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Research Papers

An In Vitro Assessment of the Cerebral Hemodynamics Through Three Patient Specific Circle of Willis Geometries

[+] Author and Article Information
Patrick Delassus

Galway Medical Technologies
Centre (GMedTech),
Department of Mechanical and
Industrial Engineering,
Galway Mayo Institute of Technology,
Dublin Road, Galway,Ireland

Peter McCarthy

Department of Diagnostic Radiology,
University Hospital,
Newcastle Road, Galway,Ireland

Sheriff Sultan

Department of Vascular and
Endovascular Surgery,
Western Vascular Institute,
University Hospital,
Newcastle Road, Galway,Ireland
Department of Vascular and
Endovascular Surgery,
Galway Clinic,
Doughiska, Galway, Ireland

Niamh Hynes

Department of Vascular and
Endovascular Surgery,
Galway Clinic,
Doughiska, Galway,Ireland

Liam Morris

Galway Medical Technologies
Centre (GMedTech),
Department of Mechanical and
Industrial Engineering,
Galway Mayo Institute of Technology,
Dublin Road, Galway,Ireland
e-mail: liam.morris@gmit.ie

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received April 23, 2013; final manuscript received October 15, 2013; accepted manuscript posted October 19, 2013; published online December 3, 2013. Assoc. Editor: Ender A. Finol.

J Biomech Eng 136(1), 011007 (Dec 03, 2013) (12 pages) Paper No: BIO-13-1202; doi: 10.1115/1.4025778 History: Received April 23, 2013; Revised October 15, 2013; Accepted October 19, 2013

The Circle of Willis (CoW) is a complex pentagonal network comprised of fourteen cerebral vessels located at the base of the brain. The collateral flow feature within the circle of Willis allows the ability to maintain cerebral perfusion of the brain. Unfortunately, this collateral flow feature can create undesirable flow impact locations due to anatomical variations within the CoW. The interaction between hemodynamic forces and the arterial wall are believed to be involved in the formation of cerebral aneurysms, especially at irregular geometries such as tortuous segments, bends, and bifurcations. The highest propensity of aneurysm formation is known to form at the anterior communicating artery (AcoA) and at the junctions of the internal carotid and posterior communicating arteries (PcoAs). Controversy still remains as to the existence of blood flow paths through the communicating arteries for a normal CoW. This paper experimentally describes the hemodynamic conditions through three thin walled patient specific models of a complete CoW based on medical images. These models were manufactured by a horizontal dip spin coating method and positioned within a custom made cerebral testing system that simulated symmetrical physiological afferent flow conditions through the internal carotid and vertebral arteries. The dip spin coating procedure produced excellent dimensional accuracy. There was an average of less than 4% variation in diameters and wall thicknesses throughout all manufactured CoW models. Our cerebral test facility demonstrated excellent cycle to cycle repeatability, with variations of less than 2% and 1% for the time and cycle averaged flow rates, respectively. The peak systolic flow rates had less than a 4% variation. Our flow visualizations showed four independent flow sources originating from all four inlet arteries impacting at and crossing the AcoA with bidirectional cross flows. The flow paths entering the left and right vertebral arteries dissipated throughout the CoW vasculature from the posterior to anterior sides, exiting through all efferent vessels. Two of the models had five flow impact locations, while the third model had an additional two impact locations within the posterior circulation caused by an additional bidirectional cross flows along the PcoAs during the accelerating and part of the decelerating phases. For a complete CoW, bidirectional cross flows exist within the AcoA and geometrical variations within the CoW geometry can either promote uni- or bidirectional cross flows along the PcoAs.

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Figures

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Fig. 1

Segmented 3D models obtained from patient-specific imaging data

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Fig. 2

Geometrical variations for all three models: (a) coronal view, (b) sagittal view showing the height difference between models 1, 2, and 3 from the distal end of the basilar artery (BA) to the tip of the anterior communicating artery (AcoA), (c) sectioned view showing the right PcoA branching off from the right ICA for models 1 and 2 and the right ICA bifurcating into the right PcoA, generating a T junction for model 3

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Fig. 3

Smoothing the ABS inner core: (a) SEM image of the model after rapid prototyping, and (b) SEM image of the model surface after smoothing with xylene and 2-propanol

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Fig. 4

Manufacturing the realistic models: (a) dip/spin rig setup, (b) dipping the model, (c) rotating the model, and (d) ABS model coated with silicone after repeated dipping

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Fig. 5

Flexible silicone models: (a) axial view, and (b) coronal view

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Fig. 6

The inputted and measured flow rates for the (a) internal carotid artery, (b) vertebral artery, (c) linear motor displacement for replicating the ICA flow rate waveform, and (d) linear motor displacement for replicating the VA waveform

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Fig. 7

The cerebral physiological flow rig: (a) systematic circuitry, and (b) flow rig mounted on a transportable frame

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Fig. 8

Measuring the geometrical accuracy of the models using fluoroscopy: (a) model 1, (b) model 2, (c) model 3, (d) magnified image of the edge for the inner diameter measurement, (e) selected sections along the model where the measurements were compared, and (f) sectioned model 3 for wall thickness measurements

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Fig. 9

Measured inlet flow and pressure waveforms for model 2: (a) right ICA, (b) left ICA, (c) right VA, and (d) left VA

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Fig. 10

Measured outlet flow and pressure waveforms for model 2: (a) right ACA, (b) left ACA, (c) right PCA, (d) left PCA, (e) right MCA, and (f) left MCA

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Fig. 11

Schematic of the flow direction and impact locations around the CoW: (a) models 1 and 2, and (b) model 3

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