Optical Coherence Tomography

Optical Coherence Tomography (OCT) is an imaging method that utilizes low-coherence light to freeze micrometer-resolution along with two-and three-dimensional images from optical scattering media such as biological tissue. OCT is generally used to create images in medical and also in industries. As it uses near-infrared light, it is based on low-coherence interferometry and since it uses relatively long-wavelength light, it can break through the scattering medium.

Optical Coherence Tomography has embarked on sub-micrometer resolution (with wide spectrum sources seeing over a ~100 nm wavelength range) however; it largely depends on the light source. OCT is classified as an optical tomographic technique. They’re available on a commercial scale and the systems are used for medicinal and art purposes with worthy mentions of ophthalmology and optometry (used to reveal micro-details of the retina). In contemporary times, OCT has been used to diagnose in cardiology and even in dermatology. Recently, frequency-domain OCT has given an upper hand to throw off better signals than before.

Introduction

Retracting to older times, Adolf Fercher is known to begin with using different types of lights for ocular eye measurements. Thereafter, the scans were presented at the ICO-15 SAT conference in 1990 which resulted in greater developments along the timeline. Later adopted and upgraded by Naohiro Tanno, it was told as heterodyne reflectance tomography. Then, it finally came upon the hands of MIT, after which it was termed as “Optical Coherence Tomography”. After that time, it has been upgraded several times to benefit the biomedical world in tissue-imaging, signal detection, and other prominent mentions using broadband lasers and ultrafast tunable lasers to improve upon the results.

OCT has also been used in the art field, where it scans several layers present in a painting. Compared to other imaging techniques in the medical world, this method has an edge. Where we see Medical ultrasonography, magnetic resonance imaging (MRI) having their benefits, these methods provide low-resolution that makes it difficult to decipher. Whereas, OCT can provide resolutions that go below 1 micrometer (sub-cellular) and this indicates how beneficial it is to the medical world.

In laser interferometry, interference of light happens over meters. However, in OCT this is less than that and since it used broad-bandwidth light sources it can be created using low sources of power. In OCT, light can be sub-divided into two arms—one is a sample arm and the other reference arm. Their combination births an interference pattern but only if both of them have identical (not exactly) optical distance. Through this, a reflectivity profile of the sample can be generated. This is called an A-scan; it has information about the dimensions and the location of structures that are present for use. B-scan (cross-sectional tomograph) can be obtained by laterally grouping A-scan.

Easy explanation

Optical Coherence Tomography is used to generate sub-surface images of translucent materials in seeable resolutions (like a microscope). It has attained popularity from the medical industry because it provides better resolutions as compared to other methods. Moreover, OCT is instant, it provides direct images, no ionizing radiation and no pre-tasks need to be done to undergo it.

It delivers high-quality images because it is based on light, unlike others based on sound and radiofrequency. An optical beam is fired at the tissue, and then the reflected light is captured. However, some light is reflected only. In other imaging techniques, the background disturbs the light therefore it produces lower quality images. In OCT, interferometry is utilized as it writes off optical path length of gathered protons that fends off most protons before it is detected. This is how OCT creates clear 3D images only from obtaining light from areas that are necessary.

Among 3-dimensional imaging techniques, OCT is an echo technique that seems similar to ultrasound imaging since other imaging techniques do not utilize echolocation principle. This method however, is restricted to imaging 1-2 mm below the surface in biological tissue as when the depth is increased the light escapes and cannot be detected properly. Moreover, the sample does not have to be prepared and there is no side-effect of this method to them.

Theory

OCT bases itself on interferometry. The optical setup is comprised of an interferometer that has a low coherence. Light divides and grouped in the reference arm and the sample arm.

Time domain

In time domain OCT the length of the path of the reference arm changes with time and low coherence interferometry is gained only when the difference in the path is within the coherence length of the light source. This is called autocorrelation in a symmetric interferometer. As path length difference changes, the envelope of the modulation changes.

The interference of two partially coherent light beams is expressed in a formula where k1+k2 is less than 1 and this shows the interferometer beam splitting ratio. And the interference envelope and carrier rely on reference arm scan or time delay (T) and the gathering point is OCT. Their function is represented as a Gaussian function.

The axial and lateral resolutions of OCT are decoupled from each other and they are defined with a formula.

Frequency Domain

In this, broadband interference is gained with spectrally separated detectors. Swept-source and spectral-domain OCT are used. In it, the imaging and speed are increased however the scanning range is limited and the full spectral bandwidth sets the axial resolution.

Spatially encoded

This takes spectral information by scattering different optical frequencies onto a detector stripe. It causes the scan to be observed in depth however, the large signal to noise advantage of FD-OCT is lessened (in response to the lower dynamic range).

The disadvantages of this are observed in a strong fall-off of the SNR. Moreover, the dispersive elements in the spectroscopic detector do not scatter the light in equal proportions but usually have an inverse dependence.

Synthetic array heterodyne detection provides another route to this issue and does not need high dispersion.

Time encoded

This domain groups the pros of standard TD and SEFD-OCT. The spectral components are not encoded by spatial separation however they are encoded in time. By accommodation of a frequency scanning light source, the optical setup can become more basic than SEFD. The disadvantages happen in the nonlinearities in the wavelength at high frequencies and sensitivities.

Full-field OCT

Claude Boccara’s team developed this imaging approach to temporal OCT. It gains cross-sections of the sample and the images of classical microscopy are here.

Interferometric images are generated by a Michelson interferometer and the path length difference is changed by a quick electric component. The images are gained by a CCD camera and they are used depending on the modulation period.

Line-field (confocal) OCT

This is an imaging method that derives itself from time-domain OCT in which a broadband laser and line detection are used along with a line-scan camera. The focus is adjusted several times in the scan. The scattered light is prevented from entering and the signal is not detected by the camera. LC-OCT has an upper edge with respect to detection sensitivity and penetration in highly scattering media like skin tissues.

Scanning schemes

The light beam is focused to a certain point on the surface of the sample and the reflected light is combined with the reference gives an interferogram with info relating to a single A-scan (Z-axis only). A linear scan receives two-dimensional data whereas an area scan receives a three-dimensional data set.

Single point

The sample is scanned in two lateral dimensions and reconstructed as a three-dimensional image using depth information that is collected by coherence-gating.

Parallel

Known as a charge-coupled device (CCD), it uses a camera to obtain a three-dimensional representation that can be made again.

Selected applications

OCT is known as a proper imaging technique in the medical world and is used widely in ophthalmology and cardiology.

Ophthalmology

Ocular OCT gives ophthalmologists and optometrists gather high-resolution images of the retina and anterior segment. OCT gives a clear result and finds out several diseases including eye diseases. Moreover, it can be used to assess the vascular health of the retina as well.

Cardiology

Coronary arteries are imaged through OCT to see the vessel walls at a resolution 10 times higher than normally used techniques. 1mm diameter fiber-optics catheters are utilized to enter the artery lumen.

It was first done in 1997, at MIT by different professors. The better imaging speed allowed usage of this practice and technology for artery imaging. OCT is now combined with fluorescence molecular imaging as it improves the capability to detect molecular/functional and tissue morphological information at the same time.

Oncology

Endoscopic OCT is utilized to detect and diagnose cancer and precancerous lesions and esophageal dysplasia.

Dermatology

First used in 1997, OCT is now used to detect various skin lesions including carcinomas. Modern technology has allowed better diagnosis of skin-related problems and allowing doctors to detect skin tumors early.

Research applications

OCT has been used to generate detailed pictures of mice’s brains. OCT is utilized also in industrial applications and cross-section imaging. The pharmaceutical industry has used OCT to check the coating of tablets. Lately, OCT has been used to identify root canals in teeth.