AMD updated - page 83

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Fundus autofluorescence in age-related macular degeneration
2.3 Fundus camera
Fundus cameras are widely used in clinical routine for
imaging the retina as fundus photographs, reflectance
photographs and fluorescein angiography.
Fundus cameras use a single flash to capture images
from large retinal areas. When confocal optics are not
available, the autofluorescent signal from all the ocular
structures with fluorescent properties reaches the cam-
era, and scattered light, anterior and posterior to the
plane of interest can influence the detected signal (25-27).
The lens contributes significantly to the autofluores­
cent signal when similar wavelengths are used in the
blue-range, as for the cSLO ( λ = 488 nm), particu-
larly in older patients with lens yellowing and nuclear
opaci­ties. Flashlight intensities and detector gain have
to be set at relatively high levels in order to obtain
reasonable FAF images. However, the signal-to-noise
ratio decreas­es simultaneously which may result in
reduced image quality. To reduce interference from
lens fluorophores which mainly emit in the range
between 510 to 670 nm, Spaide modified the exci-
tation filter (peak 580 nm, bandwidth 500-610 nm)
and the barrier filter (peak 695 nm, bandwidth 675-
715 nm). A further modification was introduced in
2007 using a slightly different filter set (excitation
bandwidth 535-580 nm, emission band­width 615-
715 nm)(28), thus improving signal-to-noise ratio and
image quality. Furthermore, as this setup of the fundus
camera uses different excitation and emission fil­ters
compared with the cSLO, it may even visualize other
retinal fluorophores. However, a systematic compari-
son of different pathologies with clinico pathologi-
cal correla­tions between cSLO and fundus camera,
particularly in patients with AMD, has not yet been
performed. Origi­nally, a fundus camera that enabled
imaging with a field of 13º was used. Recently, Spaide
has obtained images of the spatial distribution of FAF
intensities over larger retinal areas up to 50º with his
new modified fundus camera(25-28).
In the near future we may improve FAF imaging with
the aid of scientists and investigators developing filters
and some other innovations, and increased experience.
Furthermore, it is already possible to visualise differ-
ent fluorophores from the retina with the configura-
tion of the fundus camera using excitation and emis-
sion filters for the cSLO. A systematic comparison of
clinical images with different pathologies obtained by
the cSLO and the fundus camera, (especially in AMD
patients) has not been performed yet.
3. Autofluorescence imaging in the
human eye in vivo
FAF images show the spatial distribution of the intensity
of autofluorescence of each pixel in grey values (arbitrary
values from 0 to 225); low intensities are commonly
known as low pixel values (dark) and high intensities as
high pixel values (light).
3.1 Normal fundus
FAF imaging shows a consistent pattern of autofluo­
rescence distribution in normal eyes(21). Such common
findings have been reported in children as young as
four years old(29). The macular FAF signal is reduced at
the fovea because it is limited by the presence of lutein
and zeaxanthin in the neurosensory retina. The signal
is high­er in the parafoveal area and tends to increase as
we move away from it, peaking at the most peripheral
retinal ar­eas. It has been suggested that this FAF pat-
tern is caused by the melanin deposition and density of
LF granules at the different areas of the retina(18,30). The
optic nerve head typically appears dark mainly due to the
absence of RPE. The retinal vessels are associated with
a markedly reduced FAF signal because of the blocked
fluorescence (Fig.1).
The common ratios of grey intensity between the fovea
and the perifoveal area have been established(31,32). Con­
sidering these findings, any deviation from the normal
pattern in a specific location can be easily identified;
hence the qualitative description of the local changes
in the FAF is widely used. The changes in signal inten­
sity are qualitatively described as decreased, normal, or
increased as compared to the background signal of the
same eye.
3.2 FAF imaging in AMD
When examined with autofluorescence, the fundus
of patients with AMD may show a range of signal
chang­es(20,33-37) . Assuming that RPE has an important
role in the pathophysiology of AMD and that the major
fluoro­phores in the retina are located within RPE cells,
FAF imaging can show changes in the concentration and
distribution of RPE LF and hence establish the condi­
tion of RPE in patients with AMD. Therefore, atrophic
RPE typically appears as dark patches in FAF and can
be clearly delineated, even better than in normal fun-
dus photograph(21,38), (Fig. 2). All this information can