ultrasonic transducer fundamentals

Rad225/Bioe225UltrasoundFall 2019Ultrasonic Transducer

Fundamentals

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piezo effecttransducer anatomyresonance frequencymatching layer, backing material, Qmultielements/dicingaxial resolutiondemo

Rad225/Bioe225UltrasoundFall 2019

Outline

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piezo effecttransducer anatomyresonance frequencymatching layer, backing material, Qmultielements/dicingaxial resolutiondemo

Rad225/Bioe225UltrasoundFall 2019Piezo Effect

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Discovered by Pierre and Jacques Curie in 1880.

Direct Effect Reverse Effect

Rad225/Bioe225UltrasoundFall 2019PZT

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lead zirconate titinatecrystals oriented randomlyheating beyond Curie temperature (365°C for PZT) frees the microscopic crystals, allowing them to movehigh voltage produces partial alignment, or polarization, then cooled, called poling

Rad225/Bioe225UltrasoundFall 2019Bone

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bone exhibits some piezoelectric propertiescollagen exhibits the polar orientation of molecular dipolesbiological force sensorshown to stimulate resorption and growth

Rad225/Bioe225UltrasoundFall 2019

Outline

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piezo effecttransducer anatomyresonance frequencymatching layer, backing material, Qmultielements/dicingaxial resolutiondemo

Rad225/Bioe225UltrasoundFall 2019Transducer Anatomy

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Rad225/Bioe225UltrasoundFall 2019Transducer Anatomy

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Matching layer 2

Piezoelectric

Backing

Matching layer

Electrode

Rad225/Bioe225UltrasoundFall 2019

Outline

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piezo effecttransducer anatomyresonance frequencymatching layer, backing material, Qmultielements/dicingaxial resolutiondemo

Rad225/Bioe225UltrasoundFall 2019Resonant Frequency

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fo is determined by the transducer thickness

f0 =ncp2L

L = nλ2

n=odd integers

Wave going backwards experiences one round trip through crystal, then is back in phase with wave emanating from front of crystal.

Rad225/Bioe225UltrasoundFall 2019

Outline

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piezo effecttransducer anatomyresonance frequencymatching layer, backing material, Qmultielements/dicingaxial resolutiondemo

Rad225/Bioe225UltrasoundFall 2019Quarter Wave Matching Layer

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If Zm = ZstZtand thickness of matching layer = λ/4then, it appears that transducer has same impedance as soft tissue.This maximizes transmission for one frequency.Multiple matching layers can be used for multiple frequencies.

where st=soft tissue, t=transducer, m=matching layer

Rad225/Bioe225UltrasoundFall 2019Effects of Matching Layer

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Rad225/Bioe225UltrasoundFall 2019Backing Material

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Rad225/Bioe225UltrasoundFall 2019Effects of Backing

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Rad225/Bioe225UltrasoundFall 2019Quality Factor

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Q =f0BW

low Q

high Q

Imaging

Therapy

Rad225/Bioe225UltrasoundFall 2019Transducer Characterization

Experimental Bandwidth = 48.2%

Imaging Xducer, damped, low Q

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Rad225/Bioe225UltrasoundFall 2019Transducer Characterization

Sputtered Electrode

Wires

PZT

Brass Housing

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• HIFU - air backed, high Q

Rad225/Bioe225UltrasoundFall 2019Efficiency

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=Acoustic Power/Electrical Power

~ Eg. 50% for an air-backed therapy systempotential for heat for any transducer, CW system need cooling

Rad225/Bioe225UltrasoundFall 2019

Outline

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piezo effecttransducer anatomyresonance frequencymatching layer, backing material, Qmultielements/dicingaxial resolutiondemo

Rad225/Bioe225UltrasoundFall 2019Single Elements Diced into

Multiple Elements

21Duck

Rad225/Bioe225UltrasoundFall 2019

Outline

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piezo effecttransducer anatomyresonance frequencymatching layer, backing material, Qmultielements/dicingaxial resolutiondemo

Axial Resolution Lateral Resolution

Resolvable

NotResolvable

Resolvable

NotResolvable

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Axial Resolution

Pulses

Objects you want to resolve

Resolvable Not Resolvable Resolvable

How to get a smaller SPL? Higher frequency, low Q 24

Rad225/Bioe225UltrasoundFall 2019Quality Factor

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Q =f0BW

low Q

high Q

Imaging

Therapy

Resolution is determined by SPL

x

To resolve x, the spatial pulse length must be < 2x.nλ < 2x

Axial Resolution x = nλ/2

• Ex. n=3, f = 1.5 MHz, λ = 1 mm, resolution = 1.5 mm

SPL2x

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Higher FrequencyBetter Resolution

Body WallFat, muscle

Liver Capsule

Rad225/Bioe225UltrasoundFall 2019Outline

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piezo effecttransducer anatomyresonance frequencybacking material, matching layer, Qaxial resolutiondemo