On the accuracy of intracardiac flow velocimetry methods.

TL;DR: The topic of the flow pattern inside the heart and vortex imaging has been a main stream of research in echocardiography during the past decade and knowledge gained about LV fluid dynamics, and in particular the associated vortical flow motion, has introduced novel clinical indicators for LV function based on vortex dynamics.

Abstract: ‘‘Begin challenging your own assumptions. Your assumptions are your windows on the world. Scrub them off every once in a while, or the light will not come in.’’—Alan Alda [1]. The topic of the flow pattern inside the heart and vortex imaging has been a main stream of research in echocardiography during the past decade. Progress has been made to incorporate quantitative fluid dynamics into echocardiography using particle tracking algorithms [2, 3, 39] that are based mostly on the well-known optical imaging techniques of particle image velocimetry (PIV) [4–6] or color Doppler imaging [7–10]. Recent advances in understanding left ventricular (LV) fluid dynamics based on experimental methods [11–14] and numerical simulations [15–17] have shed light on many aspects of ventricular flow, such as the development of intraventricular vortices. These vortices are shown to significantly influence transmitral momentum transfer and help redirect the flow from the left atrium toward the left ventricular outflow tract (LVOT) [18, 19]. Alternatively, formation of unnatural vortices can be a sign of adverse blood flow, which may indicate progressive LV dysfunction [18–21]. The knowledge gained about LV fluid dynamics, and in particular the associated vortical flow motion, has introduced novel clinical indicators for LV function based on vortex dynamics [18, 19, 21–25]. PIV is an optical method for flow visualization used to obtain instantaneous velocity measurements and related properties in the fluids. In this technique, the fluid is seeded with tracer particles, which are assumed to faithfully follow the dynamics of flow. The motion of these seeding particles is used to compute the flow velocity. In its current form, 2D ultrasound-based PIV or 2D echocardiographic PIV (EchoPIV) was introduced by Kim et al. [2], through capturing digital B-mode images of contrast agent particles, and further used for vortex imaging by Kheradvar et al. [21]. This technique computes the velocities of the ultrasoundimaged particles based on the PIV technique, with the Dt being equal to scanning time. The number of beams and the samples along each beam define the number of pixels for each image after scan conversion. Particles used as the flow tracers are microbubbles filled with octafluoropropane encapsulated in either a lipid (DEFINITY , Lantheus Medical Imaging, Inc.) or protein (Optison, GE Healthcare) outer shell [3, 26], which are both FDA-approved for clinical use. This technique allows the velocity directions and streamlines, principal blood flow patterns, recirculation regions, and vortices to be drawn with reasonable confidence in a reproducible scheme [18, 21, 22, 27–32]. Alternatively, vector flow mapping (VFM) measures blood flow velocity by considering color Doppler imaging and ventricular wall velocity [7–10]. This method works based on combining measured axial velocities with estimated radial velocities according to the physical principles [33]. VFM ignores the three-dimensional component of the flow by assuming the flow is two-dimensional, solves the 2D continuity equation, and use ventricular wall velocity acquired by tissue tracking to improve the results [34]. & Arash Kheradvar arashkh@uci.edu

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Title
On the accuracy of intracardiac flow velocimetry methods.
Permalink
https://escholarship.org/uc/item/7f79v1xr
Journal
Journal of echocardiography, 15(2)
ISSN
1349-0222
Author
Kheradvar, Arash
Publication Date
2017-06-01
DOI
10.1007/s12574-017-0332-x
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This work is made available under the terms of a Creative Commons Attribution License,
availalbe at https://creativecommons.org/licenses/by/4.0/
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EDITORIAL
On the accuracy of intracardiac flow velocimetry methods
Arash Kheradvar
1
Received: 6 January 2017 / Accepted: 9 January 2017 / Published online: 15 February 2017
Ó Japanese Society of Echocardiography 2017
Keywords Vortex imaging  Vector flow mapping  Echo-
PIV  Velocimetry  Fluid dynamics  Flow visualization
‘‘Begin challenging your own assumptions. Your assump-
tions are your windows on the world. Scrub them off every
once in a while, or the light will not come in.’’—Alan Alda
[1].
The topic of the flow pattern inside the heart and vortex
imaging has been a main stream of research in echocar-
diography during the past decade. Progress has been made
to incorporate quantitative fluid dynamics into echocar-
diography using particle tracking algorithms [2, 3, 39] that
are based mostly on the well-known optical imaging
techniques of particle image velocimetry (PIV) [4–6]or
color Doppler imaging [7–10]. Recent advances in under-
standing left ventricular (LV) fluid dynamics based on
experimental methods [11–14] and numerical simulations
[15–17] have shed light on many aspects of ventricular
flow, such as the development of intraventricular vortices.
These vortices are shown to significantly influence trans-
mitral momentum transfer and help redirect the flow from
the left atrium towar d the left ventricular outflow tract
(LVOT) [18, 19]. Alternatively, formation of unnatural
vortices can be a sign of adverse blood flow, which may
indicate progressive LV dysfunction [18–21]. The knowl-
edge gained about LV fluid dynamics, and in particular the
associated vortical flow motion, has introduced novel
clinical indicators for LV function based on vortex
dynamics [18, 19, 21– 25].
PIV is an optical method for flow visualization used to
obtain instantaneous velocity measurements and related
properties in the fluids. In this technique, the fluid is seeded
with tracer particles, which are assumed to faithfully follow
the dynamics of flow. The motion of these seeding particles
is used to compute the flow velocity. In its current form, 2D
ultrasound-based PIV or 2D echocardiographic PIV (Echo-
PIV) was introduced by Kim et al. [ 2 ], through capturing
digital B-mode images of contrast agent particles, and
further used for vortex imaging by Kheradvar et al. [21].
This technique computes the velocities of the ultrasound-
imaged particles based on the PIV technique, with the
Dt being equal to scanning time. The number of beams and
the samples along each beam define the number of pixels
for each image after scan conversion. Particles used as the
flow tracers are microbubbles filled with octafluoropropane
encapsulated in either a lipid (DEFINITY
Ò
, Lantheus
Medical Imaging, Inc.) or protein (Optison
TM
,GE
Healthcare) outer shell [3, 26], which are both FDA-ap-
proved for clinical use. This technique allows the veloc-
ity directions and streamlines, principal blood flow
patterns, recirculation regions, and vortices to be drawn
with reasonable confidence in a reproducible scheme
[18, 21 , 22, 27–32].
Alternatively, vector flow mapping (VFM) measures
blood flow velocity by considering color Doppler imaging
and ventricular wall velocity [7–10]. This method works
based on combining measured axial velocities with esti-
mated radial velocities according to the physical principles
[33]. VFM ignores the three-dimensional component of the
flow by assuming the flow is two-dimensional, solves the
2D continuity equation, and use ventricular wall velocity
acquired by tissue tracking to improve the results [34].
& Arash Kheradvar
arashkh@uci.edu
1
The Edwards Lifesciences Center for Advanced
Cardiovascular Technology, University of California Irvine,
2410 Engineering Hall, Irvine, CA 92697-2730, USA
123
J Echocardiogr (2017) 15:67–69
DOI 10.1007/s12574-017-0332-x

In reality, any physical flow is three-dimensional.
However, some flow regimens can be considered 2D if the
out-of-plane velocity component does not (or at least
minimally) exist. A good example for such a flow regime is
laminar flow in an axisymmetric tube. In laminar flow,
there is no lateral mixing, and the nearby layers pass each
other in a totally parallel scheme. Laminar flow requires no
cross-currents perpendicular to the flow direction or eddies/
swirls in the fluid [35]. Non-uniform geomet ries, such as in
the heart chambers, increase flow three-dimensionally.
Furthermore, time-dependency and the rotational nature of
the flow minimize the application and accuracy of the
methods developed for potential flow. Principles of fluid
dynamics shoul d be properly considered and applied for
each particular flow regimen to avoid fundamental over-
sights in solving cardiovascular problems [36].
In prospect, intracardiac flow velocimetry is an emerg-
ing field in cardiac imaging. It should be considered that
intracardiac flow is principally three-dimensional, time-
dependent, and non-laminar. Modern echocardiography
systems use ultrasound probes that can capture three-di-
mensional brightness fields associated with the blood flow.
Generally, the ultrasound-based velocimetry methods are
all bounded by the limitations and constraints of echocar-
diographic acquisi tions, such as inter and intra-operator
variabilities and acoustic shadowing. Furthermore, limited
frame rate of echocardiographic acquisitions—particularly
in 3D—is currently a major obstacle for accurate assess-
ment of high-velocity values and advancement of 3D
ultrasound-based velocimetry modalities for intracardiac
flow [21, 33, 37 ]. More recent efforts may overcom e these
limitations and pave the way for routine clinical applica-
tions [33, 37–39].
Compliance with ethical standards
Conflict of interest Prof. Kheradvar holds multiple pending patents
on velocimetry methods.
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