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Biomedical Optics & Medical Imaging

High-speed, full-range optical coherence tomography

A new technique to remove complex-conjugate artifacts inherent to conventional Fourier-domain interferometry can double accessible imaging depth.
13 October 2009, SPIE Newsroom. DOI: 10.1117/2.1200909.1812

In the past decade, optical coherence tomography (OCT)1 has emerged as an important, noninvasive tool for depth-resolved imaging of biological structures. Current developments are shifting its focus toward spectral-domain and swept-source techniques, since these frequency-domain OCT (FDOCT) versions have better acquisition speed and sensitivity than time-domain imaging.2 However, a major drawback limiting FDOCT's practical application is the complex-conjugate ambiguity. The detected, real-valued spectral interferogram is Fourier transformed to localize the scatter in the sample. The Fourier transform of a real-valued function is Hermitian (i.e., its complex conjugate is equal to the original function with the variable changed in sign), so the reconstructed image is symmetrical with respect to the interferometer's zero-phase delay. This leads to ambiguity in interpreting the resulting OCT images. Overcoming this requires placing the sample entirely within positive or negative space, which leaves only half of the imaging depth available in practice. Resolving the complex-conjugate ambiguity can double the imaging depth. In turn, this provides additional flexibility to explore the most sensible measurement range near the zero-delay line.3

Researchers proposed several methods for achieving full-range complex imaging for FDOCT. In spectral-domain OCT (SDOCT), phase-shifting methods were among the first approaches attempting to achieve full-range imaging.4 However, they have limited accuracy and constrain the imaging speed and complex-conjugate rejection ratio, since several phase steps must occur at the same sampling location. Instead, scientists proposed an alternative technique. It uses a 3×3 optical coupler as a phase-shifting element and enables simultaneous detection of two quadrature phase-shifted interferograms to obtain complex, ambiguity-free images.5 This approach requires two separate detectors for image acquisition, which adds cost and complexity and thus limits its practical application. For high-speed, full-range complex imaging, we (as well as others) proposed a second alternative scheme in which a constant modulation frequency is introduced into the spatial interferogram during transverse sample scanning.6,7 Here, the complex data is obtained through either Fourier or Hilbert transformation of the B-scan (2D, cross-sectional) interferogram in the transverse direction. This enables us to achieve full-range complex imaging at high speed. Yet this method still needs a piezo stage to provide modulation in the interferograms, limiting the system's full imaging speed because of the stage's mechanical movement.

We recently introduced a new FDOCT technique for in vivo imaging, in which the carrier frequency is introduced by the galvo scanner8 used for transverse scanning of the sample (i.e., in the x direction). To achieve this, we offset the sample beam from the x scanner's pivot axis. This causes path-length modulation and thus introduces a modulation frequency into the B-scan interferograms. The main advantage of this new method is that it does not need any additional phase-shifting elements to realize frequency modulation, while the modulation frequency is given by the system itself. An additional, important advantage of using frequency modulation is that it is relatively insensitive to sample movement, which is critical for in vivo applications. These aspects may allow us to achieve high-speed, full-range complex SDOCT without additional hardware requirements, and without limiting imaging speed.

For high-speed SDOCT applications, another factor that could constrain the system's imaging speed is its data-acquisition capability, particularly its data-flow management, since the higher speed requires more computing power to rapidly capture, process, and display imaging information. To improve the system's acquisition rate, we developed custom software using the Visual C++ platform, enabling the acquired raw data to connect with the computer's random access memory directly, thus optimizing the data-streaming process.

We have implemented an SDOCT system based on the beam-offset method, in conjunction with an algorithm that exploits Hilbert transformation6 to obtain full-range complex imaging. The resulting system can use the camera's full imaging speed. We have used a line-scan camera with a maximum speed of 47,000 A scans (depth-resolved scattering profiles) per second to obtain full-range imaging. Before reconstructing the OCT image, we first converted the captured spectral data from optical wavelength to optical wavenumber space. The DC autocorrelation terms (where DC refers to the mean value of the waveform) were subsequently removed from the A scans by subtracting the reference spectrum.9 We obtained the latter from averaging the spectrogram over the B scans. Before Hilbert transformation to construct the interferogram's analytical function, we applied a Fourier filtering technique around the zero-frequency region of the time-variable interferogram to suppress self-correlation artifacts.

We have used our full-range imaging system for a wide variety of applications. We demonstrated the technique's feasibility for high-speed in vivo imaging in human anterior-eye segments, nail folds, skins, and chicken embryos at a speed of 47,000 A scans/s (see Figures 1 and 2).

Figure 1. Optical coherence tomography (OCT) images from real-time videos of (top) the anterior chamber of the human eye and (bottom) a three-day-old chicken embryo. (a) and (c) Standard Fourier-domain OCT (FDOCT). (b) and (d) Full-range complex FDOCT. The images' physical size is 2.7×4.4mm2(lateral × depth).

Figure 2. OCT images from real-time videos of (top) the palm skin and (bottom) finger nail near the nail fold region of a human volunteer in vivo. (a) and (c) Standard FDOCT. (b) and (d) Full-range complex FDOCT. The images' physical size is 2.7×4.4mm2(lateral × depth).

In summary, we have developed a novel technique to achieve full-range complex FDOCT imaging for in vivo applications, without the need for additional phase-shifting devices. Since the phase modulation necessary for complex-signal reconstruction is given by the system itself, we can use the camera's full imaging speed without any limitations. Our ongoing studies focus on developing ultrahigh-speed, full-range, complex FDOCT systems based on complementary metal oxide semiconductor (CMOS) detectors for ophthalmic applications. CMOS cameras are suitable for imaging the anterior and posterior chambers of the eye, since the high imaging speed can overcome motion artifacts, which are unavoidable in human retinal imaging. Moreover, using CMOS one can select active pixels, thus offering more freedom to choose imaging parameters and time.

Our research is currently supported by grants from the National Institutes of Health (R01 HL093140, R01 EB009682, and R01 DC010201) and the American Heart Association (0855733G).

Ruikang Wang
Oregon Health and Science University
Portland, OR

Ruikang Wang is a professor of biomedical engineering, anesthesiology, and peri-operative medicine, and head of the Biophotonics and Imaging Laboratory. His main research interests include high-resolution functional optical imaging using coherence gating and confocal gating techniques applied to healthcare, optical biopsy and functional imaging in tissue engineering, photoacoustic imaging, and light propagation in biological tissue.