Dual Frequency Ultrasound Transducers

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Dual Frequency UltrasoundTransducers

• Impedance Matching Electronics

Miaad Aliroteh Imaging Research Summer Student
Sunnybrook Health Sciences Centre Research
Institute
Tuesday, 8 July 2008
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Overview

• Why Dual Frequency?
• How Dual Frequency?
• Some terms
• The Big Questions
• Results
• Summary and Recommendations
• Future Work

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Why Dual Frequency?

• Allows two ranges of operation proximal and
  distal focus

Low Freq
High Freq

• gt4MHz (avoid bone heating)
• lt15-20MHz (maintain some depth of heating)

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Achieving Dual Frequency

• Old Design Two transducer layers that combine to
  resonate at two frequencies
• New Design One transducer fired at two harmonics
  (1st, 3rd)

THE NEW
THE OLD
Efficiency of old design
Signal GND
Signal GND
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Maximizing Power Transfer
Multiple Matching Circuits
Single Matching Circuit
Single-FMatching Circuit
Signal GND
Relay or Switch
Dual-FMatching Circuit
Transducer
VS
Transducer
Signal GND
Single-FMatching Circuit

• Bulky
• Not MRI Compatible
• Mechanical switching wear
• Simple and Straightforward

• Small foot print
• MRI Compatible
• Complex Circuit?
• Poor performance?

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ImpedanceWhat is it and Why is it Important?

• Impedance is resistance against the movement of
  alternating current (AC)
• The (standard) 50O impedance
• Signal Generator and Amplifier output
• Want Output Impedance to match Input Impedance
• maximum power transfer

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Focus Of Investigation

• Can a transducer efficiently operate at two
  frequencies using harmonics
• Can a single matching circuit be used for a dual
  frequency transducer.

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Efficiency Pre-Test
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Methodology
Characterize Impedance - To find operating
frequencies - Using an Impedance
Analyzer Circuit Design - To evaluate dual
freq., single matching cct approach - Using
various software and bench-tests Measure
Acoustic Power - To evaluate dual frequency
design with harmonics - Using Radiation Force
Balance
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Design ProcessGoal Match _at_ 5.085MHz and
14.775MHz
Conventional Single Frequency Design
New Dual Frequency Design
Goal Achieve 50/0
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Efficiency TestAnd Repeatability
PE,fwd 5W 5.06MHz ? 72.7 avg efficiency
14.8MHz ? 56.2 avg efficiency
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Summary

• Transducer has good efficiency (73 52) at
  harmonics
• A single, simple matching circuit works well
• Recommendations
• - DGDI water makes a small difference
• - low tolerance components (lt5)
• - high-slope spectrum curves
• - Define Resonance as phase0

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Future Work

• Simulations
• Best firing frequency
• Applicators (multi-element)
• Demonstrate correct operation
• Multi-channel

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Acknowledgements

• Primary Investigators
• Dr. Michael Bronskill
• Dr. Rajiv Chopra
• Senior Engineers
• Matthew Asselin
• Aaron Boyes
• Nicolas Yak

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Transducer Characterization Setup
SMA or BNC cable
Impedance Analyzer Agilent 4395A Port 1
Calibration plane
Direct Connection
Normal Tap Water and later De-gassed and
De-ionized Water
Transducer
Floppy Disk

• Harmonics shift slight in DGDI water
• Final setup SMA and DGDI water

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First Spectrum Analysis Transducer Input
Impedance

• Normal Tap Water
• No anti-reflective foam
• Circular container, transducer was placed away
  from focal point of container

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Match Qualityof Trial
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Design ProcessGoal Match _at_ 4.9MHz and 14.775MHz
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Match Qualityof Design
? 1st Harmonic ? 3rd Harmonic ? 3rd Harmonic
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Efficiency Testing Setup
RF inc
Function Generator Agilent 33250A
Amplifier NP Technologies FILTER REMOVED
Power Meter Rohde Schwarz
Matching Circuit
RF ref
DG and DI Water Tank Ultrasound Absorbent
Pads
Transducer
Mounted Transducer And Matching Circuit
Radiation Force Scale Ohmic Instruments Co
Radiation force cone connects to the scale
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Efficiency Test 1 Match Quality Acoustic Power
PE,fwd 5W _at_ lt10MHz PE,fwd .25W _at_
gt10MHz
Intended Freq 5.085MHz and 14.775MHz