Director Product Management & Marketing at Wasatch Photonics Inc
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Optical coherence tomography (OCT) is a valuable technique for non-invasive imaging in medicine and biology.
In some applications, conventional time-domain OCT (TD-OCT) has been supplanted by spectral-domain OCT
(SD-OCT); the latter uses an apparatus that contains no moving parts and can achieve orders of magnitude faster
imaging. This enhancement comes at a cost, however: the CCD array detectors required for SD-OCT are more
expensive than the simple photodiodes used in TD-OCT. We explore the possibility of extending the notion of
compressed sensing (CS) to SD-OCT, potentially allowing the use of smaller detector arrays. CS techniques can
yield accurate signal reconstructions from highly undersampled measurements, i.e., data sampled significantly
below the Nyquist rate. The Fourier relationship between the measurements and the desired signal in SD-OCT
makes it a good candidate for compressed sensing. Fourier measurements represent good linear projections for
the compressed sensing of sparse point-like signals by random under-sampling of frequency-domain data, and
axial scans in OCT are generally sparse in nature. This sparsity property has recently been used for the reduction
of speckle in OCT images. We have carried out simulations to demonstrate the usefulness of compressed sensing
for simplifying detection schemes in SD-OCT. In particular, we demonstrate the reconstruction of a sparse axial
scan by using fewer than 10 percent of the measurements required by standard SD-OCT.
Recent results show that bone vasculature is a major contributor to local tissue porosity, and therefore can be directly linked to the mechanical properties of bone tissue. With the advent of third generation synchrotron radiation (SR) sources, micro-computed tomography (μCT) with resolutions in the order of 1 μm and better has become feasible. This technique has been employed frequently to analyze trabecular architecture and local bone tissue properties, i.e. the hard or mineralized bone tissue. Nevertheless, less is known about the soft tissues in bone, mainly due to inadequate imaging capabilities. Here, we discuss three different methods and applications to visualize soft tissues. The first approach is referred to as negative imaging. In this case the material around the soft tissue provides the absorption contrast necessary for X-ray based tomography. Bone vasculature from two different mouse strains was investigated and compared qualitatively. Differences were observed in terms of local vessel number and vessel orientation. The second technique represents corrosion casting, which is principally adapted for imaging of vascular systems. The technique of corrosion casting has already been applied successfully at the Swiss Light Source. Using the technology we were able to show that pathological features reminiscent of Alzheimer’s disease could be distinguished in the brain vasculature of APP transgenic mice. The third technique discussed here is phase contrast imaging exploiting the high degree of coherence of third generation synchrotron light sources, which provide the necessary physical conditions for phase contrast. The in-line approach followed here for phase contrast retrieval is a modification of the Gerchberg-Saxton-Fienup type. Several measurements and theoretical thoughts concerning phase contrast imaging are presented, including mathematical phase retrieval. Although up-to-now only phase images have been computed, the approach is now ready to retrieve the phase for a large number of angular positions of the specimen allowing application of holotomography, which is the three-dimensional reconstruction of phase images.