Research Highlights

Optical coherence tomography (OCT) is a noninvasive, noncontact imaging modality that uses coherent gating to obtain high-resolution cross-sectional images of tissue microstructure.¹ Compared with conventional time domain OCT (TDOCT), Fourier domain OCT (FDOCT) attracted much attention recently for its potential of higher sensitivity and imaging speed.² In addition to the morphological structural image, FDOCT can also provide functional information of tissue physiology. In TDOCT systems, phase-resolved approach was demonstrated to be an effective method for high speed acquisition of functional information such as blood flow velocity and tissue birefringence.³ This approach can also be used in FDOCT system to reconstruct intensity and functional images by processing the complex signals from the sample. However, the mirror image accompanied with the structure image due to Fourier transform limits the imaging range of FDOCT. Additionally, the reflected terms from the reference mirror and sample always yield the DC and low frequency autocorrelation noises. These parasitic terms obscure the object structure and generate phase error which compromises the reconstruction of complex fringe signals.

Our recent work shows the full range complex signal can be achieved in FDOCT by complete elimination of DC and autocorrelation noises as well as the mirror image with a heterodyne technique.⁴ By choosing an appropriate carrier frequency generated by an EO phase modulator in our swept source based FDOCT system, the positive and negative frequency terms of the Fourier transformed interference fringe signal can be separated from each other and from the low frequency noise. Thus the complex signal containing both the amplitude and phase terms can be reconstructed by selecting the positive (or negative) frequency term. From the amplitude term of the complex signal, the structure image can be acquired. Fig. 1(a) and (b) show imaging of a rabbit cornea by the conventional FDOCT system and our novel FDOCT system, respectively. Using our FDOCT system with the heterodyne technique, the imaging range was doubled by canceling the overlapped mirror images. In addition, the autocorrelation noise close to the zero position was eliminated which increased the signal to noise ratio by 20 dB. To illustrate the performance of our system in acquisition of functional information, tissue birefringence was imaged by processing the phase and amplitude terms. Fig. 1(c) shows the phase retardation image of rabbit tendon. The intensity and birefringence images demonstrated the capacity of our system to acquire structure and functional information of biological tissue. In summary, we presented, to the best of our knowledge for the first time, the heterodyne technique in FDOCT for reconstruction of complex fringe interference term. The technique has the potential for ultrahigh speed and ultrahigh resolution functional FDOCT in biomedical imaging.

(a)  (b)
(c)

Figure 1: (a) Image of rabbit cornea by conventional FDOCT system. (b) Image of rabbit cornea by complex FDOCT system using the heterodyne technique. (c) Phase retardation image of rabbit tendon using complex FDOCT system.

References

1. D. Huang, et. al., Science 254, 1178-81 (1991).
2. R. Leitgeb, et. al., Opt. Express 11, 889-94 (2003).
3. H. Ren, et. al., Opt. Lett. 27, 1702 (2002).
4. J. Zhang, et. al., "Removal of mirror image and enhancement of signal to noise ratio in Fourier domain optical coherence tomography using an electro-optic phase modulator." Opt. Lett.