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)