We used a novel method of analysis for polarimetry data, intentionally computing images that visualized scattered light primarily from deeper structures in the fundus. In keeping with the hypothesis that the scattered light would dominate other optical phenomena in the depolarized light images,
10 the regions of hyperpigmentation in depolarized light images were bright, rather than having the more traditional dark appearance.
14 15 16 The average images that corresponded to typical confocal, near-infrared images visualized the hyperpigmentation typically as dark, but provided less contrast. The birefringence images often showed little contrast for hyperpigmentation, as expected, because it lies beneath the nerve fiber layer. Although the nerve fiber layer was probably thinned in these patients, reflected light from the nerve fiber layer nevertheless dominated those images computed from light that retained polarization, as previously known from tomographic measurements in glaucoma diagnosis.
19 23 24
The goal of this imaging method is to visualize small, pathologic features at high contrast, rather than to provide information about layers of tissues, such as numerical estimates of thickness.
18 Scattered light, whether due to atrophy or hyperpigmentation, is visualized as bright by our depolarized images. This could be an advantage in that a region with several bright areas may be more recognizable as abnormal than would a few discrete, focal areas of hyperpigmentation. In grading studies and clinical examination of AMD, hyperpigmentation and hypopigmentation are often combined into the single category of “pigmentary changes.”
25 However, the appearance of specific pathologic features may differ between the polarimetry method used in this study and previously used methods, involving annular confocal apertures and also longer or shorter wavelengths
11 15 or lateral displacement of the aperture with respect to the illumination.
17 Further, most previous images were collected with larger-diameter apertures, allowing more and longer range scattered light and less sectioning of layers, consequently losing spatial resolution while delineating the large scattering structures. Additional clinical experience with polarimetry may be necessary before its utility for macular disease can be assessed.
The depolarized light images visualized regions of hyperpigmentation with higher contrast than any of the three channels of the color fundus photographs. Sometimes the control regions of interest were not brighter than the focal regions of hyperpigmentation, because of typical variations in fundus pigmentation across the retina, and this experimental difficulty occurred more frequently for the color fundus photographs than for the depolarized light images. Although the color fundus photographs were collected at higher spatial resolution and increased sampling can often improve signal-to-noise ratio, this increased sample size per pixel and consequently larger data file size had fewer, not more, statistically significant results. Models of combining color information across differently pigmented fundi do not yet exist for hyperpigmentation, and therefore the combination of three channels was not tested. However, there is an inherent difficulty with the fundus photography method, in that the nerve fiber layer can obscure peripapillary hyperpigmentation in normal subjects, whereas thinning of the nerve fiber layer allows the peripapillary hyperpigmentation to be revealed. Thus, the relation between the nerve fiber layer and RPE and photoreceptor loss was concluded to be unclear with this method.
12
The pattern of hyperpigmentation that could be quantified in the color fundus images of our patients indicates two things. First, after removal of the patient with the more advanced peripapillary atrophy and hyperpigmentation, we had a patient sample with small, often multiple, regions of hyperpigmentation. We did not need a large sample size to obtain significant differences between depolarized light images and all three color separation images. With large lesions of the type we omitted, the pathologic tissues are typically even more visible. Second, our technique works on small regions of hyperpigmentation that were specifically sought by an experienced clinician, but that might in some cases be missed. Thus, the polarimetry technique provides more contrast and can be used on small regions. We tested only patients who still had sufficiently clear media that hyperpigmentation could be detected on clinical examination and with color fundus photographs. Confocal, near infrared imaging has already been shown to have the advantages of penetrating through cataract, providing a useful image in darkly pigmented fundi, and reducing long-range scatter from a variety of ocular media changes.
14 This type of instrument does not require the pupil dilatation that is necessary for the high-quality fundus photographs, and the light levels are safe and comfortable.
The polarimetry imaging method seems suited for early detection of hyperpigmentation, either in new patients or as a potential means to determine progression of peripapillary damage. Chorioretinal abnormalities, such as atrophy or hyperpigmentation, have been reported to occur in some but not all eyes with glaucomatous optic nerve damage. Ophthalmoscopically, these changes have been divided into a central beta zone and a more peripheral alpha zone. The beta zone shows visible sclera, due to a complete loss of the RPE cells and a decreased number of photoreceptors, whereas the alpha zone is characterized by uneven hypo- and hyperpigmentation that histologically correspond to pigmentary irregularities in the RPE.
13 26 27 28
Pigmentary changes, such as hyperpigmentation, result from severe damage to the RPE layer, and therefore may be an important sign of disease related damage to deeper layers of the retina.
11 12 16 This may occur, not only in glaucoma but also in other diseases that affect the RPE, particularly when the RPE is one of the primary sites, such as in AMD. For AMD, focal hyperpigmentation is a well-known risk factor for disease progression.
11 29 30 31 It is not known whether these changes are a very early sign of damage or merely quite visible, particularly once the retina is thinned.
Our novel polarimetry method has been shown to lead to enhanced contrast of subretinal features such as drusen.
10 We have shown that it will also provide information for the hyperpigmented regions around the optic nerve head. Polarimetry methods have evolved
32 so that they now distinguish the polarization retaining light from the depolarized light.
9 10 33 We intentionally compute images that visualize features in scattered light, primarily originating from deeper structures in the fundus. This is one key difference between our method and the wide variety of new polarimetry methods that are optimized for high contrast
34 or light with scatter minimized to the extent that a coherence method can be used
35 —that is, polarization retaining light.
The present results support our previous findings that different features are emphasized according to the proportion of polarization retaining light in the image, with superficial features better visualized with a high proportion of polarization containing light and deeper features visualized better for images that diminish the proportion of polarization retaining light.
9 10 33 For glaucoma management, several important features were rapidly visualized in a few key computed images, without the need for dilatation or uncomfortably bright light. These features include the optic nerve head rim and cup, pores in the lamina cribrosa, a nerve bundle defect, peripapillary hyperpigmentation, peripapillary atrophy, retinal arteries and veins, the arterioles and veins associated with the optic nerve head, and the distribution of birefringence associated with the measurement of retinal nerve fiber layer thickness. In addition to visualizing features, the depolarized light images have been used to obtain better information from beneath and within blood vessels, because the specular reflection from blood vessel surfaces is greatly minimized.
9 22 36 Other polarimetry techniques used for macular imaging have been used to improve the measurements of structures, such as the computation of retinal nerve fiber layer thickness that is based on the absolute retardance.
37 38 39
Further improvement of polarimetry methods is being advanced by the use of Mueller matrix instrumentation, which completely specifies the polarization content in an image,
34 40 41 42 and by experiments to identify factors such as individual variations in anterior segment optics.
37 38 43 Both the specular and scattered light return, which were shown in panels B versus C in
Figures 2 and 3to provide different information for the optic nerve head structures, have been quantified with Mueller matrix polarimetry (Guthrie et al.
IOVS 2004;45:ARVO Abstract 2796). Although the uses of polarimetry information are different, the mathematical treatments are closely related and well-described.
9 10 37 41 42
Our sample included patients old enough to have aging changes to the cornea, lens, or vitreous that might alter the polarization results. None of the patients had severe enough changes to prevent color fundus photography, indicating that our sample did not include the most severe media changes. Two known artifacts of the cornea include an irregular tear film, which results in variation of optical power over the surface and over time, and individual differences in birefringence of the cornea.
37 38 When the corneal compensator does not correct for corneal birefringence, then our method overestimates somewhat the amount of depolarized light. This effect is generally constant across the retina. For example, 85% to 90% of the light returning from the normal retina retains polarization.
32 Therefore, if we remove only 70% of this component due to incomplete compensation of corneal birefringence, then the contrast of the depolarized light is two times higher. The addition of uniform depolarized light reduces image contrast to some extent. As for aging changes due to the lens, the polarization effects of the normal lens and most intraocular lenses are barely measurable with present instrumentation.
43 The effect of a cataractous lens has not been measured in detail with this instrumentation, except for the comparison of before versus after cataract extraction.
44 Such studies include not only the potential scattering of the cataractous lenses but also the alteration of corneal polarization due to wound healing after surgery. Differences on long-term follow-up were, on average, in the direction of improved signal amplitude with cataract extraction, but not generally statistically significant with either scanning laser polarimetry or confocal imaging. These findings are consistent with the idea that depolarization due to neither the cornea nor the lens has a large effect on the light return from the retinal plane, and consequently the computed depolarized image, due to the use of a confocal aperture to remove long-range scatter.
In a more general model, interesting polarization retaining information and possible artifacts could arise from sources not limited to the cornea—that is, when considering light return that is not limited to only birefringence or only the cornea and nerve fiber layer. The present method differs from the use of a static, cross-polarizer to remove glints that may occur in a given plane of polarization. Further, light collected at the detector for crossed polarization, regardless of the source, has no effect on the depolarized light image, as long as it is modulated in phase with the polarization preserving light that here is attributed to the nerve fiber layer, varying systematically according to the input polarization angle. Unlike the case of a single cross-polarized element, this modulating light is removed by the computation. Alternatively, any light reaching the detector for crossed polarization that does not modulate in phase with the light attributed to the nerve fiber layer serves to reduce the computed modulation and therefore reduces the contrast of the depolarized light images. The artifact is only in one direction—decreasing contrast—and does not add spurious features that are not present. The method described here clearly visualized the features of interest, some not otherwise visible, in a clinical population. Thus, further removal of artifacts, which although potentially small are inherent in an incomplete polarimetry device, would serve to make this method even better. Besides further reducing these already identified artifacts, the potential benefits of fully specifying polarization for pathologic structures is unknown in the eye. Image contrast in computed images as yet unavailable may be high for optically active molecules, and new information may be provided.
We tested only patients with clinically visible peripapillary hyperpigmentation, but we anticipate that this method may reveal similar findings in other patients with either more overlying nerve fiber layer, ocular pigmentation, or retinal disease such as epiretinal membrane that precludes visualization of hyperpigmentation. For patients undergoing scanning laser polarimetry in the management of glaucoma,
18 19 37 38 39 there is almost no additional time needed during patient data acquisition. As we did not experience difficulties in distinguishing fundus features, despite large atrophic regions prone to scatter, this method may be extended to peripapillary atrophy, known to be concordant with visual field loss.
45 This method may not only be more sensitive than traditional methods, but it also uses far less data storage. Thus, it should be considered for further studies for early detection of peripapillary changes improving the estimation of prognosis by factoring in peripapillary hyperpigmentation.
The authors thank the Ophthalmic Consultants of Boston for their continued assistance after the death of Ruthanne B. Simmons, MD; Michael C. Cheney, MS, for technical assistance; and Russell Chipman, PhD, for discussion of his model of computations from existing methods of scanning laser polarimetry.