Swept Source Optical Coherence Tomography (SSOCT) provides non-invasive, high-resolution 2D and 3D imaging of biological tissues. Recently, Thorlabs SS-OCT system has been applied for real-time monitoring and guidance during vascular surgical procedures due to its high imaging speed and deeper light penetration into tissue at 1.3 um wavelength.
Figure 1. The 3D OCT data of in vitro porcine artery wall taken with a standard SS-OCT system. The layered structure of the blood vessel wall comprising intima (I), media (M), and adventitia (A) can be visualized.
Despite the usefulness of 3D OCT for blood vessel imaging, there are still many clinic application challenges exist. One critical requirement for OCT vascular imaging is the successful identification of the boundary between the different layers in the vessel wall. This is difficult due to the fact that the lesions, fatty material and calcified tissue deposited on the vessel wall change the layered structure, making detection more difficult using standard OCT imaging techniques. To address this challenge, a swept source polarization sensitive OCT (PSOCT) system was developed to improve image contrast and accuracy in tissue differentiation, by highlighting specific tissue types that possess optical birefringence.
PSOCT provides cross-sectional images of phase retardation, birefringence or optical axis information in the sample with high spatial resolution (~10 microns). It analyzes the polarization property of light propagation in sample to extract the birefringence-induced phase retardation using polarization modulated light and a polarization diversity detection scheme. This allows simultaneously mapping the sample structure information based on the intensity signals, and birefringence information based on phase retardation signals. PSOCT can detect intrinsic birefringence in the sample such as optical crystals or biological tissues, and serve as a useful technical to enhance image contrast.
The schematic of the fiber-based PS-OCT system is shown in figure 2. The system incorporates a broadband, high-speed frequency swept laser centered at 1325 nm (SL1325-P16), which supports an axial resolution <10 µm in tissue. This PS-OCT system uses a high-speed electro-optic modulator (Novaphase EO-PM-NR-C3) and other polarization optical elements to modulate the light polarization states of the sample illumination through a Michelson interferometer. Two orthogonal polarization states of the interference fringe signals are detected using a fiber polarization beam splitter (OFR PFS-1300-S/P/P). Advanced signal processing algorithms are used to calculate birefringence induced phase-retardation images which are acquired at the full-speed of the swept laser source (16kHz).
Figure 2. Schematic of Thorlabs swept source PSOCT system. SS: swept source, PC: polarization controller, PM: polarization modulator, CIR: circulator, FC: fiber coupler, C: collimator, DC: dispersion compensator, VA: variable attenuator, M: mirror, PBS: polarization beam splitter, D: detector, DAQ: data acquisition board, AO: analog output card, SD: XY scanners driver, CCD: CCD camera, OBJ: objective, MS: microscope, AL: Aiming laser.
The advantage of OCT combined with polarization detection was demonstrated for blood vessel imaging. One blood vessel was cut open to allow the OCT beam to image the vessel wall. A standard OCT image of the sample is shown in figure 3a. This image also shows the cross-section of a second vessel (V) buried under the first vessel. Figure 3b shows a PSOCT phase retardation image of the same sample. Here the adventitia layer of the vessel wall is more clearly identified due to the abundant birefringence of the collagen fibrils.
Figure 3. OCT (3a) and PSOCT (3b) images of the porcine artery wall. The media (M) and adventitia (A) layers can be visualized in the OCT structural images. Another vessel (V) buried in the deep adventitia layer can be also visualized. The PSOCT image highlights the adventitia layer where the collagen fibers are abundant to cause strong optical birefringence induced polarization phase retardation. Image size: 4 mm × 2 mm (width × depth).
Another PSOCT sample data set is shown in Figure 4. Figure 4a shows the standard OCT image of the porcine artery wall, while Figure 4b shows the phase retardation image of the same sample obtained simultaneously. Figure 4c is the spatially averaged phase retardation image. The strong tissue optical birefringence induced polarization phase-retardation was easily detected, due to the collagen-rich media and adventitia layers in the blood vessel wall. PS-OCT demonstrates the improved image contrast compare to standard OCT, making this technique promising for real-time monitoring tissue layers by providing high-resolution images with excellent contrast highlighting specific layers in the tissue.
Figure 4. (a) Standard OCT image of porcine artery wall showing the media (M) and adventitia (A) layers. (b) Phase retardation image acquired from same sample using PSOCT system showing strong tissue optical birefringence in the adventitia layer. (c) Spatially averaged phase retardation image showing improved image contrast in identifying different tissue types in the vessel wall.
PSOCT has been shown to be an extremely useful technique for real-time monitoring and guiding of medical procedures as well as material diagnostics. Typical applications of PSOCT include vascular imaging of blood vessel for plaque detection, early detection of caries and other dental applications, diagnosis of glaucoma and many retinal diseases, determination of burn depth in the skin, and non-invasive detection of strain in industry materials.