Retardation measurement with capillary blood flow using enhanced polarization-sensitivity optical coherence tomography (EPS-OCT)

Abstract

Polarization-Sensitive Optical Coherence Tomography (PS-OCT) has been used to measure birefringence of biological samples, namely the retinal nerve fiber layer (RNFL). The presence of blood vessels in biological samples complicates accurate measurement of tissue birefringence as a result of the Doppler shift in fringe frequency and the shadowing effect below blood vessels due to absorption and scattering of light photons by blood. We investigate phase retardation measurement with controlled capillary blood flow overlying a birefringent sample with enhanced polarization-sensitivity optical coherence tomography (EPS-OCT). The effect of blood flow on the calculation of phase retardation and tissue birefringence was studied in the polarization domain. Light propagating through an overlying moving turbid medium (blood) undergoes single or multiple forward scattering events and a Doppler shift in presence of flow. Light propagating through an overlying medium may introduce Doppler shifts of each polarization component and/or polarization shifts or retardation of light. While undergoing multiple forward scattering, each scattering event can modify the frequency or light phase delay. In successive scattering events, potential Doppler shifts and/or polarization shifts accumulate. Light propagating within the birefringent sample undergoes multiple forward scattering events leading to phase retardation between polarization components. This paper investigates phase retardation measurement underlying physiological blood flow rates (6, 12, 18, and 24μl/min) at a range of light incident angle (0-20 deg.) on the sample. With EPS-OCT, the effect of light scattering and differential Doppler shifts between the polarization modes on the measurement of phase retardation was within our speckle noise range.