16th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 09-12 July, 2012
Flow Measurements in Patient Specific Conducting Airways Models
B. Timmerman1,*, G. Gibbons1, P.J. Bryanston-Cross2
1: WMG, University of Warwick, Coventry, UK
2: OEL, School of Engineering, University of Warwick, Coventry, UK
* correspondent author: B.H.Timmermanl@warwick.ac.uk
Abstract The paper presents the development of techniques to study flows in human conducting airways in
order to improve understanding of underlying mechanisms in asthma and COPD. These include high-speed
particle image velocimetry (PIV) as well as tomo-PIV measurements in geometrically realistic transparent
airway models obtained through additive layer manufacturing (ALM) of segmented lung CT scans. This
enables study of breathing dynamics, which will allow validation and development of CFD models and in
vivo MRI velocimetry.
1. Introduction
Asthma and COPD (chronic obstructive pulmonary disease) are wide-spread, serious health
problems with asthma affecting 300 million people, while 80 million people have moderate–to–
severe COPD worldwide. They impose serious health risks with asthma estimated to cause
approximately 239,000 deaths worldwide per year and COPD predicted to be the 3rd leading cause
of death in the world by 2030 (AirPROM, 2012). In people with COPD and asthma damaged,
inflamed or obstructed airways are common, hindering breathing. Current methods to detect and
treat these conditions do not consider individual differences between airways and although targeted
approaches to treatment are being developed, often it has been unknown how to match these to
specific patients. Thus, people suffering from these conditions may not get the optimal treatment.
The EU-funded programme AirPROM aims to develop models of the airways to assess how air
flows through the lungs and why this flow becomes obstructed in people with asthma and COPD.
This will enable development and testing of new individually tailored therapies, through linking the
characteristics of different airways to a particular treatment. Furthermore this will aid monitoring
future risks to patients by helping predictions regarding the effect on the airways and the
progression of the diseases.
As part of this project, techniques are developed to study flows in human conducting airways to
improve understanding of underlying mechanisms in asthma and COPD. For this, realistic patientspecific anatomical models of the conducting airways are created through additive layer
manufacturing (ALM) of segmented lung CT scans, which allow the airflow in lungs to be studied
to understand how lung-conditions affect the patients’ breathing. The paper discusses the
preliminary results in the development of high-speed and tomographic particle image velocimetry
(PIV) to investigate breathing dynamics and flow characteristics through high order (up to 7th)
ALM airway models.
2. PIV in lungs
Understanding flows in human airways is vital for combating respiratory complications. Issues
include aerosolised drug delivery, high frequency ventilation and pulmonary disease diagnosis and
treatment (Freitas, 2008). The work presented here is focused on flow modifications seen in asthma
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16th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 09-12 July, 2012
and COPD. Patients suffering asthma have inflamed airways, making them swollen and very
sensitive. When the airways react, mucus production increases and the muscles around airways
tighten, narrowing them and thus causing less air to flow into the lungs. COPD typically involves
two separate lung conditions: chronic bronchitis and emphysema. In chronic bronchitis, the airways
(bronchi) become inflamed, congested with mucus, and narrowed, resulting in obstructed air flow.
In emphysema, the walls of the air sacs (alveoli) are destroyed, leading to fewer but larger alveoli,
making them less efficient in transferring oxygen from the lungs to the bloodstream.
Flows in human airways have been studied using silicone models based on typical airways
geometries (Adler and Brücker, 2007) derived from data from e.g. Horsfield et al (1971) and
Weibel (1963), as well as using silicone optically transparent ALM models based on patient CTdata, generally aimed at improving artificial ventilation (e.g. Vermeulen et al., 2010; Soodt et al.,
2011; S. Große et al, 2008). Typically a water-glycerine mixture is used as the working fluid,
allowing refractive index matching of fluid and model to minimise refraction effects in resulting
images. Generally planar 2C PIV measurements are obtained in different planes and combined to
find the three-dimensional flow distribution.
The flow in the trachea and upper bronchi has been found to be highly three-dimensional and
asymmetric with counter-rotating vortices in the bronchi (e.g. Große et al., 2007). There is no
consensus on the influence of different inflow conditions on the flow in the subsequent
bifurcations., e.g. to what extent the upstream flow conditions have an impact on the flow field in
sub-branches as a function of the local Reynolds number. Ball et al (2006) showed strong vortex
shedding in numerical simulation of the flow from the laryncheal region with flow impact up to 8D.
Van Ertbruggen (2005) found highly three-dimensional flow in numerical simulations, showing the
importance of non-fully developed flows in the branches due to their relatively short lengths. Thus
physically relevant studies require realistic models of the lung geometry, including effects from the
laryncheal region (e.g. Lizal et al., 2012). Studies to date have shown a quantitative evaluation of
volumetric flow will require time-resolved three-dimensional measurements.
In the experiments described here these techniques are investigated further using particle image
velocimetry (PIV) to enable study the flow dynamics in two types of optically transparent ALM
models of conducting human airways.
3. Patient-specific anatomical models
In the measurements presented two types of anatomical model have been used, which are both
optically transparent: a low-order model made by casting an optically clear elastomer around a CTbased, ALM produced core, which is subsequently removed, and a high-order CT-based model
produced through standard ALM techniques (Fig. 1). Both are based on CT's obtained in vivo.
The elastomer used in the first model (Clear Flex® 50 water clear urethane rubber, Smooth-On Inc)
possesses a level of elasticity similar to that of the cartilage in the trachea and left and right
bronchial tubes (Young's modulus ~2.47MPa vs. averages ranging from 2.5-7.7MPa for trachea
(Lambert et al., 1991)), thus allowing flow study at near-realistic compliance. The second model
provides a full airway geometry up to 7th order, enabling study of the effects of the various
branches on total and local flow dynamics.
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16th
th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal,
Portugal 09-12 July, 2012
Figure 1: Transparent ALM models of conducting human airways. Left: 7th generation model,
Right: 2nd order model obtained by dissolving an ALM core.
4. Experiments
For preliminary measurements, the flow was generated using a peristaltic pump rig providing a
controlled input flow rate. Distortions
istortions due to refractive index differences were minimised by using
rapeseed oil was as flow medium, enabling visualisation in the complex airway geometries by
matching refractive index of the models (~1.49) to that of the flow (~1.47 (Wesolowski and
Erecinska, 1998)).
). To enable velocity measurements the flow was seeded with reflective polyamide
particles of 50 µm diameter. A light sheet was generated using a 200mW green laser diode (SD(SD
301) and used to illuminate a 2D plane or thin volume in the geometr
geometry.
y. The movement of the
particles was recorded from which velocities were calculated (DaVis, LaVision).
The anatomical ALM phantoms used were mounted on a traversing system allowing fully
automated accurate scanning of the flow region of interest througho
throughout
ut the airway models (Fig. 2).
Thus, by effectively scanning the light sheet different parts of the airways can be investigated and
the turbulent flow distribution through the complete airway geometry can be built up.
Figure 2: Measurement system. Le
Left:
ft: Overview showing traversing system enabling accurate
movement of the lung model in x, y and zz-direction
direction on the left. Right: Illumination system and flow
tank with 7th order model. Note: in this picture the tanks is not filled causing distortions and
opaqueness in lung model,, which are removed when oil is added
added.
To investigate the dynamics of the flow, time
time-resolved
resolved measurements were obtained using highhigh
speed PIV (Olympus i-Speed3).
Speed3). Furthermore, to allow volumetric measurements a novel hybrid
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16th
th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal,
Portugal 09-12 July, 2012
tomo-PIV system (Elsinga, 2007) was designed based on the use of three to five synchronised
(high-speed/PIV)
speed/PIV) cameras (Fig. 3) for simultaneous capture of PIV data from a thick light sheet.
Depending on the configuration used this enables (quasi) time
time-resolved
resolved 3C volumetric
v
measurements allowing study of different aspects of flow behaviour including flow development
inside the airways (Fig. 4).
Figure 3: Tomo-PIV
PIV measurement systems. Left: 33-camera
camera arrangement. Right: Hybrid tomo/hightomo/high
speed system.
on in a cross plane exiting one of the lower
Figure 5 shows a short sequence of the flow distribution
bronchi. A clear unsteady behaviour can be seen.
Figure 4:: Velocity distribution in trachea and upper bronchi of 7th order model. Left:
instantaneous, Right: mean velocity distribution. Note the turbulence for flow directed towards
right bronchus.
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16th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 09-12 July, 2012
Figure 5 Instantaneous velocity maps of flow exiting lower bronchus. Note image of ALM model
superimposed as background.
Figure 6 shows three simultaneous views of the flow in the 7th order model. Using this data a 3D3C velocity can be extracted over a thin volume.
Figure 6 Simultaneous images of seeded flow captured using three cameras from left, centre
and right viewing direction.
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16th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 09-12 July, 2012
5. Conclusions
A measurement system capable of high-speed and tomographic PIV has been designed for
measurements in optically transparent airway models of realistic configuration. Preliminary results
have been presented, with further data being processed.
To investigate flow behaviour at realistic breathing conditions a flow rig is being designed to
produce an oscillating flow representing inspiration and expiration. The mass flow rate and
oscillation frequency of the model flows need to be corrected for properties of the flow medium by
matching the Reynolds number, Re, and Wormersley number, α, in order to mimic realistic
velocities and breathing rates respectively. The Reynolds number is matched based on the
maximum velocity in the trachea, Umax :
ܷ௠௔௫ ∙ ‫ܦ‬
ܴ݁ =
ν
where D is the diameter of the trachea and ν is the kinematic viscosity. The Wormersley number, α,
is given by:
‫߱ ܦ‬
ߙ= ට
2 ν
where ω is the angular frequency.
Typically, this implies a lower breathing frequency for liquid media compared to air (0.25Hz), as
well as a higher flow velocity for liquid versus air (Umax ~0.8m/s). The controlled pneumatic flow
rig will be used to generate flows simulating the patient specific breathing patterns.
To obtain volumetric 3D-3C data, in addition to tomo-PIV and digital holography (Claus et al,
2011a,b), the multi-plane stochastic estimation technique of Hunter et al (2012) will be further
developed to extract the time-resolved volumetric flow distribution. Further work includes potential
derivation of pressure distribution from this PIV-based velocity data (van Oudheusden et al, 2007),
which is of particular interest to understand behaviour of airway surfaces.
The visualization techniques will be further developed for use in novel multi-material ALM models
(Objet Connex 260), including endoscopic PIV and X-ray PIV (Dubsky et al, 2011; Lee et al,
2009). Methodologies will be developed for the use of ALM techniques to provide flexible
compliant boundary high order airway physical models with a mechanically-realistic description of
the airway, based on bronchial tube tissue mechanical properties. High-speed measurements will be
used to capture real time response of air flow to boundary movement, both on global large-scale as
well as localised high-resolution, providing insight into ventilation dynamics and airflow
mechanisms in human lungs in obstructive airway diseases. The integration of the large scale
analysis with the local scale analysis will allow small scale effects on flow (e.g. reduced air flow
due to partial closure of the middle lobar bronchus) to be correlated to an overall large-scale change
in flow pattern (e.g. flow field in the trachea), thus providing a true macro-large airway CFD
validation.
The anatomical ALM prototypes will subsequently be used for validation of hyperpolarised 3He
MRI, for in vivo imaging of flow in human lungs. Ultrafast imaging of 3He gas has been shown to
provide insight into ventilation dynamics in human lungs (Wild et al, 2003).
Acknowledgements
The project is funded through the EU 7th Framework Programme AirPROM "Airway Disease
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16th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 09-12 July, 2012
PRedicting Outcomes through Patient Specific Computational Modelling", contract no 270194,
http://www.airprom.eu. The authors gratefully acknowledge Olympus KeyMed Ltd. for use of a
high-speed camera and to P. Hackett for invaluable technical assistance. Further thanks to G.
Canham, B. Falconer, C. Maske, A. Patronis, T. Bradbury, S. Final and D. Carter.
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