Optical Computational Imaging

Adaptive Spectal Adopization In optical coherence tomography (OCT), as in other imaging modalities, one desires to achieve the highest possible resolution given instrument limitations. The bandwidth of the OCT source chiefly determines the useful resolution. However, if the spectrum is not smooth, then the point response in the interferogram will have large sidelobes that cause a degradation of effective resolution and introduce artifacts. As a result, a smooth, Gaussian-like spectrum is often employed to minimize sidelobes. However, there are many sources that do not produce smooth spectra but still produce a wide bandwidth, such as Ti-sapphire oscillators, nonlinear supercontinuum generation optical fibers including microstructured and tapered fibers, and ultrahigh-numerical-aperture fiber sources. We have created a method that produces an estimate of object scattering density (or reflectivity profile) that minimizes sidelobes while not overly increasing the contribution of noise. This method can make broad spectrum sources much more useful for high-resolution OCT.

Fast FPGA/DSP We have implemented a real-time, multi-dimensional, all digital, OCT acquisition and processing system for use with OCT and doppler imaging. The system uses a high speed A/D converter to acquire data from OCT optical systems that employ a high-speed delay line. Using a floating-point DSP processor and a field programmable gate array, the acquisition and processing system demodulates, filters, and computes the magnitude and phase shifts of the input signal. The data is read by a host machine and displayed on screen at real-time rates. This system offers flexible acquisition and processing parameters for a wide range of optical microscopy techniques.

Dispersion Compensation Algorithms Practical clinical optical coherence tomography systems require automatic tools for identifying and correcting flaws in OCT images. One type of flaw is the loss of image detail owing to the dispersion of the medium, which in most cases is unknown. We have developed an autofocus algorithm for estimating the delay line and material dispersion from OCT reflectance data. This autofocus algorithm can be used in conjunction with a high-speed, digital-signal-processor-based OCT acquisition system for rapid image correction.

Projected Index Computed Tomography (PICT) Projected index computed tomography (PICT) is an imaging technique that provides a computed reconstruction of the index of refraction of a sample. PICT makes use of data from standard optical coherence tomography images taken from several view angles to determine a mapping of the refractive indices of the sample. A rectilinear propagation model is assumed, so the data are understood to be related to the line integral of the refractive index in the beam paths. These data thus provide a set of angular projections of the sample. The spatial distribution of the index of the object may then be reconstructed by use of standard filtered backprojection techniques. The resultant PICT images are free of the spatial distortion that is inherent in standard optical cross-sectional images and correspond well to the manufactured dimensions of specific samples. PICT has the potential to produce high-resolution, highly sensitive, and spatially accurate images of a variety of biological and material samples.

Image Processing It is possible to achieve Doppler imaging with OCT when imaging blood vessels or any other movement. When blood flows towards or away from the transducer, the frequency of the backscattered light varies depending on the velocity of the fluid the light is incident upon. This shift can then be used to calculate the velocity and direction of the blood. This technology is especially useful in the study of microfluidics. Below is an example of Doppler imaging in microfluidics. The left image is a cross-section of a tubular microfluidic channel after Doppler OCT processing. On the right is a graph showing the flow velocity measured vs. the ideal flow profile.

Speckle Reduction in OCT Images In OCT, ultrasound, synthetic-aperture radar, and other coherent ranging methods, speckle can cause spurious details that detract from the utility of the image.  Speckle is a problem inherent to imaging densely scattering objects with limited bandwidth.  We have developed methods to minimize speckle and improve image quality while maintaining features that are consistent with the known data.

Interferometric Synthetic Aperture Microscopy (ISAM) State-of-the-art methods in high-resolution three-dimensional optical microscopy requires that the focus be scanned through the entire region of interest. However, an analysis of the physics of the light-sample interaction reveals that the Fourier space coverage is independent of depth. We have shown that be solving the inverse scattering problem for coherence microscopy, computed reconstruction yields volumes with a resolution in all planes that is equivalent to the resolution achieved only at the focal plane for conventional high-resolution microscopy. The entire illuminated volume has spatially-invariant resolution, thus eliminating the compromise between resolution and depth-of-field. This novel computational image-formation technique is called ISAM, and has the potential to broadly impact real-time 3-D microscopy and analysis in the fields of cell and tumor biology, as well as in clinical diagnosis where in vivo imaging is preferable to biopsy. We have analyzed and demonstrated this technique for rectangular Cartesian-coordinate imaging, for rotational catheter-based imaging, and for full-field imaging.