pling frequency of the system, the AK velocity esti-
mator does not experience aliasing before the OCT
signal diminishes. This corresponds to a maximum
detectable velocity limit of ±0.78 m/s in the axial di-
rection (±5.6 m/s at 82° with our endoscopic cath-
eter), which can be further increased by widening the
demodulation bandwidth, with a trade-off in SNR of
the OCT signal. The computational complexity of AK
is of the same order as TK and can be implemented
for real-time operation in software.
previous autocorrelation methods,
the Kasai esti-
mation output is linear with the ﬂow velocity. We
note that misalignments in the RSOD, wavelength-
dependent scattering and absorption, and nonlineari-
ties in the demodulation process contributed to the
background AK phase changes, which need to be sub-
tracted for visualizing the true ﬂow induced phase
changes. The TK and TKV processed from the same
data set are sensitive to slow ﬂow conditions, and the
results can serve as segmentation maps for distin-
guishing stationary versus ﬂow regions in subse-
quent AK calculations. Conversely, we are also ex-
ploring the AK as a tool for estimating the centroid
shift due to wavelength-dependent scattering and ab-
sorption in spectroscopic OCT after the segmentation
In conclusion, we described the AK algorithm as a
method for extending the velocity estimation range
on high-speed DOCT systems, to include higher ﬂow
velocities. Using the Kasai autocorrelation technique
in two dimensions by combining the AK with TK, one
can obtain sensitivity from extremely slow to fast
ﬂow velocities on the same data set. For what is for
the ﬁrst time to our knowledge, we demonstrated in
vivo transesophageal imaging of the rat aortic blood
ﬂow using the 2D Kasai technique.
Support from the Canada Research Chairs pro-
gram, Photonics Research Ontario and Canadian In-
stitutes of Health Research is acknowledged. We
thank A. Mariampillai and B. Standish for their
assistance. V. Yang’s e-mail address is
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, Science 254, 1178
2. X.-J. Wang, T. E. Milner, Z. Chen, and J. S. Nelson,
Appl. Phys. Lett. 36, 144 (1997).
3. Z. Chen, T. E. Miller, D. Dave, and J. S. Nelson, Opt.
Lett. 22, 64 (1997).
4. J. A. Izatt, M. D. Kulkarni, S. Yazdanfar, J. K. Barton,
and A. J. Welch, Opt. Lett. 22, 1439 (1997).
5. A. M. Rollins, S. Yazdanfar, J. K. Barton, and J. A.
Izatt, J. Biomed. Opt. 7, 123 (2002).
6. V. Westphal, S. Yazdanfar, A. M. Rollins, and J. A.
Izatt, Opt. Lett. 27, 34 (2002).
7. Y. Zhao, Z. Chen, C. Saxer, S. Xiang, J. F. de Boer, and
J. S. Nelson, Opt. Lett. 25, 114 (2000).
8. B. White, M. Pierce, N. Nassif, B. Cense, B. Park, G.
Tearney, B. Boumma, T. Chen, and J. de Boer, Opt.
Express 11, 3490 (2003).
9. V. X. D. Yang, M. L. Gordon, A. Mok, Y. Zhao, Z. Chen,
R. S. C. Cobbold, B. C. Wilson, and I. A. Vitkin, Opt.
Commun. 208, 209 (2002).
10. V. X. D. Yang, M. L. Gordon, B. Qi, J. Pekar, S. Lo, E.
Seng-Yue, A. Mok, B. C. Wilson, and I. A. Vitkin, Opt.
Express 11, 794 (2003).
11. A. L. Oldenburg, J. J. Reynolds, D. L. Marks, and S. A.
Boppart, Appl. Opt. 42, 22 (2003).
12. B. J. Vakoc, S. H. Yun, J. F. de Boer, G. J. Tearney, and
B. E. Bouma, Opt. Express 13, 14 (2005).
13. R. Huber, M. Wojtkowski, and J. G. Fujimoto, Opt.
Express 14, 8 (2006).
14. W. Y. Oh, S. H. Yun, G. J. Tearney, and B. E. Bouma,
Opt. Lett. 30, 23 (2005).
15. A. W. Schaefer, J. J. Reynolds, D. L. Marks, and S. A.
Boppart, IEEE Trans. Biomed. Eng. 51, 186 (2004).
16. T. Loupas, J. T. Powers, and R. W. Gill, Ferroelectr.
Freq. Control 42, 672 (1995).
17. V. X. D. Yang, M. L. Gordon, S. J. Tang, N. E. Marcon,
G. Gardiner, B. Qi, S. Bisland, E. Seng-Yue, S. Lo, J.
Pekar, B. C. Wilson, and I. A. Vitkin, Opt. Express 11,
18. A. B. Driss, J. Benessiano, P. Poitevin, B. I. Levy, and
J. B. Michel, Am. J. Physiol. Heart Circ. Physiol. 227,
Fig. 4. (Color online) In vivo M-mode images of transe-
sophageal DOCT of rate aortic blood ﬂow. A, TK results
overlaid on structural image. Doppler signals indicate aor-
tic wall motion 共ⴱ兲 , systolic rush of high-speed blood ﬂow
共ⴱⴱ兲, and regions of slow ﬂow between heart beats (white
arrows). B, AK results overlaid on the same structural im-
age, with the esophagus and aortic wall labeled. High-
speed systolic ﬂow regions consistent with large Doppler
frequency shifts are clearly visualized. The temporal ﬂow
proﬁles measured at the dotted lines of corresponding col-
ors in B are plotted in C.
February 1, 2007 / Vol. 32, No. 3 / OPTICS LETTERS 3