Optical coherence tomography

Abstract

Optical coherence tomography (OCT) is a non-contact method for imaging the topological and internal microstructure of samples in three dimensions. OCT can be configured as a conventional microscope, as an ophthalmic scanner, or using endoscopes and small diameter catheters for accessing internal biological organs. In this Primer, we describe the principles underpinning the different instrument configurations that are tailored to distinct imaging applications and explain the origin of signal, based on light scattering and propagation. Although OCT has been used for imaging inanimate objects, we focus our discussion on biological and medical imaging. We examine the signal processing methods and algorithms that make OCT exquisitely sensitive to reflections as weak as just a few photons and that reveal functional information in addition to structure. Image processing, display and interpretation, which are all critical for effective biomedical imaging, are discussed in the context of specific applications. Finally, we consider image artifacts and limitations that commonly arise and reflect on future advances and opportunities.

Keywords: Angiography; Elastography; Fourier-domain; Polarimetry; detection sensitivity; frequency-domain; interferometry; resolution; spectral-domain; spectrometer; wavelength-swept laser.

Figure 1.. Basic OCT system configuration.

Temporally incoherent (multiple wavelengths) and spatially coherent light from a source is directed through an interferometer to a sample and a controlled reference path. Backreflected light is then detected and digitized. A computer is used for signal and image processing and display.

Figure 2:. Basic OCT signal analysis:

(a) Block diagram Background subtraction is initially applied to the raw digital signal. Although some systems may intrinsically record the interferometric signal as a function of wavenumber (inverse wavelength), most systems require a step of interpolation to ensure equal k-sampling. Numerical compensation of any imbalance of the dispersion in the reference vs the sample interferometer path improves resolution (b). In fd-OCT systems, this step may be accompanied by compensation for any nonlinearity in the wavelength sweep of the laseUsing the optimal parameters of the correction function, it is possible to remove nonlinearities from the fringe signal and thus maximize the SNR and minimize the width of the PSF axial function - in this case, by compensating for dispersion, a 3.3-fold improvement in axial resolution is achieved (d). Application of a suboptimal correction function can result in the appearance of asymmetry, large side-lobs and PSF broadening even with small mismatches of this function (green dashed line) (d). A spectral window, typically a Gaussian or Hamming filter, is applied to control the shape of the axial point spread function (c)When the window (in this case Gaussian) is used optimally, a reduction in side-lobs is achieved at the expense of a broadening of the PSF and a loss of axial resolution (d). When a suboptimal window (narrower than the original spectrum) is used, removal of side-lobs is associated with a significant loss of axial resolution and SNR (d). Conversion of the fringe data to reflectance as a function of depth is performed using Fourier transformation. The large dynamic range of OCT data is preserved for display using logarithmic compression to form the final axial reflectance profile or ‘A-line’.

Figure 3:. Circular ranging concept.

The imaging properties of a CR-OCT. a. When conventional OCT is used to image a sample with a large variation in height, a significant portion of the measurement data describes “empty” space, which are the regions above the sample surface and beyond the imaging penetration depth. b. A CR-OCT image collects multiple depth locations into each measurement such that the sample appears as though it had been circularly wrapped onto itself. Because this wrapped image is a more data compact representation of the sample, it can be captured with fewer measurements. c. The upper and lower image boundaries of CR-OCT images are cuts through a continuous circular delay. This can be seen by noting the continuity of the sample when these images are tiled vertically. d. An A-line in the CR image describes the sample as a function of a looped circular depth coordinate. This depth spacing between measurements (Δz) and number of depth pixels (N) in the depth A-line are directly related to the combline spacing (Δz) and the number of comblines (N) in the optical frequency comb source used to perform CR-OCT imaging. An image of a human hand atop an optical table is depicted in the lower left panel as an x-y (transverse) projection. At any every point in this image, it is possible to display the corresponding cross-sectional image. One cross-sectional image, across the nail-fold region of the left ring finger, demonstrates the depth of penetration and resolution characteristic of state-of-the-art conventional OCT.

Figure 4:. Basic image processing steps.

Multiple A-line (depth, z) scans are acquired while the OCT beam is swept transversally across the sample. Cross-sectional (transverse - depth) images (B-scans) are computed by combining multiple A-lines and mapping the reflectance data to gray scale. Typically images are displayed with isotropic scale. Data corresponding to multiple B-scans may be acquired with a second direction of transverse scanning (x-y). The resulting data volume may be processed to compute horizontal sections or en-face projections. In some cases, three-dimensional visualization of the data may assist interpretation and further extraction of sub-volume of interest. In the case of rotational catheter scanning systems, the typical acquisition coordinates are cylindrical: r (depth) and θ-z (transverse).

Figure 5:. Polarization-sensitive OCT results.

Catheter-based PS-OCT in the left circumflex coronary artery of a 52-year-old male patient undergoing percutaneous coronary intervention. Polarization-diversity detects the signal along two linear orthogonal polarization states (h,v) in response to illumination by two input polarization states (e1, e2) (bottom left). The incoherent mean of all signals serves to visualize the conventional scattering signal of OCT (top left). The retardance of the round-trip Jones matrix (J) demonstrates tissue anisotropy but is difficult to interpret because it cumulates the effect of propagation through tissue layers (top middle). In comparison, analysis of the local Jones matrix reveals depth-resolved tissue birefringence (Δn, bottom middle) and optic axis orientation (θ, bottom right) and clearly delineates tissue layers (media, white arrow heads) with distinct anisotropy (purple arrows point to areas with increased Δn), suggestive of increased collagen and smooth muscle cell content or cholesterol crystals. Depolarization, computed using the incoherent Mueller-Stokes formalism quantifies the randomization of the detected polarization states, caused by scattering in lipid-rich plaques and macrophages, or by the absence of meaningful signal. Scale bar: 1 mm.

Figure 6:. OCT Angiography principle and results.

Steps to calculate OCTA enface maps starting from series of tomograms taken at the same slow scanning position at time intervals ΔT to enface projection views at indicated depth ranges (a); nodular basal cell carcinoma in skin, the color coded depth ranges are indicated in the color bar(reproduced from[127] (b). OCTA of a xenotransplanted U87 human glioblastoma multiforme tumor in a mouse brain. Depth-projected vasculature within the first 2 mm is color coded by depth, ranging from yellow (superficial) to red (deep). Scale bar, 500 μm (c) (reproduced from[24]);

Figure 7:. Elastography results.

OCT intensity image and corresponding elastogram (upper panels) of human breast tissue (reproduced in part from [212]. Elevated stiffness of tumor is readily detected by elastographic OCT. Measurements of shear wave speed (lower panel) provide for the identification of important structural layers within the human cornea. [Reproduced in part from [86]]

Figure 8:. Ophthalmic application.

An eye with exudative age-related macular degeneration. (A) En face structural OCT. The dotted line transects the cross section shown in panel C. (B) En face OCT angiogram with color coded flow signal (inner retina in violet and outer retina in yellow). The choroidal neovascularization is in the outer retina. (C) Cross-sectional projection-resolved OCT angiogram with reflectance shown in gray scale and flow signal color coded by depth (choroid in red). The neovascular membrane is in the outer retina (yellow) under the pigment epithelium. Intraretinal cyst (green arrow) and subretinal fluid (red arrow) are indications for an anti-neovascular injection.

Figure 9:. Catheter and endoscopic applications.

Figure 9a Representative fd-OCT images of the coronary artery after stent implantation. Upper panel shows cross sectional images and lower panel longitudinal pull-back image. Based on the baseline OCT measurements before PCI showing 2.6mm and 3.4mm lumen diameters at distal and proximal reference sites, respectively, with the lesion length of 31.8mm, the operator implanted a stent with a 3.0mm diameter and 38mm length. A lipid-laden atheroma (5 – 12 o’clock) is observed distal to the stent (A). Struts of metallic stent are identified as reflections accompanied by back shadows (white arrows, B). Minor dissection of intima is detected between stent struts (white arrowhead, C). Significant malapposition of stent struts are visualized at the proximal edge of the stent (green arrows, D). Figure 9b: Representative frame of a volumetric laser endomicroscopy (fd-OCT) scan performed on a patient with Barrett’s esophagus with high-grade dysplasia. (A) Normal squamous epithelium demonstrates a layered architecture that transitions into Barrett’s epithelium (B) characterized by effacement of the layered architecture and irregular surface. Area of Barrett’s esophagus with highgrade dysplasia (C) characterized by high-surface signal intensity and dilated, irregular epithelial gland (arrow). Scale bar = 1 cm. Figure 9c: Endobronchial fd-OCT images of pulmonary diseases, including asthma, lung cancer and interstitial lung disease. (A) Cross sectional OCT image acquired in vivo from an individual with mild allergic asthma. The airway smooth muscle (ASM) highlighted in red was segmented utilizing circumferentially oriented optic axis information from PS-OCT. (B) A longitudinal reconstruction of squamous cell carcinoma obtained using EB-OCT. Arrows point to squamous nests located within the tumor mass. e - epithelium, c - cartilage. (C) EB-OCT image obtained from a patient with IPF highlighting the presence of honeycombing (HC), fibrosis (f) and traction bronchiectasis (TB). Scale bars = 1mm.