January 15, 2000 / Vol. 25, No. 2 / OPTICS LETTERS 111Spectroscopic optical coherence tomographyU. Morgner, W. Drexler, F. X. K¨artner, X. D. Li, C. Pitris, E. P. Ippen, and J. G. FujimotoDepartment of Electrical Engineering and Computer Science and Research Laboratory of Electronics,Massachusetts Institute of Technology, Cambridge, Massachusetts 02139Received August 2, 1999Spectroscopic optical coherence tomography (OCT), an extension of conventional OCT, is demonstrated forperforming cross-sectional tomographic and spectroscopic imaging. Information on the spectral content ofbackscattered light is obtained by detection and processing of the interferometric OCT signal. This methodallows the spectrum of backscattered light to be measured over the entire available optical bandwidthsimultaneously in a single measurement. Specific spectral features can be extracted by use of digital signalprocessing without changing the measurement apparatus. An ultrabroadband femtosecond Ti:Al2O3laser wasused to achieve spectroscopic imaging over the wavelength range from 650 to 1000 nm in a simple model aswell asin vivo in the Xenopus laevis (African frog) tadpole. Multidimensional spectroscopic data are displayedby use of a novel hue-saturation false-color mapping. 2000 Optical Society of AmericaOCIS codes: 110.4500, 100.6950, 100.7410.Optical coherence tomography (OCT) has emerged as apromising medical diagnostic imaging technology fornoninvasive in situ cross-sectional imaging of biologicaltissues and materials.1Recently, extensions of OCTtechnology, including Doppler f low2,3and polarization-sensitive4,5imaging, have been developed that permitspatially resolved imaging of velocity or birefringence.The axial resolution of OCT is determined by the band-width of the low-coherence light source, and imageresolutions 10–100 times better than standard ultra-sound imaging have been achieved. Ultrashort-pulsemode-locked Ti:Al2O3laser technology has made it pos-sible to generate pulses shorter than two optical cycles,corresponding to bandwidths of 350 nm, centeredaround 800 nm.6This laser is a powerful source forultrahigh-resolution and spectroscopic OCT imaging.In vivo subcellular imaging in the Xenopus laevistadpole has been demonstrated with1 mm axial by3 mm transverse resolution, what is to our knowledgethe highest OCT resolution to date.7In this Letter, we demonstrate broadband spectro-scopic OCT, an extension of standard OCT technol-ogy. Spectroscopic and wavelength-dependent OCT isa new area of investigation, and few studies of it havebeen performed to date because sufficiently broad-bandwidth light sources have not been available. Oneprevious study demonstrated spectroscopic detectionover a bandwidth of⬃50 nm at 1.3 mm.8Other stud-ies were performed at 1.3 and 1.5 mm, combining twoseparate light sources to detect water content in tis-sue.9,10By use of state of the art femtosecond Ti:Al2O3lasers, spectroscopic information over the entire out-put bandwidth from 650 to 1000 nm can be obtained.This spectral region is important because it overlapsabsorption features in oxyhemoglobin and deoxyhe-moglobin and may permit the functional imaging of he-moglobin oxygen saturation. Spectroscopic OCT canalso be used to enhance image contrast, permitting thedifferentiation of tissue pathologies through their spec-troscopic properties or functional states. This spectro-scopic staining is somewhat analogous to histologicalstaining.OCT uses interference of low-coherence (i.e., broad-band) light in a Michelson interferometer. Light froma reference path with length z 苷 ngt, scanned continu-ously at a speed ng, is interfered with light from thesample. The beam position on the sample is scannedin the transverse x direction, creating a cross sectionthrough the object under study. By use of a broadbandlight source with a spectrumjE共v兲j2, the detected in-tensity at the output of the interferometer as a functionof t or z is given byID共z兲 苷ZjE共v兲rBS共v兲tBS共v兲关exp共ikz兲1 rS共v, z兲兴j2D共v兲dv , (1)where k is the wave number, D共v兲 is the detector re-sponse, and rS共v, z兲, rBS共v兲, and tBS共v兲 are the com-plex ref lectivity of the sample and the ref lectivity andtransmissivity of the interferometer beam splitter, re-spectively. ID共t 苷 z兾ng兲 is the oscillating output frominterference of the signal field with the Doppler-shiftedreference field. Figure 1 shows an example of datafrom a single axial scan. Each delay t correspondsto detecting light that is backscattered from a corre-sponding depth z inside the tissue, whose depth resolu-tion is given by the point spread function P 共t兲, whereP 共t兲 is obtained from ID共t兲 for rS共v兲 苷 1 by Fouriertransform F.In standard OCT imaging, only the envelope of theinterference signal ID共t兲 is detected. The OCT imageis a two-dimensional array representing the backscat-tered intensity T共x, z兲 苷 Enn关ID共z兲兴. Spectral infor-mation can be obtained by measurement of the fullinterference signal and the use of appropriate digi-tal signal processing. Spectroscopic detection is per-formed with a Morlet wavelet transform that reduces0146-9592/00/020111-03$15.00/0  2000 Optical Society of America112 OPTICS LETTERS / Vol. 25, No. 2 / January 15, 2000Fig. 1. OCT schematic. The broadband laser source iscoupled into a Michelson-type interferometer. The result-ing interferogramID共t兲 contains spectroscopic informationabout the ref lected – backscattered light. The envelope ofID共t兲 is used in standard OCT imaging to detect the inten-sity of the ref lected–backscattered light.windowing artifacts associated with other methodssuch as the short-time Fourier transform.11The Mor-let wavelet transformation is given byW 共V, t兲 苷ÇZID共t 1t兲exp关2共t兾t0兲2兴exp共iVt 兲dtÇ2苷 jF 兵ID共t 1t兲exp关2共t兾t0兲2兴其j2. (2)The advantage of this technique is that an entire spec-trum W 共V兲 can be calculated for each point 共x, t兲in the OCT image. For the purposes of demonstra-tion, and to display the spectroscopic data in a simplecolor image, we calculated the center of mass VC共x, z兲of the spectra. It is also possible to extract spec-troscopic information at a specific single frequency,within a given bandwidth, at multiple discrete wave-lengths, etc. Unlike in standard OCT, displaying thespectroscopic OCT image requires a multidimensionalmap. Hue, saturation, luminance color