Principle of Optical coherence tomography (OCT)

Optical coherence tomography is a non-destructive interferometric process allowing cross-sectional images of biological tissue to be produced with a resolution of the order of a micrometer. OCT uses a low coherence light beam, usually emitted by a superluminescent diode. Similarly to what sonar does with the backdrops (acoustically), due to the computerized analysis of the light reflected from the tissues under examination, it is possible to reconstruct the structure in two or three dimensions. The OCT can return images in color or in black and white that is with the grayscale, both with great details.

We propose to illustrate the principle. The principle of ultrasound has been applied to optics using optical coherence tomography. Will this interference method soon provide the means to examine any suspect tissue in real-time.

How to examine in its thickness an inaccessible biological tissue, such as the retina? One solution is to take an image of it by optical coherence tomography or OCT. It is invented fifteen years ago by James Fujimoto, a physicist at the Massachusetts Institute of Technology. The OCT (Optical coherence tomography) is a kind of optical ultrasound; the ultrasound has been replaced by infrared light. 

It makes it possible to view the structures with a resolution of up to a micrometer, and this in real-time, in situ, and without contact without sampling or preparation. Today in common use in ophthalmology, the OCT is promised for a whole series of applications in dermatology, gynecology, cardiology, etc.

During clinical trials, these new applications are the result of recent technical research and the development of several variants. After having explained the principle of optical coherence tomography, we will present its most advanced applications, including open field optical coherence tomography.

Optical coherence tomography compares to ultrasound for the most part. Recall what the latter consists of a probe in contact with the patient's skin emit ultrasound, which is then more or less absorbed or reflected depending on the density of the biological tissue encountered. A sensor measures the reflected ultrasonic signals. The return times and the variations in the intensity of the echoes then make it possible to reconstruct an image of the biological tissue up to several centimeters deep and with a resolution of the order of a millimeter.

In optical coherence tomography, ultrasound is replaced by light, and the reconstructed image depends on the absorption and reflection of this light by biological tissues. In principle, substitution has several advantages, like, given the short wavelengths of light, the details of the tissues are revealed in width (the direction perpendicular to the propagation of the waves). With the resolution of optical microscopy, 1,000 times that of imaging by ultrasound. As for the resolution in-depth, it depends on the brevity of the light pulses used, with the ultra-short pulses of a femtosecond laser (a femtosecond is 10–15 seconds). 

It could determine the depth of an area from which it comes from an optical echo with a micrometric resolution. In fact, in ten femtoseconds, the light travels only two micrometers in a tissue! Tissue details are revealed in width (the direction perpendicular to the wave propagation) with the resolution of optical microscopy, 1,000 times that of ultrasound imaging. As for the resolution in-depth, it depends on the brevity of the light pulses used. 

With the ultra-short pulses of a femtosecond laser (a femtosecond is 10–15 seconds), one could, in principle, determine the depth of an area from which comes from an optical echo with a micrometric resolution. In fact, in ten femtoseconds, the light travels only two micrometers in tissue. Tissue details are revealed in width (the direction perpendicular to the wave propagation) with the resolution of optical microscopy, 1,000 times that of ultrasound imaging. 

As for the resolution in-depth, it depends on the brevity of the light pulses used with the ultra-short pulses of a femtosecond laser (a femtosecond is 10–15 seconds). One could, in principle, determine the depth of an area from which comes from an optical echo with a micrometric resolution. In fact, in ten femtoseconds, the light travels only two micrometers in tissue. With the ultra-short pulses of a femtosecond laser (a femtosecond is 10–15 seconds), we could, in principle, determine the depth of an area from which an optical echo comes with a micrometric resolution. 

In fact, in ten femtoseconds, the light travels only two micrometers in a tissue! With the ultra-short pulses of a femtosecond laser (a femtosecond is 10–15 seconds), we could, in principle, determine the depth of an area from which an optical echo comes with a micrometric resolution. In fact, in ten femtoseconds, the light travels only two micrometers in tissue.

However, this promising principle of "optical ultrasound" cannot be put into practice directly because no light detector is fast enough to measure propagation times (back and forth), which are measured in femtoseconds.

- In order to obtain optical images of biological tissues in the form of a section or tomography, it is first necessary to measure its internal structures. In OCT, the first step to obtaining these images is to measure the axial distance inside the tissue. When a beam of light is directed inside the eye, it is reflected in the level of the interfaces between the different tissues and diffuses in a different way from tissues that have different optical properties. The distances and dimensions of the different ocular structures can be determined by measuring the time of delay of the echo of light that is reflected or backscattered by the different structures at vary the axial distance.

- It is defined with ΔT =V/z the temporal resolution associated with the instrument a measure the delay of the light echo; Δz is the distance covered by the echo. While v is the speed of propagation of the echo in the fabric, therefore for typical values ​​of Δz of 5 μm, with v equal to 3×10 8 m/s, there will be temporal resolutions of about 15 femtoseconds.

- The only way to detect such fast signals is to use tools of detection that exploit the principle of interferometry.

Author: Vicki Lezama