High Frequency Arrays and Beamformers

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High Frequency Arrays and Beamformers

• Christine Morton Démoré
• Jeremy Brown
• Geoff Lockwood
• Queens University, Kingston

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Outline

• Miniature Ultrasound Arrays
• Electronic Beamformer
• Ongoing research

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High Frequency Transducers

• Want
• Excellent resolution over large depth of field
• Need
• Arrays that focus throughout image
• But
• High frequency arrays are very difficult to build
• Linear arrays above 35 MHz are not available

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Transducer Design Evaluation

• Array success based on
• Radiation pattern
• How well the ultrasound energy is focused
• Lateral resolution and contrast
• Pulse-echo response
• Axial resolution

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Transducer Design Techniques

• Models used to evaluate designs
• Impulse response method
• Assume ideal piston vibration of elements
• Based on transducer geometry alone
• Finite element model
• Takes non-ideal vibrations into account
• Accurately models element interactions
• Comparison tells how design influences radiation
  pattern

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Annular Arrays
Annular Array

• Fewer and larger elements for same resolution as
  a linear array
• Beam focussed along array axis
• Axially symmetric beam
• Array must be mechanically scanned to form image

Linear Phased Array
l/2
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Annular Arrays

• Made by cutting circular kerfs in PZT ceramic
• Difficult to fabricate at high frequencies
• Poor aspect ratio of outer elements

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Kerfless Annular Array

• Array defined by electrode pattern
• Cross talk is high
• No unwanted lateral resonances

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40 MHz Kerfless Annular Array

• Pulse at 0.1 mm
• Focused to 5 mm
• Long ring down
• Large coupling between elements degrades pulse

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40 MHz Kerfless Annular Array

• Pulse at 5 mm
• Focused to 5 mm
• Well defined in time with short ring down
• Thickness mode vibrations are correlated they
  add
• Uncorrelated vibrations destructively interfere

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Radiation Pattern Cross-section
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