Ch.8 Transducers Flashcards
What is a transducer?
is any device that converts one form of energy into another
What two functions does the transducer perform?
Transmission
Reception
Transmission
electrical energy from the system is converted into sound
Reception
the reflected sound pulse is converted into electricity
Transducer Converts frm-to
ultrasound transmit
ultrasound receive
car engine
from To
electrical acoustic
acoustic electrical
chemical motion
Transducer Converts frm-to
Electric motion
Light bulb
Muscle
From To
electrical motion
electrical light & heat
chemical motion
what is the Piezoelectric effect?
describes the property of certian materials to create a voltage when they are mechanically deformed or when pressure is applied to them.
What is the reverse piezoelectric effect?
a process when piezoelectric materials change shape when a voltage is applied to them
What are piezoelectric materials?
materials which convert sound into electricity (and vice versa)
Also called:
piezoelectric
ferroelectric
What is the most commonly used piezoelectric material used in transducers ?
lead zirconate titanate (PZT)
What is the most commonly used piezoelectric material used in nature?
quartz and tourmaline
PZT synonyms 3
ceramic
active element
crystal
What are the seven components of a basic transducer?
Case
Electrical shield
Acoustic insulator
PZT or active element
Wire
Matching layer
Backing material (damping material)
Case
the cylindrical tube, constructed of metal or plastic, that protects the internal components of the transducer from damage. The case also insulates the patient from electrical shock
Electrical shield
A thin metallic barrier lining the inside of the case. The shield prevents spurious electrical signals in the air, unrelated to diagnostic information, from entering the transducer.
Electrical shield prevents
electrical noise from contaminating the clinically important electrical signals used to create diagnostic images.
Acoustic insulator
A thin barrier of cork or robber that isolates or “uncouples” the internal components of the transducer from the case.
Acoustic insulator prevents
vibrations in the case from inducing an electrical voltage in the PZT of the transducer
PZT or active element
Piezoelectric crystal shaped like a coin.
The characteristics of the sound beam emitted by the transducer are related to ________ of the ______ _______.
PZT is one-half ________ _____.
dimensions of the active element
wave length thick
Wire
provides an electrical connection between the PZT and the ultrasound system.
The active element requires electrical contact (wire) so that during transmission, the voltage from the ultrasound system can cause the crystal to ______ and produce an ______ ______.
vibrate
ultrasonic wave
During reception, the crystal’s vibration produces a _______ that must return to the system for processing into a _____.
voltage
image
Matching layer
Positioned in front of the PZT at the face of the transducer. It is one-quarter wavelength thick.
What are the two functions of the matching layer?
1) Increases the efficiency of sound energy transfer btw the active element and the body.
2) Protects the active element
Backing Material
Crystal damping enhances what?
(damping element)
Bonded to the back of the active element, it reduces the “ringing” of the PZT.
-Axial Resolution
When an electrical spike excites the PZT, the backing material restricts the _ _ _ of ______.
the PZT deformation
The emitted sound pulse is dampened; thus it is short in duration and length
Transducer layers A-P? back to front?
case
Electrical shield
Acoustic insulator
Wire
Backing material
PZT
Matching layer
Acoustic insulator
Electrical shield
How thick is the matching layer? Active element?
1/4 wavelength thick
1/2 wavelength thick
The matching layer and gel _____ the efficiency of sound transfer between the transducer’s _ _ _ and ____.
increases
PZT
skin
Label in decreasing order of impedance:
PZT
Matching layer
Skin
Gel
PZT > matching layer > gel > skin
The impedance of the PZT is about _ _ ____ greater than the impedance of ____.
20 times
skin
The matching layer is designed with an impedance that is ______ that of the _____ _______ and the _____.
between
active element
skin
The impedance of gel is _______ that of the ______ and _______.
between matching layer
biologic media
What is the function of the gel?
it further increases the percentage of sound transmitted into out of the body.
described as coupling the transducer to the patient
What are the two characteristics of the damping material?
high degree of sound absorption
acoustic impedance similar to PZT
What are the consequences related to the use of a backing material?
decreased sensitivity
wide bandwidth
low quality factor
Backing material not only _____ active element vibration during _______-, but also during _______ .
reduces
transmission
reception
Why is decreased sensitivity by the backing material undesireable?
the transducers are less responsive to the tiny reflected sound waves returning from reflectors in the body.
Transducers with damping material are less able to convert low-level ____ ______ into meaningful _____ _____.
sound reflections
electrical signals
Resonant Frequency
a long pure tone of sound that vibrates freely for a long time at a single frequency
The PZT is restricted from vibrating freely by the ____ ____ , and the pulse emitted by an imaging probe is a _____ duration sound “_____” rather than a steady ____ ____.
backing material
short
click
long
tone
Bandwidth
is the range of frequencies in the pulse. It is the difference between the highest and lowest frequencies
Imaging probes produce pulses that are identifies as ___ _______ or (_______).
Continuous wave doppler, do not use ______ ______ and produce ______ _____ pulses.
wide bandwidth
(Broadband)
backing material
narrow bandwidth pulses
Long duration events are _____ bandwidth, whereas short duration events are ____ bandwidth.
narrow
wide
Quality factor
is a unitless number that is inversely related to bandwidth.
Q-factor is the main frequency divided by the bandwidth.
Q-Factor formula
Q-factor = main frequency / bandwidth
Wide bandwidth probes have a ___ Q-factor.
Narrow bandwidth probes have a ___ Q-factor
low
high
What is the relationship between Q-factor and pulse length?
directly related
A short, damped pulse has a ____ Q-factor.
A longer, undamped pulse has a ______ Q-factor.
low
higher
What are the 6 characteristics of imaging transducers?
1) Pulses with short duration and length
2) Uses backing material to limit ringing
3) Reduced sensitivity
4) Wide bandwidth or broadband
5) Lower Q-factor
6) Improved axial resolution
What are the 6 characteristics of non-imaging transducers?
1) Creates continuos wave or pulses with long duration and length
2) No backing material
3) Increased sensitivity
4) Narrow bandwidth
5) Higher Q-factor
6) NO images created
How is PZT created?
polarization
(created by exposing the material to a strong electrical field while being heated to a substantial temprature)
What is the Curie temperature?
the temperature at which PZT is polarized
What is the importance of the curie temperature?
if the polarized PZT is heated above the Curie temperature the crystal’s piezoelectric properties are destroyed. (called depolarization)
Depolarization
the loss of piezoelectric properties (depolarized)
Which method is used to clean transducers?
transducers should be disinfected with Cidex or other cold germicides.
What determines the frequency of sound produced by a transducer?
differs depending on whether the transducer produces continuous wave sound or pulses
Continuous wave produces
a continuous electrical signal that constantly excites or drives the active element in the transducer.
The frequency of sound emitted by a continuous wave probe is equal to the?
frequency of the electrical signal
Electrical frequency = _______
Acoustic frequency
Pulse wave creates
a short duration electrical spike that travels thru the wire and strikes the PZT crystal in the transducer
The frequency of sound created by the active element of a pulsed wave transducer depends upon the characteristics of what?
the active element in the transducer
What 2 characteristics of the active element determine the frequency of sound created by a pulsed wave transducer?
1) Speed of sound in the PZT
2) Thickness of the PZT
How does the speed of sound in PZT affect frequency?
the speed of sound in PZT and the frequency of sound are directly related
What is the range of speed of sound in most piezoelectric materials?
from 4 to 6 mm/us
(4 times greater than the speed of sound in soft tissue)
How does the thickness of the PZT crystal affect frequency?
PZT thickness and frequency are inversely related
The thickness of the PZT crystal in a pulsed wave transducer is equal to ?
1/2 of the wavelength of sound in the PZT
What are the two characteristics of high frequency pulsed wave imaging transducers?
1) Thinner PZT crystals
2) PZT with higher speeds
What are the two characteristics of low frequency pulsed wave imaging transducers?
1) Thicker PZT crystals
2) PZT with lower speeds
What is the formula for frequency (MHz) ?
(MHz) = sound’s speed in PZT (mm/us) / 2 x thickness (mm)
