Speckle Reduction in Optical Coherence Tomography
by Frequency Compounding
Bio-Photonics
2003
Michael Pircher, Erich Götzinger, Rainer Leitgeb,Adolf F. Fercher and
Christoph. K. Hitzenberger
Department of Medical Physics, University of Vienna, Austria
Introduction: Optical coherence tomography (OCT),
originally developed for cross sectional imaging of rather
transparent ocular tissues1 has found an increasing
number of applications in scattering media2.
Like other coherent imaging techniques, OCT suffers
from speckle noise, which degrades contrast of images of
dense biological tissues like human skin. So far several
techniques have been introduced to reduce speckle noise,
either based on spatial compounding 2,3,4 or on digital
signal-processing algorithms2.
In this paper we introduce a frequency compounding
based speckle reduction technique. By the use of two
independent light sources with different center
wavelengths and emission bands, which do not overlap,
we are able to reduce speckle noise.
Method: The experimental setup, shown in figure 1,
consists of a Michelson Interferometer. Two light sources
can be used simultaneously and independently. The light
sources are coupled into the interferometer by a
wavelength division multiplexer (WDM) and separated
by a WDM at the interferometer exit.
By moving the translation stage with a constant velocity
a Doppler shift is introduced to the reference beam,
causing a heterodyne interferometric signal centered at
the Doppler frequency. This enables measurements with
high sensitivity.
WD M
D3
BS
ì A
æ
A2 ö
÷
exp çç A³0
ï
P ( A) = ís 2
2
s 2 ÷ø
è
(1)
ï0
otherwise,
î
where s denotes the standard deviation.
The density distribution of the envelope of the OCT
signal SOCT (usually after rectifying and low pass
filtering) can be derived as a change of scale, which
changes the mean value, but not the shape of the density
distribution:
)=
2
æ p S OCT
p S OCT
exp çç 2
2
2 S OCT
4
S OCT
è
ö
÷
÷
ø
P
NPBS
SLD - 2
D1
Lens
WD M
D2
Voice-CoilScanning-System
Results: The first aim of our study was to investigate the
speckle statistics of the OCT-image. A piece of plastic
rubber was used as the scattering test sample.
a)
S
S
ö
÷ (3)
÷
ø
1,2
1,2
1,0
1,0
0,8
0,8
0,6
0,4
Via the second moment of this distribution we obtained a
speckle contrast of 0.36, which corresponds to a speckle
contrast reduction of 1.4 in comparison to equation (2).
0,0
0,6
0,4
0,2
0
1
2
S/<S>
a)
Figure 4: OCT image of human finger tip in vivo.
a) Image taken with a single SLD with center wavelength at 1312nm ,
b) Wavelength compounded image
(the white bar indicates a distance of 1mm)
Conclusion: We derived the speckle statistics for OCT
systems and compared the theoretical results with data
obtained by a uniformly scattering test sample. These results
show a good agreement with the theoretical predictions. One
drawback of the method is the increased system complexity.
System complexity is further increased if more than two light
sources are compounded, which would be necessary for further
speckle noise reduction. The main advantage of this method, as
compared to other speckle reduction techniques, lies in the fact,
that spatial resolution is maintained. Another advantage is, that
the same system can be used to perform differential absorption
measurements, if the sample contains substances of different
absorption coefficients at the two wavelengths.
b)
Figure 2: Example of a subsection (500mm x 500mm) of the
OCT-images of the test sample
a) Subsection recorded at 1312nm,
b) compounded image
<S>P(S)
P (S OCT
) = 81 p
128
2
æ 9p S OCT
exp çç 2
16
S OCT
è
Sample
Figure 1: Experimental setup of the OCT system.
SLD-1 and SLD-2: SLD’s with center wavelengths of 1312nm
and 1488nm, respectively. BS…Beamsplitter, P…Polarizer,
NPBS…non polarizing beam splitter, HE-NE…Helium Neon
Laser, D1-D3…Detectors, WDM…Wavelength division
multiplexer
0,2
3
OCT
4
OCT
b)
Retroreflector
(2)
The contrast of this speckle pattern is 0.52. If we
incoherently superimpose two speckle fields with
distributions given by equation (2), for example by use of
non-polarized light or by use of two uncorrelated light
sources with different center wavelengths, the new
density distribution is given by a convolution of two
independent density distributions5 each of the form given
by equation (2)6:
2
NPBS
<S>P(S)
P (S OCT
a)
HE-NE
SLD - 1
Speckle statistics: Speckles arise due to a coherent
superposition of backscattered light waves from different
scattering points or areas of a sample containing densely
packed scattering particles. The electromagnetic light
waves can be described by a complex valued phasor
represented by amplitude A and phase F.The amplitude
A of a phasor sum can be described by a Rayleigh
density distribution:5
To demonstrate our method in real tissue, we recorded OCTimages in human skin across a scar of a fingertip in vivo.
Figure 4 shows the result.
The different layers show more contrast in the compounded
image. Especially in the scar region small structures appear
more separated than in the single wavelength image.
3
4
0,0
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
S/<S>
b)
Figure 3: Probability distribution of the speckle intensity
values of the test sample. a) recorded at 1312nm, b)
compounded image.
Solid line: Theoretical values according to equation (2) and
(3). Data points: measured values
References
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M. R. Hee, T. Flotte, K.
Gregory, C. A. Puliafito, J. G. Fujimoto, “Optical coherence tomography”,
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2. B. E. Bouma, G. J. Tearney “Handbook of Optical Coherence Tomography”,
Marcel Dekker, New York (2002).
3. J. M. Schmitt, S. H. Xiang, K. M. Yung, “Speckle in optical coherence
tomography”, J. Biomed. Opt. 4,
95-105 (1999).
4. M. Bashkansky, J. Reintjes, “Statistics and reduction of speckle in optical
coherence tomography”, Opt.
Lett. 25, 545-547 (2000).
5. J. W. Goodman “Statistical Optics”, John Wiley & Sons, Inc, New York (1985).
6. M. Pircher, E. Götzinger, R. Leitgeb, A. F. Fercher and C. K. Hitzenberger :
“Speckle reduction in optical coherence tomography by frequency
compounding”, J.Biomed. Opt. 8(3), 2003