Transducers: Ultrasound, Piezoelectric Effect

• A transducer is a device that converts one form of energy into another.
• Ultrasound transmitting transducers convert electrical energy into acoustic energy.
• Ultrasound reception transducers convert acoustic energy into electrical energy.
• Car engines convert chemical energy into motion.
• Electric motors convert electrical energy into motion.
• Light bulbs convert electrical energy into light and heat.
• Muscles convert chemical energy into motion.

Ultrasound Transducers

• Perform two functions.
• During transmission, electrical energy from a system is converted into sound.
• During reception, reflected sound pulses are converted to electricity.
• Made with Piezoelectric Materials, which convert sound into energy and vice versa.
• Piezoelectric materials are known as crystal, ceramic, PZT, element, or active element.

Piezoelectric Effect

• Creates voltage when certain materials are mechanically deformed.

Reverse Piezoelectric Effect

• A material changes shape when a voltage is applied, creating a mechanical wave.

Polarization

• The piezoelectric properties of a crystal are created by exposing the material to a strong electrical field while being heated to a substantial temperature.
• The Curie Temperature is the temperature at which PZT is polarized.
• Depolarization is when Piezoelectric properties of PZT can be destroyed by heating above the Curie temperature.

Cleaning Transducers

• Sterilization is the destruction of all microorganisms by exposure to extreme heat, chemical agents, or radiation.
• Disinfection is the application of a chemical agent to reduce or eliminate infectious organisms on an object.
• Transducers should be disinfected with Cidex™ or other cold germicides or a Trophon device..
• Transducers should not be exposed to extreme temperatures because it can affect piezoelectric properties, even if not above the Curie temperature, and may damage adhesives and bonding agents.
• Single-use disposable covers should be routinely used on endocavitary probes.

Basic Transducer Construction

• Key parts of a transducer include:
• Case
• Electrical Shield
• Acoustic Insulator
• Crystal / Active Element
• Matching Layer
• Backing Material

Active Element

• The active element is also called the Ceramic/Crystal/Element/ PZT (Lead Zirconate Titanate).
• It is a piezoelectric material.
• It has a thickness of 1/2 wavelength (λ of sound in the PZT).
• In pulsed wave ultrasound, sound beam characteristics are related to crystal characteristics.

Matching Layer

• Reduces impedance differences between two materials.
• Large reflections occur with large differences in impedance.
• Positioned in front of the crystal at the transducer face and is what touches the skin.
• 1/4 wavelength thick (λ of sound in the matching layer).
• Evens out impedance between the active element and the skin.
• Impedance order: PZT > matching layer > gel > skin

Backing Material

• Also called Damping material.
• Plays a key role in the creation of pulses.
• Without it, the excited PZT will ring for a long time, thus creating a large pulse.
• Enhances resolution by limiting the ringing of PZT, therefore shortening the pulse length and duration.
• Characteristics:
• High degree of sound absorption.
• Acoustic impedance similar to PZT.

Consequences of Backing Material

• Decreased Sensitivity:
• Reduces crystal vibration during both transmission and reception.
• This inhibits its ability to convert low-level sound reflections into meaningful electrical signals during reception.
• Creates a Wide Bandwidth:
• Pulses are not one single pure frequency, they range from high to low of the main one.
• Restricts PZT from vibrating freely.
• Low Quality Factor:
• Is the unitless number related to the purity of the frequency
• Quality Factor = main frequency / bandwidth
• Quality factor is directly related to pulse length. -Shorter/dampened pulse has a low Q factor while a longer/undampened pulse has a high Q factor

Imaging Transducers Vs. Non-Imaging Transducers

• Imaging transducers:
• Pulses with short duration and length.
• Uses backing material to limit ringing.
• Reduced sensitivity.
• Wide bandwidth or broadband.
• Lower Q-factor.
• Improved axial resolution
• Non-imaging transducers:
• Creates continuous wave or pulses with long duration and length.
• No backing material.
• Increased sensitivity.
• Narrow bandwidth.
• High Q-factor.
• Cannot create an image

Determining Frequency

• In continuous wave transducers the machine produces a continuous electrical signal which excites the crystal.
• The frequency of sound emitted by the probe is the same as the electrical stimulus
• In pulsed wave transducers create a short duration electrical spike that excites the PZT.
• The transducer frequency, is not determined by the electrical stimulus.
• The frequency is determined by the properties of the crystal.

Pulsed Wave Transducer Frequencies

• The MAIN or CENTER frequency is determined by two things:
• Thickness of the crystal.
• Speed of Sound in the crystal.
• The speed of sound in the crystal and the frequency of the sound are directly related.
• When the speed of sound is higher, the frequency produced will be higher.
• When the speed of sound is lower, the frequency produced will be lower.
• The PZT material ranges from 4 to 6 mm/µs.
• Thickness of the crystal and frequency are inversely related.
• Thin crystals create high frequency sound.
• Thick crystals create low frequency sound.
• PZT thickness = 1/2 wavelength of sound in the PZT.
• Ranges from 0.2 to 1 mm.
• Higher frequency transducers:
• Faster PZT propagation speed.
• Thinner crystal.
• Lower frequency transducer:
• Slower PZT propagation speed.
• Thicker crystal.