Ultrasound Physics Chapter 2: Transducers

Transducer

A device that converts energy from one form to another

- microphone

-generator

-light bulb

-electric motor

Ultrasound Transducer

converts electrical energy into mechanical energy and vice versa on the receiving side

Piezoelectric effect

the mechanism in which energy conversion occurs in an ultrasound transducer

What are the Piezoelectric natural materials?

-quartz

-Rochelle salts

-tourmaline

What are the Piezoelectric man made materials?

-lead zirconate titanate (PZT)- most common for US

-barium titanate- used less commonly

How are man made materials made Piezoelectric?

by heating them above the curie point and applying an electrical charge

What does heating the man made material above the curie point do?

helps to align the dipoles into rows allowing the material to cool with the charged plates; dipoles will maintain their position

Dipoles

molecules of positive and negative charges on opposite ends

Why should you never autoclave a transducer?

crystals or elements are polarized at temperatures above the curie level, therefore if a transducer is placed in an area above the curie temperature, it will depolarize

With continuous wave transducers, the frequency of sound is equal to:

the frequency of the alternating voltage applied

- this is in contrast to how pulsed ultrasound transducers work

Shock-excited mode

uses voltage impulses to create pressure excitation upon the crystal, which will expand and contract, creating the mechanical waves

- used for pulsed ultrasound

High frequency= _____ element

thinner

Low frequency= _____ element

thicker

The resonant or operating frequency of a transducer is determined by:

the thickness or thinness of the crystal and the propagation speed of the material the element is comprised of

The thickness of the crystal should be ___ the wavelength of the material composite in order to keep the outgoing and incoming waves in phase

1/2

What is the formula for resonance frequency?

prop speed of the crystal (mm/microsec)/2x the crystal thickness (mm)

The smaller the value of crystal thickness, the _____ the frequency

higher

The higher the speed of the crystal medium, the _____ the frequency

higher

Why does the crystal thickness have to be 1/2 the wavelength of the material composite?

- crystals emit waves in two directions

-one created from the back surface of the crystal to the front

-one created from the front surface to the back

-these two waves must be in phase

-waves will superimpose if the thickness is 1/2 the wavelength

- elements or crystals that must be thicker than the wavelength must be done in a multiple of 1/2

What are the three key internal components to a transducer?

- backing

-element

-matching

Backing/ damping material

stops the vibration after two or three times; this is what creates the number of cycles in a pulse

What are the advantages of a highly dampened transducer?

- reduces pulse duration

-reduces SPL, therefore reducing the axial resolution numerical value, which improves the quality of axial resolution

-increases the bandwidth

- decreases the quality factor

What is the disadvantage of a highly dampened transducer?

the backing material reduces the amplitude, thereby decreasing the sensitivity of the transducer to detect weak signals.

Backing material

- glued to the back side of the crystal

- prevents an echo from the crystal/backing interface, which would reenergize the crystal

Backing material decreases ___

SPL, which improves axial resolution

The backing material should have the same acoustic impedance as _____

element/ crystal

Excessive backing creates too wide of a _____

bandwidth; this decreases image quality

Bandwidth

the range of frequencies found within the ultrasound transducer

Shorter pulses have a _____ range of frequencies

broader or greater

What is responsible for determining the bandwidth and tends to increase the bandwidth?

Damping

What is the formula for bandwidth?

max frequency - min frequency

The ideal transducer has:

a dampening factor which will provide a reasonable SPL and a bandwidth which allows good imaging at different imaging depths

Quality factor

represents the ability of a transducer to emit a clean or pure pulse

Transducers want to have _____ Q factor and a _____ bandwidth

low; wide

What is the formula for Q factor?

Operating frequency/bandwidth

Element

determines the frequency/ period of the transducer

- converts electrical to mechanical and vice versa

Matching layer

located between the element and the face of the transducer

- creates a more even playing field with acoustic impedance mismatch

Without the matching layer about _____ of sound would be reflected at the skin surface without transmission into the adjacent tissues.

90%

The matching layer has an acoustic impedance about _____ between the crystal and soft tissue

half way

How many layers are used in the matching layer?

typically two

Matching layer is usually designed with a _____ wavelength technology

1/4; promotes destructive interference

What helps to further reduces this acoustic impedance mismatch in the matching layer?

acoustic gel allows for transmission of waves into the skin

Insulation Ring

Surrounds the internal components and is located inside the housing

▪Same material as damping block

▪Used to absorb energy generated from the sides of the crystal

Housing

plastic protective covering

▪moisture-proof

The _____ is the ultrasonic field

sound beam

The characteristics of the field is dependent on:

-the diameter of the crystal

-frequency bandwidth

- focusing

- attenuation

-propagation speed of the media

Diffraction

The pattern created when the waves from different regions of the sound source interfere at different points in the beam

Waves generated by small crystal diameters create _____ fields than those from wider diameters

larger

Huygen's Principle

The division of large sound source into a collection of small sound sources. Each individual source creates its own beam pattern, which interferes with those from the other sources to form a complex beam pattern.

Near Field /Near Zone

the area between the transducer and the point of narrowing and diversion ( this is referred to as the focus)

- also called Fresnel Zone

What is the formula to calculate near field length?

radius 2 / wavelength

- calculates the distance from the start of the beam to the focus

As beam diameters increase, near zone length will _____

increase

As the frequency of a transducer increases so does the _____

near zone length

The near zone is:

the distance from the start of the beam (transducer face) to the point where the beam is at its narrowest (focus)

For unfocused or naturally focused beams, the focal point is where the beam width is_____ the initial beam diameter or crystal diameter.

= to 1/2

Focus equals:

½ the beam diameter

Focus is the area where _____ resolution is the best

lateral

Far Field

Also called the Fraunhofer Zone

▪Defined as the region beyond the focal point

At approximately two times the near zone, the beam width is equal to:

the original beam diameter

The intensity of the beam weakens as it travels and become clinically useless at a distance beyond:

2 times the near field

Can focal point be placed in the far field?

NO

Divergence

the widening of the beam beyond the focal point

What is the formula for divergence?

.61 x wavelength/ radius

The greater the divergence, the poorer the _____ resolution

lateral

What is divergence dependent on?

the beam diameter and the frequency of the transducer (wavelength)

Increase in transducer frequency _____ the beam divergence or beam width

decreases

Narrower beams increase _____ resolution and _____ artifacts

lateral; decrease

Increasing the beam (crystal) diameter _____ the diverging beam width

decreases

the ideal transducer for the far field is one with _____ frequency and _____ beam diameter

high; larger

An increase in frequency or beam diameter= _____ NZL

increase

An increase in frequency or beam diameter= _____ beam divergence

decrease

Mechanical steering

accomplished through use of a motor which moved in a circular motion or wobbling back and forth.

Electronic steering

accomplished through time-delayed electrical stimulation to the elements creating the sound beam.

Focusing

The reduction of the beam width allowing for concentration and increased intensity of the beam and increasing lateral resolution

Mechanical focusing

requires an extra piece of hardware or modification in shaping of internal or external hardware

External mechanical focusing

an acoustic lens or mirror is placed in front of the crystal(element)

▪ curvature of the lens or covering

Internal mechanical focusing

shaping or curving the crystal

Does mechanical focusing allow for variable focus?

No

Electronic Focusing

The pulsar stimulates groups of elements in a sequential or fixed order by implementing time delays.

▪ Most commonly associated with phased array transducers

Does electronic focusing allow for variable focus?

Yes

- allows for variable focus or multiple focal zones which is controlled by the sonographer

The steeper the curve in focusing, the _____ the focus will be to the transducer.

closer

- the focus will be more shallow

Dynamic aperature focusing

Synonymous with electronic focusing

-Groups of elements are selected to be pulsed

Mechanical Transducer

sector or fan image shape; very narrow FOV at the top

▪contains a single, active crystal that is physically moved or rotated through a pathway

How is a mechanical transducer steered?

mechanically steered

How is a mechanical transducer focused?

Focused mechanically- ie. Though curvature of the lens, crystal or a reflecting mirror

▪ called conventional focusing

What are the advantages to a mechanical transducer?

had a smaller footprint ( more pointed) allowing access to intercostal scanning

What are the disadvantages to a mechanical transducer?

▪ increase in sidelobes because the beams diverged the further away from the source

▪ if the crystal malfunctions, the entire image goes i.e single crystal

Linear switched (flat linear) transducers

The elements are fixed, there are no moving parts.

▪Linear refers to a rectangular-shaped image

▪Elements are arranged in a row or line (similar to a keyboard)

▪ Each element is isolated from the next by a minimal amount of epoxy

How is a Linear Switched transducer steered?

w/o extended FOV- none

w extended FOV- electronic

How is a Linear Switched transducer focused?

electronic

In a Linear Switched transducer, elements are electronically fired _____?

simultaneously in groups

ex. 8-10 elements at a time

What are the methods of pulsing in Linear Array transducers?

▪ segmental

▪ sequential

▪ sequenced

▪ switched

Curved Linear/ Convex Switched arrays

▪active elements arranged in an arc

▪steering is based upon its natural arc architecture

▪Focusing is electronic

What is the advantage to curved linear transducers?

wider field of view

What is the disadvantage to curved linear transducers?

loss of contact on the edges & difficulty getting in between the ribs

Phased Array/ sector

o Elements arranged in a line; however, the elements are all electronically pulsed as a complete group.

▪ Creates a sector-shaped image

▪ Steering is accomplished by time delaying the firings of the entire group

How is a Phased Array transducer steered?

electronically

How is a Phased Array transducer focused?

electronically

Advantage to Phased array transducers?

Small footprint to get in between ribs; mainly used for echo

What is the disadvantage to Phased array transducers?

narrow FOV in the near field