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Fig. 26.16 Tubercular choroidal granuloma: hypouorescence seen corresponding to the lesion in mid-phase ICGA
A. Kulkarni et al.
(e) Serpiginous choroidopathy: On ICGA, the
26.4 Conclusions
active edges of the lesion appear hypouorescent [39]. Late hyperuorescence is a
marker of choroidal hyperpermeability and
may indicate a more rapid progression of the
illness. The boundaries of the healed lesions
are clearly dened and appear hypouorescent. The pigmentation of the lesion edges
often appears dark on ICGA due to blocked
uorescence. At the same time, the atrophy
Dye-based angiography can give detailed information about the shape, extent, and vascularity
of most retinochoroidal pathologies effectively
and affordably. It is invaluable in establishing
diagnosis, prognosticating, and managing several diseases, making it one of the most commonly used tools in an ophthalmologist’s
armamentarium.
of the RPE and choriocapillaris enables the
visualization of the underlying large- and
medium-sized choroidal arteries [7].
Funding Hyderabad Eye Research Foundation,
Hyderabad, India.
(f) Choroidal granulomas: Choroidal granulomas
can be seen in both primary and secondary
stromal choroiditis like Vogt-KoyanagiHarada disease and tubercular and sarcoid
uveitis. Choroidal granulomas displace cho-
Disclosure TYYL: Consultant: Allergan, Bayer, Boeh-
ringer Ingelheim, Iveric Bio, Novartis, Oculis, Roche;
Speaker: Alcon, Bayer, Chendu Kanghong Biotech,
Novartis, Roche.
roidal vessels and block ICG dye from lling
the area of the choroidal stroma occupied by
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ICGA, they present as hypouorescent round
lesions. The granulomas gradually get surrounded by the dye in the late phases, making
them less prominent [7]. After therapy, these
gradually shrink and eventually disappear,
which can be better identied and monitored
by ICGA [40].
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Paediatric Wide-Field Retinal
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Imaging
27
SouravPal
27.1 Introduction
The use of wide-eld imaging on paediatric subjects is primarily targeted at but is not restricted
to the detection of retinopathy of prematurity
(ROP). Early detection of the disease at stage-1,
which appears as a thin demarcation line on the
retina between the vascular and avascular region,
can largely reduce the risk of severe vision loss.
Therefore, for nding signatures of ROP at its
early stage, imaging of the extreme peripheral
regions around the ora serrata becomes far more
important than imaging the central part of the
retina.
27.2 History andStages ofROP
The disease was more apparent with the invention
of incubators in the early 1940s. ROP was rst
reported in 1942 and identied as a cause of
excess oxygen concentration inside incubators. In
1949, Owens and Owens [1] described the early
stages of ROP and its progression. With the
advancement of incubators and the increased
number of premature survivals, cases of vision
loss due to ROP signicantly increased by the
1950s. The rst attempt to deal with this problem
S. Pal (*)
Forus Health Pvt. Ltd., Bengaluru, India
e-mail: sourav@forushealth.com
universally was made by Reese etal. [2] by classifying the disease in 1953. More than 30years
went by, and many such attempts were made [3–
6] before a group of 23 ophthalmologists from 11
different counties collaborated to propose the
International Classication of Retinopathy of
Prematurity (ICROP) in 1984 [7] to dene protocols for ROP studies. The third edition, published
in 2021, is currently used for ROP screening [8].
In all practical cases, the state of vascularization is
investigated, classied, and recorded in multiple
circular zones with the optic disc at the centre.
The central zone or Zone-I is the circular area
around the optic disc with a radius twice the distance between the optic disc and macula (d).
(Fig.27.1). Zone-I boundary makes an angle of
approximately 45° at the centre of the eyeball [9]
(Fig. 27.2). This slightly varies for premature
babies with postmenstrual age (PMA) due to
rapid changes in the eyeball size [9].
Zone-II is the annular region concentric to
Zone-I and adjacent to it with an extension till
nasal ora serrata. The boundary of Zone-II closely
matches the equator [10] (Fig.27.2), at least on
the nasal side. The Zone-II boundary makes an
angle between 180°and200° at the centre of the
eyeball. Additionally, ICROP denes a narrow
annular zone inside Zone-II. This is named
Posterior Zone-II, adjacent to Zone-I, and has a
thickness twice the diameter of the optic disc. In
ICROP, this is marked as an area prone to more
severe disease than ROP.
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024
T. Das, P. Satgunam (eds.), Ophthalmic Diagnostics, https://doi.org/10.1007/978-981-97-0138-4_27
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Fig. 27.1 Schematic representation of three zones and other features of the left (LE) and right (RE) eyes
Optical Inc., Mentor, USA), a portable non-contact digital fundus camera with an FoV of 45°. In
a study, Prakalapakorn et al. [12] could image
only Zone-I and little beyond that. Results show
the possibility of detecting pre-plus or plus diseases [13], characterized by dilated and tortuous
arterioles near the posterior pole. But it failed to
capture signatures of an early stage of ROP.Hence,
a wide-eld imaging system, which gives exibility to the user and allows imaging up to the ora
serrata with no or little tilt in the device, remained
the rst preference over the years.
S. Pal
Fig. 27.2 Different zones and FoV at the centre of the
eye
Zone-III is the crescent-shaped retinal area on
the temporal side of Zone-II.By Toslak etal. [10]
and Atkinson [11], the far extent of Zone-III
makes an angle of 230° at the centre of the
eyeball.
Therefore, it is evident that a conventional fundus camera system with a eld of view (FoV)
between 40°and50° can image up to Zone-I normally. An odd tilt in the device is needed to cover
Zone-III and image ora serrata. A similar attempt
was made with the Pictor imaging system (Volk
27.3 Wide-Angle Imaging
Technology
A binocular indirect ophthalmoscope (BIO) is the
gold standard for ROP screening. But it necessitates an experienced ophthalmologist. Screening
using BIO is generally time- consuming. Thus,
ROP screening with BIO is less practical for developing countries with poor patient-to-physician
ratios; therefore, there is a need for wide-angle
digital cameras. The wide- angle camera facilitates
photo-document a larger area of the retina in a
lesser time for future reference. Additionally, a
minimally trained technician with a wide-angle
ab
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349
camera can participate in an ROP screening programme. Telemedicine- based ROP screening is
effective too [14–16]. It overcomes the barrier of
poor patient-to- physician ratio and reaches out to
larger populations to ensure timely screening and
treatment wherever necessary. Wide-angle imaging system with digital image recorder is an inseparable part of the programmes.
Fig. 27.3 System architecture adopted by RetCam,
PanoCam, and Icon
27.3.1 Currently Available WideAngle Cameras
RetCam by Natus Medical Inc. (Middleton,
USA), previously known as Clarity Medical
System Inc., is the rst paediatric retinal camera
with FoV from 30 to 130° through changeable
front attachments. Many other manufacturers,
like Forus Health Pvt. Ltd. (Bangalore, India),
Phoenix (Pleasanton, USA), and Visunex Medical
Systems (Fremont, USA) subsequently introduced wide-eld imaging devices, named 3nethra
neo, Icon and PanoCam, respectively. Devices
like RetCam, PanoCam and Icon build on a trolley, as shown in Fig. 27.3. Probe consists of
imaging and illumination optics along with an
imaging sensor. All of them has integrated display and computing system. Control unit generally holds light sources and other control circuits
that establish a communication between software
and imaging system. All of them uses a footpedal to control light intensity and focus on the
retina as and when needed. PanoCam probe has
the distinctive quality of operating wirelessly.
Conversely, 3nethra neo has a compact control
unit that holds the probe, which contains light
source, illumination, and imaging optics. It gets
connected to and uses a laptop/PC as display and
computing system. Figure 27.4 shows an
upgraded version of 3nethra neo, which facilitates uorescein imaging is named as 3nethra
neo-HD FA.
Table 27.1 presents various models offered by
the above four manufacturers and their comparison. Despite diversied features, many similarities in basic structure exist. All systems use white
Fig. 27.4 3nethra neo-HD FA by Forus Health Pvt. Ltd.; (a) probe and its control unit, (b) imaging unit/probe
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Table 27.1 Presents various models offered by the above four manufacturers and their comparison
Device Manufacturer FoV Imagesize Portability FA
RetCam shuttle Natus Medical 130° VGA Heavy No
RetCam 3 130° 2 MP Heavy Ye s
RetCam envision 130° 1.5 MP Heavy Yes
3nethra neo Forus Health 120° 4 MP Compact No
3nethra neo-HD FA 150° 20 MP Compact Ye s
Panocam LT Visunex Medical Systems 130° 8 MP Heavy –
Panocam Pro 130° 13 MP Heavy –
Icon Phoenix 100° – Heavy –
MP megapixel; VGA video graphics array
S. Pal
ring-shaped light to illuminate the retina, operate
in contact with the cornea, and obtain true colour
images.
27.4 Technology
We describe the technology using a camera system made in India. But basically, the technology
is similar across currently available cameras.
27.4.1 Optical Design ofImaging
System
Most of the wide-angle fundus cameras in paediatric imaging works in contact with the cornea to
avoid an increase in lens diameter abnormally.
Generally, a meniscus lens is selected as the contact lens. The radius of the contact surface is generally selected between 6 and 7mm to match it
with the corneal curvature of the baby. Optical
components that follow contact lens act like a
relay system; these generally form an intermediate real image of the retina before carrying it forward and shaping it appropriately for the selected
sensor. The imaging system of the 3nethra neo HD FA (Forus Health, Bangalore, India) is schematically shown in Fig.27.5. Imaging systems of
wide-angle cameras have strategically similar
designs but differ in how they distribute the optical power.
There are two groups of relay lenses.
Relay lens Group-1 forms an intermediate
image and guides rays towards the aperture of the
system. The 3nethra neo-HD FA uses a transpupillary illumination scheme. The aperture is
placed at a plane conjugate to the pupil of the eye
so that the entrance pupil of the imaging system
coincides with the pupil for reex-free imaging
(see Sect. 27.4.3.1).
Relay lens Group-2 determines the nal image
size suitable for the camera/sensor in use. Another
job of it is balancing residual aberration from the
previous group of lenses. The 3nethra neo-HD
FA uses a tunable focus lens to make the necessary correction aligned with the changes in the
refractive power of the eye. An electronically
controlled tunable focus lens enables a smooth,
vibration-free operation. Without such lenses, a
physical movement of Group-2 lenses is needed
for focus. Although contact and Group-1 lenses
decide the FoV, Group-2 lenses play an important
role in image formation without vignetting.
27.4.2 Understanding ofField
ofView
Understanding and expressing the FoV correctly
is critical when working with wide-angle fundus
cameras. Historically, the FoV is dened as the
angle subtended by the front optics of any imaging system at the pupil of the eye (Fig.27.6a).
This is usually named as external angle, θex [11].
An oversimplied expression, θin = θex/1.33 for
calculation of internal angle θin is sometimes useful but not accurate in all practical cases. A ray of
light from the retina follows a multifold path
through the vitreous humour, crystalline lens, and
xe
=+
()
()
−
n/
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Fig. 27.5 Schematic of 3nethra neo-HD FA imaging system
351
Fig. 27.6 Illustration of external angle, internal angle, and eye angle
aqueous humour, having refractive indices of
27.4.3 Illumination System ofWide-
1.33, 1.42, and 1.33, respectively, before exiting
the eye. Finding the nodal point of the eye and
mapping the external angles in relation to it is,
therefore, benecial for greater precision. The
eye angle ,θe, is the angle subtended by the retinal
area being imaged at the centre of the eyeball.
Modern-day wide-angle cameras use θe to spec-
Fundamentally, there are two types of illumination systems: (1) trans-pupillary—uses a clear
window of the pupil of the eye to throw light onto
the retina, and (2) trans-scleral—utilizes sections
of the sclera to deliver light to the retina.
ify the FoV of the system.
Figure 27.6b schematically presents θex and θe,
27.4.3.1 Trans-pupillary Illumination
in reference to the nodal point of the eye N, which
is very close to the posterior surface of the crystalline lens [17]. With these factors considered,
Yao etal. [18] arrived at the following relationship between the two angles.
1
2051 2
ee
sin.si
x
As per the above expression, an external angle
of 50° is equivalent to an eye angle of 74.89°,
which closely matches the result presented by
Atkinson [11]. As per Atkinson and Mazo,
θe=1.48θex, which is also effective in many prac-
tical cases.
A trans-pupillary illumination system is the most
widely used illumination scheme for retinal
imaging [19] (Table27.1). It complies with the
Gullstrand principle [20–22], which states that
the lighting and imaging channels must be separated near the pupil, from the corneal surface to
the crystalline lens, for glare-free retinal imaging.
In general, the illumination beam enters through
an annular zone near the margin of the pupil, and
the light scattered back from the retina is collected through the central part of the pupil
(Fig. 27.7) [19, 22]. Fig. 27.7(b) shows the
Angle Cameras
System
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S. Pal
a
Fig. 27.7 A schematic of trans-pupillary illumination
that follows the Gullstrand principle. (a) The illumination
beam enters through an annular zone near the margin of
arrangement in the 3nethra neo-HD FA. A
smaller diameter of the ring illumination system
leaves little space for the imaging beam, which
can severely affect the optical resolution and light
throughput. Given that a larger ring diameter will
require a larger pupil for better imaging, a judicial selection of ring diameter is very important.
To make the system suitable for a wider age
group, it must be designed considering the rapid
structural changes in the eye, and hence the
nature of light reex, from premature to infants.
RetCam uses a bundle of optical bre arranged
in the shape of a ring to generate a ring light. The
other end of the bre bundle is fused and is used
as a light collector from a halogen source. In contrast, 3nethra neo or 3nethra neo-HD FA uses a
waveguide structure made of acrylic material to
shape desired ring light. It uses an array of LEDs
arranged on an annular PCB (printed circuit
board) as a light source, placed near the base of
the waveguide structure. Light gets coupled into
the waveguide, bounces back from the walls due
to total internal reection, and propagates towards
the circular output end of it.
27.4.3.2 Trans-scleral Illumination
Trans-scleral illumination is an alternative to the
trans-pupillary illumination scheme that utilizes
the pupillary area fully for imaging [10, 23–28].
The history of trans-scleral illumination can be
traced back to the early 1970s when Pomerantzeff
et al. [29, 30] imaged the retina by delivering
b
the pupil; the light scattered back from the retina is collected through the central part of the pupil. (b) The tip of
the imaging probe in 3nethra neo-HD FA
light through the palpebral region of the sclera.
Named, Equator-plus, it had an extended FoV to
capture the retina from anterior to the equator,
nearly 148°.
In 2002 Panoret–1000, a trans-scleral illumination-based system, was introduced. It could
obtain images of the optic disc and ora serrata
together in a single frame. But the clinical deployment of this device was not successful. Unlike
the Equator-plus camera that delivers light
through the eyelid, the illumination system of
Panoret–1000 requires scleral contact increasing
the risk of contamination, inammation, and
abrasion to the sclera and cornea. Currently,
Panoret–1000 is commercially not available.
Pars-plana (a Latin phrase for Flat area) is a
at portion of sclera 3–4mm posterior to the limbus with a width of ~4mm. Pars-plana region has
a comparatively lower density of muscle, blood
vessels, pigmentation, and consequently more
transparent than other parts of the sclera.
Therefore, scleral illumination through the parsplana region is of utmost interest to recent investigators. Figure 27.8 schematically represents a
trans-pars-scleral illumination scheme, where
generally, a bre-optics illumination probe is
placed over the sclera on the pars-plana region to
produce a diffuse illumination over the retina.
Thus, the entire pupil area is available for imaging without any possibility of crosstalk with the
illumination beam. The transitive pars-plana area
reduces the risk of any physical hazard by
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353
enabling suitable light levels on the retina with
little input energy.
The primary benet of trans-scleral lighting is
the ability to use the entire pupil area for imaging. This allows for some degree of wide-angle
imaging even without pupil dilation. Additionally,
it gives freedom to the designer to plan optical
systems with higher resolution. Wide-angle cameras of Table27.1, having a eld of view around
130° offer an optical resolution ranging between
20 and 35lp/mm at the centre of the eld.
The biggest disadvantage of trans-scleral illumination is its usability and, in some cases, associated safety issues. This type of illumination
Fig. 27.8 Schematic representation of trans-pars-planar
illumination scheme
system is not tightly coupled with the imaging
system. Two independent movements by two
hands are needed to capture an image. Despite
many advantages, trans-scleral illumination
probably is not very popular till it becomes more
user-friendly.
27.4.4 Fluorescein Angiography
Most state-of-the-art wide-angle fundus cameras
can perform fundus uorescein angiography
(FA). FA is a tool for investigating vascular
changes not evident in a colour fundus image.
The development of digital imaging in the late
twentieth century made FA easier and more practical, and it has become an indispensable part of
ROP treatment [31]. Currently, FA is invariably
performed before laser photocoagulation to map
the retinal vascularization more precisely.
In FA, retinal images are captured with the
time of acquisition offset from the injection of
Sodium uorescein (NaFl) dye. NaFl absorbs
energy in the blue and emits it in the green region
of the visible spectrum [32]. A typical absorption
and emission spectrum of the NaFl is shown in
Fig.27.9. It has peak absorption at 495nm, which
is pH dependent and slightly shifts towards the
left when injected into blood [33]. NaFl emits
light in the approximate range of 515–600 nm,
with a peak at 540nm.
The source is typically selected with a peak
around 480nm. A typical spectrum of the source
a
Fig. 27.9 Spectrum associated with uorescein angiography (a) absorption and emission spectrum of sodium uores-
cein; (b) spectrum of illumination and imaging beam after going through optical lters
b
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