This is a repository copy of A complex flow phantom for medical imaging : ring vortex
phantom design and technical specification.
White Rose Research Online URL for this paper:
http://eprints.whiterose.ac.uk/149487/
Version: Accepted Version
Article:
Ambrogio, S., Walker, A., Narracott, A. orcid.org/0000-0002-3068-6192 et al. (3 more
authors) (2019) A complex flow phantom for medical imaging : ring vortex phantom design
and technical specification. Journal of Medical Engineering & Technology, 43 (3). pp.
190-201. ISSN 0309-1902
https://doi.org/10.1080/03091902.2019.1640309
This is an Accepted Manuscript of an article published by Taylor & Francis in Journal of
Medical Engineering and Technology on 15th July 2019, available online:
http://www.tandfonline.com/10.1080/03091902.2019.1640309
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A Complex Flow Phantom for Medical Imaging: Ring Vortex Phantom
Design and Technical Specification
1,2,3
Ambrogio Simone,
3
Walker Adrian,
1,2
Narracott Andrew,
1,2
Ferrari Simone,
4
Verma Prashant,
1,2
Fenner John
1
Medical Physics, Mathematical Modelling in Medicine Group, Department of Infection, Immunity and Cardiovascular Disease, University
of Sheffield, Sheffield, UK
2
Insigneo Institute for In Silico Medicine, University of Sheffield, Sheffield, UK
3
Leeds Test Objects Ltd., Boroughbridge, UK
4
Medical Imaging & Medical Physics, Sheffield Teaching Hospitals NHS Foundation Trust, Sheffield, UK
Corresponding Author:
Simone Ambrogio
E-mail:
simoambr@gmail.com
(Orcid 0000-0003-1571-0016)
ABSTRACT
Cardiovascular fluid dynamics exhibit complex flow patterns, such as recirculation and vortices. Quantitative
analysis of these complexities supports diagnosis, leading to early prediction of pathologies. Quality Assurance
of technologies that image such flows is challenging but essential, and to this end, a novel, cost-effective,
portable, complex flow phantom is proposed and the design specifications are provided. The vortex ring is the
flow of choice because it offers patterns comparable to physiological flows and is stable, predictable,
reproducible and controllable. This design employs a piston/cylinder system for vortex ring generation, coupled
to an imaging tank full of fluid, for vortex propagation. The phantom is motor-driven and by varying piston speed,
piston displacement and orifice size, vortex rings with different characteristics can be produced. Two
measurement methods, namely Laser-PIV and an optical/video technique, were used to test the phantom under
a combination of configurations. Vortex rings with a range of travelling velocities (approximately 1-80cm/s) and
different output-orifice diameters (10-25mm) were produced with reproducibility typically better than +/-10%.
Although ultrasound compatibility has been demonstrated, longer-term ambitions include adapting the design
to support comparative studies with different modalities, such as MRA and X-ray-CTA.
Keywords: Medical Imaging Phantom, Doppler Ultrasound Quality Assurance, Vortex Ring, Complex Flow
Phantom, Laser PIV
1. Introduction
The fluid dynamics of the cardiovascular system are
associated with many complexities and this has
provoked interest from scientists for decades.
Recirculation, turbulence, jets and vortices are
observed both in healthy and pathophysiological
conditions [1,2]. Quantitative analysis of the
distribution of blood velocity patterns can support
diagnosis of the cardiovascular system, leading to
early prediction of pathologies, improvements of
surgical outcomes and evaluation of potential
therapies [2,3]. Magnetic Resonance angiography and
Doppler Ultrasound are examples of technologies that
offer valuable flow-based assessment of the
cardiovascular system. 4D Flow MRI is a powerful
technique that provides a time-resolved 3D velocity
field. Velocity is encoded along all three spatial
dimensions in the vessel of interest (4D = 3D + time).
Unfortunately, this technique is not clinically routine
because the acquisition time exceeds that demanded
by the clinical workflow [4]. Doppler Ultrasound is
portable, non-invasive, cost effective, does not involve
ionizing radiation and provides qualitative and
quantitative real-time information about volumes (i.e
stroke volume, ejection fraction), inferred pressures
(across heart valves) and flows (i.e. cardiac output)
[5,6]. For these reasons, Ultrasound is currently the
first choice as a diagnostic modality for the
assessment of several cardiovascular pathologies [7].
Recent developments include 2D and 3D real-time
angle independent Ultrasound Doppler imaging
techniques, based on Vector Flow Imaging (VFI), and
post-processing algorithms for particle velocimetry
tracking and volume quantification
[6,7,8,9,10,11,12,13]. A specific Ultrasound VFI
technique - Transverse Oscillation (TO) - has been FDA
approved (2013) and is clinically available on
commercial scanners (eg. BK Ultrasound, Nova Scotia,
Canada, and Carestream Health, Ontario, Canada) for
real-time analysis of complex flows in valves,
bifurcations and heart chambers [6,14]. A colour
Doppler based VFI technique is also commercially
available on clinical scanners manufactured by Hitachi
(Hitachi Ltd., Tokyo, Japan), GE Healthcare (GE
Healthcare, Illinois, USA) and Mindray (Mindray
Medical International Limited, Shenzen, China). Blood
speckle imaging is available on the GE Healthcare Vivid
E95
TM
(GE Healthcare, Illinois, USA). In this domain,
improved technologies for calibration of such medical
imaging techniques are essential. Complex flows are
needed as a standard reference for the validation of
innovative flow estimation algorithms.
Recent audits concerning Ultrasound Quality Control
protocols indicate that the current flow/velocity test
objects available on the market are rather limited,
expensive, and often fail to reproduce physiological
conditions and physiological waveforms
[15,16,17,18]. Manufacturers are not clear about
tolerances and accuracy specifications [18] and the
lack of well-defined protocols makes the Quality
Control of scanners that measure flow and velocity
challenging [15,16]. For example, experimental mean
velocity measurements using a commercial string
phantom (as recommended in IPEM Report 102,
Institute of Physics and Engineering in Medicine, 2010)
for linear, curvilinear and phased array probes
reported errors exceeding 20%, 50% and 40%,
respectively [17]. These values are considerably higher
than the recommended maximum error value of +/-
5%, as declared by BS EN 61685:2002-IEC 61685:2001,
a current International standard (stability date: 2020)
for the development of a flow Doppler test object [19].
It is this context that is the motivation for the
development of a novel phantom, designed to
produce complex flows that are stable, predictable,
controllable and reproducible. The particular flow
chosen as the basis for the design is the vortex ring.
This paper describes the salient features of a
prototype ring vortex phantom providing necessary
details for construction in a form suited to calibration
of Ultrasound flow imaging systems. With its uniquely
controllable characteristics, the ring vortex is a
suitable flow reference for a flow test object.
2. Materials and Methods
The complex flow phantom design described here
uses a piston to propel a slug of fluid along a cylindrical
channel, through an orifice port that connects to an
open tank of fluid (containing water, Blood-Mimicking
Fluid etc.) where propagation of the ring may be
observed and imaged. A vortex ring naturally forms
when a column of fluid is pushed through an orifice
into a neighbouring expansive fluid environment. The
face, forming a toroidal core of vorticity that may
ultimately detach (at high enough Reynolds number)
and propagates along the axisymmetric axis of the
ring. The whole process is managed by computer
control of piston displacement that delivers the fluid
slug with a precisely specified velocity/displacement
profile. Each component of the phantom is presented
below followed by description of the functioning unit
as a whole.
2.1 Imaging Tank
The imaging tank (Figure 1) is the principal component
of the phantom since it is the environment in which
the ring vortex propagates; it is the volume used to
image the generated flow. The tank is manufactured
from clear poly(methyl methacrylate) (PMMA) which
rests on four small screw feet. These are adjustable to
allow levelling of the system via two spirit levels
placed on opposite walls of the tank box. The tank has
internal dimensions of 15 cm (W) x 35 cm (L) x 16.5 cm
(H) (Figure 1), chosen to be sufficiently large that the
walls do not influence ring vortex propagation.
Several reference markers and a ruler are laser-cut
into the surfaces of the tank walls to support
Figure 1. CAD drawing of the ring vortex tank.
Figure 2. CAD drawing of the piston cylinder system.
Figure 3. CAD drawing of the attachable/detachable orifices.
