114 OPTICS LETTERS / Vol. 25, No. 2 / January 15, 2000Phase-resolved optical coherence tomography and opticalDoppler tomography for imaging blood flow in humanskin with fast scanning speed and high velocity sensitivityYonghua Zhao, Zhongping Chen, Christopher Saxer, Shaohua Xiang, Johannes F. de Boer, and J. Stuart NelsonBeckman Laser Institute and Medical Clinic, University of California, Irvine, Irvine, California 92612Received July 30, 1999We have developed a novel phase-resolved optical coherence tomography (OCT) and optical Doppler tomography(ODT) system that uses phase information derived from a Hilbert transformation to image blood f low in humanskin with fast scanning speed and high velocity sensitivity. Using the phase change between sequentialscans to construct f low-velocity imaging, this technique decouples spatial resolution and velocity sensitivity inf low images and increases imaging speed by more than 2 orders of magnitude without compromising spatialresolution or velocity sensitivity. The minimum f low velocity that can be detected with an axial-line scanningspeed of 400 Hz and an average phase change over eight sequential scans is as low as10 mm兾s, while a spatialresolution of 10 mm is maintained. Using this technique, we present what are to our knowledge the firstphase-resolved OCT/ODT images of blood f low in human skin. 2000 Optical Society of AmericaOCIS codes: 110.4500, 170.3880, 100.5070.Optical Doppler tomography (ODT),1–4also termedDoppler optical coherence tomography, is a recentlydeveloped optical technique for imaging both the tis-sue structure and the f low velocity of moving par-ticles in highly scattering media. The noninvasivenature and exceptionally high spatial resolution ofODT have many potential applications in the clini-cal management of patients in whom imaging tissuestructure and monitoring blood-f low dynamics are es-sential. Examples include burn-depth determination,evaluation of the efficiency of laser treatment of portwine stains, photodynamic therapy monitoring, andbrain injury evaluation. We have described anin vivoODT imaging system with high spatial resolution andaccurate blood-flow velocity measurements in vesselsin rodent skin.1,2However, previously developed ODTsystems were unable to achieve simultaneously bothhigh imaging speed and high velocity sensitivity, whichare essential for measuring blood f low in humanskin.1–4We describe a novel fast-scanning ODT sys-tem that uses phase information derived from a Hilberttransformation to increase the sensitivity of f low-velocity measurements while maintaining high spa-tial resolution. The significant increases in scanningspeed and velocity sensitivity made it possible for us toimage in vivo blood flow in human skin.ODT combines the Doppler principle with OCT5toyield high-resolution tomographic images of static andmoving constituents simultaneously in highly scatter-ing biological tissues. The flow velocity of movingparticles in the sample can be determined by measure-ment of the Doppler shift of the fringe frequency witha short-time Fourier transform. Since detection of theDoppler shift requires sampling the interference fringeintensity over at least one oscillation cycle, the mini-mum detectable Doppler frequency shift共DfD兲 variesinversely with the short-time Fourier transform win-dow size 共Dtp兲 at each pixel (i.e., DfD艐 1兾Dtp). For agiven time-window size at each pixel, the velocity sen-sitivity 共nmin兲 is given bynmin苷l02n cos共u兲Dtp,(1)where l0is the light-source center wavelength, nis the sample’s refractive index, and u is the anglebetween the probing beam and the direction of f low.Therefore, the higher the value of Dtp, the higher thevelocity sensitivity. However, spatial resolution, Dxp,is proportional to the short-time Fourier transformwindow size and is given byDxp苷 VDtp, (2)where V is the one-dimensional scanning speed ofthe ODT system. Consequently, velocity sensitivityand spatial resolution are coupled. A large pixeltime-window size increases velocity sensitivity whiledecreasing spatial resolution. Increasing the imageframe rate also decreases velocity sensitivity. Forexample, for a rate of one frame per second for an imagewith 100 3 100 pixels, the maximum data-acquisitiontime for each pixel 共Dtp兲 is 1兾10,000 s. Accordingly,the minimum resolvable Doppler frequency shift is10 kHz, which corresponds to a velocity sensitivity ofapproximately 25 mm兾sforl0苷 1300 nm and u 苷 80±.To measure blood flow in small vessels in which redblood cells are moving at low velocity, one must reducethe imaging frame rate if the spectrogram methodis used. When ODT goes to real-time imaging, thetime for each axial scan (A scan) is very short. As aresult, the velocity sensitivity decreases dramatically,because the window time for each pixel is so short thata fast Fourier transform algorithm can detect any largeDoppler frequency shift.To overcome these limitations we developed amethod that uses the phase change between sequential0146-9592/00/020114-03$15.00/0  2000 Optical Society of AmericaJanuary 15, 2000 / Vol. 25, No. 2 / OPTICS LETTERS 115line scans for velocity image reconstruction. The ODTsignal phase can be determined from the complexfunction,eGODT共t兲, which is determined through ana-lytic continuation of the measured interference fringesfunction, GODT共t兲, by use of a Hilbert transformation6:eGODT共t兲 苷 GODT共t兲 1ipPZ`2`GODT共t兲t2tdt苷 A共t兲exp关if共t兲兴 , (3)where P denotes the Cauchy principle value and A共t兲and f共t兲 are the amplitude and the phase ofeGODT共t兲,respectively. The phase change in each pixel betweensequential A-line scans is then used to calculate theDoppler frequency shift:v 苷 Df兾T , (4)where T is the time interval between successive Ascans. Because T is much longer than the pixel timewindow, very small Doppler shifts can be detected withthis technique. For example, in an OCT/ODT imagewith 100 3 100 pixels, if the data-acquisition timeat each pixel is 100 ms, using the phase differencebetween sequential A-line scans increases the timewindow from 100 msto100 3 100 ms 苷 10 ms. There-fore, the frequency resolution improves from 10 kHzto 100 Hz, and the velocity sensitivity improves from3mm兾sto30 mm兾s. In addition, spatial resolutionand velocity sensitivity are decoupled. Furthermore,because two sequential A-line scans are compared atthe same location, speckle modulations in the