eye produces a wavefront, which is a complex 3-D 'surface'. This surface can be described by an infinite number of mathematical expressions called Zernike polynomials. Spherical and cylindrical refractive errors are quantified by the first two 'orders' of Zernike polynomials, and are of a parabolic and saddle-shape, respectively. The orders of aberrations are listed in Table 1.
Figure 3 shows three-dimensional representations of spherical refractive error (Figure 3A), cylindrical refractive error (Figure 3B) and trefoil (Figure 3C).
Similar to the maps produced by corneal topography, qualitative analysis of a wavefront can be displayed as a colour-coded map. The warmer colours represent the advancing portions of the wavefront and the cooler colours represent the trailing parts. An example of a wavefront from an eye that exhibits predominantly 'trefoil', assessed using the Bausch & Lomb Zywave aberrometer, is shown in Figure 4.

Clinical applications of wavefront aberrometry
Measuring wavefront aberrations using the Hartmann-Shack aberrometer has a number of applications, such as displaying the aberrations of light produced by abnormalities of the tear film, the cornea, refractive surgery and cataract.5
The most widely recognised clinical application of measuring wavefront aberration is in corneal refractive surgery, such as RK, PRK and Lasik.
However, like spectacles and contact lenses, these techniques correct only the lower-order aberrations of the eye. Until recently, these techniques have not corrected, and in some cases have actually increased, higher order aberrations.6 Such under or uncorrected higher-order aberrations are believed to contribute to subjective symptoms of glare, halos and visual difficulties in low lighting conditions.
A new technique of laser refractive surgery has, therefore, been developed in an attempt to correct both lower and higher order aberrations of the eye. Pre-operative assessment includes both corneal topography measurement, for example, Orbscan II and evaluation of the ocular aberrations using an objective aberrometer, such as Zywave.
The data from these two analyses are mathematically combined to allow an individualised excimer laser ablation to be performed.

Posterior segment imaging
In recent years it has been established that the gold standard for evaluating progression in glaucoma is observation of changes in the optic disc, since early detection of changes in the neural rim may precede visual field deterioration. Until recently, the main means of observing the changes was stereo disc photography (Figure 5) but this visualisation tends to be qualitative and rather subjective, and it relies upon clear media and a dilated pupil for good imaging. They also require a not insignificant amount of time for interpretation.
In recent years, however, a number of computerised techniques have become available for quantifying optic disc changes.7,8 The Heidelberg Retina Tomograph, the Optical Coherence Tomographer and the GDx Nerve Fibre Analyser are among these. They each work in a different way, and have advantages and disadvantages, which will be outlined below.

The Heidelberg Retina Tomograph
The Heidelberg Retina Tomograph is a confocal laser scanning system (670nm) for the acquisition and analysis of 3-D images of the posterior segment of the eye. The data created, serve to quantitatively describe the retinal topography and progressive topographic change. The main application of the HRT is the assessment of the glaucomatous optic nerve head.
A 3-D image acquired with the HRT is made up of a series of 32 optical sections at different locations of the focal plane, as shown in Figure 6. From this layered 3-D image, a topography image is computed which consists of more than 65,000 local measurements of the retinal surface height (Figure 7a). The topography image is colour coded, with dark colours representing elevated structures and light colours representing depressed structures (Figure 7b).
In the case of optic nerve head analysis, after the definition of the disk margin, the HRT software computes a set of stereometric parameters that quantitatively describe the shape of the optic nerve. The results of the topographic description are used to classify an optic nerve head, sector by sector, as being normal (green tick) or outside normal (red cross) limits.
More than a decade of research with the Heidelberg Retina Tomograph and similar instruments showed that quantification of the optic nerve head topography provides an important tool for glaucoma detection and follow-up.
Therefore, the HRT II, a more user-friendly clinical instrument was developed, specifically for topographic optic nerve head analysis in an almost completely automated system.
After image acquisition, the examination results, (classification as shown in Figure 8a and Figure 8b, and progression analysis as shown in Figure 9), are presented, and the total examination time is only a few minutes.
The only manual step required in the analysis process, is the definition of the optic disc margin, after which, the automatic analysis continues with the computation of the stereometric parameters, the classification of the eye, a comparison to previous exams (if applicable), and the presentation of the results.
Recent research is showing new and exciting uses for the HRT II, when additional software modules are added, in the examination of the macular area, and in studies of retinal disease.

GDx Nerve Fibre Analyser
The GDx is a scanning laser polarimeter, which assesses the peripapillary retinal nerve fibre layer.
It takes advantage of the polarising nature of the retinal nerve fibre layer (Figure 10) by using a polarised diode light source (780nm) to allow visualisation of the profile of the layer (Figure 11).
The results obtained with the GDx are illustrated below for a patient with a normal optic nerve head (Figure 12), and for a patient with optic disc asymmetry in the right eye.
The data, which is collected in a novel, if indirect method, is well presented with the GDx, and the instrument itself is simple to operate and does not require the patient's pupil to be dilated.
Like the HRT, the software has been designed to provide information regarding progression of glaucoma. One potentially significant disadvantage however, has to be interference with the results caused by the structures anterior to the retina, since the cornea and the lens also affect the polarisation of the light, and by varying degrees between subjects. This has been balanced by specialised software modification in recent models.

Optical Coherence Tomography
This commercial instrument utilises yet another principle. It measures the echo time-delay of light, back-scattered from different layers in the retina, using low-coherence interferometry.
The instrument allows the retinal nerve fibre layer to be observed in cross-section as shown in the image in Figure 14. The Optical Coherence Tomography has the advantage of being capable of analysing the retina as well as the optic nerve head and therefore has many uses, such as allowing detailed examination of patients with macular holes, with cystoid macular oedema, or with diabetic maculopathy, for example.

Anterior segment imaging (Scheimpflug photography)
An imaging technique for the anterior segment is the Nidek EAS-1000 Anterior Eye Segment Analysis System, which has been in use over the past decade, primarily in research as a non-invasive tool for in vivo examination of the crystalline lens.9
It provides a unique slit image of the anterior segment, a retroillumination image of the lens, and it performs analysis of different parameters of these images. The slit-image is acquired using the Scheimpflug principle (Figure 15) and can be captured, at any angle, by rotation of the system. If 60 of these images are taken at 3¡ intervals over the whole of the lens, it is possible to reconstruct a 3-D image of the lens.10

The Scheimpflug principle
The Scheimpflug camera (camera based on the Scheimpflug principle) forms an image of the anterior segment of the back scattered light from the full thickness of the lens. The exact mechanism is illustrated in Figure 15.
The slit beam plane and the image plane meet at one point S and are bisected by an objective plane containing the camera lens (that is: q1 = q2). A non-coherent light source (S) illuminates the lens and forms a slit image in the image plane by the objective lens.
If the planes are arranged as illustrated in Figure 15, the image will contain the backscattered light from the cornea and the full thickness of the lens.
Analysis of the slit images, as shown in Figure 16 for a normal anterior segment, includes linear and multilinear biometric analysis, (performed as shown in Figure 17 by the Nidek EAS-1000), including the radius of curvature of the anterior corneal surface, pachymetry, anterior chamber depth, anterior chamber angle and lens thickness.
Further analysis enables assessment of the lens and includes densitometric analysis of different points within the lens, average density between two points of the lens and average density of an integral area of the lens. This is of interest for analysing lenticular opacities as seen in Figure 18.
Further analysis of lens opacities can be performed using the retroillumination images. A histogram displaying the areas of opacification to areas of transparence can be displayed as well as a colour-coded map illustrating the differences in densities. Analysis of intraocular lenses includes the degree of tilt and decentration of intraocular lens implants, computed by taking two images perpendicular to each other and comparing the optical axis of the eye to the optical axis of the lens implant.
As with much of the innovative instrumentation described in this article, the cost of the Nidek EAS-1000 Anterior Eye Segment Analysis System may be considered prohibitive in terms of regular use in ophthalmic clinics.
However, the potential for exciting clinical research, which may ultimately dictate the future management of cataract patients can clearly be seen.


References
1 Collins MJ, Wildsoet CF, Atchison DA. Monochromatic aberrations and myopia. Vision Res, 1995; 35(9):1157-63.
2 Thibos LN. Wavefront data reporting and terminology. J Refract Surg, 2001; 17; S578-583.
3 Howland HC. The history and methods of ophthalmic wavefront sensing. J Refract Surg (2000) 16; S552-3.
4 Thibos LN. Principles of Hartmann-Shack aberrometry. J Refract Surg, 2000; 16; S563-565.
5 Thibos LN, Hong X. Clinical applications of the Hartmann-Shack aberrometer. Optom & Vis Sci, 1999; 76:817-825.
6 Applegate RA, Howland HC, Sharp RP, Cottingham AJ, Yee RW. Corneal aberrations, visual performance and refractive keratectomy. J Refract Surg, 1998; 14;397-407.
7 Zangwill LM, Bowd C, Berry CC, Williams J, Blumenthal EZ, Sanchez-Galeana CA, Vasile C, Weinreb RN. Discriminating between normal and glaucomatous eyes using the Heidelberg Retina Tomograph, GDx Nerve Fiber Analyzer, and Optical Coherence Tomograph. Arch Ophthalmol, 2001; 119(7):985-993.
8 Bowd C, Zangwill LM, Blumenthal EZ, Vasile C, Boehm AG, Gokhale PA, Mohammadi K, Amini P, Sankary TM, Weinreb RN. Imagin of the optic disc and retinal nerve fiber layer; the effects of age, optic disc area, refractive error and gender. J Opt Soc Am, 2002; 19 (1):197-207.
9 Sasaki K, Sakamoto Y, Shibata T et al. The Multi-Purpose Camera: A New Anterior Eye Segment Analysis System. Ophthal Res, 1990; 22(supp 1):3-8.
10 Masters BR. Three-dimensional microscopic tomographic imaging of the cataract in a human lens in vivo. Optics Express, 1998; 3(9):332-338.


Acknowledgements
The author would like to acknowledge Professor Charles McGhee, Associate Professor Helen Danesh-Meyer and Dr Nisha Sachdev of the Department of Ophthalmology, University of Aukland, for advice. This article is based on a presentation at the OEL Roadshow last autumn.


Dr Jennifer P Craig is a senior lecturer in ophthalmology at the University of Auckland, New Zealand