Hongwu Ren

High-resolution optical coherence tomography over a large depth range with an axicon lens

In optical coherence tomography, axial and lateral resolutions are determined by the source coherence length and the numerical aperture of the sampling lens, respectively. Whereas axial resolution can be improved by use of a broadband light source, there is a trade-off between lateral resolution and focusing depth when conventional optical elements are used. We report on the incorporation of an axicon lens into the sample arm of an interferometer to overcome this limitation. Using an axicon lens with a top angle of 160 degrees , we maintained 10-microm or better lateral resolution over a focusing depth of at least 6 mm. In addition to having high lateral resolution, the focusing spot has an intensity that is approximately constant over a greater depth range than when a conventional lens is used.

Real-time phase-resolved functional optical coherence tomography by use of optical Hilbert transformation

We have developed a novel real-time phase-resolved functional optical coherence tomography system that uses optical Hilbert transformation. When we use a resonant scanner in the reference arm of the interferometer, with an axial scanning speed of 4 kHz, the frame rate of both structural and Doppler blood-flow imaging with a size of 100 by 100 pixels is 10 Hz. The system has high sensitivity and a larger dynamic range for measuring the Doppler frequency shift that is due to moving red blood cells. Real-time images of in vivo blood flow in human skin obtained with this interferometer are presented.

Real-time phase-resolved optical coherence tomography and optical Doppler tomography

We have developed a novel real-time phase-resolved optical coherence tomography (OCT) and optical Doppler tomography (ODT) system using optical Hilbert transformation. By combining circularly polarized reference and linearly polarized sample signals, in-phase and quadrature interference components are produced in separate channels and treated as the real and imaginary parts of a complex signal to obtain the phase information directly. Using a resonant scanner at an axial scanning speed of 4 kHz in the reference arm of the interferometer, both structure and blood flow velocity images with 200 axial scans can be acquired at 20 frames per second with high sensitivity and large dynamic range. Real-time videos of in vivo blood flow in the chick chorioallantoic membrane using this interferometer are presented.

Phase-resolved functional optical coherence tomography: simultaneous imaging of in situ tissue structure, blood flow velocity, standard deviation, birefringence, and Stokes vectors in human skin

We describe a phase-resolved functional optical coherence tomography system that can simultaneously yieldin situ images of tissue structure, blood flow velocity, standard deviation, birefringence, and the Stokes vectors in human skin. Multifunctional images were obtained by processing of analytical interference fringe signals derived from two perpendicular polarization-detection channels. The blood flow velocity and standard deviation images were obtained by comparison of the phases from pairs of analytical signals in neighboring A-lines in the same polarization state. The analytical signals from two polarization-diversity detection channels were used to determine the four Stokes vectors for four reference polarization states. From the four Stokes vectors, the birefringence image, which is not sensitive to the orientation of the optical axis in the sample, was obtained. Multifunctional in situ images of a port wine stain birthmark in human skin are presented.

Three-dimensional reconstruction of in vivo blood vessels in human skin using phase-resolved optical Doppler tomography

Phase-resolved optical Doppler tomography (ODT) has very high sensitivity while maintaining a fast axial scanning rate, making it possible to reconstruct blood flow in three dimensions. Here we demonstrate an ODT system that employed novel signal-processing techniques to remove aliasing effects and artifacts caused by lateral scanning and target movement. The results show that these techniques not only simplified ODT, but also improved lateral scanning speed. Using these signal-processing techniques, three-dimensional images of in vivo blood vessels in human skin were then reconstructed.

Imaging and quantifying transverse flow velocity with the Doppler bandwidth in a phase-resolved functional optical coherence tomography

The Doppler bandwidth extracted from the standard deviation of the frequency shift in phase-resolved functional optical coherence tomography (F-OCT) was used to image the velocity component that is transverse to the optical probing beam. It was found that above a certain threshold level the Doppler bandwidth is a linear function of flow velocity and that the effective numerical aperture of the optical objective in the sample arm determines the slope of this dependence. The Doppler bandwidth permits accurate measurement of flow velocity without the need for precise determination of flow direction when the Doppler flow angle is within +/-15 degrees perpendicular to the probing beam. Such an approach extends the dynamic range of flow velocity measurements obtained with the phase-resolved F-OCT.

Accurate measurement of blood vessel depth in port wine stained human skin in vivo using pulsed photothermal radiometry

We report on application of pulsed photothermal radiometry (PPTR) to determine the depth of port wine stain (PWS) blood vessels in human skin. When blood vessels are deep in the PWS skin (>100 microm), conventional PPTR depth profiling can be used to determine PWS depth with sufficient accuracy. When blood vessels are close or partially overlap the epidermal melanin layer, a modified PPTR technique using two-wavelength (585 and 600 nm) excitation is a superior method to determine PWS depth. A direct difference approach in which PWS depth is determined from a weighted difference of temperature profiles reconstructed independently from two-wavelength excitation is demonstrated to be appropriate for a wider range of PWS patients with various blood volume fractions, blood vessel sizes, and depth distribution. The most superficial PWS depths determined in vivo by PPTR are in good agreement with those measured using optical Doppler tomography (ODT).

Power optical Doppler tomography imaging of blood vessel in human skin and M-mode Doppler imaging of blood flow in chick chrioallantoic membrane

We describe power optical Doppler tomography (ODT) imaging in phase-resolved optical coherence tomography (OCT) capable of providing the precise location of blood flow in human skin. The power Doppler signal is the squared amplitude of the Doppler signal. By properly setting the intensity threshold and priority displaying Doppler power in phase-resolved OCT, we obtained a Doppler power tomography image of the blood flow in human skin. Power Doppler tomography uses only the amplitude information, so it is not susceptible to aliasing and Doppler flow angle and provides more accurate and smooth imaging of the location of the blood vessels in human skin than Doppler velocimetry. We also modified the phase-resolved algorithm we published before and used it to do Doppler tomography and M-mode Doppler imaging. The dynamic of blood flow in chick chorioallantoic membrane (CAM) was studied using M-mode Doppler imaging.

Phase-resolved polarization sensitive optical coherence tomography imaging of tendon and muscle

We describe a phase-resolved polarization sensitive optical coherence tomography system that can obtain the Stokes vectors, polarization diversity intensity, and birefringence images of rat-tail tendon and muscle. The Stokes vectors were obtained by processing the analytical interference fringe signals from two perpendicular polarization-detection channels for the same reference polarization state. From the four Stokes vectors, the birefringence image, which is insensitive to orientation of the optical axis in the sample, and the polarization diversity intensity image, in which speckle noise is greatly reduced, were obtained. The birefringence changes in the rat muscle caused by freezing were investigated using phase-resolved polarization sensitive optical coherence tomography. It was found that freezing degrades birefringence in rat muscle.

High-resolution optical coherence tomography using self-adaptive FFT and array detection

We developed a novel optical coherence tomographic (OCT) system which utilized broadband continuum generation for high axial resolution and a high numeric-aperture (N.A.) Objective for high lateral resolution (<5 micrometers ). The optimal focusing point was dynamically compensated during axial scanning so that it can be kept at the same position as the point that has an equal optical path length as that in the reference arm. This gives us uniform focusing size (<5 mum) at different depths. A new self-adaptive fast Fourier transform (FFT) algorithm was developed to digitally demodulate the interference fringes. The system employed a four-channel detector array for speckle reduction that significantly improved the images signal-to-noise ratio.