Ultrahigh velocity resolution imaging of the microcirculation in vivo using color Doppler optical coherence tomography

Color Doppler optical coherence tomography (CDOCT) is a method for noninvasive cross-sectional imaging of blood flow in vivo. In previous implementations, velocity estimates were obtained by measuring the frequency shift of discrete depth-resolved backscatter spectra, resulting in a velocity resolution on the order of 1 mm/s. We present a novel processing method that detects Doppler shifts calculated across sequential axial scans, enabling ultrahigh velocity resolution (∼1 micron/s) flow measurement in scattering media. This method of sequential scan processing was calibrated with a moving mirror mounted on a precision motorized translator. Latex microspheres suspended in deuterium oxide were used as a highly scattering test phantom. Laminar flow profiles down to ∼15 micron/s centerline velocity (0.02 cc/hr) were observed with a sensitivity of 1.2 micron/s. Finally, vessels on the order of 10 microns in diameter were imaged in living human skin, with a relative frequency sensitivity less than 4 × 10-5. To our knowledge, these results are the lowest velocities ever measured with CDOCT.