Interferometric Synthetic Aperture Microscopy: Inverse Scattering for Optical Coherence Tomography

Abstract

L coherence interferometry has a long history for accurately probing depths in biological media.1 In an effort to better visualize the resulting scattering maps, we used optical coherence tomography (OCT), which provided crosssectional images and three-dimensional volumes using only oneor two-dimensional scanning, respectively. Over the past 15 years, OCT has been shown to be useful in many biomedical applications, including ophthalmology, cardiology, developmental biology and microscopy.2 With the use of near-infrared light, OCT facilitates the noninvasive monitoring of cellular and nuclear organization, proliferation and functionality. A strength of OCT is its ability to recreate object structure based on first-order object scattering, provided that only the data in the confocal region of the lens was used. OCT has vastly improved the visualization of biological processes. However, it remains unable to resolve features imaged outside of the confocal region. OCT signals are akin to echo pulses from strip-map radar or sonar, where raw data are plotted in adjacent columns. OCT To demonstrate this technique, we present a collection of scatterers having a mean diameter of 2 μm suspended in silicone and imaged with cross-sectional ISAM. Our ISAM system is similar to that for spectral-domain OCT,5 except with additional instrumentation for phase stability and tighter focusing. The multiplexed raw OCT data set must maintain phase stability to ensure proper reconstruction. The figure displays (a) the original and (b) the reconstruction of an imaging area of 500 μm (transverse) by 1,000 μm (axial), where the bandwidth is 100 nm, and the spot size is 6 μm. The image resolution of point scatterers outside of the confocal region for the original experimental image data is not constant. However, for the reconstruction, the resolution is constant along the entire image with only amplitude variations. The interference between the light scattered from a group of adjacent particles (boxed) is evident in the original image (top magnified). Our method properly rephases the signal from scatterers to produce a well-resolved image (bottom magnified). This method will be extended to imaging biological samples to achieve high, spatially invariant resolution in 3D. t