Article

Wide dynamic range detection of bi-directional flow in Doppler OCT using 2-dimensional Kasai estimator

Department of Physics, Ryerson University, Toronto, Ontario, Canada
Optics Letters (Impact Factor: 3.29). 02/2007; 32(3):253-5. DOI: 10.1364/OL.32.000253
Source: PubMed

ABSTRACT

We demonstrate extended axial flow velocity detection range in a time-domain Doppler optical coherence tomography (DOCT) system using a modified Kasai velocity estimator with computations in both the axial and transverse directions. For a DOCT system with an 8 kHz rapid-scanning optical delay line, bidirectional flow experiments showed a maximum detectable speed of >56 cm/s using the axial Kasai estimator without the occurrence of aliasing, while the transverse Kasai estimator preserved the approximately 7 microm/s minimum detectable velocity to slow flow. By using a combination of transverse Kasai and axial Kasai estimators, the velocity detection dynamic range was over 100 dB. Through a fiber-optic endoscopic catheter, in vivoM-mode transesophageal imaging of the pulsatile blood flow in rat aorta was demonstrated, for what is for the first time to our knowledge, with measured peak systolic blood flow velocity of >1 m/s, while maintaining good sensitivity to detect aortic wall motion at <2 mm/s, using this 2D Kasai technique.

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Available from: Victor X D Yang
Wide dynamic range detection of bidirectional flow
in Doppler optical coherence tomography
using a two-dimensional Kasai estimator
Darren Morofke and Michael C. Kolios
Department of Physics, Ryerson University, Toronto, Canada
I. Alex Vitkin
Ontario Cancer Institute/University Health Network, Toronto, Canada
Victor X. D. Yang
Imaging Research, Sunnybrook Health Sciences Center, Toronto, Canada
Received August 30, 2006; accepted October 16, 2006;
posted October 27, 2006 (Doc. ID 74498); published January 12, 2007
We demonstrate extended axial flow velocity detection range in a time-domain Doppler optical coherence
tomography (DOCT) system using a modified Kasai velocity estimator with computations in both the axial
and transverse directions. For a DOCT system with an
8 kHz rapid-scanning optical delay line, bidirectional
flow experiments showed a maximum detectable speed of
56 cm/s using the axial Kasai estimator without
the occurrence of aliasing, while the transverse Kasai estimator preserved the
7
m/s minimum detect-
able velocity to slow flow. By using a combination of transverse Kasai and axial Kasai estimators, the ve-
locity detection dynamic range was over
100 dB. Through a fiber-optic endoscopic catheter, in vivo M-mode
transesophageal imaging of the pulsatile blood flow in rat aorta was demonstrated, for what is for the first
time to our knowledge, with measured peak systolic blood flow velocity of
1 m/s, while maintaining good
sensitivity to detect aortic wall motion at
2mm/s, using this 2D Kasai technique. © 2007 Optical Society
of America
OCIS codes: 170.3880, 170.4500, 110.4500, 170.3340, 170.2150, 100.2000.
Optical coherence tomography (OCT) can acquire of
high-resolution images of subsurface tissue structure
and function.
1–4
By use of autocorrelation,
5,6
phase
sensitive detection,
7,8
and a Kasai velocity
estimator,
9,10
Doppler frequency shifts can now be es-
timated in real time with Doppler OCT (DOCT). Flow
velocity can be determined via phase detection with
high sensitivity.
6–10
The transverse Kasai (TK) auto-
correlation estimator is suitable for imaging slow bi-
directional flows representative of microcirculation.
Aliasing due to the axial scan (a-scan) frequency,
however, limits the maximum TK detected nona-
liased axial flow speed to 4 mm/s in OCT where
rapid scanning optical delay (RSOD) lines operate at
8 to 15 kHz.
7,10,11
This upper limit is increased to
8 mm/s on spectral domain
8
or swept-source OCT
systems
12
with higher effective a-scan rates. New
swept-source systems
13,14
with effective a-scan rates
of 115 to 290 kHz can theoretically have an aliasing
limit of 7 cm/s. Phase-unwrapping techniques can
extend the velocity detection range; however, at high
flow rates, separation between aliasing rings can be-
come smaller than the spatial resolution of the imag-
ing system, making phase unwrapping unreliable.
Digital hardware autocorrelation with time delays
less than the a-scan period
5
and Hilbert transform
techniques
15
can provide higher aliasing limits up to
35 cm/s with reduced sensitivity to low flow speed.
However, in applications such as coronary imaging,
flow velocity estimation in the range of meters per
second is required. In addition, blood flow velocity in
the microvasculature of atheroma can be orders of
magnitude lower than that in the lumen and both
can be present in the OCT field of view.
In this Letter we report Kasai autocorrelation per-
formed in both the axial and transverse directions, on
the same data set, which results in an extended axial
velocity estimation range. It is based on the 2D Kasai
algorithm proposed by Loupas et al.
16
for ultrasound
imaging. We previously reported TK estimation of
flow-induced frequency shift.
10
The aliasing limits
are ±1/2f
a
, the a-scan frequency, which is typically in
the kilohertz range. Sampling rate in the axial direc-
tion, however, is in the megahertz range. To take ad-
vantage of the larger bandwidth, we propose the
axial Kasai (AK) algorithm, computed as
f
AK
=
f
s
2
tan
−1
m=0
M−2
n=0
N−1
Q
m,n
I
m+1,n
I
m,n
Q
m+1,m
m=0
M−2
n=0
N−1
I
m,n
I
m+1,n
+ Q
m,n
Q
m+1,n
, 1
where f
s
is the sampling rate and provides the alias-
ing limits at ±1/ 2f
s
. I and Q are in-phase and quadra-
ture components of the signal, and m and n are the
axial and transverse indices. When a stationary AK
result is subtracted from a moving source, the re-
maining signal is the Doppler shift induced by the
February 1, 2007 / Vol. 32, No. 3 / OPTICS LETTERS 1
0146-9592/07/030001-0/$15.00 © 2007 Optical Society of America
Page 1
motion of the scatterers. The change in frequency is
related to velocity by v=
0
f /2n
t
cos
, where v is the
velocity at a specic point,
0
is the center wave-
length of the light, n
t
is the refractive index of the
sample,
is the Doppler angle, and f is the change
in frequency due to Doppler shift estimated by TK or
moving AK subtracted from the stationary AK.
A ow phantom experiment was performed using
an infusion pump with calibrated ow rate control
and 1% Intralipid uid pumped through a glass cap-
illary 0.5 mm inner diameter at a Doppler angle of
=59°. Images were acquired using a previously de-
scribed time-domain DOCT system
10
containing a
5 mW broadband light source centered at 1.3
m
with 63 nm bandwidth with an 8 Hz a-scan frequency
f
a
. Transverse Kasai variance (TKV) processing,
10
which computed the variance of the estimated mean
Doppler shift, provided segmentation between ow
and no-ow regions, similar to standard deviation
Doppler imaging.
7
Stationary background AK phase
change was then subtracted for AK ow visualiza-
tion. The fastest experimentally achievable peak ow
velocity was 2 m/s, with a Reynolds number of 730.
The calculated entrance length was 13 mm, shorter
than the capillary tube used. Laminar parabolic ow
was assumed for all ow rates in this experiment.
Different ow speeds were analyzed using Eq. (1),
and the AK frequency results were shown in Fig. 1A.
Bidirectional velocity was obtained by subtraction of
the stationary signal, shown in Fig. 1B. The esti-
mated peak velocities from the AK and TK (mean and
standard deviation over 1000 lines) were plotted in
Fig. 2, which showed good agreement between the
measured and expected velocities. We separated the
ow regimes into Zone I, with velocities estimated by
TK; Zone II, where the spatial dimensions of the
aliasing rings are larger than the spatial resolution
of the system and phase unwrapping can be reliably
applied; and Zone III, where TK aliasing rings are
smaller than the axial resolution and the TKV ap-
proaches f
a
; so phase unwrapping cannot be reliably
performed, and the velocity estimation relies on AK,
as shown in Fig. 3. In Zones I and II, the TK exhib-
ited better velocity resolution than AK. Beyond them,
the phase unwrapped TK underestimated the true
velocities, where the AK was still able to estimate ve-
locities with good agreement with set ow rates.
Hence it is possible to use the full 2D Kasai estimator
(TK and AK) to accurately measure across a wide
range of ow velocities, spanning from
(7
m/s to ±57 cm/s, which is over 100 dB.
In vivo transesophageal M-mode DOCT imaging of
a rat aorta was performed using an endoscopic
catheter.
17
Motion artifacts were removed by a-scan
alignment using the aortic wall to blood interface.
The heart rate to be 230 beats per min or 0.26 s per
beat. A temporal smoothing lter set at 0.025 s in
length (10% of the cardiac cycle), was used to im-
prove the signal-to-noise ratio (SNR) while still pre-
serving the temporal resolution and allowing visual-
ization of the cardiac cycle. The Doppler angle was
approximately 82°. The peak systolic velocity
through the aorta was estimated to be 1 m/s (Fig.
4C), in good agreement with literature.
18
Comparing
Figs. 4A and 4B, it is evident that the TK is sensitive
to a slower ow, detecting the pulsating motion of the
aortic wall (velocity 2 mm/ s), while the AK is ca-
pable of estimating high ow velocities 1 m/s
without aliasing. We are exploring the use of TKV to
aid merging of the TK and AK results to yield a single
wide dynamic range velocity image.
The physical limiting factor in the maximum de-
tectable Doppler shift using AK in our system is the
bandwidth of the hardware demodulation circuit,
which is ±1.6 MHz (3 dB point) around the carrier
frequency. Since this is much smaller than the sam-
Fig. 1. A, Change in the AK estimated frequency as a
function of ow velocities from 57 to 34 cm/s in capillary
tube. B, When the no-ow AK result is subtracted from the
owing conditions, parabolic proles are obtained.
Fig. 2. TK and AK estimated peak velocities (mean and
SD) are compared to expected ow velocities. A, Peak ve-
locities derived from TK and AK algorithms recorded over a
large range of velocities. B, detailed view of TK (with phase
unwrapping) and AK estimated velocities for lower ow ve-
locities. See text for Zone I, II, and III denitions.
Fig. 3. (Color online) M-mode images of the three detec-
tion zones showing structural, TK, and AK color maps after
ow segmentation. In (I), TK accurately measures velocity,
and the aliasing rings in (II) can be unwrapped reliably. In
(III), the aliasing rings cannot be accurately unwrapped.
The AK is able to accurately measure ow in (III), but not
in (I) and (II). Aggregation of Intralipid produced artifacts,
especially at low shear rate regions.
2 OPTICS LETTERS / Vol. 32, No. 3 / February 1, 2007
Page 2
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.
10
Compared with
previous autocorrelation methods,
5,6
the Kasai esti-
mation output is linear with the ow 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 ow induced phase
changes. The TK and TKV processed from the same
data set are sensitive to slow ow conditions, and the
results can serve as segmentation maps for distin-
guishing stationary versus ow 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
process.
In conclusion, we described the AK algorithm as a
method for extending the velocity estimation range
on high-speed DOCT systems, to include higher ow
velocities. Using the Kasai autocorrelation technique
in two dimensions by combining the AK with TK, one
can obtain sensitivity from extremely slow to fast
ow velocities on the same data set. For what is for
the rst time to our knowledge, we demonstrated in
vivo transesophageal imaging of the rat aortic blood
ow 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. Yangs e-mail address is
victor.yang@utoronto.ca.
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Fig. 4. (Color online) In vivo M-mode images of transe-
sophageal DOCT of rate aortic blood ow. A, TK results
overlaid on structural image. Doppler signals indicate aor-
tic wall motion , systolic rush of high-speed blood ow
ⴱⴱ, and regions of slow ow 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 ow regions consistent with large Doppler
frequency shifts are clearly visualized. The temporal ow
proles 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
Page 3
  • Source
    • "The CGD probe with confocal optics design enable us to target a 0.4 mm capillary (Fig. 4A) and differentiate blood vessels that are only 1 mm apart (Fig. 5). On the other hand, Doppler optical coherence tomography (DOCT) has high spatial resolution (~10 μm), high temporal resolution (40 Hz) and wide flow speed dynamic range (7 μm/s to 52 cm/s) [16], but it requires extensive post processing and an expensive system to obtain the high resolution images. In contrast, CGD is a simple, robust and low-cost sensing system, which provides an audio signal that is rich in content, yet is easy for the operator to interpret (Fig. 5 ( "
    [Show abstract] [Hide abstract] ABSTRACT: Miniature optical sensors that can detect blood vessels in front of advancing instruments will significantly benefit many interventional procedures. Towards this end, we developed a thin and flexible coherence-gated Doppler (CGD) fiber probe (O.D. = 0.125 mm) that can be integrated with minimally-invasive tools to provide real-time audio feedback of blood flow at precise locations in front of the probe. Coherence-gated Doppler (CGD) is a hybrid technology with features of laser Doppler flowmetry (LDF) and Doppler optical coherence tomography (DOCT). Because of its confocal optical design and coherence-gating capabilities, CGD provides higher spatial resolution than LDF. And compared to DOCT imaging systems, CGD is simpler and less costly to produce. In vivo studies of rat femoral vessels using CGD demonstrate its ability to distinguish between artery, vein and bulk movement of the surrounding soft tissue. Finally, by placing the CGD probe inside a 30-gauge needle and advancing it into the brain of an anesthetized sheep, we demonstrate that it is capable of detecting vessels in front of advancing probes during simulated stereotactic neurosurgical procedures. Using simultaneous ultrasound (US) monitoring from the surface of the brain we show that CGD can detect at-risk blood vessels up to 3 mm in front of the advancing probe. The improved spatial resolution afforded by coherence gating combined with the simplicity, minute size and robustness of the CGD probe suggest it may benefit many minimally invasive procedures and enable it to be embedded into a variety of surgical instruments.
    Full-text · Article · May 2013 · Biomedical Optics Express
  • Source
    • "The maximum detectable velocity, when multiple aliasing rings are visible, is affected by a combination of factors including SNR, spatial resolution, and the performance of the phase unwrapping techniques. In principle, phase unwrapping technique breaks down when the velocity gradient equivalent to 2π occurs over a spatial dimension comparable to the resolution of the OCT system [25]. In practice, reduced SNR due to the low scattering flush fluid (1.5% blood in saline) will further decrease the maximal detectable velocity. "
    [Show abstract] [Hide abstract] ABSTRACT: Feasibility of detecting intravascular flow using a catheter based endovascular optical coherence tomography (OCT) system is demonstrated in a porcine carotid model in vivo. The effects of A-line density, radial distance, signal-to-noise ratio, non-uniform rotational distortion (NURD), phase stability of the swept wavelength laser and interferometer system on Doppler shift detection limit were investigated in stationary and flow phantoms. Techniques for NURD induced phase shift artifact removal were developed by tracking the catheter sheath. Detection of high flow velocity (~51 cm/s) present in the porcine carotid artery was obtained by phase unwrapping techniques and compared to numerical simulation, taking into consideration flow profile distortion by the eccentrically positioned imaging catheter. Using diluted blood in saline mixture as clearing agent, simultaneous Doppler OCT imaging of intravascular flow and structural OCT imaging of the carotid artery wall was feasible. To our knowledge, this is the first in vivo demonstration of Doppler imaging and absolute measurement of intravascular flow using a rotating fiber catheter in carotid artery.
    Full-text · Article · Sep 2012 · Biomedical Optics Express
  • Source
    • "These capabilities could be valuable for screening the vessels posing high risk in neurosurgery. We acknowledge that the aliasing problem may hinder using DOCT signal to quantify blood flow; however, we can work around this problem by using velocity variance [37] or axial Kasai algorithm [40]. Also, the high speed Fourier domain mode locking laser should be able to increase the velocity detection limit from one to two orders [41,42]. "
    [Show abstract] [Hide abstract] ABSTRACT: A forward-imaging needle-type optical coherence tomography (OCT) probe with Doppler OCT (DOCT) capability has the potential to solve critical challenges in interventional procedures. A case in point is stereotactic neurosurgery where probes are advanced into the brain based on predetermined coordinates. Laceration of blood vessels in front of the advancing probe is an unavoidable complication with current methods. Moreover, cerebrospinal fluid (CSF) leakage during surgery can shift the brain rendering the predetermined coordinates unreliable. In order to address these challenges, we developed a forward-imaging OCT probe (740 μm O.D.) using a gradient-index (GRIN) rod lens that can provide real-time imaging feedback for avoiding at-risk vessels (8 frames/s with 1024 A-scans per frame for OCT/DOCT dual imaging) and guiding the instrument to specific targets with 12 μm axial resolution (100 frames/s with 160 A-scans per frame for OCT imaging only). The high signal-to-background characteristic of DOCT provides exceptional sensitivity in detecting and quantifying the blood flow within the sheep brain parenchyma in real time. The OCT/DOCT dual imaging also demonstrated its capability to differentiate the vessel type (artery/vein) on rat's femoral vessels. We also demonstrated in ex vivo human brain that the location of the tip of the OCT probe can be inferred from micro-anatomical landmarks in OCT images. These findings demonstrate the suitability of OCT guidance during stereotactic procedures in the brain and its potential for reducing the risk of cerebral hemorrhage.
    Full-text · Article · Dec 2011 · Optics Express
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