Fundus autofluorescence beyond lipofuscin: lesson learned from ex vivo fluorescence lifetime imaging in porcine eyes

Fluorescence Lifetime Imaging Ophthalmoscopy of Mouse Models of Age-related Macular Degeneration

Purpose

To investigate fluorescence lifetime of mouse models of age-related macular degeneration (AMD) by fluorescence lifetime imaging ophthalmoscopy (FLIO).

Methods

Two AMD mouse models, apolipoprotein E knockout (ApoE-/-) mice and NF-E2-related factor-2 knockout (Nrf2-/-) mice, and their wild-type mice underwent monthly ophthalmic examinations including FLIO from 3 months of age. After euthanasia at the age of 6 or 11 months, blood plasma was collected to determine total antioxidant capacity and eyes were enucleated for Oil red O (ORO) lipid staining of chorioretinal tissue.

Results

In FLIO, the mean fluorescence lifetime (τm) of wild type shortened with age in both spectral channels. In short spectral channel, τm shortening was observed in both AMD models as well, but its rate was more pronounced in ApoE-/- mice and significantly different from the other strains as months of age progressed. In contrast, in long spectral channel, both model strains showed completely opposite trends, with τm becoming shorter in ApoE-/- and longer in Nrf2-/- mice than the others. Oil red O staining at Bruch's membrane was significantly stronger in ApoE-/- mice at 11 months than the other strains. Plasma total antioxidant capacity was highest in ApoE-/- mice at both 6 and 11 months.

Conclusions

The two AMD mouse models exhibited largely different fundus fluorescence lifetime, which might be related to the different systemic metabolic state. FLIO might be able to indicate different metabolic states of eyes at risk for AMD.

Translational Relevance

This animal study may provide new insights into the relationship between early AMD-associated metabolic changes and FLIO findings.

Spectral and lifetime resolution of fundus autofluorescence in advanced age-related macular degeneration revealing different signal sources

PURPOSE

To determine the fundus autofluorescence (FAF) lifetimes and spectral characteristics of individual drusen and hyperpigmentation independent of those with retinal pigment epithelium (RPE) in geographic atrophy (GA) areas in late-stage age-related macular degeneration (AMD).

METHODS

Three consecutive patients with complete RPE and outer retinal atrophy (cRORA) exhibiting drusen that were calcified or associated with hyperpigmentation were investigated with multimodal non-invasive ophthalmic imaging including colour fundus photography (CFP), optical coherence tomography (OCT), near-infrared reflectance (NIR), blue FAF and fluorescence lifetime imaging ophthalmoscopy (FLIO). Fluorescence lifetimes were measured in two spectral channels (short-wavelength spectral channel (SSC): 500-560 nm and long-wavelength spectral channel (LSC): 560-720 nm).

RESULTS

Drusen lacking RPE coverage, as confirmed by CFP and OCT, had longer FAF lifetimes than surrounding cRORA by 127 ± 66 ps (SSC) and 113 ± 48 ps (LSC, both p = 0.008 in Wilcoxon test, N = 9) and by 209 ± 100 ps (SSC) and 121 ± 56 ps (LSC, p < 0.001, N = 14) in two patients. Hyperpigmentation in CFP in a third patient shows strong FAF with prolonged lifetimes. In the SSC, persistent FAF was found inside cRORA. A crescent-shaped hyperfluorescence in an area of continuous RPE but lacking outer retina was seen in one eye with a history of anti-VEGF treatment.

CONCLUSIONS

Short-wavelength fluorescence in cRORA points to fluorophores beyond RPE organelles. Fluorescence properties of drusen within cRORA differ from in vivo drusen covered by RPE. These limited findings from three patients give new insight into the sources of FAF that can be further elucidated in larger cohorts.

Adaptive optics fluorescence lifetime imaging ophthalmoscopy of in vivo human retinal pigment epithelium

The intrinsic fluorescence properties of lipofuscin - naturally occurring granules that accumulate in the retinal pigment epithelium - are a potential biomarker for the health of the eye. A new modality is described here which combines adaptive optics technology with fluorescence lifetime detection, allowing for the investigation of functional and compositional differences within the eye and between subjects. This new adaptive optics fluorescence lifetime imaging ophthalmoscope was demonstrated in 6 subjects. Repeated measurements between visits had a minimum intraclass correlation coefficient of 0.59 Although the light levels were well below maximum permissible exposures, the safety of the imaging paradigm was tested using clinical measures; no concerns were raised. This new technology allows for in vivo adaptive optics fluorescence lifetime imaging of the human RPE mosaic.

Adaptive optics two-photon excited fluorescence lifetime imaging ophthalmoscopy of photoreceptors and retinal pigment epithelium in the living non-human primate eye

Fluorescence lifetime imaging has demonstrated promise as a quantitative measure of cell health. Adaptive optics two-photon excited fluorescence (TPEF) ophthalmoscopy enables excitation of intrinsic retinal fluorophores involved in cellular metabolism and the visual cycle, providing in vivo visualization of retinal structure and function at the cellular scale. Combining these technologies revealed that macaque cones had a significantly longer mean TPEF lifetime than rods at 730 nm excitation. At 900 nm excitation, macaque photoreceptors had a significantly longer mean TPEF lifetime than the retinal pigment epithelium layer. AOFLIO can measure the fluorescence lifetime of intrinsic retinal fluorophores on a cellular scale, revealing differences in lifetime between retinal cell classes.

Ionized Radiation-Mediated Retinoid Oxidation in the Retina and Retinal Pigment Epithelium of the Murine Eye

The present study evaluated the effects of proton and gamma-ray ionizing radiation on the mouse eye. The aim of this work was to analyze radiation-mediated retinoid oxidation in the retina and retinal pigment epithelium (RPE). The findings from this analysis can be used to develop a noninvasive method for rapid assessment of the effects of ionizing radiation. Comparative fluorescence and chromatographic analyses of retinoids before and after irradiations were performed. The fluorescent properties of chloroform extracts from irradiated mouse retina and RPE exhibited an increase in fluorescence intensity in the short-wave region of the spectrum (λ < 550 nm). This change is due to increased retinal and RPE retinoid oxidation and degradation products after radiation exposure. Comparative analyses of radiation effects demonstrated that the effect of proton exposure on the retina and RPE was higher than that of gamma-ray exposure. The present study revealed a new approach to assessing the level of radiation exposure in ocular tissues.

Spectral fundus autofluorescence peak emission wavelength in ageing and AMD

PURPOSE

To investigate the spectral characteristics of fundus autofluorescence (FAF) in AMD patients and controls.

METHODS

Fundus autofluorescence spectral characteristics was described by the peak emission wavelength (PEW) of the spectra. Peak emission wavelength (PEW) was derived from the ratio of FAF recordings in two spectral channels at 500-560 nm and 560-720 nm by fluorescence lifetime imaging ophthalmoscopy. The ratio of FAF intensity in both channels was related to PEW by a calibration procedure. Peak emission wavelength (PEW) measurements were done in 44 young (mean age: 24.0 ± 3.8 years) and 18 elderly (mean age: 67.5 ± 10.2 years) healthy subjects as well as 63 patients with AMD (mean age: 74.0 ± 7.3 years) in each pixel of a 30° imaging field. The values were averaged over the central area, the inner and the outer ring of the ETDRS grid.

RESULTS

There was no significant difference between PEW in young and elderly controls. However, PEW was significantly shorter in AMD patients (ETDRS grid centre: 571 ± 26 nm versus 599 ± 17 nm for elderly controls, inner ring: 596 ± 17 nm versus 611 ± 11 nm, outer ring: 602 ± 16 nm versus 614 ± 11 nm). After a mean follow-up time of 50.8 ± 10.8 months, the PEW in the patients decreased significantly by 9 ± 19 nm in the inner ring of the grid. Patients, showing progression to atrophic AMD in the follow up, had significantly (p ≤ 0.018) shorter PEW at baseline than non-progressing patients.

CONCLUSIONS

Peak emission wavelength (PEW) is related to AMD pathology and might be a diagnostic marker in AMD. Possibly, a short PEW can predict progression to retinal and/or pigment epithelium atrophy.

In Situ Morphologic and Spectral Characterization of Retinal Pigment Epithelium Organelles in Mice Using Multicolor Confocal Fluorescence Imaging

Purpose

To investigate the major organelles of the retinal pigment epithelium (RPE) in wild-type (WT, control) mice and their changes in pigmented Abca4 knockout (Abca4^−/−) mice with in situ morphologic, spatial, and spectral characterization of live ex vivo flat-mounted RPE using multicolor confocal fluorescence microscopy (MCFM).

Methods

In situ imaging of RPE flat-mounts of agouti Abca4^−/− (129S4), agouti WT (129S1/SvlmJ) controls, and B6 albino mice (C57BL/6J-Tyr^c-Brd) was performed with a Nikon A1 confocal microscope. High-resolution confocal image z-stacks of the RPE cell mosaic were acquired with four different excitation wavelengths (405 nm, 488 nm, 561 nm, and 640 nm). The autofluorescence images of RPE, including voxel-by-voxel emission spectra, were acquired and processed with Nikon NIS-AR Elements software.

Results

The 3-dimensional multicolor confocal images provided a detailed visualization of the RPE cell mosaic, including its melanosomes and lipofuscin granules, and their varying characteristics in the different mice strains. The autofluorescence spectra, spatial distribution, and morphologic features of melanosomes and lipofuscin granules were measured. Increased numbers of lipofuscin and reduced numbers of melanosomes were observed in the RPE of Abca4^−/− mice relative to controls.

Conclusions

A detailed assessment of the RPE autofluorescent granules and their changes ex vivo was possible with MCFM. For all excitation wavelengths, autofluorescence from the RPE cells was predominantly contributed by lipofuscin granules, while melanosomes were found to be essentially nonfluorescent. The red shift of the emission peak confirmed the presence of multiple chromophores within lipofuscin granules. The elevated autofluorescence levels in Abca4^−/− mice correlated well with the increased number of lipofuscin granules.

Fluorescence lifetime imaging ophthalmoscopy: autofluorescence imaging and beyond

Fluorescence lifetime imaging ophthalmoscopy, FLIO, has gained large interest in the scientific community in the recent years. It is a noninvasive imaging modality that has been shown to provide additional information to conventional imaging modalities. The FLIO device is based on a Heidelberg Engineering Spectralis system. Autofluorescence lifetimes are excited at 473 nm and recorded in two spectral wavelength channels, a short spectral channel (SSC, 498-560 nm) and a long spectral channel (LSC, 560-720 nm). Typically, mean autofluorescence lifetimes in a 30° retinal field are investigated. FLIO shows a clear benefit for imaging different retinal diseases. For example, in age-related macular degeneration (AMD), ring patterns of prolonged FLIO lifetimes 1.5-3.0 mm from the fovea can be appreciated. Macular telangiectasia type 2 (MacTel) shows a different pattern, with prolonged FLIO lifetimes within the typical MacTel zone. In Stargardt disease, retinal flecks can be appreciated even before they are visible with other imaging modalities. Early hydroxychloroquine toxicity appears to be detectable with FLIO. This technique has more potential that has yet to be discovered. This review article focuses on current knowledge as well as pitfalls of this technology. It highlights clinical benefits of FLIO imaging in different ophthalmic and systemic diseases, and provides an outlook with perspectives from the authors.

FLIM data analysis based on Laguerre polynomial decomposition and machine-learning

SIGNIFICANCE

The potential of fluorescence lifetime imaging microscopy (FLIM) is recently being recognized, especially in biological studies. However, FLIM does not directly measure the lifetimes, rather it records the fluorescence decay traces. The lifetimes and/or abundances have to be estimated from these traces during the phase of data processing. To precisely estimate these parameters is challenging and requires a well-designed computer program. Conventionally employed methods, which are based on curve fitting, are computationally expensive and limited in performance especially for highly noisy FLIM data. The graphical analysis, while free of fit, requires calibration samples for a quantitative analysis.

AIM

We propose to extract the lifetimes and abundances directly from the decay traces through machine learning (ML).

APPROACH

The ML-based approach was verified with simulated testing data in which the lifetimes and abundances were known exactly. Thereafter, we compared its performance with the commercial software SPCImage based on datasets measured from biological samples on a time-correlated single photon counting system. We reconstructed the decay traces using the lifetime and abundance values estimated by ML and SPCImage methods and utilized the root-mean-squared-error (RMSE) as marker.

RESULTS

The RMSE, which represents the difference between the reconstructed and measured decay traces, was observed to be lower for ML than for SPCImage. In addition, we could demonstrate with a three-component analysis the high potential and flexibility of the ML method to deal with more than two lifetime components.

CONCLUSIONS

The ML-based approach shows great performance in FLIM data analysis.

Autofluorescence Lifetimes Measured with Fluorescence Lifetime Imaging Ophthalmoscopy (FLIO) Are Affected by Age, but Not by Pigmentation or Gender

Purpose

Fluorescence lifetime imaging ophthalmoscopy (FLIO) is a novel modality to investigate the human retina. This study aims to characterize the effects of age, pigmentation, and gender in FLIO.

Methods

A total of 97 eyes from 97 healthy subjects (mean age 37 ± 18 years, range 9-85 years) were investigated in this study. This study included 47 (49%) females and 50 males. The pigmentation analysis was a substudy including 64 subjects aged 18 to 40 years (mean age 29 ± 6 years). These were categorized in groups A (darkly pigmented, 8), B (medium pigmented, 20), and C (lightly pigmented, 36). Subjects received Heidelberg Engineering FLIO and optical coherence tomography imaging. Retinal autofluorescence lifetimes were detected in two spectral channels (short spectral channel [SSC]: 498-560 nm; long spectral channel [LSC]: 560-720 nm), and amplitude-weighted mean fluorescence lifetimes (τ_m) were calculated. Additionally, autofluorescence lifetimes of melanin were measured in a cuvette.

Results

Age significantly affected FLIO lifetimes, and age-related FLIO changes in the SSC start at approximately age 35 years, whereas the LSC shows a consistent prolongation with age from childhood. There were no gender- or pigmentation-specific significant differences of autofluorescence lifetimes.

Conclusions

This study confirms age-effects in FLIO but shows that the two channels are affected differently. The LSC appears to show the lifelong accumulation of lipofuscin. Furthermore, it is important to know that neither gender nor pigmentation significantly affect FLIO lifetimes.

Translational Relevance

This study helps to understand the FLIO technology better, which will aid in conducting future clinical studies.

Fundus Autofluorescence Lifetimes and Spectral Features of Soft Drusen and Hyperpigmentation in Age-Related Macular Degeneration

Purpose

To investigate the autofluorescence lifetimes as well as spectral characteristics of soft drusen and retinal hyperpigmentation in age-related macular degeneration (AMD).

Methods

Forty-three eyes with nonexudative AMD were included in this study. Fluorescence lifetime imaging ophthalmoscopy (FLIO), which detects autofluorescence decay over time in the short (SSC) and long (LSC) wavelength channel, was performed. The mean autofluorescence lifetime (τ_m) and the spectral ratio (sr) of autofluorescence emission in the SSC and LSC were recorded and analyzed. In total, 2760 soft drusen and 265 hyperpigmented areas were identified from color fundus photographs and spectral domain optical coherence tomography (SD-OCT) images and superimposed onto their respective AF images. τ_m and sr of these lesions were compared with fundus areas without drusen. For clearly hyperfluorescent drusen, the local differences compared to fundus areas without drusen were determined for lifetimes and sr.

Results

Hyperpigmentation showed significantly longer τ_m (SSC: 341 ± 81 vs. 289 ± 70 ps, P < 0.001; LSC: 406 ± 42 vs. 343 ± 42 ps, P < 0.001) and higher sr (0.621 ± 0.077 vs. 0.539 ± 0.083, P < 0.001) compared to fundus areas without hyperpigmentation or drusen. No significant difference in τ_m was found between soft drusen and fundus areas without drusen. However, the sr was significantly higher in soft drusen (0.555 ± 0.077 vs. 0.539 ± 0.081, P < 0.0005). Hyperfluorescent drusen showed longer τ_m than surrounding fundus areas without drusen (SSC: 18 ± 42 ps, P = 0.074; LSC: 16 ± 29 ps, P = 0.020).

Conclusions

FLIO can quantitatively characterize the autofluorescence of the fundus, drusen, and hyperpigmentation in AMD.

Translational Relevance

The experimental FLIO technique was applied in a clinical investigation. As FLIO yields information on molecular changes in AMD, it might support future diagnostics.

Fluorescence Lifetime Imaging Ophthalmoscopy (FLIO) in Eyes With Pigment Epithelial Detachments Due to Age-Related Macular Degeneration

Purpose

To investigate fluorescence lifetime imaging ophthalmoscopy (FLIO) in neovascular AMD and pigment epithelial detachments (PEDs).

Methods

A total of 46 eyes with PEDs (>350 μm) as well as age-matched healthy controls were included in this study. We found 28 eyes showed neovascular AMD (nvAMD), and 17 had nonneovascular (dry) AMD (dAMD). The Heidelberg Engineering FLIO excited fluorescence at 473 nm. Fluorescence decays were detected in two spectral channels (498–560 nm; 560–720 nm) to determine fluorescence lifetimes of endogenous fluorophores in their specific spectral emission ranges. Mean fluorescence lifetimes (τ_m) were investigated. Multimodal imaging was reviewed by two ophthalmologists who circumscribed and classified PEDs as either serous (n = 4), hemorrhagic (n = 4), fibrovascular (n = 16), drusenoid (n = 17), or mixed (n = 5). Blood samples from a healthy subject and a patient with PED were investigated in a quartz cuvette.

Results

Eyes with nvAMD show similar FLIO patterns to dAMD: ring-shaped prolongations of τ_m 3 to 6 mm from the fovea. Different PED-forms show characteristic τ_m, while serous and hemorrhagic PEDs exhibit shortened τ_m, drusenoid PEDs show prolonged τ_m, and τ_m in fibrovascular PEDs is variable. Areas corresponding to sub-/intraretinal fluid display shortened τ_m. Ex vivo studies of blood also show short τ_m.

Conclusions

The previously described dAMD-related FLIO pattern is also present in nvAMD. Short τ_m in serous, fibrovascular, and hemorrhagic PEDs as well as sub/intraretinal fluid may disrupt this pattern. FLIO appears to differentiate between PEDs, hemorrhage, and fluid. Additionally, ex vivo studies of human blood help to better interpret FLIO images.

Fluorescence Lifetime Imaging Ophthalmoscopy of the Retinal Pigment Epithelium During Wound Healing After Laser Irradiation

Purpose

To investigate the change in fluorescence lifetime of retinal pigment epithelium (RPE) after laser irradiation by using an organ culture model.

Methods

Porcine RPE-choroid-sclera explants were irradiated with selective retina treatment laser (wavelength: 527 nm, beam diameter: 200 μm, energy: 80–150 μJ). At 24 and 72 hours after irradiation, the mean fluorescence lifetime (τ_m) was measured with fluorescence lifetime imaging ophthalmoscopy (FLIO) (excitation wavelength: 473 nm, emission: short spectral channel: 498-560 nm, long spectral channel: 560–720 nm). For every laser spot, central damaged zone (zone 1: 120 × 120 μm), area including wound rim (280 × 280 μm except zone 1), and environmental zone (440 × 440 μm except zone 1 and 2) were analyzed. Peripheral zone at a distance from laser spots longer than 2000 μm was examined for comparison. Cell viability was evaluated with calcein-acetoxymethyl ester and morphology with fluorescence microscopy for filamentous-actin.

Results

The RPE defect after selective retina treatment was mostly closed within 72 hours. FLIO clearly demarcated the irradiated region, with prolonged τ_m at the center of the defect decreasing with eccentricity. In short spectral channel, but not in long spectral channel, τ_m in the environmental zone after 72 hours was still significantly longer than in the peripheral zone.

Conclusions

FLIO may clearly demarcate the RPE defect, demonstrate its closure, and, moreover, indicate the induced metabolic changes of surrounding cells during wound healing.

Translational Relevance

This ex vivo study showed that FLIO may be used to evaluate the extent and quality of restoration of the damaged RPE and to detect its metabolic change in human fundus noninvasively.

Bleaching effects and fluorescence lifetime imaging ophthalmoscopy

This study investigates the influence of photopigment bleaching on autofluorescence lifetimes in the fundus in 21 young healthy volunteers. Three measurements of 30° retinal fields in two spectral channels (SSC: 498-560 nm, LSC: 560-720 nm) were obtained for each volunteer using fluorescence lifetime imaging ophthalmoscopy (FLIO). After dark-adaptation by wearing a custom-made lightproof mask for 30 minutes, the first FLIO-measurement was recorded (dark-adapted state). Subsequently, the eye was bleached for 1 minute (luminance: 3200 cd/m²), followed by a second FLIO-measurement (bleached state). Following an additional 10 minute dark adaptation using the mask, a final FLIO-measurement was recorded (recovered state). Average values of the fluorescence lifetimes were calculated from within different areas of a standardized early treatment diabetic retinopathy study (ETDRS) grid (central area, inner and outer rings). The acquisition time in the bleached state was significantly shortened by approximately 20%. The SSC did not show any significant changes in fluorescence lifetimes with photopigment bleaching, only the LSC showed small but significant bleaching-related changes in the fluorescence lifetimes τ₁ and τ₂ from all regions, as well as the mean fluorescence lifetime in the central area. The fluorescence lifetime differences caused by bleaching were by far less significant than pathological changes caused by eye diseases. The magnitudes of fluorescence lifetime changes are <10% and do not interfere with healthy or disease related FLIO patterns. Thus, we conclude that bleaching is not a relevant confounder in current clinical applications of FLIO.