Module 1 - Transducers 2

Transducer

Any device that converts one form of energy into another form

Piezoelectric effect

Bidirectional ability to convert electrical energy to mechanical energy

Crystals

PZT

PZT

Piezoelectric material used in transducer

PureWave Crystals

Philips technology for higher efficiency PZT

Currie point

300C

the temperature at which dipoles of piezoelectric materials can be adjusted

Electromagnetic coupling efficiency

Bidirectional efficiency of converting electrical energy to mechanical energy

Dipoles

Material with charged ends

Polarization

Alignment of dipoles within a piezoelectric material to create an overall direction (positive and negative end)

Sensitivity

Amplitude of a signal that can be detected by a transducer

Why do U/S transducers use PZT?

High coupling coefficient

High frequency of natural resonance

Stable design

What are the frequency determinants for PW applications?

Crystal thickness

Propagation speed of sound in the crystal

Drive voltage

Electrical stimulation applied to the crystal must be at or near the operating frequency

Impulse response

Response of a crystal to a single short-duration pulse

Name an advantage and disadvantage to crystal resonation.

Advantage: efficient voltage excitation

Disadvantage: crystals resonate longer than desired

Composition and location of damping material on a transducer?

Metal powder and plastic or epoxy resin

Attached to rear face of the crystal

Explain how the PD and SPL of a transducer is controlled.

Backing material limits the number of cycles in a pulse through limiting the impulse response

What are some effects of backing material?

↓PD → ↑BW

↓SPL → ↑axial resolution

↓electromagnetic coupling efficiency

Describe the thickness of the matching layer. Why is this thickness chosen?

¼ of the wavelength of sound leaving the crystal.

¼ = 90° → GRT makes it 180°

180° creates destructive interference of reverberation artifact caused by the matching layer → no reverberation artifact from matching layer

Describe the ideal Z value of the matching layer.

Z_crystal > Z_{matching layer} > Z_medium

Explain how transducers improve sound transmission across the BW of the transducer.

Multiple matching layers to improve transmission over the large range of frequencies emitted during a PW.

Draw a simple transducer block diagram.

Bandwidth

Range of frequencies over which a device can operate

What is the expected PD with backing material?

n=2-3

Draw the BW of the given PDs. Label which PD would be used for the following: 2D imaging, CD and PW, and CW.

Provide some advantages of a wider BW.

Multi-Hertz operation

Dynamic frequency tuning

Harmonic imaging

Frequency compounding (frequency tuning)

Describe the expected BW for Doppler applications and explain why.

Narrow BW, high sensitivity.

Doppler application require receiving signals from Rayleigh scatter, which is extremely faint.

In a patient with large body habitus, would you prefer the transducer has a wide or narrow BW? Explain.

A narrow BW would be more beneficial because of the increased need for penetration.

↑penetration → ↑attenuation → ↓returning signal strength

Decreasing the BW allows for more sensitivity, which is necessary for detecting the returning signals

Draw a representation of dynamic frequency tuning.

BW =

f_max - f_min

fractional BW (FBW) =

BW / f_center

A broad BW is considered to be ___% FBW.

80

A high QF indicates ___ BW whereas a low QF indicates____ BW.

narrow, broad

Is 3MHz equivalent on a L12-3 transducer and a C5-1 transdicer?

No

Transducers are more effective closer to their center frequency.

List the advantages of a high electromechanical coupling coefficient.

Increases penetrability of SW

Improves SNR and overall contrast resolution

Aids in tissue harmonic production

List the advantages of high sensitivity.

Detect echoes from farther away → increased penetration

Detect more Rayleigh scatter → Doppler imaging improvements

List some goals for improving PZT.

Reduce acoustic impedance to better match soft tissue

Increase efficiency

Wider BW without sensitivity comprimise

Describe a method for reducing PZT’s acoustic impedence.

Piezoceramics

• break PZT block into hundreds of components and set in polymer epoxy

• Z-value = 8-12MRayls

Describe a method for improving PZT efficiency.

Poling: aligning regions of positive and negative magnetic charges

List the steps associated with poling a piezoelectric material.

1. crystals are heated above Currie temperature (300)

2. magnetic field applied to align dipoles

3. crystals cooled while magnetic field is still applied

4. dipoles set in a new orientation

What is the difference between piezoceramics and Philip’s PureWave Crystal Technology?

Piezoceramic alignment ~70%

PureWave alignment ~100%

List some benefits to PureWave technology.

Improved electromechanical efficiency

Improved sensitivity

Improved penetration and image quality

Wider BW without compromising sensitivity