Transducers - Basic Ultrasound Physics (Chapter 8)

A transducer is a device that converts

one form of energy into another

Ultrasound transducers perform two functions:

1. Electrical energy to sound energy (during transmission)

2. Sound energy to electrical energy (during reception)

Piezoelectric effect is the property of certain materials that create ____ when they are ____ ____ or ____ by some ____

voltage ; mechanically deformed ; applied ; pressure

Reverse piezoelectric effect is a process of ____

changing material’s shape when the voltage is applied to them

What’s a synonym with piezoelectric?

Ferroelectric

What are Piezoelectric materials?

Materials that are responsible for converting sound energy into electrical energy and vice versa.

What piezoelectric materials can be found in nature?

1. Quartz

2. Tourmaline

What is the name of the synthetic piezoelectric materials that are used in clinical settings?

Lead Zirconate Titanate (PZT)

What are 3 synonyms for PZT?

1. Ceramic

2. Active element

3. Crystal

There are seven components of a basic transducer:

1. Case

2.Electrical shield

3.Acoustic insulator

4.PZT

5.Wire

6.Matching layer

7.Backing material

A case is a ____ tube made of ___ or ___.

Cylindrical ; metal ; plastic

What are 2 functions of the case?

1. Protects the internal components

2. Provides insulation from electrical shock

The electrical shield is a thin ___ ___

metallic barrier

What is the function of the electrical shield?

Prevents the electrical noise from entering the transducer

What is the acoustic insulator made of?

A thin barrier of cork or rubber

What are the two functions of the acoustic insulator?

1. Isolation of internal components

2. Prevention from vibration

How thick is the PZT element?

½ a wavelength thick

Where is the PZT located?

In between backing material and matching layer

Characteristics of the sound beam are related to the dimensions of the ___ ____.

Active Element

What is the connection between the PZT and the ultrasound system?

Wire

Where is the matching layer located?

In front of the PZT

How thick is the matching layer?

¼ of a wavelength thick

What are the 2 functions of the matching layer?

1. Increases the efficiency of the sound energy

2. Protects the active element

The backing material is also known as the ___ ___

damping element

Where is the backing material located?

At the back of the active element

What is backing material made of?

Epoxy resin impregnated with tungsten filaments

What is the function of backing material? (3 relationships)

1. Reduces ringing of PZT

2. Restricts the PZT deformation

3. Enhances axial resolution

The matching layer is designed with an impedance between ____ ____ and ____.

Active element ; skin

What is the function of the matching layer?

• Increases the percentage of transmission between matching layer and skin

• Decreases reflection

The backing material is also known as ___

damping element

What is the function of the backing material?

• ØReduces the ringing of PZT

  ØRestricts the PZT deformation

  ØEnhances axial resolution

Without backing material, PZT will ____

ring for a longer time

What are 2 characteristics of damping material?

1. High degree of sound absorption

2. Acoustic impedance similar to PZT

What are the consequences of backing material?

1. Decreased sensitivity

2. Wide bandwidth

3. Low Q Factor

Define bandwidth

Range of frequencies in a pulse

Imaging probes have (wide/narrow) bandwidth and are also called ____.

wide broadband

Q-factor is a ____ number that is (directly/inversely) related to bandwidth.

unitless ; inversely

What is the equation for Q factor?

Q factor = (main frequency/bandwidth)

Continuous wave and therapeutic doppler do not contain ___ ____ and have a ___ bandwidth and ____ Q-factor.

backing material ; narrow ; high

Frequency of sound in pulsed wave is dependent on 2 characteristics of the PZT:

1. Speed of sound of the PZT

2. Thickness of the PZT

Arrange in decreasing order of impedance:

• Gel

• PZT

• Skin

• Matching Layer

PZT > matching layer > gel > skin

Define polarization

It is a process of creation of the PZT properties by exposing the active element to a strong electrical field while being heated to a substantial temperature.

Define Curie Point

The temperature at which the PZT is polarized

Define Curie Point:

The temperature at which the PZT is polarized.

Define Depolarization

When the polarized PZT is heated above the curie point, the PZT properties are destroyed.  The loss of PZT properties is called depolarization.

Speed of sound in most piezoelectric materials range from ___ to ____ mm/us

4-6 mm/us