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Modelling and Fabrication of High Frequency Ultrasound Transducer q y
Arrays for Medical Applications
Robert T. SsekitolekoDTC in Medical Devices, University of Strathclyde
G HGerry HarveyWeildlinger Associates, Ltd (PzFlex Europe)
Christine E.M. Démoré, Zhen Qiu, Muhammad. R. Sadiq, Sandy CochranI tit t f M di l S i d T h l U i it f D dInstitute of Medical Science and Technology, University of Dundee
UIA 2011: Glasgow, UK, 23-25 May, 2011
OutlineOut eBackground
• Ultrasound Systems and Applications• High Frequency Imaging• High Frequency Imaging• Ultrasonic Device Development
Miniature Array Design• Design Specification
Fabrication Techniques• Bonding and Interconnects
Single Crystal Piezoelectric MaterialsPi l t i M t i l• Piezoelectric Material Characterisation
Virtual Prototyping• Preliminary Results• 15 MHz virtual prototyped transducer
Basic Fabricated PrototypeSummary & ConclusionsFurther WorkFurther Work
Ultrasound Systems and ApplicationsOutput to clinicians
y pp
Applications
Systems
Software
Instruments
DevicesDevices
Materials
Input from technologists
Ultrasound Systems and ApplicationsOutput to clinicians
y pp
Applications
Systems
Software
Instruments
DevicesDevices
Materials
Input from technologists
High Frequency Imaging
Increasing operational frequency improves image resolutionimproves image resolution
• 15 MHz and above give image resolution of less than 0.5 mm
Main applications include:Intravascular imagingSmall animal imagingOphthalmologyDermatology
BUT
Skin layers imaged with 32 MHz transducer. The epidermis (E), Dermis (D) and subcutaneous (S) layers are clear BUT
Increasing frequency reduces depth of penetration due to frequency dependent attenuation
( ) y
attenuation[1] R. T. Ssekitoleko et al:Progress In High Resolution
Ultrasound Towards In Vivo Pathology: SMIT 2010
Transducer on Interventional Tool
Conventional Ultrasound Probe
Miniature array on a biopsy needleUltrasound Probe biopsy needle
Transducer on Interventional ToolTransducer on Interventional Tool
Transducer size reduces with increasing frequency15 MH h di i ll h• 15 MHz array has dimensions small enough to fit in a tool with a diameter of 2 mm
Imaging transducers on interventional toolsImaging transducers on interventional tools overcomes the attenuation problem
• Can be placed at the tissue region of interestBiopsy needle
Accurate and quick characterisation of tissues with high resolution images
• Miniature arrays enable real-time imaging with electronic scanning of ultrasound beamwith electronic scanning of ultrasound beam
Potential for ultrasound in vivo pathology• Accurately positioned tissue biopsies for
TransducerAccurately positioned tissue biopsies for staging disease
Ultrasonic Device Development
Clinical Need/
p
Performance
Device D i
Fabrication Techniques
Packaging in Design TechniquesInstrument
Piezoelectric
Materials
Miniature Array Designy g
Clinical Need/Clinical Need/ Performance
Device D i
Fabrication
T h i
Packaging in Design Technique
sInstrument
Piezoelectric Materials
Design Specificationg p
Application: Imaging of small, deep-seated tumours
Interventional 2 mm diameter biopsy needletool:
p y
Array Type: Linear array for electronic scanningSide viewing for rectilinear image
Image resolution: 100 µm or better
Imaging 15 MH hi hImaging frequency:
15 MHz or higher
Design Specificationg p
Initial Array SpecificationPiezoelectric layer: 1-3 piezocomposite with single crystal
PMN-PT and hard-setting epoxyMatching layer: Alumina loaded epoxy
Backing layer: Tungsten loaded epoxy
Array elements: 64 elements
InterconnectsImaging Array
Array elements: 64 elements100 µm pitch ( λ at 15 MHz)
Array dimensions: 0.8 mm wide6 4 mm long6.4 mm long
Cabling: Flex circuit cabling to fit in core of 2 mm biopsy needle
Basic Array Implementationy p
• Process diagram is fully defined
• Technical challenges have been outlined and solutions investigated
P ki t h i f th dl• Packing technique for the needle orientation is established
Fabrication Techniquesq
ClinicalClinical Performance
Device D i
Fabrication
T h i
Packaging in Design Technique
sInstrument
Piezoelectric Materials
Array Fabricationy
1-3 Piezocomposite is made from fragile PMN-PT piezocrystalsp y
Composites made through dice and fill to enhance properties
Lapped and polished to a known thickness for the desired frequency
Matching and backing layers are cast on to avoid
1-3 piezocomposite
Matching and backing layers are cast on to avoid bondlines
Interconnect
Imaging array
Array Element dicingPrototype assembly
Bonding and Interconnects
Fragile PMN-PT materials need low temperature and pressure for bonding electrical interconnects
Conductive silver-loaded epoxy cures at room temperature
Electromagnetic anisotropic UV curable epoxy has a good potential
Photolithographic patterning of flex‐circuit with dry film photoresist and copper etching
190 µm track width and 110 µm separation achieved as first prototype
Technique shows promising results for prototypes with finer pitch tracks
Single Crystal Piezoelectric Materialsg y
ClinicalClinical Performance
Device D i
Fabrication
T h i
Packaging in Design Technique
sInstrument
Piezoelectric Materials
PMN-PT Single crystalPMN PT Single crystal
Benefits
Improved properties compared to conventional ceramics
• For imaging Parameter Symbol PZT‐5H PMN‐PT– Improved coupling coefficient
– High permittivityRelative permittivity at constant strain
εs33 1470 3026
Drawbacks
Fragile in fabrication
Expensive
Electromechanical coupling coefficient
kt 0.51 0.62
Difficult to characterise
– Properties vary even if from the same batch
Curie temperature Tc 195 oC 150 oC
Makes modelling more challenging
Piezoelectric Material Characterisation
IEEE standard characterisation technique
• Multiple specimen geometries of plates and bars to isolate multiple resonant modes
• Electrical impedance spectroscopy of specimens analysed with PRAP (TASI Technical Software Inc., Kingston, Canada)
• Full set of elastic, piezoelectric and dielectric properties can be extracted
Uniformity of Material Properties
1E+05Length Thickness Extensional Resonances
Uniformity of Material Properties
1E+03
1E+04
Impe
danc
e sample 1
sample 2
sample 3
sample 4
sample 5 PMN PT material from the same supplier
1E+01
1E+02
60 80 100 120Frequency (KHz)
sample 5 PMN‐PT material from the same supplier
Varied material propertiesFrequency (KHz)
For example relative permittivity at constant strain varies by ~80%
Z.Qui et al., “Characterisation of piezocrystals for practical configurations with temperature and pressure-dependent electrical impedance spectroscopy.” IEEE Trans. Ultrason., Ferroelect. and Freq. Control, in press 2011.
Transducer Designith Vi t l P t t iwith Virtual Prototyping
Clinical Performanc
Desig
Device Design
Performance
Fabrication
Techniqu
Packaging in
Design Cycle
n
Build
Design
Piezoelectric Materials
TechniquesInstrument
Test
Vi t lVirtual Prototyping
Materials Characterisation
Virtual Prototypingyp g
Finite element analysis (FEA) allows realistic and cost-effective analysis of device performancedevice performance
Virtual prototyping enables rapid testing of ranges of design parameters
PZFlex a time domain FEA software utilised for the designPZFlex, a time domain FEA software utilised for the design
However, FEA requires accurate material properties for a realistic mode
2D Composite with matching 3D Composite
Virtual Prototypingyp g
• First model:• Monolithic block of piezoelectric materialMonolithic block of piezoelectric material • Material properties are modified to match simulation to experimentally
obtained electrical impedance spectra
• Second model:• Second model:• 1-3 piezocomposite block • Check electrical impedance from simulation and experiment match for
confidence in full model
• Third model:• Full array with 1-3 composite, matching and backing layers• Can also model bond lines or electrode thicknesses to determine effects
of fabrication processes• Used to optimise design
2D Block 2D Composite
Preliminary Results: Model 1y
The model was validated with a 2D plate resonating at low frequency 1000
10000
g q y
PTZ-5H (CTS 3203HD)
10x10x1 mm 10
100
1000
Mag
nitu
de (O
hms) Experiment
Model
10x10x1 mm
1
10
1 2 3 4 5 6 7 8Frequency(MHz)
PMN‐PT Model was more challenging to match to the experiment matches with experimental test
1000
10000
hms)
ExperimentModel (a)Model (b)Model (c)test
2D plate model was used to extract properties 10
100M
agni
tude
(Oh ( )
10x10x1 mm plate 11 1.5 2 2.5 3 3.5
Frequency (MHz)
Preliminary Results: Model 1y
1000 ExperimentM d l
10000
10
100
danc
e (O
hms)
Model
100
1000
nitu
de (O
hms)
Experiment
Model
1
10
1.5 2 2.5 3 3.5
Impe
d
1
10
1 1.5 2 2.5 3 3.5
Mag
n
Supplier A Supplier B
Frequency (MHz)Frequency (MHz)
• It is more challenging to get a one model fits all for piezocrystal
Preliminary Results: Model 2y1-3 PMN-PT/Epoxy Composite
3d unit cell model3d unit cell model
5 MHz resonant frequency
Some properties were increased by up to 80%to obtain a reasonable match
10000 Experiment1200
100
1000
tude
(Ohm
s)
After re-polingModel
600
800
1000
1200
ude
(Ohm
s)
ExperimentAfter re-polingModel
1
10
2 4 6 8
Mag
nit
0
200
400
600
Mag
nitu
Fitted curves on linear scale Fitted curve on log scale
2 4 6 8Frequency (MHz)2 4 6 8
Frequency (MHz)
15 MHz virtual prototyped transducerp yp
• Active layer thickness is 80 µm• Matching layer thickness is 50
µm• Backing thickness is 800 µm
• 36% piezocrystal volume fraction in the composite has 10000
100000
higher impedance magnitude at the electrical resonant frequency than the 49%.
10
100
1000
Mag
nitu
de (O
hms)
49% Volume fractionWith 50 um matchingWith matching and backingWith 60 um Matching
18 13 18 23
Frequency (MHz)
Basic Fabricated Prototypeyp
Single element transducer integrated intoSingle element transducer integrated into small tube
15 MHz transducer with matching and Transducer
Tube
backing layers
Microfabrication techniques such as lapping and micro-dicing were usedlapping and micro dicing were used
Transducer element: 1 mm x 5 mmMetal tube diameter: 2 mm
Coaxial cable
ConnectorCoaxial cable diameter: 0.34 mm
Connector
Summary & Conclusionsy
High frequency transducers and arrays for high resolution imaginghave significant clinical potentialhave significant clinical potential
• Miniature arrays enable integration with interventional tools• Design of device and probe can optimised for specificDesign of device and probe can optimised for specific
applications
Fabrication of a prototype array integrated into a biopsy needle is in progressprogress
Comprehensive characterisation of single crystal piezoelectric materials is required for more accurate results in virtual prototypingmaterials is required for more accurate results in virtual prototyping
Virtual prototyping is used to save time and expense in optimising the design of high frequency transducers
Further Work
• Prototype testingI d l i• Impedance analysis
• Pulse-echo test• Imaging
• More material characterisation
• More device fabrication working through the process diagram• Fabrication of small pitch flexi-circuit
• Ex-vivo tissue testing
Acknowledgementsg
Prof. George Corner and Dr. Elaine Henry g y(Ninewells Hospital, Dundee, UK)
MicroSystems Engineering Centre y g g(Heriot-Watt University, Edinburgh, UK)
Dr. Jeff Bamber and Nigel Bush g(Joint Department of Physics, Institute for Cancer Research, London)
Loadpoint Ltd. (Swindon, UK)
AFM Ltd. (Birmingham, UK)
Logitech Ltd. (Glasgow, UK)