Research On Spectral Optical Coherence Tomography

Optical coherence tomography (OCT) is a new imaging modality developed to image the highly scattering medium, such as biological tissue. The core of the OCT system is a Michelson interferometer illuminated by a broadband light source. It was demonstrated that this technique provides depth-resolved image of the biological tissue with high resolution in a non-contact and noninvasive manner. The original development was based on passively scanning the optical pathlength to provide the depth-resolved imaging capability, so called the time domain OCT. In this method the length of the reference arm in an interferometer is scanned over a distance to provide ranging in the biological tissue. The mechanical scanning however limits the imaging acquisition speed that in turn refrains a number of applications, for example in vivo imaging of a live subject. In recent years a novel OCT system is proposed by a number of groups that operates in frequency domain, i.e. spectral OCT. Spectral OCT inherits all advantages of its counterpart. More important, it achieves imaging by parallel acquisition of depth information, thus imaging speed is increased dramatically. Spectral OCT is gradually taking over the traditional time domain OCT and attracting increasingly more attention.In this thesis, the basic theory of time domain OCT was first introduced on the basis of the spectral OCT theory. In spectral OCT, spectrometer was used to acquire interference spectrum between reference arm and sampling arm, then Fourier transformation was used to retrieve the depth information, thus it avoids depth scanning. However, the results of Fourier transformation not only contain signal term but also noise terms. In order to eliminate those terms, phase shifting methods were introduced. Here, arbitrary three-phase-shifting method was proposed to eliminate all noise terms to realize the full range spectral OCT. Two-channel acquisition technique was proposed to improve acquisition speed of phase-shifting method.High speed high resolution spectral OCT system was built in our lab and basic schematic of the system was introduced. Normally, spectral OCT uses linear CCD camera as the detect unit, however, the quantum efficiency across the detector array is not uniform that will introduce a pattern noise in the final reconstructed OCT image. To minimize this effect, a B-scan spectra average method was proposed to eliminate the pattern noise present in the OCT image. In addition, the dispersion mismatch between reference arm and sampling arm would deteriorate system axial resolution. To eliminate this effect in the system, a spectrum coordinate calibration method was proposed to compensate dispersion. To evaluate the performance of our system, sensitivity of spectral OCT was analyzed and was measured at different depths.Phase-resolved flow measurement technique based on spectral OCT was introduced. Phase stability of the system was measured. We found that the lateral scanning over the sample and motion of the sample will introduce severe phase noise artifacts. To cope with this, a double-pass compensation method was proposed to eliminate phase-noise artifacts introduced by the lateral scan. In the experimental work that used the mouse brain as the imaging target, we proposed a new bulk motion compensation method to minimize all the phase artifacts. We built three spectral OCT systems in our laboratory and the main difference among them was the center wavelength of the light source used, i.e.766nm, 840nm and 1300nm. Comparison between 840nm system and 1300nm system was performed on the early chicken embryo. The results demonstrated that the 1300nm system was more suitable for chicken embryo heart out-flow tract blood flow measurement.