OCT is noncontact noninvasive technique for imaging
In 1990, Fercher presented cross-sectional
topographic image of the retinal pigment epithelium
(RPE) of a human eye.
The first commercial instrument, OCT 1,was launched
Scattering is a fundamental property of a
heterogeneous medium, and occurs because of
variations in the refractive index within tissue.
Principle of OCT
Interferometry is the
technique of superimposing
(interfering ) two or more
waves, to detect differences
because two waves with the
same frequency that have
the same phase will add
each other while two waves
that have opposite phase
Light from a source is
directed onto a partially
reflecting mirror and is
split into a reference
and a measurement
The measurement beam
reflected from the
specimen with different
time delays according to
The light in the reference
beam is reflected from a
reference mirror at a
variable distance which
produces a variable time
The light from the
specimen, consisting of
multiple echoes, and the
light from the reference
mirror, consisting of a
single echo at a known
delay are combined and
Formation of OCT image
A-SCAN, B-SCAN, 3D-SCAN
Time Domain OCT
The Michelson interferometer splits the light from the
broadband source into two paths, the reference and
The interference signal between the reflected reference
wave and the backscattered sample wave is then recorded.
The axial optical sectioning ability of the technique is
inversely proportional to its optical bandwidth.
Time Domain OCT
Transverse scanning of the sample is achieved via
rotation of a sample arm galvonometer mirror.
In order to measure the time delays of light echoes
coming from different structures within the eye, the
position of the reference mirror is changed so that the
time delay of the reference light pulse is adjusted
Fourier domain OCT
In FD-OCT ,the detector arm of the Michelson
interferometer uses a spectrometer instead a single
The spectrometer measures spectral modulations
produced by interference between the sample and
No physical scanning of the reference mirror is
required; thus, FD-OCT can be much faster than
The simultaneous detection of reflections from a
broad range of depths is much more efficient than
TD-OCT, in which signals from various depths are
FD-OCT is also fast enough for sequential image
frames to track the pulsation of blood vessels during
the cardiac cycle.
Time domain Optical
Fourier domain Optical
Time vs Fourier domain OCT
Time domain OCT
A scan generated
sequentially, one pixel at a
time of 1.6 seconds
Moving reference mirror
Resolution – 10 micron
Slower than eye movement
Fourier domain OCT
Entire A scan is generated at
once based on Fourier
Stationary reference mirror
Resolution – 5 micron
Faster than eye movement
Limitation of OCT technology was difficulty in
accurately localizing the cross-sectional images
and correlating them with a conventional en face
view of the fundus.
To localize and visually interpret the images,
integrating a scanning laser ophthalmoscopy
(SLO) into the OCT was needed.
This rationale was used by OTI technologies
(Toronto, Canada) to develop the Spectral
The Spectral OCT/SLO is a computerized optical
scanner device providing high-resolution, high-definition
images of the fundus anatomy.
It integrats SLO’s confocal imaging principles with
OCT’s high resolution tomographic images.
The system simultaneously produces SLO and OCT
images that are created through the same optical path,
and therefore correspond pixel to pixel.
It produces a new image format called as C scan
The technique has already become
established as a standard imaging modality
for imaging of the eye.
The application of OCT imaging to other
biomedical areas such as endoscopic imaging
of gastro-intestinal and cardiovascular
systems is currently an active field of
In a first approach towards tomographic imaging a cross-sectional topographic image of the retinal pigment epithelium (RPE) of a human eye obtained in vivo by the dual beam LCI technique was presented at the ICO-15 SAT conference by Fercher (1990) and published by Hitzenberger (1991).
OCT using fibre optic Michelson LCI was pioneered by Fujimoto and co-workers (Huang et al 1991).
First in vivo tomograms of the human retina were published by Fercher et al (1993a) and Swanson et al (1993).
Later Chinn et al (1997) used wavelength tuning interferometry (WTI) to synthesize OCT images, whereas H¨ausler and Lindner (1998) generated OCT images using spectral interferometry.
For a review of early work in LCI and OCT see the selection of key papers published by Masters (2001).
Based on the principle of low-coherence interferometry where distance information concerning various ocular structures is extracted from time delays of reflected signals
Light incident onto a scattering or turbid medium such as tissue is either transmitted, absorbed, or scattered.
Absorbed light is converted into heat in the tissue and is effectively removed from the incident beam.
Optical coherence tomography (OCT) is an imaging technique which works similar to ultrasound, simply using light waves instead of sound waves.
By using the time information contained in the light waves which have been reflected from different depths inside a sample, an OCT system can reconstruct a depth-profile of the sample structure.
Three-dimensional images can then be created by scanning the light beam laterally across the sample surface.
Whilst the lateral resolution is determined by the spot size of the light beam, the depth (or axial) resolution depends primarily on the optical bandwidth of the light source.