Piezoelectric Effect

Sound waves striking a PZ material
produce an electrical signal

Can be used to detect sound (and echoes)!
Piezoelectric Effect

Sound waves
striking a PZ
material produce
an electrical signal

Can be used to
detect sound (and
echoes)!
Reverse Piezoelectric Effect

Applying an
electrical signal
causes the PZ
element to vibrate

Produces a sound
wave
Transducer
Define?
 Many types of transducers exist

– Pressure transducers
– Air flow transducers, etc.

What is function of transducer?
convert electrical signals to sound
waves, and vice versa.
Ultrasound Transducer
Materials

Quartz (naturally piezoelectric)
– First used as a stable resonator in time
measurement devices
– Used in some laboratory ultrasound
applications

Most current applications use
piezoelectric ceramics (ie, lead
zirconate titanate; barium titanate)
– Lower “Q” (good for short pulses)
– Good sensitivity
– Many shapes are possible
Miniature quartz
tuning fork; 32,768
Hz.
Polarizing a Piezoelectric
Element

Most ultrasound transducer materials
are not ‘naturally’ piezoelectric
– Lead zirconate titanate
– Microscopic crystals, randomly oriented

Must be polarized
– Heat to ~350oC (Curie Temperature)
– Apply strong voltage across crystal
– Cool while voltage is still applied
Polarization
Single Element Transducers


Uses
– Simple A-mode
machines
– Mechanical scanning
transducers
The design serves as a
useful example of general
construction methods
Single element
transducer construction
Matching layers, lens
Ultrasound
Transducers
Piezoelectric (PZT) ceramic
elements
Backing layer
½ wavelength resonance
d
Resonance frequency corresponds to
the thickness = ½ wavelength
 Speed of sound in Piezoelectric
material ~ 4,620 m/s
 What thickness is required for a 3 MHz
frequency transducer?

c
4,620m / s
 
 0.00154m  1.54mm
f 3,000,000 / s
 / 2  0.77mm
½ wavelength resonance
d
Resonance frequency corresponds to
the thickness = ½ wavelength
 Speed of sound in Piezoelectric
material ~ 4,620 m/s
 What thickness is required for a 3 MHz
frequency transducer?

c
4,620m / s
 
 0.00154m  1.54mm
f 3,000,000 / s
 / 2  0.77mm
½ wavelength resonance
d
Resonance frequency corresponds to
the thickness = ½ wavelength
 Speed of sound in Piezoelectric
material ~ 4,620 m/s
 What thickness is required for a 5 MHz
frequency transducer?

c
4,620m / s
 
 0.000924m  0.924mm
f 5,000,000 / s
 / 2  0.462mm
Resonance Frequency
PZT (c=4620m/s) Thickness vs
5
Frequency
Element Thickness (mm)
4.5
4
3.5
3
2.5
2
Series1
1.5
1
0.5
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14
Frequency (MHz)
Backing (Damping) Layer

Need short duration pulses for decent
axial resolution (we will discuss this later)
 Backing
layer helps to reduce vibrations
of the element following excitation
– Like placing your hand on a bell to stop the
ringing!
Matching Layers

Thin layer of material
– ¼ wavelength thick
– Impedance is between that of the element
(quite high) and that of tissue
Provides better sound transmission
from the transducer-patient-transducer
 Improves sensitivity

Focusing, Methods


Focusing reduces the beam width in the
focal zone
Methods
– Lens
– Curved element
– Electronic
20 mm
In Most Applications, Beams Are Focused
- curved element
- lens
- electronic (arrays)
Improves lateral resolution near the focal distance
Higher frequencies produce narrower beams
20 mm
2.5 MHz
Dr.Awad Elkhadir
5.0 MHz
Short pulse (50% bw)
2.5 MHz
5.0 MHz
CW
5.0 MHz
20 mm
10 mm
- Previous diagrams exhibit sidelobes
- Must be eliminated for good image quality
- Pulsing reduces (or even eliminates) side lobes
2.5 MHz
Array Transducer



“Scanhead” containing
many small PZT
elements
Element, along with a
transmit-receive circuit
in the machine is a
channel.
128 channels are
common.
Beam Forming (Transmit)
Group also permits electronic beam steering
and electronic focusing.
Curvilinear
Phased Array
Linear-Phased
(“Virtual Convex”)

Linear array
– Rectangular FOV,
defined by transducer
footprint

VC adds beam
steering to expand
imaged region at
edges
Annular