Deep Tissue Imaging with Dual Axis Optical Coherence Tomography

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2018

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Abstract

Optical imaging techniques generally offer shallow penetration depths due to high scattering in biological tissue. We have recently developed frequency domain multispectral multiple scattering low coherence interferometry (ms2/LCI) for deep tissue imaging. The ms2/LCI system offers unique spatial and angular rejection of out-of-focus photons by utilizing an off-axis interferometric setup. Multiply forward scattered light is preferentially detected for imaging at extended depths. Using tissue-mimicking phantoms that match the full scattering phase function of human dermal tissue, we demonstrate that the ms2/LCI system can provide a signal/noise ratio (SNR) improvement of 15.4 dB over conventional OCT at an imaging depth of 1 mm.

In vivo imaging is challenging for the ms2/LCI system due to its slow acquisition speed. To enable fast image acquisition, we have developed dual-axis optical coherence tomography (DA-OCT), which offers a 100-fold speed increase compared to the ms2/LCI system. Two DA-OCT systems were designed and constructed, operating at a center wavelength of 800 nm and 1300 nm respectively. The 1300 nm DA-OCT system offers up to 2 mm depth penetration in skin imaging, which is unprecedented for an OCT system. This significant improvement in penetration depth opens the door for various exciting applications in fields where conventional OCT imaging was limited by poor penetration depths.

Deep features revealed by DA-OCT can be confounded by speckle noise. Speckle is an intrinsic noise of interferometric signals which reduces contrast and degrades the quality of images. A novel frequency compounding speckle reduction technique using the Dual Window (DW) method was recently presented. Using the DW method, speckle noise is reduced without the need to acquire multiple frames. A ~25% improvement in the contrast-to-noise ratio (CNR) was achieved using the DW speckle reduction method with only minimal loss in resolution. The DW speckle reduction method can work on any existing OCT instrument without further system modification or extra components. This makes it applicable both in real-time imaging systems and during post-processing.

Finally, functional information was extracted from the raw interferometric data for diagnostic purposes. Depth-resolved spectra were calculated by a time-frequency analysis, which carry valuable localized tissue information. The spectroscopic information was first used to objectively evaluate burn injuries in an in vivo mouse model. Significant spectral differences were observed and correlated with the depth of the injury as determined by histopathology. Later, spectroscopic DA-OCT was used for the assessment of flap viability in an in vivo Macfarlane rat flap model, which exhibited a gradient in tissue perfusion along the length of the flap. These results suggest that the DA-OCT system can be used for objective evaluation of skin injuries at extended depths.

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Zhao, Yang (2018). Deep Tissue Imaging with Dual Axis Optical Coherence Tomography. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/17500.

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