Development of endoscope-compatible fiber-optic probes and polarization-sensitive detection schemes have each independently expanded the utility of optical coherence tomography. Several application areas have emerged which require polarization-sensitive measurements to be combined with endoscopic imaging techniques, in order to proceed to in vivo studies. Endoscopic-OCT typically requires a section of the sample arm fiber to be scanned during image acquisition, which produces a dynamically changing polarization state of light incident on the sample. Here, we demonstrate the effects of linear-scanning, and rotary-scanning probes in the sample arm of a PS-OCT system, and demonstrate the necessary modifications to be made for successful endoscopic PS-OCT imaging.
Osteoarthritis is a prevalent medical condition that presents a diagnostic and therapeutic challenge to physicians today because of the inability to assess the integrity of the articular cartilage early in the disease. Polarization sensitive optical coherence tomography (PS-OCT) is a high resolution, non-contact imaging modality that provides cross-sectional images with additional information regarding the integrity of the collagen matrix. Using PS-OCT to image provides information regarding thickness of the articular cartilage and gives an index of biochemical changes based on alterations in optical properties (i.e. birefringence) of the tissue. We demonstrate initial experiments performed on specimens collected following total knee replacement surgery. Articular cartilage was imaged using a 1310 nm PS-OCT system where both intensity and phase images were acquired. PS-OCT images were compared with histology, and the changes in tissue optical properties were characterized. Analysis of the intensity images demonstrates differences between healthy and diseased cartilage surface and thickness. Phase maps of the tissue demonstrated distinct differences between healthy and diseased tissue. PS-OCT was able to image a gradual loss of birefringence as the tissue became more diseased. In this way, determining the rate of change of the phase provides a quantitative measure of pathology. Thus, imaging and evaluation of osteoarthritis using PS-OCT can be a useful means of quantitative assessment of the disease.
Properties and applications of Spectral Domain OCT have recently been explored, demonstrating improved signal to noise ratios and the potential for high-speed acquisition. In this presentation, we demonstrate in-vivo measurements of a human retina using a Spectral Domain system with acquisition rates of 10,000, 20,000, and 30,000 A-lines per second and 580 μW incident on the eye. Images consisting of 1000 depth profiles and ranging in width from 4 to 12 mm were acquired. The dynamic ranges within an image at 104, 2x104, and 3x104 lps were 40 dB, 39 dB and 28 dB respectively. These values are comparable to that of time domain Optical Coherence tomography yet are achieved at acquisition rates over 50 times faster, demonstrating video-rate OCT imaging at up to 30 frames/sec with 1000 A-lines per image.
This work is more thoroughly described in the following publications:
1) B. R. White, M. C. Pierce, N. Nassif, B. Cense, B. H. Park, G. J. Tearney, B. E. Bouma, T. C. Chen,J. F. de Boer, "In vivo dynamic human retinal blood flow imaging using ultra-high speed spectral domain optical doppler tomography," Opt. Express 11, 3490-3497 (2003).
2) N. A. Nassif, B. Cense, B. H. Park, M. C. Pierce, S. H. Yun, B. E. Bouma, G. J. Tearney, T. C. Chen, and J. F. de Boer, "In vivo high-resolution video-rate spectral-domain optical coherence tomography of the human retina and optic nerve," Opt. Express 12, 367-376 (2004).
3) N. Nassif, B. Cense, B. H. Park, S. H. Yun, T. C. Chen, B. E. Bouma, G. J. Tearney,J. F. de Boer, "In-vivo human retinal imaging by ultra high-speed spectral domain optical coherence tomography," Optics Letters 29, 480-482 (2004).
We have developed a scanning fiber-optic probe compatible with high-speed polarization-sensitive OCT imaging. The effect of sample arm fiber motion on the polarization state of light incident on a sample is demonstrated by the evolution of incident Stokes vectors during the course of acquiring a single image. By referencing the polarization state of light backscattered from within a sample to the measured surface state, effects of motion-induced birefringence can be isolated. Conventional and polarization-sensitive images of human tissues are presented.
We present phase resolved digital signal processing techniques for Optical Coherence Tomography to correct for the non Gaussian shape of source spectra and for Group Delay Dispersion (GDD). A broadband source centered at 820 nm was synthesized by combining the spectra of two superluminescent diodes to improve axial image resolution in an optical coherence tomography (OCT) system. Spectral shaping was used to reduce the side lobes (ringing) in the axial point spread function due to the non-Gaussian shape of the spectra. Images of onion cells taken with each individual source and the combined sources, respectively, show the improved resolution and quality enhancement in a turbid biological sample. An OCT system operating at 1310 nm was used to demonstrate that the broadening effect of group delay dispersion (GDD) on the coherence function could be eliminated completely by introducing a quadratic phase shift in the Fourier domain of the interferometric signal. The technique is demonstrated by images of human skin grafts with group delay dispersion mismatch between sample and reference arm before and after digital processing.
Optical coherence tomography (OCT) was used to obtain cross sectional images of the internal structure of the cochlea in guinea pigs following sacrifice. The 1310 nm source (bandwidth (lambda) equals75 nm) allowed a penetration depth of approximately 1.5 mm. Cross-sectional images (1.87 x 2.00 mm, 10 x 10 micrometers /pixel) were acquired at a frame rate of 1 Hz. Access to the middle ear space was obtained by removing the mastoid bulla. Imaging was performed in situ and also in ex vivo temporal bones. The scala vestibuli, scala media, scala tympani, modiolus and all four and a half turns of the cochlea were identified. These images demonstrate the potential value of OCT for use in determining the internal structures of the cochlea with near-microscopic resolution and at near-real time frame rates.