Mechanism and Nomenclature
The contrast from OCT imaging arises from the direct reflection (backscatter) of light as it passes through structures with different optical densities. The depths of these boundaries cannot be measured by recording the time taken between the emitted light and reflected light, as is the case with sound in ultrasound, because the speed of light is too fast to be measured directly in this way. Instead the depth can be measured indirectly using a technique called interferometry. This measures the depth according to the amount of disruption to the coherence of the laser light beam as it passes through a sample and is compared with a reference light beam. The reference beam and the beam directed into the sample are split from the original laser beam so initially have the same coherence. After passing through the sample, the beams are recombined. If the distances travelled by the light in the reference beam and reflected sample beam are identical, constructive interference occurs and the signal increases. This allows the depth of the signal (the interface between two structures with different optical densities) to be measured as long as it is within the distance that the light beam itself remains coherent (termed coherence length) (Figure 1B, C).
The terminology used to describe OCT technology can be confusing. OCT imaging can be realized by three main techniques: physically changing the length of the reference arm (time-domain OCT), measuring the interference in the spectrum of light (spectral-domain OCT) or using the interference from a laser source that is varying or 'sweeping' over a wide frequency band (swept-source or optical frequency-domain OCT). The latter two techniques measure changes across multiple wavelengths, requiring Fourier transformation for analysis. They are therefore also classified as Fourier-domain OCT.[9] HD-OCT, or high-definition OCT, was originally a manufacturer-defined term for a device (SKINTELL®) that had a greater horizontal and axial resolution than standard OCT. It was a variant of Fourier-domain OCT and used 'full-field' illumination to acquire a whole en face image at once. This technique was termed full-field optical coherence microscopy, but that particular device is no longer available.[10] Speckle variance OCT and angiographic OCT (also known as dynamic OCT) use the principle of speckle variance to detect blood flow by measuring small variations in the signal intensity between two consecutive images taken in rapid succession.[11–14]
Angiographic OCT can achieve high-resolution two-dimensional and three-dimensional images of combined vascular structures within the skin structural organization (Figure 2). Polarization-sensitive OCT measures the state of polarity of the backscattered light from the sample, which can be used to assess changes in cutaneous collagen, as it is highly polarizing. The main attraction of OCT compared with its current clinical competitor, RCM, is its significantly increased depth of imaging (up to 2 mm vs. < 0·2 mm). This permits imaging to the level of the reticular dermis and generates horizontally orientated images (called B scans) that are similar to the standard histological orientation of tissue on slides. Unfortunately this increased depth of imaging is at the expense of reduced image resolution.
Figure 2.
In vivo two-dimensional (2D) and three-dimensional (3D) angiographic optical coherence tomography (OCT) images from healthy human skin. Angiographic OCT is a noninvasive optical imaging technique that uses low-power infrared laser light to image human skin in vivo to a depth of 2 mm. OCT produces both 2D and 3D images. (A) The 2D image is a greyscale image showing the structure of the skin, with epidermis at the top and dermis at the bottom. Red areas indicate blood flow motion in dermal vessels detected by angiographic OCT scanning (a). (b) En face projection (XZ plane) of angiographic OCT of the same image shows clearly branching vessels of the scanned area: hand palm skin. (B) OCT scan can also be represented in a XYZ plane, thus in 3D images, including all the scans acquired, such as 500 frames (a, b). The 3D angiographic OCT images show the structure of the skin epidermis at the top and dermis at the bottom. Red areas in the dermis show the blood flow motion (a, b). (c) A 3D en face projection of angiographic OCT (XYZ plane) shows the precise structure and shape of the epidermis of the hand palm from the top view and suggests dermal branching vessels by transparency. (d) Analysis of the 3D angiographic OCT of the blood vessels without the skin structure represented shows the branching-vessel system of hand palm skin. Angiographic OCT images were acquired by VivoSight Dx from the hand palm of healthy skin from a 29-year-old healthy donor (en face view). 3D reconstructions were generated using Amira® software. Image size 6 × 6 mm; scale bar = 300 μm.
Line-field confocal OCT (LC-OCT) is a new technique that has been developed to address this. It is most similar to time-domain OCT but acquires a whole B scan image at once instead of generating a B scan by combining many recordings from single points. This allows imaging at greater resolution than other OCT techniques, enabling some cytological evaluation, but with lower depth, positioning it between RCM and speckle variance OCT. The most noticeable difference compared with other OCT techniques is that the skin must remain in contact with the probe for LC-OCT to function.[10,15–17]
The first application of OCT tissue imaging was in ophthalmology[18–20] and it is now an established method for imaging of the retina and anterior segment.[21] Other applications have included the imaging of coronary arteries during percutaneous intervention procedures,[22] renal imaging[23] and endoscopic imaging of the gastrointestinal tract.[24]
The British Journal of Dermatology. 2021;184(6):1014-1022. © 2021 Blackwell Publishing
