Flow velocity estimation in optical Doppler tomography and a preliminary study on radiation detection for hybrid optical coherence tomography/scintigraphy

Coherent artifacts can severely degrade OCT image quality by introducing false targets, cancellation of the true targets or display of incorrect echo amplitudes of the targets. In this thesis, we demonstrate that a non-linear deconvolution algorithm, CLEAN, can be used to reduce coherent artifacts in OCT system. We have modified CLEAN and adapted it to a conventional OCT system, and have shown that the artifacts can be effectively reduced to background noise level. ^ In our OCT system modification, a rapid scanning optical delay line based on Littrow-mounting of diffraction grating is introduced. This Littrow-mounting configuration has the minimum loss of reference light power among grating-based delay lines; and for a balanced detection OCT setup, the resulted SNR improvement is about 3dB compared with other grating-based delay lines. ^ The clinical importance of optical Doppler tomgraphy (ODT) is localizing directional blood flow with high spatial resolution and noninvasive detection. The existing blood flow estimation techniques in ODT detect only axial or transverse velocity component by using Doppler shift or Doppler bandwidth measurement individually. However, accurate estimation of blood flow velocity requires the knowledge of Doppler angle, which is not available in general clinical applications. In this thesis, we describe a new method, which simultaneously calculates Doppler angle and flow velocity with a conventional single-beam ODT system by combining Doppler shift and Doppler bandwidth measurements. Evaluations of this method by numerical simulation and experiment show that beyond certain signal-to-noise ratio (SNR), the Doppler angle and flow velocity can be well recovered. In vivo study on human lip microvascularization demonstrates that this method is capable of providing both flow speed and flow direction information. ^ Optical coherence tomography (OCT) has limited functional imaging capability notwithstanding it can provide high-resolution subsurface imaging. Nuclear imaging gives tissue functional activity information while with lower spatial resolution. Combining OCT with nuclear imaging could obtain high-resolution and high-contrast detection of blood-vessel wall structure and function simultaneously. In this thesis, we present a prototype setup for combined circumferential OCT imaging and single channel radiation counting to demonstrate the feasibility of hybrid OCT/scintigraphy.