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Real-Time Optical Coherence Tomography of the Anterior Segment at 1310 nm
Sunita Radhakrishnan, MBBS, DO;
Andrew M. Rollins, PhD;
Jonathan E. Roth, BS;
Siavash Yazdanfar, MS;
Volker Westphal, MS;
David S. Bardenstein, MD;
Joseph A. Izatt, PhD
Arch Ophthalmol. 2001;119:1179-1185.
ABSTRACT
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Background Recent advances in high-speed scanning technology have enabled a new
generation of optical coherence tomographic (OCT) systems to perform imaging
at video rate. Here, a handheld OCT probe capable of imaging the anterior
segment of the eye at high frame rates is demonstrated for the first time.
Objective To demonstrate real-time OCT imaging of anterior segment structures.
Design Survey of anterior segment structures in normal human subjects.
Setting Laboratory.
Main Outcome Measures Achieving real-time imaging of the anterior segment, satisfactory image
quality, and convenience of a handheld probe.
Results Optical coherence tomographic imaging of the anterior segment of the
eyes of human subjects was performed using 1310-nm wavelength light with an
image rate of 8 frames per second. Imaging trials demonstrated clear resolution
of corneal epithelium and stroma, sclerocorneal junction, sclera, iris pigment
epithelium and stroma, and anterior lens capsule. The anterior chamber angle
was clearly visualized. Limited imaging of the ciliary body was performed.
Real-time imaging of pupillary constriction in response to light stimulus
was also performed.
Conclusion High-speed OCT at 1310-nm wavelength is a potentially useful technique
for noninvasive assessment of anterior segment structures.
Clinical Relevance Our results suggest that real-time OCT has potential applications in
glaucoma evaluation and refractive surgery.
INTRODUCTION
THERE ARE many situations in clinical ophthalmology that require precise
understanding of the spatial relationships and dimensions of various structures
in the anterior segment of the eye. A technique that is capable of anterior
segment imaging with micrometer scale resolution would be valuable in obtaining
such information, enabling superior delineation of anterior ocular morphology
and highly precise biometry. In clinical situations such as refractive corneal
surgery and glaucoma evaluation it would be desirable if high-resolution images
of dynamic events could be obtained and displayed as they occur in real time.
Several techniques have been reported for high-resolution noninvasive
or marginally invasive examination of the anterior eye. Ultrasound biomicroscopy1-2 provides resolution ranging from 20
to 60 µm with a depth of penetration of about 4 mm and can obtain images
of structures concealed by opaque media. Imaging time is 8 frames per second,
allowing in vivo observation of movements of ocular structures in real time.
However, this technique requires immersion of the eye in a water bath solution
and it is difficult to determine the exact location of the examined area.
Confocal microscopy3-4 has been
used for high-resolution imaging in transparent tissues of the anterior segment
and provides en face images of the corneal layers. Video-rate imaging has
been achieved with scanning slit confocal microscopy,5
which has a lateral resolution of 0.8 µm and scans optical sections
that are 10 µm in thickness (with a x50 objective lens). The technique
requires the use of an index matching gel to optically couple the tip of the
microscope objective to the cornea. Also, micrometer scale resolution measurements
using confocal microscopy are limited to the cornea. Optical interferometric
techniques6-8 have
been described for ocular biometry, and continuous corneal pachymetry9 has been demonstrated with optical low-coherence reflectometry.
Image information obtained from these techniques is, however, limited to one
dimension. Magnetic resonance imaging using special surface receiver coils
has achieved an in vivo intraocular imaging resolution of 230 µm in
human eyes,10 with an acquisition time of 6.5
minutes per sequence. Recently, cryogenically cooled surface coils have been
used in anesthetized rabbits to obtain an intraocular resolution of 117 µm.11 To obtain magnetic resonance images of good quality,
patients have to maintain eye and head positions for the duration of imaging,
and chemical-shift artifacts have to be considered before interpretation of
the images. This technique may be useful for elucidation of physiologic or
pathologic disease mechanisms; however, it is impractical in many clinical
situations.
Optical coherence tomography12 (OCT)
is a high-resolution imaging modality that can overcome many of the limitations
of current techniques used to image the anterior eye. It is a completely noninvasive
technique that uses low-coherence interferometry to provide in vivo cross-sectional
images of tissue structure with a spatial resolution of 10 to 20 µm.
Optical coherence tomography has been predominantly used for imaging of the
posterior segment of the eye.13-15
Anterior segment imaging using OCT was first demonstrated in 199416 using light with a wavelength of 830 nm. In recent
studies OCT has been used to evaluate anatomic outcomes of refractive surgery17 and has also been implemented as a slitlamp-adapted
system for routine clinical examination of the anterior segment.18
Other studies of the anterior segment include comparison of in vivo OCT imaging
of cataracts with histopathologic findings19
and the use of OCT for in situ monitoring of laser interactions in biological
tissue.20 Recently, transcleral OCT with 1310-nm
wavelength light21 has been described. However,
all of these OCT systems have been relatively slow, with acquisition times
of 1 to 5 seconds per image. A system capable of imaging in real time would
reduce misalignment and patient motion artifacts and enable imaging of dynamic
ocular events. The requirements for real-time imaging with OCT are that scans
must be performed, processed, acquired, and displayed quickly and the optical
source power must be increased with the frame rate so that the signal-to-noise
ratio is maintained. High-speed OCT at 4 to 32 frames per second has already
been achieved.22-23 Near real-time
OCT systems for endoscopic imaging of the gastrointestinal tract are currently
in clinical trials.24-25 We have
developed a high-speed (4-16 frames per second) OCT system coupled to a handheld
probe, suitable for ophthalmic examination. To image the anterior segment
at a high frame rate we used high-speed Fourier domain optical depth scanning
technology and an efficient interferometer design described previously.22, 26 To enable higher optical power incident
on the eye, we used a semiconductor optical amplifier light source operating
at 1310-nm wavelength. In this wavelength region, the transmittance of ocular
media is reduced27 such that higher-power illumination
may be used without danger to the eye.28 The
added advantage of using illumination at this longer wavelength is that the
amount of scattering in ocular tissue is less than at 830 nm. The reason for
this is that absorption and scattering in most tissue constituents is a decreasing
function of wavelength in the near infrared spectrum29
whereas absorption in water (the primary constituent of vitreous humor) increases
sharply, being approximately an order of magnitude higher at 1300 nm than
at 830 nm. Thus, using 1310-nm rather than 830-nm illumination for anterior
segment OCT allows for increased penetration in scattering tissues, such as
the sclera and iris, while simultaneously permitting sufficient illumination
power to be used to enable high-speed imaging. In this article we demonstrate
the application of high-speed OCT to perform real-time cross-sectional imaging
of the anterior eye in vivo for the first time to our knowledge. Real-time
visualization of the anterior eye was achieved, with better morphological
detail than previously possible.
PATIENTS AND METHODS
Our study was approved by the institutional review board of the University
Hospitals of Cleveland. All subjects underwent scanning after giving informed
consent. Five healthy volunteers with clear ocular media and uncorrected or
best-corrected visual acuity of 20/20 were examined. One subject wore soft
contact lenses.
The real-time OCT system used in this study was similar to a system
we have previously reported for high-speed endoscopic imaging in the gastrointestinal
tract.22, 25-26 The
system employed a semiconductor optical amplifier light source capable of
emitting 22 mW of low-coherence light with a central wavelength of 1310 nm
and a spectral bandwidth of 68 nm full width at half maximum. The source had
a free-space coherence length of 11.1 µm; thus the imaging depth resolution
was 8.1 µm (following division by an assumed tissue refractive index
of 1.38).30 High-speed scanning was achieved
using a rapid scan Fourier-domain delay line, which generated longitudinal
depth reflectivity scans (A scans) at a rate of 4 kHz. These A scans were
subdivided into B scans according to the desired image rate. The system was
further optimized for high-speed operation by employing a novel power-conserving
interferometer design using an optical circulator26
and dual-balanced detection for excess noise reduction.25
Image data were displayed in real time on a computer monitor and simultaneously
archived to high-quality (S-VHS) videotape for later review. Selected image
frames could be frozen by the operator and saved in digital format. Computer
"hot-keys" allowed for selection of gray or false-color display scale, frame
averaging, image freezing, and saving of frozen images.
A novel aspect of the OCT system developed for anterior segment imaging
is the use of a handheld OCT probe (Figure
1). The probe employs a miniature computer-controlled galvanometer
scanner to provide a single axis of lateral scanning of the 1310-nm light
on the subject. The probe optics transfers a magnified image of the sample
arm fiber tip into the anterior segment while simultaneously imaging the galvanometer
mirror plane into the plane of the objective lens to avoid vignetting during
galvanometer mirror motion. The probe provides lateral scanning of up to 5
mm at imaging rates adjustable between 4 and 16 frames per second, and has
a focal spot size of 15 µm full width at half maximum.
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Figure 1. Schematic diagram of the handheld
probe used for imaging of the anterior segment. The optical coherence tomographic
system allows for real-time imaging (4-16 frames per second) with a pixel
resolution of 8 µm (depth) x 15 µm (lateral) over a field
of view of 4.25 mm of free-space depth (3.08 mm in tissue) by 5 mm of lateral
range.
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Figure 2 illustrates the use
of the handheld probe for anterior segment imaging. The examination was done
with the subject in the sitting position. The OCT probe was held with both
hands as shown in the figure, with the examiner's fingers resting on the subject's
face for support. Alternatively, in some cases, the subjects held the probe
themselves. The distance between the probe and the eye being examined (the
working distance) was 10 mm. Scanning was done by placing the probe close
to the area of interest and observing the resulting image on the screen. Different
structures in the anterior segment were viewed by either moving the probe
itself or by asking the subject to move his or her eyes. Because of the fast
acquisition rate of real-time OCT, involuntary changes in eye position did
not cause deterioration of image quality. Thus, image processing for removal
of A-scan artifacts (as performed in a previous study of low-speed anterior
segment OCT16) was not required. Subjects were
allowed to blink during the examination.
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Figure 2. The use of real-time optical coherence
tomography (OCT) for anterior segment imaging. The OCT probe was held approximately
1 cm from the subject's eye, and was positioned interactively while viewing
the image on the computer screen. Computer "hot-keys" provided capabilities
for image freezing, archiving, averaging, and color/gray-scale selection.
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The optical power used in our 1310-nm OCT system was in accordance with
the American National Standards Institute maximum permissible exposure of
15.4 mW for intrabeam viewing through a 7-mm pupil for exposure times of up
to 8 hours.28 This is substantially higher
than the maximum permissible exposure of 700 µW at 830 nm, the wavelength
used in previous OCT systems. The maximum permissible exposure is much higher
at 1310 nm because transmittance of the ocular media is less by approximately
an order of magnitude at this wavelength than it is at 830 nm.27
RESULTS
Healthy subjects were examined with the high-speed OCT system using
a handheld probe. Each examination took approximately 10 minutes. All images
presented are single digitized frames acquired from continuous real-time output
at 8 frames per second, each originally 520 (vertical) x 312 (horizontal)
image pixels. The digitized images were smoothed using a 3 x 3 blur
filter prior to final printing. The use of 1310-nm illumination provided deeper
penetration in highly scattering tissue such as the sclera, yielding images
with better morphological detail than those obtained with the conventional
830-nm OCT system.
CORNEA
Figure 3 shows an in vivo
OCT image of the cornea. Two different zones can be visualized. The posterior
zone measures 433 µm in thickness and includes the corneal stroma, the
Descemet membrane, and the endothelium, which could not be individually resolved.
The anterior zone, consisting of a surface interface reflection and a hyporeflective
(dark) region measures 55 µm in thickness and represents the corneal
epithelial structures. Because of the corneal curvature, the OCT beam is not
perpendicular to the corneal surface over the whole scanning range, leading
to refraction of the scanning beam at the air-cornea interface. Corneal measurements
are therefore accurate only at points where the incident beam is perpendicular
to the corneal surface. At all other points, accurate measurements can be
made only after correcting for refraction. Development of an algorithm for
refractive correction in anterior segment OCT images is currently underway.
In Figure 3, the corneal thickness
values were obtained at the perpendicular point assuming a corneal refractive
index of 1.38.30 Figure 4 shows an OCT image of the angle region and part of the
cornea of a subject wearing a soft contact lens. The anterior surface of the
contact lens is visualized as a highly reflective line. The posterior surface
of the contact lens and the tear film under the soft contact lens are not
separately resolved.
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Figure 3. Real-time optical coherence tomographic
image of the cornea. The corneal epithelial and stromal layers are delineated;
their thicknesses were directly measured from the image data.
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Figure 4. Real-time optical coherence tomographic
image of the angle region. The root of the iris, the angle recess, the ciliary
body (CB), scleral spur (SS), and the trabecular meshwork (TM) are observed.
The strongest reflections in this image of a darkly pigmented iris are from
the anterior limiting layer.
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IRIS
With high-speed OCT, the different layers of the iris that can be delineated
include the iris pigment epithelium, the iris stroma, and the anterior limiting
layer. The reflectivity of these layers differs according to the amount of
pigmentation in the eye. Figure 5
shows OCT images of a lightly pigmented eye. The largest reflected signals
are from the posteriorly placed pigment epithelium; the anterior limiting
layer is not clearly visualized. In a darkly pigmented eye (Figure 4), however, the largest reflected signals are from the anterior
limiting layer of the iris. This is consistent with the larger amount of scattering
pigment present in the anterior limiting layer and stroma of darker irides.
Additional morphological details visible by OCT imaging include the iris crypt
(Figure 4), the iris sphincter (Figure 5), and the pupillary ruff (Figure 6), which is formed by the iris pigment
epithelium turning anteriorly at the pupillary margin.
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Figure 5. Imaging of the pupillary light
reflex. Following a sudden light stimulus, constriction of the iris sphincter
was observed in real time as a stretching of the iris profile as well as a
thickening of iris tissue in the pupillary region. The strongest reflections
in this image of a lightly pigmented iris are from the posteriorly placed
iris pigment epithelium.
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Figure 6. Real-time optical coherence tomographic
image of the pupillary region. The anterior capsule of the lens is clearly
seen. The pupillary ruff, which is formed by the iris pigment epithelium turning
anteriorly at the pupillary margin, can be well discerned.
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Real-time OCT enables imaging of dynamic events such as pupillary constriction. Figure 5 demonstrates the contractile response
of the iris to light (the pupillary light reflex). Following a sudden light
stimulus (shining a flashlight into the eye), the iris sphincter constricted
and this was observed in real time as a stretching of the iris profile. High-speed
OCT could also be useful in real-time evaluation of the change in peripheral
iris configuration in response to darkness (the dark-room provocative test
for assessing angle occludability).31
CILIARY BODY
The ciliary body is visualized with better detail than in previous anterior
segment OCT imaging studies. This is because of the deeper penetration of
1310-nm wavelength relative to 830-nm wavelength used in previous ophthalmic
OCT systems. In Figure 7 the pars
plicata of the ciliary body is seen as a hyporeflective structure lying just
beneath the sclera. The pars plicata, which contains the ciliary processes,
is the region of maximum radial thickness of the ciliary body. Since attenuation
of light by the sclera leads to decreased reflectivity with increasing depth,
the outline of this portion of the ciliary body is poorly delineated. However,
the part of the ciliary body that forms the angle of the anterior chamber
is clearly visible (Figure 4).
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Figure 7. Real-time optical coherence tomographic
image of the angle region of a darkly pigmented eye. The corneal epithelium
and stroma, sclera, iris, and ciliary body are shown. The anterior chamber
angle is clearly seen and readily quantified. The ciliary body is visible
although its outline is not well defined because of the overlying highly scattering
sclera.
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Figure 8 shows the posterior
portion of the ciliary body. The triangular ciliary muscle is seen on the
right-hand side of the image and tapers posteriorly into the pars plana. In
contrast to the pars plicata, the pars plana is thin, which allows the inner
boundary formed by the ciliary epithelium to be clearly discerned. The ciliary
epithelium is continuous anteriorly with the iris pigment epithelium and posteriorly
with the retinal pigment epithelium.
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Figure 8. Real-time optical coherence tomographic
image of the region just posterior to the limbus. The ciliary muscle and the
ciliary epithelium are defined. Structures superficial to the sclera, such
as the episcleral blood vessels and the rectus muscle with its insertion into
the sclera, can also be seen.
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SCLERA
The sclera in OCT images appears as a highly reflective structure. The
sclerocorneal junction or the limbus is clearly outlined by the difference
in reflectivity between the highly scattering sclera and the weakly scattering
cornea (Figure 4 and Figure 7). The scleral spur, which is an important landmark in determining
angle configuration, is identified as the pointed inward projection from the
inner surface of the sclera. The scleral spur also gives attachment to the
ciliary body. Figure 8 is an image
of the region just posterior to the corneoscleral limbus. Superficial structures
such as the episcleral vessels and the rectus muscle insertion into the sclera
are clearly visualized. However, the episclera and sclera are not differentiated
and appear as a single highly reflective complex.
ANGLE OF ANTERIOR CHAMBER
Full-thickness visualization of the angle structures was achieved with
real-time OCT. Figure 7 is an OCT
image of the angle region of a darkly pigmented eye. The angle of the anterior
chamber is clearly defined throughout its radial extent. Figure 4 is an image of another darkly pigmented eye and illustrates
further details of the angle region. Structures that can be discerned are
the root of the iris, the angle recess, the anterior surface of the ciliary
body, the scleral spur, and the trabecular meshwork. The angle region is better
delineated in darkly pigmented eyes probably because of the higher amount
of scattering pigment. Although the canal of Schlemm is not visible in this
image, it was visualized during real-time imaging. Measurement of the angle
of the anterior chamber was made directly from the image in Figure 7.
LENS REGION
Figure 6 shows an OCT image
of the pupillary region. The anterior lens capsule is imaged within the pupil
as a highly reflective line. The retroiridial portion of the lens capsule
was not seen because of the highly backscattering iris pigment epithelium.
Although the lens cortex is not visible in this image, it could be seen sometimes
in the pupillary area. The retroiridial position of the lens cortex as well
as the zonular fibers were, however, not resolved.
COMMENT
The OCT systems used in research and commercial ophthalmic applications
thus far have used 830-nm wavelength, with image acquisition times of 1 to
5 seconds. Very recently, ophthalmic OCT at 1310-nm wavelength has been described,
with an acquisition time of 3.3 seconds. All of these systems require image-processing
techniques to remove artifacts caused by patient motion during data acquisition.32 A system capable of faster data acquisition would
not be affected by involuntary eye movement and would allow real-time display.
To achieve real-time imaging with OCT, the requirements are that images must
be acquired rapidly and any increase in the rate of image acquisition must
be accompanied by a proportional increase in source optical power so that
the signal-to-noise ratio is maintained. We have achieved an image acquisition
rate of 8 frames per second by using high-speed Fourier domain optical depth
scanning technology. To obtain higher optical power, we used a semiconductor
optical amplifier light source operating at a wavelength of 1310 nm. In addition
to enabling the use of higher power without damage to the eye resulting, this
wavelength also has the advantage of increased penetration in scattering tissue
such as the sclera and iris.
The high-speed system was coupled to a handheld probe suitable for examination
of the anterior segment of the eye. For steadier handling, the probe was held
with both hands with the fingers resting on the subject's face for support.
A disadvantage to this method is that the examiner's hands are not free for
other maneuvers—a problem that could be overcome by incorporating the
real-time scanning system into a conventional slitlamp. However, the handheld
probe allows convenient scanning in any position and would be advantageous
in situations in which positioning at the slitlamp is difficult, such as in
the pediatric age group and in patients whose general condition is poor.
In contrast to other high-speed micrometer resolution imaging techniques
such as ultrasound biomicroscopy and confocal microscopy, real-time OCT is
completely noninvasive and does not require contact with the eye or immersion
of the eye in a water bath. The depth resolution of 8.1 µm provided
by real-time OCT approaches that obtained by confocal microscopy, with the
additional benefit given to axial resolution by coherence properties of the
source, and does not depend on the available numerical aperture or the quality
of the beam focus. The cross-sectional imaging capability of real-time OCT
is similar to ultrasound biomicroscopy, with the added advantage of better
spatial resolution. However, OCT cannot obtain images through opaque media
and it provides limited penetration of the ciliary body. In addition, delineation
of angle structures with OCT is poorer in lightly pigmented eyes.
The imaging capability of the high-speed OCT system coupled with the
real-time display allows several potential applications in the anterior segment,
especially in evaluation of the anterior chamber angle and the cornea. Clinical
examination of the anterior chamber angle is routinely performed by direct
visualization by gonioscopy, a technique that provides limited information
about structures behind the iris. High-speed OCT provides in vivo cross-sectional
images of the anterior eye, similar to histological sections, and could be
a potential tool for noninvasive evaluation of the anterior chamber angle.
The images shown in Figure 4 and Figure 7 are comparable to those obtained
by ultrasound biomicroscopy, a technique that has been extensively used for
elucidation of the etiopathogenesis of various types of glaucoma.33-35 The micrometer resolution
imaging capability of OCT would be useful in evaluating the structural causes
of angle-closure glaucoma syndromes such as plateau iris syndrome, malignant
glaucoma, and pupillary block glaucoma. An important part of the glaucoma
workup is to study the alterations in anatomical configuration of angle structures
in response to light and accommodation. These structural alterations effect
a rise in intraocular pressure in conditions such as plateau iris syndrome,
pigmentary glaucoma, and primary angle-closure glaucoma. Real-time OCT imaging
of the angle could be especially useful for assessment of these conditions,
including performing provocative tests for assessing angle occludability,
such as the dark-room and the prone provocative tests.36
Another application of OCT in the imaging of the angle would be in cases of
anterior segment trauma. Since it is noninvasive, OCT can be used safely in
situations in which ocular tissue has been lacerated or punctured—conditions
under which gonioscopy cannot be performed because of the danger of aqueous
leakage. Optical coherence tomography would also be useful in angle assessment
in the pediatric age group.
Real-time imaging of the corneal layers with high-speed OCT could be
applied in the field of keratorefractive surgery, especially in laser in situ
keratomileusis (LASIK).37 Corneal flap thickness
is an important parameter in LASIK because it determines the amount of residual
stroma available for ablation. Currently there is no technique that directly
measures corneal flap thickness and it has been demonstrated that there are
noteworthy discrepancies between intended and actual flap thickness values.17, 38 A technique that can directly measure
corneal flap thickness intraoperatively could potentially improve the predictability
of LASIK. Real-time OCT is ideally suited for this purpose. For intraoperative
use of our high-speed OCT system it would be necessary to increase the lateral
field of view by increasing the limiting aperture of the probe. Another application
of OCT in keratorefractive surgery would be to determine the corneal ablation
rate. Real-time cross-sectional imaging of the corneal microstructures can
allow continuous monitoring of the ablation process. Variations in corneal
ablation rates caused by factors such as stromal hydration39
may be responsible for the differences observed between planned and actual
ablation depths in LASIK17, 38
and ablation rates could potentially be studied with high-speed OCT. With
its superior resolution of morphological detail, this technique can also be
used for postoperative assessment of the anatomical correlates of the refractive
outcome.
High-speed OCT at 1310-nm wavelength demonstrates better morphological
detail of nontransparent ocular structures than OCT at 830-nm wavelength.
This makes it a useful modality for the assessment of tumors and cysts of
the iris and the ciliary body. Optical coherence tomography would be helpful
in accurate localization of tumors of the iris and ciliary body, measurement
of tumor size, and evaluation of factors such as depth of penetration and
extrascleral extension. It would also be useful in assessing peripheral iris
pigment epithelial cysts (iridociliary cysts), which are difficult to visualize
by conventional techniques such as slitlamp biomicroscopy and gonioscopy.
In conclusion, we have demonstrated real-time imaging of the anterior
eye using high-speed OCT. Fast data acquisition allowed real-time display
of high-quality images in which delineation of the corneal layers and full-thickness
visualization of angle structures was possible. Measurements of corneal epithelial
and stromal thickness and of the anterior chamber angle were made. These preliminary
results suggest the potential use of real-time OCT in the clinical assessment
of the anterior segment, especially as an adjunct in glaucoma evaluation and
for intraoperative monitoring of corneal changes during keratorefractive surgery.
AUTHOR INFORMATION
Accepted for publication February 1, 2001.
This research was supported by grant R24 EY 13015-01 from the National
Institutes of Health, Bethesda, Md (Drs Izatt and Radhakrishnan) and by Research
to Prevent Blindness Inc, New York, NY (Dr Bardenstein).
We acknowledge the technical help of Brian A. Wolf.
Corresponding author and reprints: Joseph A. Izatt, University Hospitals
of Cleveland, Division of Gastroenterology, 11100 Euclid Ave, Cleveland, OH
44106-5066 (e-mail: jai3{at}po.cwru.edu).
From the Departments of Medicine (Drs Radhakrishnan and Rollins), Biomedical
Engineering (Dr Izatt and Messrs Roth, Yazdanfar, and Westphal) and Ophthalmology
and Pathology (Dr Bardenstein), Case Western Reserve University, Cleveland,
Ohio.
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