Download Imaging of Intraocular Foreign Body

Diagnostics
Diagnostics
Imaging of Intraocular
Foreign Body
Supriya Arora
MS
Supriya Arora MS, Richa Pyare MBBS, Prateeksha Sharma MBBS, Gauri Bhushan MS,
Meenakshi Thakar MD, FRCS, Basudeb Ghosh MD, MNAMS
Vitreoretinal Services, Guru Nanak Eye Centre, New Delhi
T
raumatic intraocular foreign bodies (IOFBs) are
a particularly significant and distinct subset of
open globe injuries, because of the increased risk for
endophthalmitis and toxicity by the IOFB material, as
well as the considerations specific to its surgical removal.
Because the diagnosis of traumatic IOFB encompasses
any foreign material from the environment that is found
within the walls of the eye as a consequence of an open
globe injury, presentation varies widely. The IOFB may
be unaccompanied by any significant intraocular damage
outside of its entry-site laceration in the eye wall, or may
be associated with massive internal damage in any or all
compartments of the eye.
The most common age group affected by IOFB injuries is
middle age (20-40 years) which is the most economically
productive age group and most injuries occur at work
place using various tools with metal striking metal such
as hammer and chisel. These particular aspects of IOFBs
should alert the ophthalmologist to their medico-legal
significance and all efforts must be made to ensure full and
detailed documentation of the diagnosis and management.
Diagnosis
High clinical suspicion combined with judicious use
of radio-diagnostic imaging is essential for prompt and
accurate diagnosis of retained IOFBs.
History should elicit how and when the penetrating trauma
occurred, and the material that penetrated. Slit lamp
examination can locate IOFB in anterior chamber and
inside the lens and also helps in picking up other suggestive
subtle ocular signs like a self-sealed corneal perforation.
Conjunctivo-scleral site of entry may be indicated by
localized subconjunctival haemorrhage or chemosis or
migration of uveal pigment to the surface. Other signs such
as an iris hole, localized cataract or defect in anterior and
posterior capsule of lens or a tract through the lens may
be visualized, a fibrous strand maybe visualized in the
anterior vitreous. Gonioscopy is required to diagnose IOFB
in angles. Indirect ophthalmoscopy localizes IOFB in the
posterior segment. In the presence of media haze due to
hyphaema, total cataract or vitreous haemorrhage, imaging
helps in diagnosis and exact localization of IOFB.
Electrical Induction Methods for Localization of
IOFB
The Berman and Roper-Hall localizers are purely of historic
importance. Based on the principle of induction, when the
instrument approached a metallic foreign body, a difference
in potential is created in the secondary circuit resulting in a
flow of current. An audio signal in the form of a continuous
sound is heard for an iron foreign body and a discontinuous
sound for a nonferrous metallic foreign body.
Imaging
X-Rays
Radiography is the first-step for metallic foreign body due
to its accessibility and low cost.
There are many methods of localization of IOFBs using
X-rays.
Direct Localisation
Two exposures at right angle (AP and lateral views) are taken
(Figure 1). For lateral view the affected side is towards the
film with infra-orbital line at right angles to the film. The AP
view or nose-chin position allows good view of maxillary
region since the bony shadow of petrous temporal bone is
excluded.
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Diagnostics
Ultrasound (USG)
It is the most common screening modality used these days
as it is non-invasive, inexpensive and easily performed. It
is 98% sensitive in detecting IOFBs in appropriate clinical
settings3,4.
Features of IOFB on USG
A scan
•
Steeply rising wide echo spike seen. It is noted along
the baseline between the initial spike and ocular wall
spike.
•
The reflectivity of the lesion spike is extremely high
(100%) which persists on low gain.
•
The distance between the IOFB and the adjacent sclera
is accurately measured at lower system sensitivity.
•
Sound attenuation is very strong.
B scan
•
It appears acoustically opaque contrasting with the
acoustically clear vitreous.
•
It remains displayed even when the system sensitivity
is decreased by 20-30 db.
•
Localization of the IOFB in different quadrants can
be determined. The proximity to adjacent intraocular
tissues is evaluated.
Methods depending on the Rotation of the globe
•
The head and the X-ray remain fixed, while the eye moves
in different directions, straight gaze, up and down. The
position of the IOFB is calculated from the direction and the
amount of displacement referred to the center of rotation of
the globe.
Mobility of the IOFB can be assessed. Topographic and
kinetic echography will show if the FB is adherent to
the retina or if it is floating in the vitreous.
•
Sound attenuation is very strong. The IOFB causes
shadowing of the ocular and orbital tissues behind
it as it totally reflects the sound beams preventing its
propagation within tissues behind it (Figure 2).
•
Associated intraocular damage like vitreous
haemorrhage, vitreous bands, fibrosis, retinal
detachment, choroidal detachment and even scleral
entry wounds can be assessed.
Figure 1: IOFB as viewed in AP view on X-Ray
Methods Using Radio-Opaque Markers
A metallic ring made of either silver or steel of suitable
diameter is sutured to the limbus. X-rays are taken in lateral
position, in the straight gaze, up and down gaze. An AP
view is also taken. The limbal ring is imaged as a straight
line corresponding to the limbus. Three such lines will be
seen corresponding to the three positions of the eyeball.
The position and movement of the IOFB in correspondence
to the limbal ring is then used to localize the IOFB1.
Other radio opaque markers such as contact lens with 4
radio opaque dots incorporated in it have been used.
However, plain radiography is not considered an adequate
modality. In one study, it failed to identify foreign material
in 60% of eyes with a known IOFB2, it does not show exact
localization of IOFB in relation to soft tissues and does not
detect non-metallic IOFB. In today’s setting, plain film x-ray
has a documentary and medico-legal role.
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Quantitative echography
The reflectivity of foreign body echo spikes is extremely
high. This special technique allows a comparison with
the scleral signal. A horizontal marker line is displayed
at a definite height. This line is kept in the same position
through out the procedure. The maximal lesion spike is
identified. The system sensitivity is then decreased until the
peak of the lesion spike just touches the horizontal marker
line without exceeding it. This is called ‘lesion sensitivity’.
The maximal scleral spike is displayed. The beam has to
bypass the IOFB to avoid shadowing. The system sensitivity
is then decreased until the peak of the scleral spike just
touches the horizontal marker line without exceeding it.
Diagnostics
•
If the IOFB is close to the ocular coats, it cannot be
made out well.
•
It cannot be performed in an open globe.
•
It is operator dependent.
Ultrasound Biomicroscopy (UBM)
Figure 2: USG B scan shows highly reflective mobile
appearing foreign body with back shadowing. On A
scan, the spike is persisting at low gain
It is an imaging technique that uses high frequency (50
MHZ) sound waves to produce high resolution, crosssectional images of anterior segment to a depth of around
5 mm. It can also visualize angle recession, cyclodialysis,
hyphema, scleral laceration, and lenticular foreign bodies.
High frequency (50 MHZ) ultrasound shows appearance
of foreign body, surrounding tissue, exact location, size
and the nature of IOFB much better than conventional
low frequency (10 MHZ) ultrasound5. UBM is useful
for detection and localization of small superficial nonmetallic foreign bodies that are usually missed on CT and
conventional USG (Figure 3).
Anterior Segment Optical Coherence Tomography
It has been used in identification of IOFBs along internal
lining of cornea, angle and iris.
Computerised Tomography (CT)
Conventional CT is regarded as the state-of-the-art method
for the detection and localization of metallic intraocular
foreign bodies, providing cross-sectional images with
a sensitivity and specificity that is superior to plain film
radiography and echography2-7.
Advantages
Figure 3: Composite ultrasound biomicroscopy
image shows a highly reflective intraocular
foreign body (arrow) in the inferior angle with
reverberation echoes seen posteriorly
This is called ‘scleral sensitivity’. The sensitivity of the two
systems is in decibels and their difference is also expressed
in decibels. A difference of 6 dB is characteristic of an IOFB
signal, no biological interface in the posterior segment of
the eye produces as strong a signal as the sclera.
•
Is safely done in severely traumatized patients or open
globe injuries.
•
CT with 1 mm sections (and no contrast) can detect
almost 100% of metallic IOFBs greater than 0.05 mm3,
although sensitivity may be lower for non-metallic
material6.
•
An exact localization of the foreign bodies is possible
due to cross-referencing of the multiple planes as
volume scanning allows reconstruction of images at any
position within the scanned volume in an overlapping
fashion8.
•
Delineation from surrounding soft tissues and
determination of shape in case of the metal foreign
body is improved as imaging artifacts are reduced in
the reconstructed coronal and sagittal planes8.
•
Localization with respect to sclera, lens and optic
nerve are well documented on CT in metallic, glass
and plastic foreign bodies7-9, although some report the
difficulties of locating glass foreign bodies located near
the crystalline lens as the radio-densities are similar10.
Disadvantages
•
•
It yields low-resolution images in near field and is
likely to miss the diagnosis of foreign bodies in anterior
segment, anterior orbit and deeply located foreign
bodies in the orbit.
Glass and vegetative matter (radiolucent) are more
challenging, but they also produce bright signals on
B-scan and tall reflective on A-scan.
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Diagnostics
repositioning that was required to image in more than
one plane.
•
CT has low sensitivity when used for organic IOFBs.
For example, wood has density similar to air and fat on
CT and can be difficult to distinguish from surrounding
soft tissue12.
•
CT is inferior to USG in the assessment of associated
vitreoretinal damage.
However, Spiral or Helical CT that has now replaced
conventional CT, the entire scan volume is imaged
continuously in one imaging plane (usually axial sections)
without a time gap in slice/image acquisition. This
provides short total scanning times and hence reduced
patient cooperation is required, minimized motion
artifacts, reduced radiation exposure and better multiplanar
reconstruction capability8.
Figure 4: Helical computed tomography axial image of a
metallic intraocular foreign body. Location of the metallic
foreign body (arrow) with respect to the optic nerve is welldemonstrated. The surrounding hyperdensity corresponds
to vitreous hemorrhage (small arrow)
•
Multiple foreign bodies can be detected and their
location with respect to each other determined.
•
Estimation of the composition of the foreign body on CT
is possible, metallic foreign bodies are seen as hyperdense structures with pronounced streak artifacts;
glass foreign bodies appear as oval-shaped structure of
similar density as that of cortical bone, without streak
artifacts. In contrast to both the metal and glass foreign
bodies, the density of the plastic foreign body is lower
than that of cortical bone7.
•
Evaluation of size by distance measurement in CT
leads to overestimation when performed on soft tissue
window displays and improves when performed
on bone window displays in both the helical and
conventional CT studies8.
•
CT is not operator dependent like ultrasonography.
•
In patients with visual loss after injury, CT may help
determine the potential for reversibility. If the optic
nerve is partially or completely avulsed, aggressive
measures are not indicated11 (Figure 4).
Disadvantages
•
Patient cooperation is required.
•
It is an expensive procedure.
•
Radiation exposure is greater than in other imaging
techniques.
•
Motion artefacts due to long scanning times and
54 l DOS Times - Vol. 19, No. 9 March, 2014
MRI
It is not a general screening tool for retained IOFBs.
Advantages
•
Wood and plastic are almost always detected. T1weighted images demonstrated wood better than T2weighted images and required less scanning time than
either proton density or T2 - weighted images13. MRI
shows well-delineated, low intensity lesions in these
cases.
Disadvantages
•
MRI employs strong magnetic forces hence torsional
forces are applied to ferromagnetic substances, which
can lead to sudden movements in metallic foreign
bodies leading to further destruction of ocular structures
and is therefore only used once metallic foreign bodies
have been ruled out on CT14.
•
Severe artifacts prevent diagnosis of iron, glass and
graphite on MRI13.
Even in the cases where IOFBs are well visualized clinically,
appropriate imaging must be done to rule out multiple
IOFBs and for medico - legal documentation. Estimation
of size and nature of IOFB along with exact localization
will also help in planning the surgical approach employed
in removal of IOFB. In cases of severely traumatized open
globe, CT scan must be done to confirm the presence
or absence of foreign bodies, their anatomic location
and integrity of orbital and ocular structures to allow an
appropriate surgical approach. Judicious and well planned
out use of the various radio-diagnostic modalities at our
disposal is essential for optimal management of IOFBs.
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