Dual Axis Optical Coherence Interferometry

Dual Axis Optical Coherence Interferometry (DA-OCT) is a derivative of optical coherence tomography (OCT) capable of collecting low-order scattered photons to achieve greater depth range than traditional OCT. The general principles of DA-OCT arise from the tendency for multiple scattered photons to emerge further away from the illumination path than ballistic photons. While OCT is capable of achieving high resolution images, it is limited to imaging only superficial layers of tissue due to its limited penetration depth.. By spatially separating the illumination and collection beams, DA-OCT can extend beyond superficial tissue layers

Theory

History of Development

Multispectral multiple scattering low coherence interferometry (m2/LCI) was an early predecessor to DA-OCT that achieved the detection of photons that had undergone multiple scattering events by combining the principles of coherence gating and an off-axis illumination and detection scheme .

Photons that have been forward scattered in a medium with high anisotropy still contain valuable structural information about the tissue. Unfortunately, traditional OCT is incapable of detecting these photons due to the principles of coherence gating. Other methods such as diffuse optical tomography are able to achieve imaging depths in the range of centimeters by detecting these multiple scattered photons but suffer from major trade-offs in resolution.

DA-OCT is an advancement of this m2/LCI method that significantly improves imaging speed by implementing a microelectro-mechanical system (MEMS) mirror to laterally scan across the tissue surface. Additionally, the point at which the detection and illumination beams are intersected lies much shallower in the tissue, allowing for easier detection of ballistic photons that are essential to OCT modalities.

Monte Carlo Simulation Study

While DA-OCT has produced improved OCT images at depth, the mechanisms behind it are not well quantified. To address this, numerical analysis in the form of Monte Carlo simulations have been conducted. These studies dumped a billion photons along different illumination paths and observed the location and angle at which multiple scattered photons emerged from the tissue. When orienting the illumination and collection beams at the correct location and angle, the DA-OCT signal-to-background ratio can be optimized and offer improvement over traditional OCT

Extended Depth of Focus Method

While simulations and qualitative analysis have shown that DA-OCT offers improved imaging at extended depths, some quantitative analysis has shown that it offers little improvement over OCT. One reason for this is due to the limited depth of focus (DOF) that inherently arises from the dual axis geometry of DA-OCT. To address this, a recent study has added a method of scanning the center of focus through the sample using a tunable lens. This method has shown significant improvements in the contrast-to-noise ratio when compared to an OCT system using most of the same system components.

References

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