A complex flow phantom for medical imaging : ring vortex phantom design and technical specification
Summary (4 min read)
1. Introduction
- The fluid dynamics of the cardiovascular system are associated with many complexities and this has provoked interest from scientists for decades.
- Magnetic Resonance angiography and Doppler Ultrasound are examples of technologies that offer valuable flow-based assessment of the cardiovascular system.
- Blood speckle imaging is available on the GE Healthcare Vivid E95TM (GE Healthcare, Illinois, USA).
- 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].
- 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 aノ┌キS ヮヴラヮWノノWS デエヴラ┌ェエ デエW ラヴキaキIW さヴラノノゲ ┌ヮざ デエW ラヴキaキIW 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.
2.1 Imaging Tank
- The imaging tank 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.
- Several reference markers and a ruler are laser-cut into the surfaces of the tank walls to support positioning and measurement.
- The top of the tank is open, offering easy access for Ultrasound imaging probes.
2.2 Piston Cylinder System
- The generation of vortex rings relies on three components: a cylindrical channel, a plunger and an orifice that connects the channel to the imaging tank.
- The cylindrical channel used here is of internal diameter 70+/-0.10 mm, reamed from a solid cylinder manufactured from PMMA .
- One end of the chamber includes a screw thread for leak-proof coupling with the orifice and the imaging tank.
- The piston head of the plunger incorporates an O-ring to provide a leak-proof seal between piston and the cylindrical chamber .
- These generate rings of several centimetres diameter with characteristics that are relevant to human physiology.
2.3 Programmable Actuator
- The nut/lead screw arrangement of the piston enables it to be driven by a linear stepper motor - this constitutes the actuator of the ring vortex generator.
- A Nema 23 external linear stepper motor drives the threaded screw of length 150 mm (Nema 23 external linear actuator, OMC Corporation Limited, Nanjing, China).
- Power is delivered through a digital stepper driver (OMC Corporation Limited, Nanjing, China) which has connections to a switching 150 W power supply (OMC Corporation Limited, Nanjing, China).
- The digital system relies on CMOS logic (0-5V), with a square-wave signal of varying frequency (from 0.5 kHz to 1 kHz) but fixed duty cycle (50%) controlling the digital motor displacement.
- The total cost of all the electronic components is less than one hundred Euros at the time of writing (2019).
2.4 The Assembled System
- The interconnection of these components provides the system depicted in Figure 5.
- The actuator is programmed to deliver a preconfigured displacement profile (eg. top-hat) that moves the piston through the cylinder and propels a slug of fluid through the orifice (e.g. 3 cm3 of displaced fluid in 50 ms).
- This results in a controlled, propagating ring vortex of an orificedependent size that travels steadily (with velocities < 1 ms-1) along the length of the tank .
- Images captured by an imaging system can be referenced to the features of the flow, permitting characterisation of imaging system performance.
- Bulk features (ring speed, size) are readily established through simple optical/video methods [20] whereas finer details of the flow benefit from technologies such as particle image velocimetry (PIV) or even computational fluid dynamics (CFD) [21].
3. Demonstration and Application
- Reynolds number calculations are based on flow through the orifice generating the ring (Re = Vd/; - density, V-velocity, d-diameter, -viscosity).
- In order to confirm the functionality of the phantom design, two different measurement methods were used to characterise the generated flows, namely optical/video measurements and Laser-PIV; details are provided in sections 3.1 and 3.2 below.
- Optical/video measurements were accomplished on the premises of Leeds Test Objects Ltd (Leeds Test Objects Ltd, Boroughbridge, United Kingdom) as previously described by Ferrari et al [20].
- The PIV methodology followed that described by Wieneke [24], with calibration procedures performed under the supervision of a LaVisionUK Ltd (LaVisionUK Ltd, Bicester, United Kingdom) application consultant.
3.1 Optical/video acquisition
- Vortex progress was captured at 25 frames per second using a Sony camera HDR-PJ220E (Sony Corporation, Tokyo, Japan).
- The camera was placed on a tripod at a distance of 3.8 m from the phantom, obtaining a clear view through the sides of the transparent tank.
- This zone was chosen as the region of interest for all measurements (including PIV).
- In addition, a 1/100sec universal counter-timer (RS Components Ltd, Stock No 612-445) was placed within the field of view of the images to provide visible timing data in support of the measurements.
- Distance measurements from still frames enabled characterisation of vortex ring position and translational speed.
3.2 Laser-PIV acquisition
- The Laser-PIV setup required neutrally buoyant scattering particles mixed throughout the tank/cylinder system to provide satisfactory conditions for flow capture using the stereo Laser-PIV technique.
- Fluorescent particles of size 10-20 micron were mixed within the volume of water, which was illuminated by a laser sheet that cut the propagating ring through its centre in the vertical plane.
- Reconstruction of particle displacement using two camera projections (stereoscopic view) and digital image correlation allowed the reconstruction of the flow field [24] at a spatial resolution of 0.4 mm and temporal steps of 0.071 s (14 Hz).
- Accuracy of the instantaneous velocities of the PIV system is declared by LaVisionUK Ltd (LaVisionUK Ltd, Bicester, United Kingdom) to be better than +/-0.1%.
- Mapping the flow field of the Vz components at each frame and knowing the frame rate of the acquisition (14 fps), the translational velocity of the ring was calculated.
4.1 Optical/Video measurements
- Average vortex ring translational velocities (Vtrans with standard deviation expressed by the error bars) as a function of vortex ring position are plotted in figs 6,7 (see Table 1 for the configurations tested).
- For improved clarity, results for the configurations that produced faster ring vortices (from 15 cm/s to 80 cm/s) are shown in Figure 6 while results from slower configurations (from 0 cm/s to 15 cm/s) are shown in Figure 7.
- In order to compare the optical data directly with the Laser-PIV translational velocity measurements, each selected configuration (Table 1) corresponds to a specific colour and marker-shape combination.
- In each case, variability within 10 runs is presented as +/-1SD on the plots and cited as coefficient of variation (%) within the text.
- Such errors calculated with the optical/video method are always less than +/-10%.
4.2 Laser-PIV
- For consistency, average PIV vortex ring translational velocities with standard deviation are also plotted (dashed lines) as a function of vortex ring position .
- These are calculated from 10 vortex ring acquisitions for each of the settings listed in Table 1 (as for the optical/video method).
- Relative errors were calculated as for the optical/video experiment, and the varying configurations confirm very similar behaviour to the optical results.
- Notably, the error increases for slower vortex ring configurations (e.g. Configuration 10, Table 1).
5. Discussion
- Bulk flow performance has been assessed using two identical systems, undertaken at two different places (Leeds and Sheffield), two weeks apart.
- In respect of ring vortex performance, Gharib observed that vortex ring generation depends on a non-dimensional parameter, described [25] as formation time or formation number.
- Importantly, the measurement processes described in this paper indicate that realistic tolerances for these reproducible flows can be asserted に hence the utility of this design as a complex flow phantom.
- With a longer-term ambition to compare flow imaging performance between different flow medical imaging modalities (Ultrasound, MRI and CT) - both for clinical and research purposes - the ring vortex phantom has been entirely manufactured from PMMA (except for the motor).
- In an environment where the quality control of flow medical imaging scanners is still challenging and confusing, this technology offers support for both research and clinical imaging activities.
6. Conclusion
- A technical specification for the design of a novel, cost-effective, portable complex flow test object currently compatible with ultrasound modalities has been provided.
- Early experience indicates that the technology has the characteristics suited to basic and more advanced ultrasound quality control checks at research and clinical level.
- The phantom as described, demonstrates that it can be reproducibly manufactured to deliver consistent performance within specified tolerances.
- The authors report no conflict of interest.
- This work is funded by the European Commission through the H2020 Marie Sklodowska-Curie European VPH-CaSE Training Network (www.vph-case.eu), GA No. 642612.
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"A complex flow phantom for medical ..." refers background in this paper
...For these reasons, ultrasound is currently the first choice as a diagnostic modality for the assessment of several cardiovascular pathologies [7]....
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...The ring vortex flow is well suited to its role as a reference flow because it offers characteristics like stability, predictability, reproducibility and controllability [20,21,27,28]....
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...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]....
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Q2. What are the future works in "A complex flow phantom for medical imaging: ring vortex phantom design and technical specification" ?
It is worth noting that their analysis has focussed on bulk flow characteristics ( eg. translational vortex ring speed Vtrans ) but for completeness, further work is needed to assess the micro-flow environment in addition to the macro-flow characteristics described here. Future work will extend the assessment presented here to include comparative studies between medical imaging modalities ( Ultrasound, CT, and with some adaptation MRI ) and optical modalities ( Laser-PIV, Laser-diode ) to further assess reliability, long-term stability and detailed flow performance. Currently, four identical phantom systems have been manufactured and are currently being evaluated in the United Kingdom and France within both research and clinical environments, in order to identify potential improvements to the design.