Even in aged eyes without dilated pupils, the main features of the fundus are readily visible in near-infrared light, including retinal blood vessels, the foveal region, exudative lesions, and the optic nerve head when in the field of view. Images differing in polarization content emphasized different features or emphasized different aspects of features. For a normal subject whose age corresponded to our patient sample, the birefringence image and the cardinal directions color maps for the maximum phase of the crossed detector and the parallel detector showed a macular cross (), which indicates the amplitude of the modulation in retardance as a function of input polarization. This cross is produced by the interaction of corneal and retinal birefringence. The cardinal directions color maps for the maximum phase indicate that there is a systematic radial change in birefringence, i.e., good radial symmetry of the polarization input angle producing the maximum retardance about the fovea. This is expected, since the form birefringence assumed to produce this cross is radially symmetric about the fovea, and the path through the cornea is constant. For this individual, the birefringence signal from the inferior nerve fiber bundle was stronger than that from the macular cross, indicated by a more saturated green color for the nerve fiber bundle ( and ). This is not surprising, as the nerve fiber bundle in the field of view in is at one of its thickest points in the retina and can be seen to vary in . While this result is found in some other normal subjects, it may not be typical for all ages and disease states. It is expected that the relative strength of the birefringence signal will vary across location on the nerve fiber bundle and across individuals, particularly if there is damage to the nerve fiber bundles due to diseases such as glaucoma. This normal subject has a large scleral crescent, the bright area next to the optic nerve head, in which there is a strong light return because there are no photoreceptors or RPE. There are also pigmentary changes next to the optic nerve, not unusual in older subjects. There is a strong birefringence signal for the scleral crescent in both the gray-scale and cardinal directions scale map, showing variation in the strength of return as a function of polarization input angle, similar to that found in atrophy in AMD (not shown).
For patients with exudative AMD, components of new vessel membranes, pools of fluid, striae indicating traction lines, abnormal retinal vasculature, exudates, and atrophic regions were readily visible, but they varied in contrast according to polarization image type (Figs. 1, 3, 4, 7, 9, and 11). Some lesions had components that extended to more than the 15 deg visual angle image size, and thus we confined our quantitative analysis to components of membranes. For well-defined membranes, the border of the new vessel membrane could be seen entirely or partially in most types of computed images (Figs. 1 and 11), with the depolarized light image particularly emphasizing the border of the membrane. Additionally, there were focal changes not always visible with other imaging modalities.
For a patient with a well-established neovascular membrane, several components are readily seen (, , , and ). The depolarized light images show the highly disrupted tissues associated with the choroidal neovascularization. Although it is difficult to determine where the fovea was prior to the severe exudative changes, the birefringence image does not show the usual macular cross. Instead, there are large changes as a function of polarization input angle that are associated with the exudative components of the lesion, as shown by comparing the FA data in with the cardinal directions scale maps in . There is an unusually strong birefringence signal in the gray-scale birefringence image and the cardinal directions scale maps of the phase of the maximum of the crossed and parallel detectors. There are several different birefringence maxima separated according to location on the lesion, with some components having little or no birefringence signal. For those with strong signals, the modulation of signal amplitude over all input polarization angles at a region of interest in the exudative portion reached 245±6.8, with the gray scale ranging from 0 to 255. This remarkable but atypical variation occurred even though the signal did not saturate over large portions of the image in either detector. The mean of this region for the crossed detector was 109±5.4 and 188±14.2 for the crossed and parallel detectors, respectively. shows an unusually high signal for the crossed detector, which in a normal eye might occur in connection with a saturated signal in the parallel detector.
The parallel polarized light image emphasizes a pool of fluid that is more superficial than the subretinal region that well-defined neovascular membranes invade and, in proximity to retinal vessel abnormalities, one type of RVAC.10,11
This pool of fluid is of narrower diameter than the larger exudative lesion, lying above on the nasal (left) side of the main leakage in the images and partially blocking the fluorescence of the deeper component of exudation (). The retina is normally under tension, and as there is clear-cut leakage from the larger component, tension can then increase and can lead to the type of striae shown in the average and parallel polarized images (). With such mechanical constraints, the superficial fluid is likely to be a thin pool, adding a small signal to the height or intensity changes from the deeper components, with thickness changes due to RVAC shown to be associated with poorer visual prognosis.9
In the parallel polarized image, the top and middle regions of interest in , both within the area of the pool of fluid, have the same gray-scale levels (p
= 0.7). In contrast, the bottom sample is not within the pool of fluid, and the top region of interest is significantly darker than the bottom region of interest (p
< 0.01). In the depolarized light image, the top and bottom samples are the same (p
= 0.4), while the top is now brighter than the middle sample (p
Another type of birefringence change, fringes surrounding the lesion, is shown for a patient with a different type of lesion: a PED with a large amount of fluid and a comparatively small choroidal neovascular component (, top). To date, this finding is not limited to AMD or age or racial group and is found in many types of exudative changes associated with large, fluid-filled lesions. For this patient, the fringes display largely amplitude changes. That is, the fringes do not indicate differing lesion components returning more light as a function of input polarization angle. The corresponding data from ocular coherence tomography (OCT) show that there is a significant amount of fluid that elevates the retina (, bottom).
Fig. 9 (Upper panels) Retinal images of a 48 year old Japanese male with a PED. (Top left) Single panel from the crossed detector, showing a strong return at specific locations that do not correspond to regions of strong light return in the other image types (more ...)
A well-defined CNV membrane with active leakage around the edges ( and ) often has a border outside the membrane that blocks fluorescence in FA and ICG (). Well-defined CNV has a strong signal in several polarimetry image types (). The assumed border of the new vessel membrane could be seen in both the computed depolarized light image and the average image.
Fig. 10 Retinal images showing well-defined CNV in a 51 year old Caucasian female, using the Heidelberg retinal angiograph. (Top left) Early-phase FA, showing the early leakage and a dark border around the CNV membrane that blocks fluorescence. A central component (more ...)
Fig. 11 Polarimetry images of the patient from on a scale sufficiently small so as not to include the lesion components outside the main well-defined lesion. (Top left) Depolarized light image, emphasizing deeper features and with retinal vessels seen (more ...)
The very bright appearance of the CNV border was not due to merely a higher index of refraction change at the border, for instance, that associated with fibrin or other protein, lipid, or cellular debris, because the ratio of the minimum of the crossed detector to the average for both detectors was significantly greater at the rim of the neovascular membrane versus a control region of interest. This ratio was 1.21, on average, and significantly >1 (t =13.3, p < 0.0001). This ratio was >1 for 13 of 19 patients. As we used a conservative measure for comparison, i.e., only one region of interest on the rim, which was selected from the average image and not the depolarized image, the data as presented might underestimate the strength of the depolarized light signature of the CNV rim. That is, the ratio might increase for individual patients or be significant for all the patients if the operator could have selected a region of interest based on the depolarized light image or taken multiple samples from the rim, which was for some patients so thin in the average image as unlikely to permit optimal sampling. Alternatively, a higher ratio may indicate a different tissue status for some of the lesions, with other lesions with different molecular constituents having a lower ratio.