Design and Construction of Ultrasonic Doppler Transducers For

7/30/2019 Design and Construction of Ultrasonic Doppler Transducers For

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Design and construction of ultrasonic Doppler transducers forblood flow measurement using Finite Element Analysis

Israel Sncheza, Hendrik Faustmann

b, Pedro Acevedo

a, Martn Fuentes

a

aUniversidad Nacional Autnoma de Mxico, DISCA-IIMAS

Apdo. Postal 20-726 Admon. No. 20, Del. Alvaro Obregn, 01000 Mxico D.F., Mxico

bUniversity of Applied Science, Coburg

Friedrich-Streib-Str. 2, 96450 Coburg, Germany

[email protected] , [email protected],[email protected],[email protected] ,

Abstract

This paper describes the design and construction of two ultrasonic Doppler transducers using FiniteElement Analysis for blood flow measurement. These transducers were constructed using as sensingelement, commercial 8 MHz PIC255 piezoceramic discs, EPO-TEK 301 and Rexolite 4422 as backingand matching layer materials respectively. The objective of this work was to design and construct twosingle element transducers for pulsed wave Doppler blood flow measurement, one with an angle of30 to 45 and another with no angle (0), both resonating at approximately 5 MHz. These transducersshould work with the associated electronics using a quadrate demodulation for the blood flow Dopplerultrasonic signal detection.

Keywords: Finite Element Analysis, Doppler transducer, Blood flow measurement, ANSYS.

Introduction

An ultrasonic transducer converts electricalenergy into acoustic energy duringtransmission when its active element isexcited by a voltage or current signal.Conversely, during reception, the acousticenergy of the returned ultrasound isconverted into an electrical signal. The

active elements of transducers used indiagnostic medicine depend onpiezoelectric effect for their operation. Themost commonly used materials for theseactive elements are piezoelectric ceramicsfor example made of lead ZirconateTitanate (PZT) [1].

Different design and construction modelswith these piezoelectric elements aredescribed in this paper regarding theapplication of a pulsed Doppler blood flowdetector device for a bidirectional Doppler

ultrasonic system for the evaluation ofcoronary bypasses.

Design

The goal is to design and construct a singleelement transducer for pulsed waveDoppler blood flow measurement withincluded known angle of 30 to 45 and thefrequency of 8 MHz. This transducer shouldwork with the electronic device using aquadrate demodulation for the blood flow

Doppler ultrasonic signal detection.

The detection device brings as output theDoppler signals I (In phase) and Q (inQuadrate) in audible range. A personalcomputer sound card line in is fed withthese signals for its processing. Thesampling volume is limited to carry out thesuperficial monitoring on arteries of 2 4mm of diameter for coronary bypasses.

Special attention should be taken to theeasy construction with available materials.

Therefore materials are introduced anddescribed regarding the use in thetransducer.

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Ultrasonic matching materials

The coupling of energy between anultrasonic transducer and sometransmission medium (e. g. blood in vesselsor water) is maximized when the medium

and transducer have the same specificacoustic impedance. Any mismatch inimpedance results, in the case ofultrasound generation, in a proportion of thegenerated energy being reflected back intothe transducer and being lost throughabsorption and leakage from the back faceof the transducer. A similar energy loss inthe medium occurs where the transduceracts as a receiver of ultrasound.

An ultrasonic matching layer is a passivelayer, which is fixed to the front face of an

ultrasonic transducer in order to improvethe coupling of energy to and from thetransmission medium. Under narrow-bandconditions, coupling is maximized when thethickness of the matching layer is equal toone quarter of the wavelength (or an oddmultiple of a quarter wavelength) of theenergy being transmitted (see figure 2).

Fig 1. ultrasonic transducer with backing and frontmatching material

Wide bandwidth and sensitive ultrasonictransducers are needed in numerousmedical and non-destructive applications.

As the characteristic impedance of the PZT(Z = 33 MRayl) is greater than that of wateror blood (Z = 1.5 MRayl), the 0 transducermust be matched.

In order to match the piezoelectrictransducer to a vein or artery, matchinglayers must be used. Theoretically,solutions proposed in the literature consist

in using one or more quarter-wave layerson the front face of the transducer.

Without having the technology for the /4matching, the research for materials isfocused on impedance matching betweenthe piezoceramic made of PZT and thehuman vessel. In addition some other

materials for the construction areintroduced; the need of easy handling andforming makes epoxies an attractivematerial to work with.

Ultrasonic backing materials

The backing is most commonly a highlyattenuate and very dense material and isused to control the vibration of thetransducer crystal by absorbing the energythat radiates from the back face of thepiezoelectric element. When the acoustic

impedance of the backing material matchesthat of the piezoelectric crystal, the result isa highly damped transducer with excellentresolution. By varying the backing materialin order to vary the difference in impedancebetween the backing and the piezoelectriccrystal, a transducer will suffer somewhatand resolution may be much higher insignal amplitude or sensitivity.

Materials

A. Piezoceramics

Piezoelectric ceramics belong to the groupof ferroelectric materials. Ferroelectricmaterials are crystals witch are polarwithout an electric field being applied. Thisstate is the thermo-dynamically stablereversibility of the axis of polarization underthe influence of an electric field, describedgraphically by a hysteresis loop. Thereversibility of the polarization, and thecoupling between mechanical and electricaleffects are of crucial significance for the

wide technological utilization ofpiezoceramics.

The main piezoceramics in use today aremade of PbTiO2, they are synthesized fromthe oxides of lead, titanium and PbTiO2zirconium. Special doping of these lead-zirconate-titanate ceramics (PZT), forexample used in PIC 155 and PIC 255 [2],make it possible to adjust individualpiezoelectric and dielectric parameters asrequired.

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PIC 255

PIC 255 is a modified PZT material withextremely high Curie temperature, highpermittivity, high coupling factor and highcharge constant. The material has been

optimized for actuator applications underdynamic conditions and high ambienttemperatures. The high coupling factor, lowmechanical quality factor and lowtemperature coefficient make this materialparticularly suitable for low-power ultrasonictransducers.

PIC155

PIC 155 is a modification of the PIC 255material distinguished by high piezoelectricstress coefficients and lower frequencyconstants. It is used in applications where ahigh g-constant is required, such as inmicrophones.

Table1 piezoelectric elements used; from PI ceramics

B. Front matching materials

Rexolite 4422

Rexolite has outstanding dielectricproperties with a dielectric constant of 2.53(up to 500GHz) together with extreme lowdissipation factor and excellent acousticimpedance close to water. Velocity93x10"/second. Its valuable for microwavelenses, microwave circuitry, antennae,coaxial cable connectors, soundtransducers, TV satellite dishes and sonarlenses. Other uses include non-destructivematerial testing devices, surveillanceequipment, radar windows, radomes andmissile guidance system housings. One

interesting application is for radar lenseswhich are used for mapping the earth's

surface from fast high-flying aircraft.[3], [4],[5], [6], [7], [8], [9], [10]

Insulcast 501 American SafetyTech.,Roseland,NJDensity 3.86 g/cm3Velocity 1900 m/sImpedance 7.3 MRaylsLoss 13.8 dB/mm

EPO-TEK 301 Epoxy Tech., Billerica, MADensity 1.15 g/cm3Velocity 2650 m/sImpedance 3.05 MRaylsLoss 9.5 dB/mm

Parylene Specialty Coating systems,IndianapolisDensity 1.18 g/cm3Velocity 2200 m/sImpedance 2.6 MRaylsAraldite 502Density 1.16 g/cm3Velocity 2620 m/sImpedance 3.04 MRayls

C. Backing materials

E-SOLDER 3022 Conductive epoxy VonRoll Isola Inc.,New Haven CTDensity 3.20 g/cm3Velocity 1850 m/sImpedance 5.92 MRaylsLoss 110 dB/mm

Conductive Epoxy CW2400The acoustic properties of the conductiveEpoxy CW2400are not known.

Simulations

To check the materials used in theconstruction, ANSYS simulations weredone and measurements of the ceramics,the backing material and the front matchingmaterial were carried out using thefrequency analyzer, One example of thesimulation is shown in figure 3.

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Fig.2. Frequency and Phase representation of thePIC255 piezoceramic disc using the ANSYS tool.

Using the ANSYS tool, the vibration modeof the PIC255 piezoceramic disc wassimulated, figure 2 shows the displacementof the disc when this is excited(intermediate displacements, minimum andmaximum stress).

(a)

(b)

(c)

(d)

Fig.3.- ANSYS simulation of the vibration mode of thePIC255 piezoceramic disc, a) minimum stress, b), c)intermediate displacements and d) maximum stress.

Construction

Two different models of construction arepresented. These two transducers basedon different design ideas were constructedand tested. Both show a certain functiontogether with the hard- and software devicefor the blood flow measurement in thephantom as well as in tests directly at thevein of the arm through the skin. To get thefull characterization about the transduceramongst others the Impedance and phaseover frequency has to be analyzed.

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Transducer

In figure 4, the two different modelsincluding the design ideas are shown. Bothconsist of two tubes, one inner plastic tubeas carrier material for the ceramic and one

outer metallic tube as protection case.The 0 transducer uses an undefined non-conductive epoxy as backing material. Theconnection between the (+) electrode andthe coaxial wire is bonded with a droplet ofconductive epoxy. The front electrode isalso bonded with conductive epoxy to awire, witch is guided through a channel onone site from the plastic tube to the insidecoaxial wire. The backing material of the35 transducer is completely made ofconductive epoxy. This allows easyhandling of the connection to the

piezoceramics and to the plastic tube aswell as the connection to the coaxial wire.

Fig 4. models of the two ultrasonic transducersconstructed

Results

After construction both transducers were

tested using an impedance analyzer, afrequency sweep from 1 to 10 MHz wasdone to verify their oscillating frequency.Results (see figure 5) showed that thesimulated performance of both transducerswas achieved. The observed oscillatingfrequency was 5.83 MHz, the frequencyshift was probably due to the soft-layer andthe backing material used in theconstruction (see figure 6) of bothtransducers.

a)

b)

Fig. 5 Frequency and phase response using theimpedance analyser from a) 0 transducer and b) 30transducer.

Fig 6. Ultrasonic transducers

To improve the acoustic properties andconsequently the sensibility of thetransducer a front matching is needed. Forexample Rexolite can fill out the spacebetween the ceramic and the vessel; sothat the acoustic waves are guided in this

front matching material and just tworeflection sites exist. Figures 7 and 8 show

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tests of both the 0 and the 35transducers.

Fig 7. test of the 0 - ultrasound transducer in thephantom

Fig 8. test of the 35 ultrasonic transducer at the veinof the arm.

Conclusions

The design and construction of two differenttransducers were presented. These twotransducers based on finite elementanalysis were constructed and tested. Bothshowed a good performance when workingwith the associated electronics for bloodflow measurement in the phantom as well

as in tests directly with real veins. To get afull characterization of the transducers, theimpedance and phase over a frequencyrange was analyzed.

As already mentioned in the results section,after construction both transducers weretested using an impedance analyzer, afrequency sweep from 1 to 10 MHz wasdone to verify their oscillating frequency.Results showed that the simulatedperformance of both transducers wasachieved. The observed oscillating

frequency was 5.83 MHz, the frequencyshift was probably due to the soft-layer andthe backing material used in theconstruction of both transducers.

Results show that the overall performance

of the transducers fulfils the requirements oftheir design using Finite Element Analysisand it is possible to use them in blood flowmeasurement applications.

Acknowledgements

The authors acknowledge the support ofDGAPA-UNAM (PAPIIT IN-109207) and thetechnical help from J. Durn.

References

[1]Snchez Domnguez, Israel, Detector Ultrasnicode Flujo Sanguneo, Tesis de Licenciatura, Fac. deIngeniera, UNAM. Mxico. 1994

[2] http://www.physikinstrumente.com/

[3]www.gii.upv.es/.../treballs%20cursos%20anteriors-TIM-IIN-NYPAYPD/TRABAJO%20transductores-margaix.pdf

[4]http://www.insulcast.com/technical/epoxies_2comp/insulcast501.html

[5]http://www.epotek.com/SSCDocs/MSDS/301-2_msds.PDF

[6]http://www.signal-processing.com/tech/us_data_solid.htm

[7]http://www.matweb.com/search/DataSheet.aspx?MatGUID=88205b271b7746cc9ed10fe10426a2aa

[8]http://ieeexplore.ieee.org/iel5/9696/30605/01418145.pdf

[9]http://www.chemtronics.com/products/americas/MSDS/4002A.pdf

[10]http://www.chemtronics.com/products/americas/MSDS/4002B.pdf

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