Ultrasound Physics & Instrumentation
4th Edition
Volume I
Companion Presentation
Frank R. Miele
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License Agreement
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Volume I Outline
 Chapter 1: Mathematics
 Chapter 2: Waves
 Chapter 3: Attenuation
 Chapter 4: Pulsed Wave
 Chapter 5: Transducers
 Level 1
 Level 2
 Chapter 6: System Operation
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Chapter 5: Transducers - Level 1
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Chapter 5: Transducers
A transducer is any device which converts one form of energy to another
form of energy.
 nerves
 lights
 speakers
 heaters
 etc.
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Piezoelectric Effect
Ultrasound transducers use the piezoelectric effect to convert electrical
energy into mechanical energy and mechanical energy back into
electrical energy.
Electrical to acoustic
transformation
Acoustic to electrical
transformation
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Piezoelectric Effect
Fig. 2a Expansion
Fig. 2b Contraction
(Pg 235)
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Fig. 2c At Rest
Block Analogy of Crystal Oscillation
This analogy is useful to illustrate the concept of the piezoelectric effect.
A: At Rest
A
B: Stretched
C: Recoiled
D
B
D: Oscillate
E: At Rest
E
C
Fig. 3: (Pg 236)
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Crystal Thickness (t) and PW
Operating Frequency
Longer Period
(Lower Frequency)
Thickness (t)
A thicker crystal “vibrates” at
a lower frequency when
driven in a pulsed mode.
There is therefore an inverse
relationship between crystal
thickness and operating
frequency in a pulsed mode
operation.
Shorter Period
(Higher Frequency)
fo 
Thickness (t)
Fig. 4: (Pg 238)
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1
thickness
PW Operating Frequency (Animation)
(Pg 238)
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PW Operating Frequency Equation
th ickn ess
th ickn ess
t 
 t 2
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CW Operating Frequency
5 MHz Voltage
5 MHz Acoustic
In a CW mode of operation, the
frequency at which the crystal
vibrates is related to the
frequency of the electrical drive
signal (as visualized in the
animation of the next slide).
Fig. 5: (Pg 239)
10 MHz Voltage
10 MHz Acoustic
f o  D rive V oltage F requency
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CW Frequency (Animation)
(Pg 239)
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CW Operating Frequency Equation
5 MHz Voltage
5 MHz Acoustic
f o  T ransducer F requency  D rive V oltage F requency
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Pulse Response
Like a bell when rung, a single pulse produces multiple cycles of ringing
(the “impulse response”). The following two figures represent the
impulse response for a 2 MHz and a 4 MHz transducer design.
2 MHz
4 MHz
Fig. 6: (Pg 240)
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Single Crystal Dimensions
PZT Crystal
Diameter (D)
The diameter of a crystal affects the beamwidth and hence, the focus.
The thickness of the crystal affects the operating frequency. These two
parameters should not be confused.
thickness (t)
Fig. 7: (Pg 241)
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CW Beamshape
Lateral
PZT Crystal
Since CW is continuously transmitting, the wave exists at all locations
simultaneously producing a beam similar to that of a flashlight. (As
visualized in the animation of the next slide)
Depth
Fig. 8: (Pg 241)
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CW Beamshape (Animation)
(Pg 241)
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Lateral
Lateral
T1
Depth
T1
Depth
Lateral
Lateral
T1
Depth
Unlike CW mode, in PW,
the transmit is turned on
and off. The beamshape is
therefore a “description” of
the shape of the path the
sound wave travels over
time (as visualized in the
animation of the next slide).
Lateral
T1
Depth
T1
Depth
Depth
Lateral
PZT CRYSTAL
PZT CRYSTAL
PZT CRYSTAL
PZT CRYSTAL
PZT CRYSTAL
Fig. 9: (Pg 242)
PZT CRYSTAL
PW Beamshape
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PW Beamshape (Animation)
(Pg 242)
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Basic Beam Characteristics
Although greatly simplified, the basic beamshape is helpful in roughly
describing the beam parameters. Notice that the beam is approximately
half as wide as the crystal diameter at the focus and the same width as
the crystal diameter at the twice the focal depth.
PZT Crystal
D1
Fresnel Zone
Fraunhoefer Zone
Natural Focus
D/2
NZL = D2 • f0
6
2 • Near Zone Length
Fig. 10: (Pg 243)
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D
A larger crystal
diameter results in a
deeper focus for the
same operating
frequency.
PZT Crystal
D1
Crystal Diameter and Focus
PZT Crystal
D2
Deeper Focus
Shallower Focus
Fig. 11: (Pg 244)
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Aperture Effects on Beam Diameter
Notice that increasing the crystal aperture by a factor of 2 increases the
focal depth by a factor of 4.
D2/2
D1 D2
D2
D1/2
NZL2
NZL1
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D1
Quarter Wavelength Matching Layer
The ideal matching layer thickness
is one fourth the wavelength
(quarter wavelength). With quarter
wavelength thickness, the energy
that reflects back from the front
surface is 180 degrees out of
phase with the reflection from the
front surface, resulting in
destructive interference. This is
beneficial since reflections from the
matching layer would otherwise
obscure the actual desired image
from the patient.
Fig. 12: (Pg 246)
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Axial Resolution
The roundtrip effect helps separate by a factor of 2 the echoes returning in
the time. Therefore, the resolution in the depth direction is better (less)
than the spatial pulse length by a factor of 2.
Fig. 13: (Pg 247)
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Axial Resolution Further Defined
*Higher frequencies have shorter wavelengths improving axial resolution.
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Lateral Resolution and Beamwidth
The lateral resolution equals the lateral
beamwidth dimension. If the beam is
wider than the distance between two
structures, the echo from both structures
will overlap, making it impossible to
distinguish between the two structures
laterally.
Lateral Resolution  Beamwidth
Fig. 14: (Pg 248)
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Lateral Resolution
*Higher frequencies form narrower beams improving lateral resolution.
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Techniques for Changing Focus
There are four techniques which can be used to change the focus from
the natural focus of a crystal design
1. Lenses
2. Curved elements
3. Electronic focusing
4. Mirrors
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Use of Lenses for Focusing
PZT Crystal
Lens
Fig. 15: (Pg 249)
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Curved Surface for Focusing
PZT Crystal
Concave Surface
Fig. 16: (Pg 250)
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Techniques for Changing Focus
Mirrors were rarely used and not used currently so will not be further
discussed. Electronic focusing is discussed in level 2.
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Simple Transducer Block Diagram
The simple block diagram is useful since it illustrates the principal
transducer components. You should be able to describe each
components purpose and function.
Lens
_
Matching Layer
Wires
Piezoelectric
Crystal
+
Backing
Material
Fig. 17: (Pg 250)
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Notes:
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