Repeatability of autofluorescence lifetime imaging at the human fundus in healthy volunteers

Suppression of natural lens fluorescence in fundus autofluorescence measurements: review of hardware solutions

Fluorescence lifetime imaging ophthalmoscopy (FLIO), a technique for investigating metabolic changes in the eye ground, can reveal the first signs of diseases related to metabolism. The fluorescence of the natural lens overlies the fundus fluorescence. Although the influence of natural lens fluorescence can be somewhat decreased with mathematical models, excluding this influence during the measurement by using hardware enables more exact estimation of the fundus fluorescence. Here, we analyze four 1-photon excitation hardware solutions to suppress the influence of natural lens fluorescence: aperture stop separation, confocal scanning laser ophthalmoscopy, combined confocal scanning laser ophthalmoscopy and aperture stop separation, and dual point confocal scanning laser ophthalmoscopy. The effect of each principle is demonstrated in examples. The best suppression is provided by the dual point principle, realized with a confocal scanning laser ophthalmoscope. In this case, in addition to the fluorescence of the whole eye, the fluorescence of the anterior part of the eye is detected from a non-excited spot of the fundus. The intensity and time-resolved fluorescence spectral data of the fundus are derived through the subtraction of the simultaneously measured fluorescence of the excited and non-excited spots. Advantages of future 2-photon fluorescence excitation are also discussed. This study provides the first quantitative evaluation of hardware principles to suppress the fluorescence of the natural lens during measurements of fundus autofluorescence.

The Development and Clinical Application of Innovative Optical Ophthalmic Imaging Techniques

The field of ophthalmic imaging has grown substantially over the last years. Massive improvements in image processing and computer hardware have allowed the emergence of multiple imaging techniques of the eye that can transform patient care. The purpose of this review is to describe the most recent advances in eye imaging and explain how new technologies and imaging methods can be utilized in a clinical setting. The introduction of optical coherence tomography (OCT) was a revolution in eye imaging and has since become the standard of care for a plethora of conditions. Its most recent iterations, OCT angiography, and visible light OCT, as well as imaging modalities, such as fluorescent lifetime imaging ophthalmoscopy, would allow a more thorough evaluation of patients and provide additional information on disease processes. Toward that goal, the application of adaptive optics (AO) and full-field scanning to a variety of eye imaging techniques has further allowed the histologic study of single cells in the retina and anterior segment. Toward the goal of remote eye care and more accessible eye imaging, methods such as handheld OCT devices and imaging through smartphones, have emerged. Finally, incorporating artificial intelligence (AI) in eye images has the potential to become a new milestone for eye imaging while also contributing in social aspects of eye care.

IMAGING ARTIFACTS IN FLUORESCENCE LIFETIME IMAGING OPHTHALMOSCOPY

PURPOSE

To investigate and quantify the influence of imaging artifacts on retinal fluorescence lifetime (FLIO) values and to provide helpful hints and tricks to avoid imaging artifacts and to improve FLIO image acquisition quality.

METHODS

A systematic analysis of potential parameters influencing FLIO quality and/or fluorescence lifetime values was performed in a prospective systematic experimental imaging study in five eyes of five healthy subjects. For image acquisition, a fluorescence lifetime imaging ophthalmoscope (Heidelberg Engineering) was used. Quantitative analysis of FLIO lifetime changes due to imaging artifacts was performed.

RESULTS

Imaging artifacts with significant influence on fluorescence lifetimes included too short image acquisition time, insufficient illumination, ocular surface problems, and image defocus. Prior use of systemic or topical fluorescein makes analysis of retinal fluorescence lifetimes impossible.

CONCLUSION

Awareness of possible sources of imaging artifacts is important for FLIO image acquisition and analysis. Therefore, standardized imaging and analysis procedure in FLIO is crucial for high-quality image acquisition and the possibility for systematic quantitative fluorescence lifetime analysis.

LONGITUDINAL FLUORESCENCE LIFETIME IMAGING OPHTHALMOSCOPY ANALYSIS IN PATIENTS WITH MACULAR TELANGIECTASIA TYPE 2 (MacTel)

PURPOSE

Fluorescence lifetime imaging ophthalmoscopy (FLIO) shows characteristic patterns in macular telangiectasia Type 2 (MacTel). This study investigates FLIO changes over time to better understand disease progression.

METHODS

Thirty-three patients with MacTel (age 60 ± 15 years) were followed at the Moran Eye Center with a prototype Heidelberg Engineering FLIO. The mean follow-up time was 19 ± 8 months (range 6-34 months). Fundus autofluorescence was excited at 473 nm, and FLIO lifetimes were recorded in in short (498-560 nm) and long (560-720 nm) spectral wavelengths channels.

RESULTS

Autofluorescence lifetimes imaging ophthalmoscopy lifetimes from the MacTel area prolonged significantly over time (subfield T1, baseline: short spectral channel 210 ± 54 ps, long spectral channel 269 ± 58 ps; follow-up: short spectral channel 225 ± 59 ps, P < 0.001, long spectral channel 282 ± 64 ps, P < 0.01). The average 12-months prolongation of FLIO lifetimes was 9 ps (short spectral channel) and 8 ps (long spectral channel). Autofluorescence lifetimes changes correlated positively with ellipsoid zone loss and negatively with changes in retinal thickness.

CONCLUSION

Autofluorescence lifetimes in MacTel slowly prolong over time, and temporal patterns progress to full rings. Detailed knowledge about FLIO changes will aid in understanding disease development and progression.

Comparing Fluorescence Lifetime Imaging Ophthalmoscopy in Atrophic Areas of Retinal Diseases

Purpose

Fluorescence lifetime imaging ophthalmoscopy (FLIO) is a non-invasive imaging modality to investigate the human retina. This study compares FLIO lifetimes in different degenerative retinal diseases.

Methods

Included were eyes with retinal pigment epithelium (RPE) and/or photoreceptor atrophy due to Stargardt disease (n = 66), pattern dystrophy (n = 18), macular telangiectasia type 2 (n = 49), retinitis pigmentosa (n = 28), choroideremia (n = 26), and geographic atrophy (n = 32) in age-related macular degeneration, as well as 37 eyes of 37 age-matched healthy controls. Subjects received Heidelberg Engineering FLIO, autofluorescence intensity, and optical coherence tomography imaging. Amplitude-weighted mean FLIO lifetimes (τ_m) were calculated and analyzed.

Results

Retinal FLIO lifetimes show significant differences depending on the disease. Atrophic areas in geographic atrophy and choroideremia showed longest mean FLIO lifetimes. τ_m values within areas of RPE and outer nuclear layer atrophy were significantly longer than within areas with preserved outer nuclear layer (P < 0.001) or non-atrophic areas (P < 0.001).

Conclusions

FLIO is able to contribute additional information regarding differences in chronic degenerative retinal diseases. Although it cannot replace conventional autofluorescence imaging, FLIO adds to the knowledge in these diseases and may help with the correct differentiation between them. This may lead to a more in-depth understanding of the pathomechanisms related to atrophy and types of progression.

Translational Relevance

Differences between atrophic retinal diseases highlighted by FLIO may indicate separate pathomechanisms leading to atrophy and disease progression.

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.

Comparison of algorithms to suppress artifacts from the natural lens in fluorescence lifetime imaging ophthalmoscopy (FLIO)

Fluorescence lifetime imaging ophthalmoscopy (FLIO) has developed as a new diagnostic tool in ophthalmology. FLIO measurements are taken from 30° retinal fields in two spectral channels (short spectral channel (SSC): 498-560 nm, long spectral channel (LSC): 560-720 nm). Because of the layered structure of the eye, the detected signal is an interaction of the fluorescence decay of the anterior part and of the fundus. By comparing FLIO measurements before and after cataract surgery, the impact of the natural lens was proven, despite the application of a confocal laser scanning (cSLO) technique. The goal of this work was to determine the best algorithmic solution to isolate the sole fundus fluorescence lifetime from the measured signal, suppressing artifacts from the natural lens. Three principles based on a tri-exponential model were investigated: a tailfit, a layer-based approach with a temporally shifted component, and the inclusion of a separately measured fluorescence decay of the natural lens. The mean fluorescence lifetime τ_m,12 is calculated using only the shortest and the intermediate exponential component. τ_m,all is calculated using all three exponential components. The results of tri-exponential tailfit after cataract surgery were considered as a reference, because the implanted artificial lens can be assumed as non-fluorescent. In SSC, the best accordance of τ_m,all of the reference was determined with τ_m,12 of the tailfit before surgery. If high-quality natural lens measurements are available, the correspondence of τ_m,12 is best with τ_m,all of the reference. In LSC, there is a good accordance for all models between τ_m,12 before and after surgery. To study the pure fundus fluorescence decay in eyes with natural lenses, we advise to utilize fluorescence lifetime τ_m,12 of a triple-exponential tailfit, as it corresponds well with the mean fluorescence lifetime τ_m,all of eyes with fluorescence-less artificial intraocular lenses.

Influence of Lens Fluorescence on Fluorescence Lifetime Imaging Ophthalmoscopy (FLIO) Fundus Imaging and Strategies for Its Compensation

Purpose

To explore the contribution of crystalline lens fluorescence to fluorescence lifetimes measured with fluorescence lifetime imaging ophthalmoscopy (FLIO) and to propose a computational model to reduce the lens influence.

Methods

FLIO, which detects autofluorescence decay over time in a short-wavelength spectral channel (SSC, 498–560 nm) and a long-wavelength spectral channel (LSC, 560–720 nm), was performed on 32 patients before and after cataract extraction. The mean autofluorescence lifetime (τ_m) of the fundus was determined from a three-exponential fit of the postoperative fluorescence decays. The preoperative measurements were fit with series of exponential functions in which one fluorescence component was time-shifted in order to represent lens fluorescence.

Results

Postoperatively, τ_m was 185 ± 22 ps in the SSC and 209 ± 34 ps in the LSC at the posterior pole. These values were best reproduced by fitting the postoperative measurements with a three-exponential model with a time-shifted third fluorescence component (SSC, 203 ± 45 ps; LSC, 215 ± 29 ps), whereas disregarding time-shifted lens fluorescence resulted in significantly (P < 0.001) longer τ_m values (SSC, 474 ± 206 ps; LSC, 215 ± 29 ps). The fluorescence of the cataract lens contributed to the total fluorescence by 54.2 ± 10.6% (SSC) and 29.5 ± 9.9% (LSC).

Conclusions

Cataract lens fluorescence greatly alters fluorescence lifetimes measured at the fundus by FLIO, resulting in an overestimation of the lifetimes; however, this may be compensated for considerably by taking lens influence into account in the fitting model.

Translational Relevance

This study investigates cataract fluorescence in FLIO and a mathematical model for compensation of this influence.

FLUORESCENCE LIFETIME IMAGING OPHTHALMOSCOPY (FLIO) PATTERNS IN CLINICALLY UNAFFECTED CHILDREN OF MACULAR TELANGIECTASIA TYPE 2 (MACTEL) PATIENTS

PURPOSE

Macular telangiectasia Type 2 (MacTel) is an inherited retinal disease following an autosomal dominant pattern with late onset and reduced penetrance. Fluorescence lifetime imaging ophthalmoscopy (FLIO) enhances diagnosis by showing distinct changes in MacTel. This study investigates FLIO-associated changes in clinically unaffected family members.

METHODS

Eighty-one patients with MacTel (61 ± 12 years), 33 clinically healthy children under age 40 years of these MacTel patients (MacTel-C; 31 ± 6 years), 27 other family members (children over age 40 years, siblings, and parents) and 30 controls were investigated with the Heidelberg FLIO. All subjects underwent multimodal conventional imaging, including optical coherence tomography, blue-light reflectance, fluorescein angiography, and macular pigment imaging.

RESULTS

All 81 patients with MacTel showed typical FLIO patterns. Of the 33 investigated MacTel-C with completely normal eye examinations and conventional imaging, 12 (36%) show FLIO patterns consistent with early MacTel.

CONCLUSION

Prolonged FLIO lifetimes in the parafoveal area within the short spectral channel, especially temporally, are MacTel-specific. Fluorescence lifetime imaging ophthalmoscopy detects these lifetime patterns in over one-third of clinically unaffected MacTel-C. Although further studies will be necessary to determine the specificity of FLIO, it may help diagnose MacTel before conventional imaging modalities show changes or patients experience visual disturbances. Early detection may facilitate future gene discovery studies and interventional trials.

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.

Repeatability of Fluorescence Lifetime Imaging Ophthalmoscopy in Normal Subjects With Mydriasis

Purpose

We evaluate the repeatability of fluorescence lifetime imaging ophthalmoscopy (FLIO) in normal subjects with mydriasis and explore factors that influence FLIO imaging.

Method

Thirty-two healthy participants (63 eyes) were enrolled in this prospective study. The Heidelberg Engineering FLIO system uses a 473 nm blue laser light and the emitted fluorescence is detected in two wavelength channels, short and long spectral channels (SSC, LSC). The mean fluorescence lifetime (τ_m) values were computed for the entire scan area as well as in five regions of interest (ROI, 1 × 1 mm) at the fovea and superior, nasal, inferior, and temporal portions of the macula. Intraclass correlation coefficients (ICC) and coefficients of variation (CV) were used to assess the repeatability. Age, macular thickness, and vascular density also were correlated with τ_m.

Results

The repeatability was good for both channels (ICC, 0.956∼0.995; CV, 9∼16%). The τ_m for the entire scan was 367.8 ± 58.1 picoseconds (ps) in SSC and 322.5 ± 34.0 ps in LSC. τ_m was the shortest in the fovea and significantly shorter in the temporal region compared to other regions. τ_m was positively correlated with age (r = 0.588 for SSC and r = 0.584 for LSC, P = 0.000) and retinal thickness (r = 0.298 for SSC and r = 0.322 for LSC, P = 0.000), and negatively correlated with vascular density (r = −0.112, P = 0.055 for SSC and r = −0.119, P = 0.040 for LSC).

Conclusion

Repeatable fluorescence lifetime values can be obtained with FLIO, but the lifetimes are affected by age, retinal thickness, vessel density, and macular location.

Translational Relevance

Establishing repeatability of FLIO can introduce fluorescence lifetime imaging technique, which is used in basic science for analysis of excitation and emission wavelength spectrum of fixed and living cells into clinical practice.

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.

Impact of mydriasis in fluorescence lifetime imaging ophthalmoscopy

Fluorescence lifetime imaging ophthalmoscopy (FLIO) is a novel technique that measures in vivo autofluorescence intensity decay over time of endogenous fluorophores in the retina. The Heidelberg Engineering FLIO system was used to obtain two 30 degree scans centered on the fovea of both eyes. The FLIO system uses a 473nm blue scanning laser light source and the emitted fluorescence is detected in two wavelengths channels, short and long spectral channels (SSC, LSC). Since the mydriatic status influence the FLIO result, the impact of mydriasis on FLIO need to be clarified. In this prospective, observational study, the impact of mydriasis on measurements from fluorescence lifetime imaging ophthalmoscope (FLIO) images in normal subjects were evaluated. 12 healthy participants (24 eyes) were volunteered and all subjects were scanned twice and the mean fluorescence lifetime (τ_m) values were computed with dilation and without dilation on different days. Intraclass correlation coefficients (ICC) and coefficients of variation (CV) were calculated from the measured τ_m in dilated, nondilated and between the dilated and non-dilated setting. Test duration was also compared and correlated with lifetimes in both settings. Repeatability was excellent for both the dilation and non-dilation settings (ICC; 0.967–0.996; 0.926–0.986, respectively). The agreement between the dilation and non-dilation settings, however, were lower (ICC; 0.688–0.970). The τ_m in the non-dilation setting was significantly longer than in the dilation setting for the SSC (P<0.05). The FLIO test duration in the non-dilation setting was significantly longer than with dilation for the SSC (P <0.05). Although good repeatability in τ_m measurements between imaging sessions were observed both with and without dilation, the agreement was not as good when comparing dilated with non-dilated measurements. Since FLIO without mydriasis results in longer τ_m in the SSC and takes a longer time for image acquisition, maximal dilation is recommended for FLIO testing.

Review of clinical approaches in fluorescence lifetime imaging ophthalmoscopy

Autofluorescence-based imaging techniques have become very important in the ophthalmological field. Being noninvasive and very sensitive, they are broadly used in clinical routines. Conventional autofluorescence intensity imaging is largely influenced by the strong fluorescence of lipofuscin, a fluorophore that can be found at the level of the retinal pigment epithelium. However, different endogenous retinal fluorophores can be altered in various diseases. Fluorescence lifetime imaging ophthalmoscopy (FLIO) is an imaging modality to investigate the autofluorescence of the human fundus in vivo. It expands the level of information, as an addition to investigating the fluorescence intensity, and autofluorescence lifetimes are captured. The Heidelberg Engineering Spectralis-based fluorescence lifetime imaging ophthalmoscope is used to investigate a 30-deg retinal field centered at the fovea. It detects FAF decays in short [498 to 560 nm, short spectral channel (SSC) and long (560 to 720 nm, long spectral channel (LSC)] spectral channels, the mean fluorescence lifetimes (τm) are calculated using bi- or triexponential approaches. These are meant to be relatively independent of the fluorophore's intensity; therefore, fluorophores with less intense fluorescence can be detected. As an example, FLIO detects the fluorescence of macular pigment, retinal carotenoids that help protect the human fundus from light damages. Furthermore, FLIO is able to detect changes related to various retinal diseases, such as age-related macular degeneration, albinism, Alzheimer's disease, diabetic retinopathy, macular telangiectasia type 2, retinitis pigmentosa, and Stargardt disease. Some of these changes can already be found in healthy eyes and may indicate a risk to developing such diseases. Other changes in already affected eyes seem to indicate disease progression. This review article focuses on providing detailed information on the clinical findings of FLIO. This technique detects not only structural changes at very early stages but also metabolic and disease-related alterations. Therefore, it is a very promising tool that might soon be used for early diagnostics.

Peripapillary fluorescence lifetime reveals age-dependent changes using fluorescence lifetime imaging ophthalmoscopy in rats

Many fundus diseases accompany fundus autofluorescence change. Fluorescence lifetime imaging ophthalmoscope (FLIO) is a latest technique in imaging fundus autofluorescence. With FLIO, the fundus fluorescence lifetime (FLT) is recorded topographically, assisting to diagnose and monitor multiple fundus diseases. The purpose of this study was to evaluate the repeatability of FLT using FLIO on adult rats and to analyze the age-dependency of the peripapillary FLT of the fundus in a short spectral channel (498-560 nm) and a long spectral channel (560-720 nm). Sprague Dawley rats (n of eyes = 10) were used for repeatability experiments. Age-dependent changes were investigated in young (two months old, n of eyes = 20) and old (eight months old, n of eyes = 10) rats. Repeatability experiments showed highly corresponding data for all segments in both spectral channel, with higher repeatability in the short spectral channel. FLT decreased significantly in all areas in the short (young: 991 ± 29 ps; old: 547 ± 42 ps) and long (young: 382 ± 28 ps; old: 261 ± 16 ps) spectral channels, indicating an overall metabolic change of the fundus in old animals. FLT of veins increased in the short spectral channel (young: 385 ± 43 ps; old: 424 ± 25 ps) and no change was observed in the long spectral channel (young: 274 ± 9 ps; old: 269 ± 24 ps). FLIO represents a highly repeatable and sensitive method to detect changes of the FLT in aged eyes for monitoring the degeneration of the rodent retinae.

Characterization of Retinitis Pigmentosa Using Fluorescence Lifetime Imaging Ophthalmoscopy (FLIO)

PURPOSE

We investigated fundus autofluorescence (FAF) lifetimes in patients with retinitis pigmentosa (RP) using fluorescence lifetime imaging ophthalmoscopy (FLIO).

METHODS

A total of 33 patients (mean age, 40.0 ± 17.0 years) with RP and an age-matched healthy group were included. The Heidelberg FLIO was used to detect FAF decays in short (SSC; 498-560 nm) and long (LSC; 560-720 nm) spectral channels. We investigated a 30° retinal field and calculated the amplitude-weighted mean fluorescence lifetime (τm). Additionally, macular pigment measurements, macular optical coherence tomography (OCT) scans, fundus photographs, visual fields, and fluorescein angiograms were recorded. Genetic studies were performed on nearly all patients.

RESULTS

In RP, FLIO shows a typical pattern of prolonged τm in atrophic regions in the outer macula (SSC, 419 ± 195 ps; LSC, 401 ± 111 ps). Within the relatively preserved retina in the macular region, ring-shaped patterns were found, most distinctive in patients with autosomal dominant RP inheritance. Mean FAF lifetimes were shortened in rings in the LSC. Central areas remained relatively unaffected.

CONCLUSIONS

FLIO uniquely presents a distinct and specific signature in eyes affected with RP. The ring patterns show variations that indicate genetically determined pathologic processes. Shortening of FAF lifetimes in the LSC may indicate disease progression, as was previously demonstrated for Stargardt disease. Therefore, FLIO might be able to indicate disease progression in RP as well.

TRANSLATIONAL RELEVANCE

Hyperfluorescent FLIO rings with short FAF lifetimes may provide insight into the pathophysiologic disease status of RP-affected retinas potentially providing a more detailed assessment of disease progression.

Fluorescence Lifetime Imaging Ophthalmoscopy: A Novel Way to Assess Macular Telangiectasia Type 2

PURPOSE

Macular Telangiectasia Type 2 (MacTel) is an uncommon, late-onset complex retinal disease that leads to central vision loss. No causative gene(s) have been identified so far, resulting in a challenging clinical diagnostic dilemma because retinal changes of early stages are often subtle. The objective of this study was to investigate the benefit of fluorescence lifetime imaging ophthalmoscopy (FLIO) for retinal imaging in patients with MacTel.

DESIGN

Cross-sectional study from a tertiary-care retinal referral practice.

SUBJECTS AND CONTROLS

42 eyes of 21 patients (mean age 60.5±13.3 years) with MacTel as well as an age-matched healthy control group (42 eyes of 25 subjects, mean age 60.8±13.4 years).

METHODS

A 30° retinal field centered at the fovea was investigated using FLIO. This camera is based on a Heidelberg Engineering Spectralis system. Fundus autofluorescence (FAF) decays were detected in short (498-560 nm, SSC) and long (560-720 nm, LSC) spectral channels. The mean fluorescence lifetime, τm, was calculated from a 3-exponential approximation of the FAF decays. For MacTel patients, macular pigment (MP), OCT, blue light reflectance, fluorescein angiography, as well as fundus photography, were also recorded.

MAIN OUTCOME MEASURES

Mean FAF lifetime (τm) images.

RESULTS

FLIO of MacTel patients shows a unique pattern of prolonged τm at the temporal side of the fovea in patients with MacTel in the "MacTel area" within 5-6° of the foveal center. In early stages, this region appears crescent-shaped, while advanced stages show a ring-like pattern. This pattern corresponds well with other imaging modalities and gives an especially high contrast of the affected region even in minimally affected individuals. Additionally, FLIO provides a novel means to monitor the abnormal MP distribution. In one case, FLIO showed changes suggestive of MacTel within a clinically normal parent of two MacTel patients.

CONCLUSIONS

FLIO detects retinal changes in patients with MacTel with high contrast, presenting a distinctive signature that is a characteristic finding of the disease. The non-invasive properties of this novel imaging modality provide a valuable addition to clinical assessment of early changes in the disease that could lead to more accurate diagnosis of MacTel.

Fluorescence Lifetime Imaging Ophthalmoscopy (FLIO) of Macular Pigment

Purpose

To describe different patterns of macular pigment (MP) seen in fluorescence lifetime imaging ophthalmoscopy (FLIO) and to analyze ex vivo fluorescence characteristics of carotenoids.

Methods

A total of 31 eyes of young healthy subjects, 4 eyes from patients with albinism, 36 eyes with macular telangiectasia type 2 (MacTel), 24 eyes with retinitis pigmentosa, and 1 eye with a macular hole were included in this clinic-based, cross-sectional study. All subjects underwent Heidelberg Engineering FLIO and MP measurements (dual-wavelength autofluorescence). Fundus autofluorescence (FAF) lifetimes of a 30° retinal field were detected in two spectral channels (SSC: 498-560 nm; LSC: 560-720 nm), and amplitude-weighted mean fluorescence lifetimes (τm) were calculated. Additionally, autofluorescence lifetimes of known dilutions of lutein and zeaxanthin were measured in a cuvette in free- and protein-associated states.

Results

MP shows a significant inverse correlation to foveal FAF lifetimes measured with FLIO (SSC: r = -0.608; P < 0.001). Different distribution patterns can be assigned to specific disease-related changes. Two patients with albinism, who did not have MP, were found to be missing short FAF lifetimes. In solvent, lutein and zeaxanthin show very short autofluorescence lifetimes (∼50-60 ps; SSC), as do their respective binding proteins (∼40-50 ps; SSC). When combining carotenoids with their specific binding proteins, the decay times shift to longer means (∼70-90 ps; SSC).

Conclusions

This study expands upon previous findings of an impact of MP on short FAF lifetimes by describing ex vivo autofluorescence lifetimes of carotenoids and different in vivo autofluorescence patterns that can be associated with certain diseases.

Monitoring foveal sparing in geographic atrophy with fluorescence lifetime imaging ophthalmoscopy - a novel approach

PURPOSE

To investigate fundus autofluorescence (FAF) lifetimes in geographic atrophy (GA) with a focus on macular pigment (MP) and foveal sparing.

METHODS

The study included 35 eyes from 28 patients (mean age 79.2 ± 8.0 years) with GA. A 30° retinal field, centred at the macula, was investigated using fluorescence lifetime imaging ophthalmoscopy (FLIO). The FLIO technology is based on a Heidelberg Engineering Spectralis system. Decays of FAF were detected in a short (498-560 nm, SSC) and long (560-720 nm, LSC) spectral channel. The mean fluorescence lifetime, τ_m , was calculated from a three-exponential approximation of the FAF decays. Macular optical coherence tomography (OCT) scans as well as fundus photography were recorded.

RESULTS

Review of FLIO data reveals specific patterns of significantly prolonged τ_m in regions of GA (SSC 616 ± 343 ps, LSC 615 ± 154 ps) as compared to non-atrophic regions. Large τ_m differences between the fovea and atrophic areas correlate with better visual acuity (VA). Shorter τ_m at the fovea than within other non-atrophic regions indicates sparing, which was identified in 16 eyes. Seventy per cent of patients treated with lutein supplementation showed foveal sparing, whereas the rate among non-supplemented patients was 22%.

CONCLUSION

Using FLIO, we present a novel way to detect foveal sparing, investigate MP, and analyse variability of τ_m in different foveal regions (including the prognostic valuable border region) in GA. These findings support the potential utility of FLIO in monitoring disease progression. The findings also highlight the possibly protective effect of lutein supplementation, with implication in recording the presence and distributional pattern of MP.

Patterns of Fundus Autofluorescence Lifetimes In Eyes of Individuals With Nonexudative Age-Related Macular Degeneration

Purpose

To investigate fundus autofluorescence (FAF) lifetimes in patients with nonexudative AMD.

Methods

A total of 150 eyes of 110 patients (mean age: 73.2 ± 10.7 years) with nonexudative AMD, as well as a healthy group of 57 eyes in 38 subjects (mean age: 66.5 ± 8.7 years), were included. Investigations were conducted at the University Eye Clinic in Jena, Germany, as well as the Moran Eye Center in Salt Lake City, Utah, USA, using the Heidelberg Engineering Spectralis-based fluorescence lifetime imaging ophthalmoscope (FLIO). A 30° retinal field centered at the fovea was investigated. FAF decays were detected in short (498-560 nm) and long (560-720 nm, LSC) spectral channels. The mean fluorescence lifetimes (τm) were calculated. Optical coherence tomography scans and fundus photographs were also recorded.

Results

In patients with nonexudative AMD, FLIO shows a ring-shaped pattern of prolonged τm in the LSC. This pattern occurs in all patients with AMD (including very early stages) and in one-third of the healthy controls. FAF lifetimes were longer with more advanced stages. The presence of drusen is associated with prolonged τm when compared with the healthy fundus, but drusen identification is difficult with FLIO only.

Conclusions

FLIO detects a clear pattern of changes within the fundus, which appears to be AMD-associated. These changes are already visible in early AMD stages and not masked by the presence of other coexisting retinal diseases. These findings may be useful for the early diagnosis of AMD and to distinguish AMD from other retinal diseases.

Fundus autofluorescence applications in retinal imaging

Fundus autofluorescence (FAF) is a relatively new imaging technique that can be used to study retinal diseases. It provides information on retinal metabolism and health. Several different pathologies can be detected. Peculiar AF alterations can help the clinician to monitor disease progression and to better understand its pathogenesis. In the present article, we review FAF principles and clinical applications.

Correction-free remotely scanned two-photon in vivo mouse retinal imaging

Non-invasive fluorescence retinal imaging in small animals is an important requirement for an array of translational vision applications. The in vivo two-photon imaging of the mouse retina may enable the long-term investigation of the structure and function of healthy and diseased retinal tissue. However, to date, this has only been possible using relatively complex adaptive-optics systems. Here, the optical modeling of the murine eye and of the imaging system is used to achieve correction-free two-photon microscopy through the pupil of a mouse eye to yield high-quality, optically sectioned fundus images. By remotely scanning the focus using an electronically tunable lens, high-resolution three-dimensional fluorescein angiograms and cellular-scale images are acquired, thus introducing a correction-free baseline performance level for two-photon in vivo retinal imaging. Moreover, the system enables functional calcium imaging of repeated retinal responses to light stimulation using the genetically encoded indicator, GCaMP6s. These results and the simplicity of the new add-on optics are an important step toward several structural, functional, and multimodal imaging applications that will benefit from the tight optical sectioning and the use of near-infrared light.

FLIMX: A Software Package to Determine and Analyze the Fluorescence Lifetime in Time-Resolved Fluorescence Data from the Human Eye

Fluorescence lifetime imaging ophthalmoscopy (FLIO) is a new technique for measuring the in vivo autofluorescence intensity decays generated by endogenous fluorophores in the ocular fundus. Here, we present a software package called FLIM eXplorer (FLIMX) for analyzing FLIO data. Specifically, we introduce a new adaptive binning approach as an optimal tradeoff between the spatial resolution and the number of photons required per pixel. We also expand existing decay models (multi-exponential, stretched exponential, spectral global analysis, incomplete decay) to account for the layered structure of the eye and present a method to correct for the influence of the crystalline lens fluorescence on the retina fluorescence. Subsequently, the Holm-Bonferroni method is applied to FLIO measurements to allow for group comparisons between patients and controls on the basis of fluorescence lifetime parameters. The performance of the new approaches was evaluated in five experiments. Specifically, we evaluated static and adaptive binning in a diabetes mellitus patient, we compared the different decay models in a healthy volunteer and performed a group comparison between diabetes patients and controls. An overview of the visualization capabilities and a comparison of static and adaptive binning is shown for a patient with macular hole. FLIMX’s applicability to fluorescence lifetime imaging microscopy is shown in the ganglion cell layer of a porcine retina sample, obtained by a laser scanning microscope using two-photon excitation.

Retinal imaging in the twenty-first century: state of the art and future directions

Assessment of chorioretinal disease is dependent on the ability to visualize pathologic changes occurring in the posterior segment of the eye using optical instruments, termed ophthalmoscopy. Ophthalmoscopy, in turn, has been enhanced greatly by the development of techniques that allow recording of these changes, termed retinal imaging. As well as documenting pathologic features, retinal and fundal imaging facilitates the identification of morphologic features not visible to the clinician on biomicroscopy. As such, advances in retinal imaging have proven fundamental to many paradigm shifts in our understanding and treatment of ocular disease. In the 1950s, with the advent of electronic flashes and 35-mm cameras, the field of modern fundus photography was born. Similarly, in the 1960s and 1970s, the introduction of fluorescein and indocyanine green angiography revolutionized our ability to assess the integrity of the chorioretinal vasculature. More recently, in the 1990s, the introduction of a wholly new form of noninvasive cross-sectional imaging, optical coherence tomography, has greatly facilitated use of emerging pharmacotherapies in diagnosing and monitoring chorioretinal disease. In this translational science review, we provide an overview of current, state-of-the-art retinal imaging technologies, as well as highlight many emerging imaging technologies that we believe are likely to transform the provision of eye care in the 21st century.