Doppler optical coherence tomography (DOCT) provides a novel method to measure blood flow velocity in vessels with diameter at micrometer scale. In this study, a developed spectral domain DOCT system is applied to monitor cerebral blood flow velocity changes in a rat. An animal model with a cranial window is used, and by application of a drug, light, and electric stimulations, changes in blood flow velocity of the pial artery in sensory cortex are measured in real time. The results show significant differences in blood flow velocity before and after drug administration or light and electric stimulations, demonstrating the feasibility of DOCT in cerebral microcirculation study. Given its noninvasive nature, high spatial resolution, high velocity sensitivity, and high imaging speed, DOCT shows great promise in brain research by imaging blood flow changes at micrometer scale vessels, which helps to understand the pathogenesis of cerebral diseases and neurodegenerative diseases.
We develop a high speed multi-functional spectral domain (SD-OCT) system, using a broadband light
source centered at 835nm and a custom-built spectrometer with a spectral resolution of 0.0674nm.
After precise spectral calibration of the spectrometer, non-uniform discrete Fourier transform (NDFT)
of the acquired spectral data is adopted for image reconstruction. In vivo imaging of human finger from
volunteer at A-scan rate of 29 kHz with different reconstruction methods reveal that with the NDFT
method, improved sensitivity fall-off especially at large depth is achieved in contrast to the
conventional discrete Fourier transform (DFT) with interpolation method. And the phase information of
the spectral data is used for functional extension to velocity mapping, Doppler images of rats' cerebral
arteries at A-scan rate of 20 kHz are demonstrated. Furthermore, based on the spatial sinusoidal
modulation in the reference arm, the complex spectral interferogram can be retrieved by harmonics
analysis and digital synchronous demodulation which is free of mirror image and the coherence noise.
Double imaging depth range on fresh shrimp at A-scan rate of 10 kHz is achieved and the resulting
complex conjugate rejection ratio is up to 41dB.
A full range complex spectral domain optical coherence tomography system based on spatial sinusoidal phase
modulation is presented. The quadrature components of an interferogram are obtained by harmonic detection, and the
complex-conjugate ambiguity, DC term and autocorrelation noise are eliminated through Fourier transform of the
complex interferogram. A complex-conjugate rejection ratio of about 35 dB is demonstrated. In practical applications,
high speed complex-conjugate resolved imaging can be performed by combining sinusoidal phase modulation with
transversal scanning.
Wavelength-encoded imaging uses wavelength division multiplexing to produce cross-sectional images without
mechanical scanning, and could be of a great interest in endoscope applications. In this paper, a scheme for
wavelength-encoded imaging using time-encoded Frequency-domain optical coherence tomography (swept-source
optical coherence tomography, SS-OCT) is presented. This approach implements swept broadband source with
narrow instantaneous spectral width and low dispersion diffraction grating to simultaneously produce depth-lateral
imaging of the sample. Compared to previous spectral-domain wavelength-encoded endoscope, the proposed
method enables high-speed and high dynamic range detection. System design such as image resolvable points,
imaging resolution as well as theoretical analysis of the interference signal pattern are developed.
We present a high-speed spectral domain optical coherence tomography (SD-OCT) system at 830
nm wavelength which is consisted of a fiber based Michelson interferometer and a custom-built
spectrometer. The designed resolution of the spectrometer is about 67.4pm which limits the
maximum detection depth 2.56mm in air. And the 35us exposure time of the high speed line scan
CCD makes real-time imaging possible. Furthermore, a novel method of spectrometer calibration
is put forward. The method can remove the influence of dispersion mismatch, thus accurately
determine the distribution of wavelength on the line scan CCD, which leads to a precise
interpolation and a subsequent better contrast image.
KEYWORDS: Optical coherence tomography, Phase modulation, Scanners, Demodulation, Signal detection, Heterodyning, Modulation, Modulators, Numerical analysis, Signal to noise ratio
Optical coherence tomography (OCT) is an emerging technology for high-resolution bio-medical imaging. The rapid
scanning optical delay line (RSOD) has developed basically for depth scanning and dispersion compensation. If the
pivot center of the galvamirror in RSOD is offset, phase modulation can be realized, providing a high carrier frequency
required for heterodyne detection. However, the size of galvamirror should be increased, leading to lower response for
its oscillation and hence a low scanning rate. In this paper we propose to apply a sinusoidal waveform (resonant scan)
instead of a triangle waveform (linear scan) to the galvamirror. Numerical analysis on phase modulation by RSOD and
successive envelop demodulation under different driving waveform is conducted. We demonstrate that it is possible to
improve OCT frame rate by resonant scan without compromising the signal-to-noise performance if complete and
accurate envelope demodulation and appropriate signal processing are adopted.
Accurate measurement of tissue optical properties becomes more and more important in biomedical applications such as
diagnostic imaging for tissue pathology. Taking account of multiple scattering effects in OCT model, an algorithm is
developed to extract depth-resolved tissue optical properties including scattering coefficient and anisotropy factor. By
adding absorption coefficient as the third fitting parameter in the algorithm, it is possible to reduce fitting errors
introduced by sample absorption. With an adapted algorithm based on the extended model, absorption coefficient can
also be extracted. The validity of those algorithms is verified by sample model of solution of polyethylene sphere (PS).
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