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Optical Coherence TomographyOptical coherence tomography (OCT) is an interferometric imaging technique offering non-invasive millimeter penetration depths of a sample under test with sub-micrometer axial and lateral resolution. The technique was first demonstrated in 1991, and since then has rapidly developed as the high resolution imaging modality of choice in ophthalmology and other biomedical applications. Introduction With micrometer resolution and cross-sectional imaging capabilities, optical coherence tomography (OCT)1 has become a prominent biomedical imaging technique, particularly suited to ophthalmic applications. In tissue imaging, requiring micrometer resolution and millimetre penetration depth, optical coherence tomography (OCT) has critical advantages over other medical imaging systems. Medical ultrasonography, magnetic resonance imaging (MRI) and confocal microscopy are not suited to morphological tissue imaging; the former two having poor resolution; the latter lacking millimetre penetration depth2,3. The fundamentals behind OCT lie in low-coherence interferometry4. The recombination of backscattered and reference light from a sample and mirror, respectively, gives rise to an interference pattern from which point-spatial dimension and location microstructures can be determined. A cross-sectional tomograph (B-scan) may be achieved by laterally combining a series of these axial depth scans (A-scan). En face imaging (C-scan) at an acquired depth is possible depending on the imaging engine used. Theory The principal of OCT is low coherence interferometry. The optical setup typically consists of a Michelson interferometer (Fig. 1) with a low coherence light source, light being split into and recombined from reference and sample arms. The pathlength of one arm is translated longitudinally in time. A property of low coherence interferometry is that interference, i.e. the series of dark and bright fringes, is only achieved when the position of the translating arm is inside the coherence gate of the light source. This creates an optical carrier amplitude modulated envelope as pathlength is varied, where the peak of the envelope corresponds to pathlength matching. It is this coherence gating feature that allows OCT to resolve sample microstructure in depth. Therefore translating one arm of the interferometer has two functions; depth scanning and a Doppler-shifted optical carrier are accomplished by pathlength variation. The interference of two partially coherent light beams can be expressed in terms of the source intensity, , as -
where represents the interferometer beam splitting ratio, and is called the complex degree of coherence, i.e. the interference envelope and carrier dependent on reference arm scan or time delay , and whose recovery of interest in OCT. Due to the coherence gating effect of OCT the complex degree of coherence is represented as a Gaussian function expressed as5 -
where represents the spectral width of the source in the optical frequency domain, and is the centre optical frequency of the source. In equation (2), the Gaussian envelope is amplitude modulated by an optical carrier. The peak of this envelope represents the location of sample under test microstructure, with an amplitude dependent on the reflectivity of the surface. The optical carrier is due to the Doppler effect resulting from scanning one arm of the interferometer, and the frequency of this modulation is controlled by the speed of scanning. Therefore translating one arm of the interferometer has two functions; depth scanning and a Doppler-shifted optical carrier are accomplished by pathlength variation. The axial and lateral resolutions of OCT are decoupled from one another; the former being a equivalent to the coherence length of the light source and the latter being a function of the optics. The coherence length of a source and hence the axial resolution of OCT is defined as -
| /dd> | | math> \, {l_c} | | | | | Imaging Schemes Focusing the light beam to a point on the surface of the sample under test, and recombining the reflected light with the reference will yield a interferogram with sample information corresponding to a single A-scan (Z axis only). Scanning of the sample can be accomplished by either scanning the light on the sample, or by moving the sample under test. A linear scan will yield a two-dimensional data set corresponding to a cross-sectional image (X-Z axes scan), whereas an area scan achieves a three-dimensional data set corresponding to a volumetric image (X-Y-Z axes scan), also called full-field OCT. Single point OCT Systems based on single point, or flying-spot time domain OCT, must scan the sample in two lateral dimensions and reconstruct a three-dimensional image using depth information obtained by coherence-gating through an axially scanning reference arm. Two-dimensional lateral scanning has been electromechanically implemented by moving the sample6 using a translation stage, and using a novel micro-electro-mechanical system scanner7. Parallel OCT Parallel OCT using a charge-coupled device (CCD) camera has been used in which the sample is full-field illuminated and en face imaged with the CCD, hence eliminating the electromechanical lateral scan. By stepping the reference mirror and recording successive en face images a three-dimensional representation can be reconstructed. Three-dimensional OCT using a CCD camera was demonstrated in a phase-stepped technique8, using geometric phase-shifting with a Linnik interferometer9, utilising a pair of CCDs and heterodyne detection10, and in a Linnik interferometer with an oscillating reference mirror and axial translation stage11. Central to the CCD approach is the necessity for either very fast CCDs or carrier generation separate to the stepping reference mirror to track the high frequency OCT carrier. Full-field OCT A two-dimensional smart detector array, fabricated using a 2m complementary metal-oxide-semiconductor (CMOS) process, was used to demonstrate full-field OCT12. Featuring an uncomplicated optical setup, each pixel of the 58x58 pixel smart detector array acted as an individual photodiode and included its own hardware demodulation circuitry. In recent developments OCT was demonstrated using a commercial programmable direct read-out CMOS camera. Using the random access capability of the camera a small region of interest (ROI) was sampled very fast and using carrier-based detection a three-dimensional surface visualisation of an industrial sample was achieved. References 1. D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito and J. G. Fujimoto, "Optical Coherence Tomography", Science, 254: 1178-1181, 1991. 2. S. C. Kaufman, D. C. Musch, M. W. Belin, E. J. Cohen, D. M. Meisler, W. J. Reinhart, I. J. Udell and W. S. V. Meter, "Confocal Microscopy: A Report by the American Academy of Ophthalmology", Ophthalmology, 111(2): 396-496, 2004. 3. S. J. Riederer, Current technical development of magnetic resonance imaging, Engineering in Medicine and Biology Magazine. 19: 34-41, 2000. 4. M. Born and E. Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light, Cambridge, Cambridge University Press, 1999. 5. J. M. Schmitt, "Optical Coherence Tomography (OCT): A Review", Selected Topics in Quantum Electronics, 5(4): 1205-1215, 1999. 6. J. M. Herrmann, M. E. Brezinski, B. E. Bouma, S. A. Boppart, C. Pitris, J. F. Southern and J. G. Fujimoto, "Two- and three-dimensional high-resolution imaging of the human oviduct with optical coherence tomography," Fertility and Sterility, 70(1), 155-158, (1998). 7. J. T. W. Yeow, V. X. D. Yang, A. Chahwan, M. L. Gordon, B. Qi, I. A. Vitkin, B. C. Wilson and A. A. Goldenberg, "Micromachined 2-D scanner for 3-D optical coherence tomography," Sensors and Actuators A, 117, 331-340, (2004). 8. C. Dunsby, Y. Gu and P. M. W. French, "Single-shot phase-stepped wide-field coherence gated imaging," Optics Express, 11(2), 105-115, (2003). 9. M. Roy, P. Svahn, L. Cherel and C. J. R. Sheppard, "Geometric phase-shifting for low-coherence interference microscopy," Optics and Lasers in Engineering, 37, 631-641, (2002). 10. M. Akiba, K. P. Chan and N. Tanno, "Full-field optical coherence tomography by two-dimensional heterodyne detection with a pair of CCD cameras," Optics Letters, 28(10), 816-818, (2003). 11. A. Dubois, G. Moneron, K. Grieve and A. C. Boccara, "Three-dimensional cellular-level imaging using full-field optical coherence tomography," Physics in Medicine and Biology, 49, 1227-1234, (2004). 12. S. Bourquin, P. Seitz and R. P. Salath, "Optical coherence tomography based on a two-dimensional smart detector array," Optics Letters, 26(8), 512-514, (2001). See Also External Links
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