The ability to quantify and objectively assess the distribution of the ocular fundus pigments could aid in the assessment and monitoring of pathological conditions of the retina and choroid.
One of the potential roles for the technique described is to provide a reproducible, simple, and objective measurement of MP. The MP confers the characteristic yellow tint around the foveal area in the primate retina; it has a sharp peak at the centre of the fovea and declines exponentially with increasing eccentricity, reaching negligible levels at 4–6° of the visual angle.5, 2, 3
Measurement of MP in vivo
is possible and has been advocated as a means of assessing the risk of development of age-related macular degeneration in later years.6
MP is mainly of dietary origin4
and is thought to have a protective role by acting as an optical filter to shorter wavelengths and as a consequence of its antioxidant properties.7, 8, 9, 10, 11
Several techniques have been adopted to measure MP in vivo
, including Raman detection,12
scanning laser ophthalmoscope,13
reflectometry,14, 15, 16, 17
and heterochromatic flicker photometry using ‘Maxwellian' and ‘freeview' systems,19, 20
but there are limitations to all these techniques; Raman spectroscopy, scanning laser ophthalmoscopy, reflectometry, and autofluorescence all require complex equipment and have been developed as research tools. The more widespread techniques used in clinical practice to measure MP rely on colour matching or heterochromatic flicker photometry, both of which are dependent on subjective interpretation of an end point by the individual performing the test and this may be difficult in elderly subjects, especially if fixation is poor; there is therefore a need for a simple objective technique that could be applied in a clinical setting.21, 22
Another potential role of the technique described is the assessment of retinal perfusion; detecting variations in the distribution of retinal blood is an essential part of the fundus examination, especially when pathology is concerned. Sometimes it can be evaluated only by fundus fluorescein angiography, an invasive technique that is potentially life-threatening (anaphylactic shock) and an alternative non-invasive method would therefore be of great benefit.
Other groups have used multispectral imaging for the quantification of retinal haemoglobins (for example, see Mordant et al23
), but MRIA offers a completely different approach in that it interprets the images by means of a comparison of the images with a computer-generated reflectance model of the fundus and it reconstructs ‘parametric maps' from the data available, potentially providing the clinician with pixel-by-pixel concentration values.
The results from this study suggest that the described method can potentially be used as a non-invasive tool in quantifying MP, retinal haemoglobins and possibly other retinal and choroidal parameters in normal subjects.
While the results are encouraging, further development is required in several areas.
First, not all model parameters can be recovered equally well at present; this is thought to be due to the high level of non-linearity of the method used in generating the model and inverse model,1, 24, 25
hence more exhaustive mathematical analysis is needed and more effective methods for parameter recovery are being investigated.
Second, one of the main drawbacks of the method is the image acquisition system used; the exposure time was too long and image movement artefacts were inevitably introduced; some of these artefacts cannot be compensated for by simple registering of the six images, as constancy of fundus illumination, essential for image normalisation, is inevitably compromised (for further details, see Styles et al1
). Modifications to the equipment have been designed and completed26, 27
so that faster image acquisition is possible and these artefacts minimised and this adapted equipment is currently being tested; the investigators are currently using the improved image acquisition system to validate MRIA for MP quantification against the current clinical gold standard, heterochromatic flicker photometry, and also to prove MRIA has an acceptable test–retest variability.
Third, yellowing of the human lens with ageing is a major problem that all fundus imaging techniques need to overcome and this has particular significance in the measurement of MP, which also absorbs in the yellow part of the spectrum. The model accounts for attenuation of light transmission by the lens as a function of age,28, 29
but subjects with significant cataract, that is, compromised view of the posterior pole on slit-lamp biomicroscopy, cannot be assessed. The investigators are currently investigating a solution to this problem.
Finally, the need for empirical scaling to ensure that model and image data were coincident is probably the most concerning factor; although improvement in the imaging system used is likely to solve this issue in part, other potential problems with the model itself have been identified, and these require careful consideration in further studies.
- The model assumes that the individual layers in the fundus are of constant thickness and are the same in all subjects. This assumption is not correct and variations in the layer thicknesses may well lead to significant changes in the model.
- Although the major optical properties of the ocular tissues were taken into account, not all the possible variations within them were considered.
While the model appears to capture the correct trends, it does not include the full variation of normal eyes. This is a consequence of two factors: first, knowledge of in vivo ocular tissue is not exhaustive, especially with regards to the scattering properties of the various layers of the fundus; second, an introduction of too many parameters in the computer simulation would require many more images to be taken and would significantly increase computation times and margin of error. Further investigations, however, are necessary in order to determine the precise cause of the discrepancy between normal data and model data, and other fundus features, for example foveal architecture, may need to be added to the model to obtain reliable and repeatable results.
In conclusion, work presented in this paper demonstrates that clinically relevant histological parameters can be computed from multispectral images of the ocular fundus. The method has been successfully used to map the distribution of MP and haemoglobins in the retina.
There is considerable scope for further development of the methodology described here. The investigators are currently investigating the use of a new imaging system,26, 27
and formulating new techniques for analysing the data that take into account unknown factors that may affect the image formation process.
Once these issues have been addressed, MRIA may be applicable to a number of clinical scenarios. Most promising are those retinal and macular disorders, which require monitoring of subtle changes. MRIA's ability to quantify pigments and their distribution may help to identify individuals at risk of developing visual loss, or help ophthalmologists to make informed decisions on the continuation or termination of treatment.