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Intra- and Inter-Frame Differential Doppler Optical Coherence Tomography

Intra- and Inter-Frame Differential Doppler Optical Coherence Tomography

Tilman Schmoll, Ioana R. Ivascu, Amardeep S. G. Singh, Cedric Blatter, Rainer A. Leitgeb
Key words: optical coherence tomography; phase shifting; Doppler OCT; optical angiography; functional imaging.
2015, volume 7, issue 1, page 34.

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Spectrometer based optical coherence tomography suffers from fringe washout for fast flow, a drawback for flow visualization, which is of interest for both lable-free optical angiography and flow quantification. We presented a method, which can be used to contrast very fast flows, while maintaining relatively low A-scan rates. It is based on introducing a phase shift of 01.jpg during acquisition such that the interference fringes associated to moving sample structures are recovered depending on the axial velocity. This enables the use of slower line scan cameras for measuring the fast blood flows within the large vessels in the region of the optic nerve head, while keeping good sensitivity. Furthermore it can be used to contrast the fast moving structures by attenuating the static and slower moving tissue signals. A drawback is still the narrow velocity bandwidth, which is not optimal for providing optical angiography for the full vascular plexus. Nevertheless, it could be of value for following fast dynamic changes, as for example in optical elastography. We introduce different variants of this method, based on intra-frame phase switching, as well as between consecutive A-scans and B-scans, which are then pairwise summed. A phase shift of 01.jpg between summed scans is in fact equivalent to subtraction of both in general complex valued scans. Analyzing complex signal differences between successive B-scans allows in particular contrasting very slow flows of the capillary network. After setting the theoretical framework we show proof-of-principle measurements with a piezo mirror, as well as in vivo measurements of the human retina for the different intra-frame phase shifting schemes. We further show the capability to contrast the parafoveal capillary structure with the differential inter B-scan method and discuss its limitations. Improvement of the sensitivity might be achieved by increasing the number of B-scans to be used for calculating the signal differences, as well as by increasing the sampling density. Care must then be taken for in vivo imaging to keep the total measurement time still sufficiently small, typically a few seconds. The method may proof as a valuable diagnostic tool, as several retinal diseases will manifest at an early stage in capillary disorders.


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