Chapter 8 Flashcards
A _ is any device that converts one
form of energy into another
Transducer
Ultrasound transducers perform two
functions: During transmission _,
During reception _
During transmission, electrical energy from the system is converted into sound. During reception, the reflected sound pulse is converted into electricity.
the property of
certain materials to create a voltage
when they are mechanically deformed
or when pressure is applied to them.
The piezoelectric effect
a process
in which piezoelectric materials change
shape when a voltage is applied to
them.
Reverse Piezoelectric Effect
Materials which convert sound into electricity
and vise versa
Piezoelectric Materials
Piezoelectric Materials AKA
ferroelectric
_ is a synthetic
material used in clinical transducers
Lead zirconate titanate or PZT
PZT AKA
The ceramic
Active element
Crystal
The _ of the transducer protects the internal
components of the transducer from damage
case
The _ of the transducer insulates the patient from electrical shock
case
The case is made of _
Plastic or metal
A thin metallic barrier lining inside the case
Electric shield
The _ prevents electrical signals that are unrelated to
diagnostic information from entering the transducer
Electrical shield
The _ prevents electrical noise from contaminating the clinically
important electrical signals
Electrical shield
A thin barrier of cork or rubber that isolates the
internal components of the transducer from the
case.
Acoustic insulator
The acoustic insulator prevents _
vibrations in the case from inducing an
electrical voltage in the PZT of the transducer.
In a simple probe, the PZT is shaped like a _
coin
The characteristics of the sound beam emitted by the transducer
are related to
the dimensions of the active element
The PZT is _ wavelength thick
1/2
Provides an electrical connection between the PZT and the
ultrasound system
Wire
During _, the voltage from the US system causes
the crystal to vibrate and produce an ultrasonic wave.
Transmission
During _, the crystal’s vibration produces a voltage
that must return to the system for processing into an image.
Reception
Increases the efficiency of sound energy transfer
between the active element and the body
Matching layer
Matching layer protects _
The active element
The matching layer is _ wavelength thick
1/4
Matching layer: Differences in impedance result in
reflections at
boundaries
Larger reflections occur with
greatest
impedance differences
The impedance of PZT is about _ times greater than the
impedance of skin
20
The matching layer is designed with an impedance in
between
that of the active element and the skin
The impedance of gel is between that of
the matching
layer and biologic media.
Multiple matching layers of
different impedances further
_ the percent of sound
transmission.
increases
Usually around _ matching layers
[with different impedances] are
used, in some cases _ layers.
2
3
The matching layer and gel increase the efficiency of
sound transfer between the PZT and the skin
Decreasing order of impedence
PZT > matching layer > gel > Skin
The thickness of the matching layer is _ of the wavelength of sound in the matching layer
1/4
The thickness of the active element is _ of the
wavelength of sound in the active element
1/2
Component: Active element
Thickness:
1/2 wavelength
Component: Matching layer
Thickness:
1/4 wavelength
Bonded to the back of the PZT
Backing material
Reduces “ringing” of the PZT
Backing material
When an electrical spike excites the PZT, the backing material_. The emitted sound pulse
is _ which makes it short in duration and length.
restricts the extent of PZT deformation.
Dampened
Plays an essential role in optimizing the pulses
created by imaging transducers.
Backing material
Without the damping material, the crystal will
vibrate longer and create pulses that are long in
length and time.
Backing material substantially _ the vibration of the PZT. This creates pulses that are
decreases
short in length and
duration which improves axial resolution.
Characteristics of Backing Material
High degree of sound absorption
Acoustic impedance similar to PZT
Made of metal powder and epoxy resin
There are three consequences of
using backing material in
transducers:
- Decreased sensitivity
- Wide bandwidth
- Low quality factor
The backing material not only decreases
vibration of the active element on
transmission but also _. This
makes the transducer
during reception
less responsive to the
reflected sound waves returning from the
body.
During reception, transducers with damping
material are less able to
convert low-level
sound reflections into meaningful electrical
signals.
the range of frequencies in the
pulse; the difference between the highest and
lowest frequencies
Bandwidth
A single frequency produced is called a
resonant
frequency.
Backing material prevents PZT from vibrating
freely which causes _. Although the click is short, it contains _
the pulse from the machine
to be more like a click than a steady long tone.
sound at many
different frequencies (both above and below the
transducer’s main frequency).
the main frequency of a
transducer divided by the bandwidth
Quality factor
Relationship between qualityy factor and bandwidth
Inversely
Quality factor is commonly called
Q factor
Wide bandwidth probes have _ Q factor
Low
Narrow bandwidth probes have _ Q factor
High
Because imaging probes use backing material
and have a wide bandwidth, they are often
referred to as
Low-Q
Shorter pulse= _ Q factor
lower
Longer pulse= _ Q factor
Higher
A 3 MHz transducer with a bandwidth of 4 MHz
has a Q-Factor of
3/4 or 0.75
The piezoelectric properties of lead zirconate
titanate (PZT) are created by
exposing the
material to a strong electrical field while being
heated to a substantial temperature. This process
is called polarization
The temperature at which PZT is polarized is
called the
Curie temperature or the Curie point
Piezoelectric properties can be destroyed by
exposure to a high temperature (depolarization).
The frequency of sound emitted by a continuous
wave probe is equal to
the frequency of the
electrical signal
A pulsed wave system creates _ that travels
through the wire and strikes the PZT
crystal in the transducer
a short
duration electrical spike
The frequency of sound created by the
active element of a pulsed wave
transducer depends upon
the
characteristics of the active element of
the transducer.
Two characteristics of the PZT combine
to determine the frequency of sound
Speed of sound in the PZT
Thickness of the PZT
The speed of sound in PZT and the frequency of
sound are _ related.
Directly
Higher speed of sound in PZT= _ frequency
Higher
Lower speed of sound in PZT = _ frequency
Lower
The speed of sound in most piezoelectric
materials
from 4 to 6 mm/μs
_ Is a characteristic of each piece of PZT and
cannot be changed
Speed
PZT thickness and frequency are _
related
inversely
Thinner active elements create pulses with
_ frequency, _ wavelength cycles.
Higher
Shorter
Thicker active elements create pulses with _ frequency, _ wavelength cycles
Lower
Longer
Thickness of PZT crystals in diagnostic
imaging transducers range from
0.2 to 1
mm.
The thickness of the PZT crystal in a
pulsed wave transducer is equal to
one-
half of the wavelength of sound in the
PZT.
Frequency (MHz) =
sounds speed in PZT (mm/us)/ 2 x thickness (mm)
Characteristics of high frequency pulsed wave imaging transducers
Thinner PZT
PZT with higher speeds
Characteristics of low frequency pulsed wave imaging transducers
Thicker PZT
PZT with lower speeds
Advantages of backing material
Reduces # of cycles in a pulse
Reduces pulse duration and SPL
Improves resolution
Disadvantages of backing material
reduces amplitude
decreased sensitivity
wide bandwidth
low Q-facto
