Introduction

OCT is noncontact noninvasive technique for imaging biological tissues.

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 in 1996.

Scattering is a fundamental property of a heterogeneous medium, and occurs because of variations in the refractive index within tissue.

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Principle of OCT

Interferometry is the technique of superimposing (interfering ) two or more waves, to detect differences between them.

Interferometry works because two waves with the same frequency that have the same phase will add each other while two waves that have opposite phase will subtract.

Light from a source is directed onto a partially reflecting mirror and is split into a reference and a measurement beam.

The measurement beam reflected from the specimen with different time delays according to its internal microstructure.

The light in the reference beam is reflected from a reference mirror at a variable distance which produces a variable time delay.

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 detected.

Formation of OCT image

A-SCAN, B-SCAN, 3D-SCAN

A-scan

B-scan

Time Domain OCT

The Michelson interferometer splits the light from the broadband source into two paths, the reference and sample arms.

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 accordingly

Fourier domain OCT

In FD-OCT ,the detector arm of the Michelson interferometer uses a spectrometer instead a single detector.

The spectrometer measures spectral modulations produced by interference between the sample and reference reflections.

No physical scanning of the reference mirror is required; thus, FD-OCT can be much faster than TDOCT.

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 scanned sequentially.

FD-OCT is also fast enough for sequential image frames to track the pulsation of blood vessels during the cardiac cycle.

Time domain Optical Coherence tomography:

Fourier domain Optical Coherence tomography:

Time vs Fourier domain OCTTime domain OCT

A scan generated sequentially, one pixel at a time of 1.6 seconds

Moving reference mirror

400 scans/sec

Resolution – 10 micron

Slower than eye movement

Fourier domain OCT Entire A scan is generated at

once based on Fourier transformation of spectrometer analysis

Stationary reference mirror

26,000 scans/sec

Resolution – 5 micron

Faster than eye movement

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Spectral OCT/SLO

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 OCT/SLO.

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 research.