Ultrasound physics Ch. 8

Transducers convert

one form of energy into another

transmission

electrical energy converted to sound

reception

reflected sound is converted to electricity

Transducer crystal property

voltage created when mechanically deformed or P (pressure) applied

Mechanical energy into electrical energy

reverse piezoelectric effect

apply voltage, crystal mechanically deforms. Electrical energy to mechanical energy

materials change shape when voltage applied

piezoelectric or ferroelectric materials

materials that convert sound into electricity (or the reverse)

piezoelectric effect may be found in

nature; quartz; tourmaline

synthetic lead zirconate titanate or PZT

manufactured PZT

-PZT properties are created
-material exposed to a strong electrical field while heated
-process of polarization

curie temp or curie point when

polarization occurs

properties destroyed if heating is

greater than curie temp

depolarization

loss of piezoelectric properties

transducer case

plastic or metal housing, protects internal components and protects patient from electrical shock

transducer electrical shield

thin metallic lining of inside case. Prevents outside electrical signals (static) from entering transducer. shields electrical noise from contaminating US signals

transducer acoustic insulator

-cork or rubber inner layer
-prevents vibrations within case that could induce electrical voltage to PZT

transducer wire

electrical connection between PZT and US system for transmission and reception
-each crystal has a wire hook-up

PZT also known as

-ceramic
-active element
-crystal

Transducer components: PZT

-coin shaped in a simple probe
-characteristics of sound beam related to crystal dimensions

transducer components: matching layer

-lies between crystal and skin
-increases efficiency of transmitting sound between crystal and skin
-protect the crystal

acoustic gel "Z" is between matching layer and skin to

optimize sound transmission

Impedances in decreasing order

(50Z)PZT > (40Z)matching layer > (30Z)gel > (20Z)skin

matching layer=

1/4 of wavelength

active element=

1/2 of wavelength

transducer components: backing material

-bonded to back side of crystal
-reduces ringing, restricts PZT deformation, shortens pulse

backing material also called

damping element

shorter pulses create

a better image

consequences of backing material

-decreased sensitivity
-wide bandwidth
-low quality factor

backing material decreased sensitivity

-transducers with backing material are less sensitive to low level sound reflections (during receptions_

damping material reduces crystal vibration during

transmission and also during reception

wide bandwidth

-Backing material prevents PZT from ringing freely.
-Abbreviated "ring" contains sound with many different frequencies.

bandwidth speaks to

range of frequencies or frequencies above and below resonant (main) f.

range of F is

bandwidth or broadband

imaging transducers have __________ bandwidth than therapeutic transducers

wider

damping material shortens pulse and increases

band width

quality factor

-Unitless
-describes transducer's ability to emit a clean pulse with a narrow bandwidth

quality factor and bandwidth relationship

inverse

quality factor equation

resonant F (MHz) / bandwidth (MHz)

imaging a _________ Q factor than therapeutic or non imaging transducers (CW)

lower

sterilization

destruction of all microorganisms by extreme heat (autoclave), chemical agents (Glutahaldaryde), or radiation. High heat can cause depolarization of crystal.

disinfection

chemical agent to reduce or eliminate infectious organisms on an object.

Transducers that penetrate skin or mucous membranes have highest potential for

transmitting infections and require sterilization. (TEE, transvag. transrectal)

what does not accurately describe an imaging transducer

High sensitivity

F of transducer depends on

whether transducer produces pulsed or CW sound

CW transducer F is determined by

electrical F applied to crystal
-electric=acoustic

in PW F determined only by

characteristic of PZT
-thickness of crystal
-Prop speed of sound in crystal

Crystal thickness and F relationship

inverse

PW crystal thickness =

1/2 wavelength

Prop speed and F relationship

direct

PW F (MHz) equation

PZT prop speed / 2 x thickness

T/F The purpose of using ultrasound gel is to avoid a big reflection.

true

If two crystals are made from the same material, the thicker transducer crystal will ________CW frequency?

have no affect

T/F Damping material of a transducer will improve image quality.

true

Describe the relationship between the damping material and the Q factor.

Damping material will decrease the Q factor.

T/F If PRF is increased, then f of US produced by the transducer is increased.

False: not related

A pulse has a primary f of 6 MHz, the lowest f is 4 MHz, the highest f is 7 MHz, what is the bandwidth and Q factor of this transducer?

3MHz, 2.0

Q-factor is ______________related to bandwidth:

inversely

T/F A pulse with a long spatial pulse length is likely to have a narrow bandwidth.

true

Imaging transducers have a __________ than therapeutic transducers:

greater bandwidth

What determines the frequency of a continuous wave transducer?

electrical F

If a transducer experiences heat above the Curie temperature, what happens?

depolarization

PZT has an impedance of 1,700,000 Rayls and the impedance of the skin is 1,200,000 Rayls. Matching layer Z should be:

1,500,000 Z

matching layer and gel increase

efficiency of sound transfer between transducer's PZT and skin

long duration events are ___________ bandwidth

narrow

short duration events are _________ bandwidth

wide (broadband)

characteristics of high F pulsed wave imaging transducers

-thinner PZT crystals
-PZT with higher speeds

characteristics of low F pulsed wave imaging transducers

-thicker PZT crystals
-PZT with lower speeds

US transducer converts (transmits)

electrical into sound (acoustic)

US transducer converts (receive)

sound (acoustic) to electrical

a shorter, dampened pulse has a _________ Q-factor

lower

A longer, undampened pulse has a ____________ Q factor.

higher

imaging transducers have pulses with short _________ and _________

duration, length

imaging transducers use __________ to limit ringing

backing material

imaging transducers have reduced or increased sensitivity

reduced

imaging transducers have a ____________ bandwidth/broadband

wide

imaging transducers have a _________ Q-factor

lower

what transducer has an improved axial resolution

imaging

non imaging transducers create

CW or pulses with long duration and length

T/F non imaging transducers use backing material

False

does non imaging transducers have reduced or increased sensitivity

increased

non imaging transducers have a _________ bandwidth

narrow

non imaging transducers have a __________ Q-factor

higher

T/F non imaging transducers cannot produce and image

true

transducer used in therapeutic US

non imaging transducer

acoustic insulation

backing material

matching layer

Crystals

electrical shield

transducer case

characteristics of damping material

-high degree of sound absorption
-acoustic impedance similar to PZT

the impedance of PZT is about _______ times greater than the impedance of skin

20

the F of sound emitted by a continuous wave probe is equal to

the F of the electrical signal

electrical=acoustic

the speed of sound in most piezoelectric material ranges from

4-6mm/us

PZT thickness and F relationship

inverse

thickness of PZT crystals in diagnostic imaging TRX range from

.2 to 1mm

piezoelectric/ferroelectric materials commonly used in clinical TRX

lead zirconate titanate or PZT
(synthetic/man-made)