Optical Coherence Tomography (OCT)

 

Optical Coherence Tomography (OCT) is a non-invasive optical imaging technique that uses diffusive light propagation to provide high-resolution, depth-resolved information about materials such as human tissues. This technique has developed rapidly since its introduction in 1991 by Huang, Fujimoto and others in the research laboratory at MIT (Massachusetts Institute of Technology). 1

Nowadays, OCT is being used to diagnose and monitor eye diseases, prevent blindness and even to determine pearl authenticity. There are other applications of this technology, such as reducing the need for painful biopsies, guiding surgeons to improve the precision of their interventions and correspondingly reducing post-operative care, and obtaining high-resolution images of teeth without the secondary effects of X-rays. OCT is an innocuous method. 1

interfometro-OCCT

Twyman-Green interferometer

Optical coherence tomography is based on an interferometric principle, the Twyman-Green interferometer, which is used to select light reflected at a given depth in tissue among all collected contributions. The selected depth is variable and the examined position can be moved over the depth range of interest, while the light beam is moved laterally. 1

Ultrasound and OCT are analogous in the sense that both imaging modalities obtain information about the inside of tissues by sending waves into them and analysing the time-of-flight and intensity of echoes scattered back into the system. Thus, sectional images and 3D reconstructions of the microscopic tissue anatomy can be generated by using data acquisition and imaging reconstruction techniques. OCT provides real-time imaging with a resolution typically higher than 10-20 µm, although imaging with a depth resolution of 1 µm or less has been demonstrated. The penetration depth is highly tissue-dependent and typically limited to a few millimetres. 2

OCT resolution is closely linked to the type of light source used in the system. It varies from 15µm for relatively standard sources to 1µm for ones with very high bandwidth emission spectra. This resolution is normally enough to provide detailed information about the histological nature of human tissues and a diagnosis of their condition. There is a physical limit to the penetration depth that can be achieved, which is motivated by the exponential attenuation of light in this type of media. For most tissues this value lies in the 0.7-2.0 mm range and varies according to the properties of the tissue, the intensity and wavelength range of the source, image acquisition speed and the mode in which OCT is implemented. 3, 4

The origin of this technique is closely related to optical communication technology, and more precisely to the true predecessor of OCT, time-domain optical reflectometry, which is used to analyse optical fibre channels. The evolution from one to two dimensions was the fundamental concept behind coining the term optical coherence tomography as the standard name for the technique. 3, 4

OCT systems are essentially divided into three categories: time domain (TD OCT), Fourier domain (FD OCT) and swept source (SS OCT). The first class –time domain– is the most intuitive and corresponds most literally to the description given above. A variable delay element in the interferometric system is used to select optical signals with a given optical path length. Traditionally, the construction of this type of variable delay element was purely mechanical and based on moving mirrors. Data acquisition speeds are a few hundred lines per second, corresponding to one or two images per second. This frame rate is not sufficient to avoid movement artefacts in many applications. 5

image

OCT system

Fourier domain systems use the relation between the frequency spectrum of the collected light and the spatial distribution of back-scattered intensity to remove the variable delay element. This can be obtained using a high-resolution optical spectrograph and applying a Fourier transform to the acquired data. This modality obtains higher image speeds than mechanical time-domain systems and the theory demonstrates a sensitivity advantage leading to a slight increase in penetration depth due to an intrinsic increase in the signal-to-noise ratio. However, achieving this advantage involves greater effort in the implementation due to an uncertainty in the inverse transform of the spectrum, leading to the need for very high resolution in the spectrograph. Swept source systems make use of lasers with a variable wavelength that can be swept over a spectral range of interest. Conceptually they are very similar to Fourier domain implementations with the particularity that wavelength selection is done in the source rather than at the detector. 5

DermaLumics obtains ground-breaking technological advantages by combining both the Fourier domain and Time domain techniques, thereby having a radical impact on manufacturing costs, reliability and size of the final systems. DermaLumics has developed a unique photonic technology platform that allows a dramatic miniaturization in the optical components and system for OCT without compromising performance. This is done by using integrated optics supported by a micro- fabrication process. A complete system including focusing optics can be assembled in a matchbox-sized butterfly package. The components, subsystems and products comprising these technological advantages are protected by a series of patents at different stages of the application procedure. These technological assets also enable new products and markets with high growth potentials. 5

In general terms, optical coherence tomography systems use intensive signal and image processing, while the necessary electronics are suitable for implementation using digital signal processors and system-on-chip concepts. 5

Optical coherence tomography can be used to obtain information about a variety of human tissue characteristics; the most significant of which are structural histologic information, flow speed in body fluids (most notably, blood), polarisation effects (birefringence), elastography and spectroscopy. In comparison with x-ray based computer tomography systems, OCT is cheaper, more precise, faster in terms of image acquisition and totally harmless for the patient. Compared to diffusion optical tomography, resolution is much higher and the studied volumes are smaller and more precisely defined. In contrast to ultrasound, OCT has a very significant advantage in terms of resolution. Other important strong points of OCT are the ability to generate images without the need for physical contact between probe and tissue (contrary to ultrasound), and the highly intuitive nature of the images obtained. These can be understood immediately by clinicians and patients, whereby the technique affords a significant didactic value. 5

GraficoConfocalvsOCTvsUltrasound2

Although OCT is a relatively new technique that began development in the medical area in 1991, it is a very promising imaging tool. The most successful clinical applications can be found in the study of the retina in ophthalmology and very recent intravascular imaging in cardiology or in dermatology, which have quickly expanded market growth. After initial experiences with various inflammatory diseases in the field of dermatology, such as like psoriasis and dermatitis, OCT began to be applied to the investigation of skin cancer. 5

DermaLumics is focused on dermatology, and more specifically on the diagnosis of NMSC and guidance of tumour edges. 5

 

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