Doppler standard deviation imaging for clinical monitoring of in vivo human skin blood flow

High speed, wide velocity dynamic range Doppler optical coherence tomography (Part I): System design, signal processing, and performance

Improvements in real-time Doppler optical coherence tomography (DOCT), acquiring up to 32 frames per second at 250 x 512 pixels per image, are reported using signal processing techniques commonly employed in Doppler ultrasound imaging. The ability to measure a wide range of flow velocities, ranging from less than 20 microm/s to more than 10 cm/s, is demonstrated using an 1.3 microm DOCT system with flow phantoms in steady and pulsatile flow conditions. Based on full implementation of a coherent demodulator, four different modes of flow visualization are demonstrated: color Doppler, velocity variance, Doppler spectrum, and power Doppler. The performance of the former two, which are computationally suitable for real-time imaging, are analyzed in detail under various signal-to-noise and frame-rate conditions. The results serve as a guideline for choosing appropriate imaging parameters for detecting in vivo blood flow.

Quantitative assessment of flow velocity-estimation algorithms for optical Doppler tomography imaging

We present a quantitative comparison of three categories of velocity estimation algorithms, including centroid techniques (the adaptive centroid technique and the weighted centroid technique), the sliding-window filtering technique, and correlation techniques (autocorrelation and cross correlation). We introduce, among these five algorithms, two new algorithms: weighted centroid and sliding-window filtering. Simulations and in vivo blood flow data are used to assess the velocity estimation accuracies of these algorithms. These comparisons demonstrate that the sliding-window filtering technique is superior to the other techniques in terms of velocity estimation accuracy and robustness to noise.

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.

Long-wavelength continuum generation about the second dispersion zero of a tapered fiber

A dispersion zero near 1300 nm of a narrow-diameter tapered fiber is used to generate broadband, near-infrared light. When femtosecond pulses at 1260 nm with 750 pJ of energy are launched in proximity to the second zero-dispersion wavelength, a continuum spanning 1000-1700 nm is produced.

Simultaneous intensity, birefringence, and flow measurements with high-speed fiber-based optical coherence tomography

We demonstrate that tissue structure, birefringence, and blood flow can be imaged simultaneously by use of techniques of polarization-sensitive optical coherence tomography and phase-resolved optical Doppler tomography. An efficient data-acquisition procedure is implemented that optimizes the concurrent processing and display of all three image types. Images of in vivo human skin acquired with a high-speed fiber-based system are presented.

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.

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.

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.

Active skin cooling in conjunction with laser dermatologic surgery

The clinical objective in the laser treatment of patients with specific dermatoses is to maximize thermal damage to the target chromophore while minimizing injury to the normal skin. Unfortunately, for some lesions, the threshold incident light dosage for epidermal injury can be very close to the threshold for permanent removal of the target chromophore, thus precluding the use of higher light dosages. An important method of overcoming the aforementioned problem is to selectively cool the most superficial layers of the skin. Although melanin absorption will result in heat production during laser exposure, cooling the epidermis can prevent its temperature elevation from exceeding the threshold for thermal injury. Spatially selective cooling can be achieved by active cooling using a cryogen spray or cold sapphire contact handpieces. These devices promote rapid and spatially selective epidermal cooling to low temperatures without affecting the target chromophore temperature before the laser pulse is delivered. Cooling has become an Integral part in the emerging discipline of laser dermatologic surgery. Attend almost any academic dermatology conference and you are likely to find many lectures that relate to cooling during dermatologic laser surgery. Although cooling in conjunction with laser therapy has become the clinical standard for many laser procedures, considerable controversy surrounds this methodology. We present herewith an overview of currently used techniques for active cooling of human skin and explore their advantages and disadvantages in relationship to specific dermatoses amenable to laser therapy.