A new method is presented, called Spectral Contrast Optical Coherence Tomography, which utilizes the visible spectrum of blood instead of doppler or speckle contrast to locate blood vessels. This is seen as a significant improvement for OCT angiography, since sample motion no longer affects vessel contrast, repetitive scanning is not required, and non-flowing blood can be imaged. A visible Optical Coherence Tomography system from 500-700 nm was used and the differential spectral intensity of two short time Fourier Transform Kaiser sampling windows centered at 557 nm and 620 nm provided contrast revealing blood vessel location. This approach allows for single-scan endogenous contrast angiography all the way down to the capillary level. We demonstrate the method by imaging the vasculature of human oral mucosa and the lymphatics and vasculature of freshly sacrificed mouse tissue.
Alterations to nanoscale structures, lymphatics, and microvasculature are early hallmarks of neoplasia as well as a variety of other diseases. Unfortunately, nanoscale alterations and microvasculature function, such as oxygen saturation, cannot be probed by histology. Furthermore, properly evaluating lymphatic and microvasculature organization can be challenging with histological slices. Optical Coherence Tomography (OCT) offers a promising noninvasive solution to evaluating these biomarkers in 3D in vivo.
OCT has shown the ability to provide 3D maps of vasculature with flow rate and blood oxygenation, as well as, lymphatic organization with a resolution on the order of 1-10 microns. Our group has established Inverse Spectroscopic OCT (ISOCT), which measures nanoscale mass density tissue fluctuations and can distinguish between histologically normal cancerous and noncancerous tissue. However, the most influential underlying assumption that allows the distinction between subdiffractional structural alterations in tissue is that the region of interest (ROI) includes a homogenous tissue type with similar scattering and absorption properties. Therefore, the highly absorbing blood and low scattering lymphatics must be excluded from analysis.
Traditional OCT techniques to isolate vasculature and its spectra require timely repetitive scanning protocols, and the commonly utilized near infrared operating bandwidths require vessel-like filters to locate lymphatics. Herein we show how vasculature location and spectra can be extracted with a single visible OCT scan. Additionally, we demonstrate the high image contrast from visible OCT allows lymphatic location to be well defined. Finally, we show ultrastructural metrics fall within physiologically reasonable ranges after excluding vasculature and lymphatics from the ROI.
Many of the earliest structural changes associated with neoplasia occur on the micro and nanometer scale, and thus appear histologically normal. Our group has established Inverse Spectroscopic OCT (ISOCT), a spectral based technique to extract nanoscale sensitive metrics derived from the OCT signal. Thus, there is a need to model light transport through relatively large volumes (< 50 um^3) of media with nanoscale level resolution.
Finite Difference Time Domain (FDTD) is an iterative approach which directly solves Maxwell’s equations to robustly estimate the electric and magnetic fields propagating through a sample. The sample’s refractive index for every spatial voxel and wavelength are specified upon a grid with voxel sizes on the order of λ/20, making it an ideal modelling technique for nanoscale structure analysis.
Here, we utilize the FDTD technique to validate the nanoscale sensing ability of ISOCT. The use of FDTD for OCT modelling requires three components: calculating the source beam as it propagates through the optical system, computing the sample’s scattered field using FDTD, and finally propagating the scattered field back through the optical system. The principles of Fourier optics are employed to focus this interference field through a 4f optical system and onto the detector.
Three-dimensional numerical samples are generated from a given refractive index correlation function with known parameters, and subsequent OCT images and mass density correlation function metrics are computed. We show that while the resolvability of the OCT image remains diffraction limited, spectral analysis allows nanoscale sensitive metrics to be extracted.