What Is Optical Coherence Tomography?

Optical Coherence Tomography (OCT) is one of the most promising new tomography technologies that has developed rapidly in recent years. Especially, it has attractive application prospects in biological detection and imaging of living tissues. It has been tried The application in the clinical diagnosis of ophthalmology, dentistry and dermatology is another major technological breakthrough after X-CT and MRI technology, which has been rapidly developed in recent years.

Chinese name: Optical coherence tomography

Foreign name: optical coherence tomography, OCT
profession: Optical technology

Optical coherence tomography technology background

With the advancement of science, today's medical imaging technology has played an important role in medical diagnosis. Various detection methods and display methods tend to be more accurate, more intuitive, and more complete to help people observe biological tissues and understand material structures. Its development is the result of a combination of physics, mathematics, electronics, computer science, and biomedicine.

From the invention of the microscope to the application of X-rays in medicine, people have observed morphological structures that cannot be seen directly by the naked eye in the form of images, which has promoted the development of medical diagnosis. At present, various medical imaging technologies are continuously developed for research in the field of biomedicine. Different imaging principles can be used to observe different organ tissues, not only giving the shape of the tissue, but also identifying and detecting the characteristics of the tissue.

Among various imaging technologies, optical coherence tomography / optical coherence tomography ^[1] (Optical Coherence Tomography) is an emerging optical imaging technology. When the ballistic photon and snake photon returned from the scattering medium and the light of the reference light The path difference occurs within the coherence length of the light source, and the optical path difference between the diffused photon and the reference light is greater than the coherence length of the light source. Interference cannot occur, so that the ballistic photons and snake photons with the information of the measured sample are extracted For imaging, it can achieve high-resolution non-invasive tomographic measurement of biological tissues, and has a wide range of application prospects. Optical coherence tomography is developed from an optical coherence domain reflectometer (or optical low-coherence reflectometer). In 1991, David Huang and others at the Massachusetts Institute of Technology (MIT) first reported optical coherence in Science. Tomography (OCT) technology. Schmitt et al. Later applied this technique to the measurement of optical characteristics of biological tissues and achieved very good results. In 1996, Carl Zeiss Meditec Inc. of California introduced the OCT system for ophthalmology into the market as a clinical medical device.

Since the advent of OCT technology, various research institutions have done a lot of research work in order to expand its application range and improve performance, and many new methods have appeared, laying the foundation for the wide application of OCT technology in the medical field.

Introduction to optical coherence tomography

Optical coherence tomography (optical coherence tomography ^[2] , Optical Coherence Tomography, OCT) is an imaging technology developed rapidly in the past decade. It uses the basic principle of weak coherence optical interferometer to detect different depths of biological tissues. The back reflection of the incident weakly coherent light or the scattering signal several times can be obtained by scanning the two-dimensional or three-dimensional structure image of the biological tissue.

How optical coherence tomography works

OCT professional full name is also called optical correlation tomography. It is a new technology applied to ophthalmology in recent years. OCT is a non-contact, high-resolution tomography and biological microscope imaging device. It can be used for in vivo viewing, axial tomography, and measurement of the structure of the posterior segment of the eye (including the retina, retinal nerve fiber layer, macula, and optic disc). It is specifically used to help detect and manage eye diseases (including but not limited to macular holes, macular sac Edema, diabetic retinopathy, age-related macular degeneration, and glaucoma). OCT is now divided into two categories: time domain and frequency domain. In fact, each has its own advantages and disadvantages. Time-domain OCT is cost-effective enough to complete most fundus and glaucoma examinations. And the technology is relatively mature.

OCT ^[3] Michelson-based interferometry, using super light-emitting diode light-emitting body as the light source. After entering the fiber coupler through the optical fiber, the light beam is divided into two beams, one beam passes through the eye's refractive medium and hits the retina, and the other beam enters the reference system. The reflected or backscattered light in the two optical paths is re-integrated into a beam at the fiber coupler and detected by the detector, and the backscattering intensity and delay time generated by tissues of different depths are measured. The image is obtained by real-time display of grayscale values of pseudo-colors. Bright colors such as red, yellow, and bright green represent areas of strong emission, while dark colors such as blue and black represent low reflection areas, and green represents medium reflection areas.

The distance between the reference mirror and the light source can be adjusted. Interference occurs only when the optical path difference between the two optical paths matches the coherent wavelength of the light source, so the axial resolution is determined by the coherent wavelength of the light source and is inversely proportional to the spectral bandwidth of the light source. . The lateral resolution of OCT is not only affected by wavelength, but pupil diameter and lateral pixel density are also important factors.

In addition, when the OCT technology is extended to image biological tissues, it uses near-infrared and optical interference principles for imaging. Simply put, the light emitted by the light source is divided into two beams, one beam is emitted to the measured object (vascular tissue), this beam is called the signal arm, and the other beam is to the reference mirror, called the reference arm. Then the two light signals reflected from the tissue (signal arm) and from the reflector (reference arm) are superimposed. When the length of the signal arm and the reference arm are the same, interference occurs. The light signal reflected from the tissue shows different strengths and weaknesses depending on the shape of the tissue. When it is superimposed with the reference light signal reflected from the reflector, the signal will increase (increasing interference) when the fixed point of the light wave is consistent, and the signal will weaken (reducing interference) when the fixed direction of the light wave is opposite. The conditions for interference are the same frequency and constant phase difference. Using the principle of interference, OCT compares standard light sources with reflected signals to enhance single reflection and attenuate the emission of scattered light. Since the interference only occurs when the length of the signal arm and the reference arm are the same, changing the position of the reflector changes the length of the reference arm, and signals of tissues of different depths can be obtained. These light signals can be processed by computer to obtain tissue tomographic images.

OCT is currently divided into two categories: time-domain OCT (TD-OCT) and frequency-domain OCT (FD-OCT). The most common form of intra-coronary OCT is time domain OCT (TD-OCT). In the time domain OCT, the optical signal reflected from the tissue at the same time is superimposed with the optical signal reflected from the reference mirror, interfered, and then imaged. The characteristic of the frequency domain OCT is that the reference mirror of the reference arm is fixed, and the signal interference is achieved by changing the frequency of the light wave of the light source. FD-OCT is divided into two types: (1) laser scanning OCT (SS-OCT), this OCT uses a variable wavelength laser light source to emit light waves of different wavelengths; (2) spectral OCT (SD-OCT), which uses Resolution spectrophotometer to separate light waves of different wavelengths. In the Chinese market, there is only TD-OCT, which is M2-OCT. It has two light sources, the main light source is a super-bright LED, which emits broadband near-infrared (center wavelength 1310um, bandwidth 40-50um). The near-infrared emitted from the light source reaches the human tissue through the optical fiber and the probe. The light waves backscattered by the tissue are collected by the probe, combined with the light wave signals of the reference arm to form interference, and then analyzed by a computer to construct a high-resolution image showing the internal microstructure of the tissue. The biggest limitation of M2-OCT is that the penetration depth is only about 1.5mm. In addition, because near-infrared light is difficult to pass through red blood cells, OCT needs to block blood flow or flush blood vessels to exclude blood from blood vessels. The disadvantage of this method is that it causes myocardial ischemia and the operation is complicated, which limits the clinical application of OCT. The new-generation OCT imaging system-FD-OCT has the highest priority for higher-speed scanning. The number of scanning frames per second is 100 frames and the retraction speed is 20mm / s, so the crown can be completed by only injecting a contrast agent once. The imaging of veins and vessels completely abandons the method of balloon blocking blood flow, which greatly improves the safety of operation. FD-OCT increases the scanning speed and the resolution of the image at the same time. The fine structure of the lesion can be seen more clearly. FD-OCT broadens the indications for OCT, and satisfactory images can be obtained for left main lesions and open lesions.

Compared with the time domain, the frequency domain OCT technology can improve the sensitivity of the system and significantly increase the sampling speed. In the spectral domain OCT, all depth structures (A scans) are acquired simultaneously without the need for deep scans. Its core components are Michelson interferometers and spectrometers illuminated by broadband light sources. The acquisition speed is limited only by the readout speed of the CCD camera in the spectrometer, and the intensity of the recorded backscattered light is only used as the spectral frequency and not time function. At the same time, the spectral domain OCT signal is sampled in the spectral density, and as a result of Fourier reconstruction, the signal-to-noise ratio SNR is improved.

OCT was first invented by Carl Zeiss and the 1990s, and now has 3-5 generations. Its products are classified into OCT in the front and OCT in the back according to function, and are classified into OCT in the time domain and OCT in the frequency domain according to technology.

At present, the most widely recognized OCTs in the world are the frequency-domain OCT produced by the American OPTOVUE company and the frequency-domain OCT produced by the Polish OPTOPOL company. At the same time by many domestic professors and experts praise.

Application of optical coherence tomography

Application of optical coherence tomography in ophthalmology

OCT is a new optical diagnostic technology that can perform non-contact, non-invasive tomography of the structure of living eye tissue microscope. OCT is an optical analog of ultrasound, but its axial resolution depends on the coherence characteristics of the light source, up to 10um, and the penetration depth is almost not limited by the transparent refractive medium of the eye. The morphological structure of the segment has good application prospects in the diagnosis, follow-up observation and treatment effect evaluation of intraocular diseases, especially retinal diseases.

The specific application of OCT in medicine is as follows. OCT is a new type of non-contact non-invasive optical imaging diagnostic technology developed in the early 1990s. It uses the different reflectance of different tissues in the eye to light (using 830 nm near-infrared light) and uses low-coherence optical interferometers. Compare the reflected light wave and the reference light wave to determine the delay time and reflection intensity of the emitted light wave, analyze the structure of different tissues and their distance, calculate and process the imaging, and display the cross-sectional structure of the tissue in a pseudo-color form. Axial resolution up to 10 microns. It has important application value for the diagnosis of macular diseases. However, the resolution of OCT is based on the different luminous properties of the tissue structure to distinguish the tissues. In the retinal tomography, there are neuroepithelial light bands, pigment epithelial light bands, and choroidal light bands. The structure between neuroepithelial layers is still difficult. Distinguish.

The scanning methods of OCT include horizontal, vertical, circular, radial, and linear scanning at different angles. The examiner can choose the appropriate scanning method according to the location, nature of the lesion and the purpose of the examination. Because the OCT lateral resolution is related to the scan length, the longer the scan line, the lower the resolution. In order to facilitate the comparison of data and the specification of data collection, a fixed scan length and a fixed scan order can be selected. For example, for the yellow scan, a linear scan with a scan line length of 4mm or 4.5mm and an interval of 45 ° can be selected as the basic scan. ^[4]

Application of optical coherence tomography in pathology

One of the most important applications of OCT technology is the detection of early cancerous changes in human soft tissue. Early diagnosis of cancer is the key to saving patients' lives. The only confirmed diagnosis method is biopsy. The problem is that it takes a certain amount of time to diagnose. The conclusions given are highly related to subjective factors such as the experience of analysts and are accurate. Determining the boundaries of cancerous areas is even more difficult. OCT is based on the fact that cancerous tissues have different spectral characteristics and structures from healthy tissues to obtain a clear image of the tissues, thereby real-time and accurate diagnosis. Because a computer is used for signal processing, the results obtained have nothing to do with the subjective factors of the operator. In addition, OCT technology will become an authoritative method for real-time diagnosis of subcutaneous tissue lesions without biopsy. However, more clinical trials are needed to reveal its advantages and problems to be solved. ^[5]

Application of optical coherence tomography in non-medical fields

The original purpose of OCT research was tomography for biomedicine, and medical applications continue to dominate. In addition to the application in the medical field, with the development of OCT technology, OCT technology is advancing to other fields, especially in the field of industrial measurement, such as displacement sensors, thickness measurement of thin film, and other measurement objects that can be converted into displacement.

Recently, low-coherence technology has become a key technology for high-density data storage. OCT technology can also be used to measure the residual porosity, fiber structure, and structural integrity of highly scattering polymer molecules. It can also be used to measure the coating of materials. OCT technology can also be used in materials science, and JPDunkers et al. Used OCT technology for non-destructive testing of composite materials. M.Bashkansky et al. Used the OCT system to test ceramic materials, expanding the application range of OCT technology. SRChinn et al. Also studied the application of OCT in high-density data storage to achieve multilayer optical storage and high detection sensitivity.

Conclusion of optical coherence tomography

OCT technology has the advantages of non-contact and non-destructiveness, high detection sensitivity and noise suppression ability, high resolution without damage, and no radiation to living tissues in vivo, as well as low cost and simple structure It has important application value and broad development prospects in nondestructive testing in the fields of materials science and biomedicine ^[6] .