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Fundus Autofluorescence Imaging

Fundus autofluorescence (FAF) imaging using a confocal scanning laser ophthalmoscope (cSLO) was initially described by von Rückmann et al. in 1995. Pioneering work on the spectral analysis of the origin of the autofluorescence signal was performed by Delori and coworkers in parallel. In 2003, Spaide introduced a modified fundus camera (using longer wavelengths as originally applied) for FAF imaging. Further, near-infrared autofluorescence imaging has been described.

Today, fundus autofluorescence imaging is the gold standard for detection of atrophic areas and identification of prognostic marker in atrophic age-related macular degeneration and other retinal diseases. Semi-automated image analysis software allows for a precise, reliable and robust quantification of atrophy and its progression over time.

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What is FAF?

Fundus autofluorescence imaging is an in-vivo imaging method for metabolic mapping of naturally occurring fluorophores of the ocular fundus. The dominant source are fluorophores, such as A2-E (N-retinylidene-N-retinylethanol-amine) in lipofuscin, that accumulate in the retinal pigment epithelium (RPE) as a byproduct from the incomplete degradation of photoreceptor outer segments. The topographic distribution of FAF intensities is altered in the presence of excessive accumulation or loss of lipofuscin/RPE cells. Additional intrinsic fluorophores may occur with disease in the outer retina and the subneurosensory space. Minor fluorophores, such as collagen and elastin in choroidal blood vessel walls, may become visible secondary to RPE atrophy. Finally, pathological alterations in the inner retina at the central macula, where the FAF signal is usually partially masked by luteal pigment (lutein and zeaxanthin), may result in manifest variations in FAF intensities.

Recording of FAF is easy, requires little time, and is noninvasive. FAF signals are emitted across a broad band from 500 to 800 nm. Excitation is usually induced in the blue range (λ = 488 nm), and a barrier filter above 500 nm is used to detect emission of the autofluorescence signal (Holz FG et al., Am J Ophthalmol 1998;125:227). The intensity of the signal is about 2 orders of magnitude lower than the background of a fluorescein angiogram. Further difficulties in detecting FAF result from absorption of the excitation light by and autofluorescence properties of anatomic structures anterior to the retina, including optical media and in particular the crystalline lens. As a result, adjustments and modifications of existing camera systems or sophisticated new imaging devices are required to record FAF. Confocal scanning laser ophthalmoscopy (cSLO) addresses the limitations of low intensity of the FAF signal and the interference of the crystalline lens and represents the state-of-the art imaging technologies for FAF recordings. The confocal optics, together with the enhanced sensitivity of the cSLO and averaging of several individual scans following automated alignment, ensure the acquisition of meaningful images in most patients.

 

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