Transducers
Transducer
-
device that converts one form of energy
into another form of energy
 Dx. US transducers:



Converts electrical energy into acoustic
energy (sound) during transmission
Coverts acoustic energy to electrical energy
during reception
Conversion is accomplished through the
piezoelectric effect
Piezoelectric Effect
piezo is Greek for ‘to press’ & elektron is Greek for ‘amber’
- property of certain crystals to emit
electrical energy when pressure is applied
 In US, the crystal expands & contracts with
a returning sound wave causing an electrical
voltage to be emitted
 Returning sound wave are converted into
electrical signals
Reverse Piezoelectric Effect
- the property of certain crystals to expand or
contract when positive or negative electrical
current is applied

In US, voltage applied to opposite sides of the
crystal cause it to expand; polarity is reversed
(AC current) causing the crystal to contract

Constant change from expansion to
contraction, contraction to expansion, results in
mechanical waves (sound) being produced

Thus, the electrical signal is converted into a
sound wave
Curie Point
- temperature the crystals are heated to
while in the presence of a strong electrical
field (Curie temperature ranges from
approximately 300°C - 400°C).
- If a crystal gets heated above its Curie
point, it loses its piezoelectric properties.
We never autoclave a transducer;
the autoclaving renders the transducer
useless
Transducer Element Characteristics
1.
2.
•
The crystal (piezoelectric element) emits the
sound beam & receives echoes.
Natural piezoelectric material such as quartz,
tourmaline, & Rochelle salt
Man-made piezoelectric ceramic material: lead
zirconate titanate (PZT), barium titanate, lead
metaniobate, & polyvinylidene difluoride (PVF2).
PVF2 crystals are being developed to have an
acoustic impedance closer to that of soft tissue.
These ceramics are not naturally piezoelectric –
the heating process in a strong electrical field
causes that effect.
Synthetic Crystals




Man-made crystals:
less expensive
more durable
more efficient in converting mechanical energy to
electrical energy
often combined with non-piezoelectric polymer to
create a material called piezo-composites
These composites have lower impedance,
improved bandwidth, sensitivity & resolution.
Lead Zirconate Titanate (PZT)
- is the most common piezoelectric
material found in diagnostic
imaging transducers
Operating Frequency
- the “resonant” or “natural” frequency
of the crystal
Operating frequency - depends on 2
factors:
 Crystal thickness (inversely related to
frequency)
 Crystal
propagation speed (directly
related to the frequency)
Crystal Thickness
thicker crystal – lower frequency
thinner crystal – higher frequency
crystal thickness = ½  for the frequency
Typical diagnostic pulsed ultrasound
elements are .2 – 1 mm thick
Propagation Speed of the Crystal
higher propagation speed – higher frequency
slower propagation speed – lower frequency
Typical propagation speeds of 4-6 mm/s
Frequency (MHz) = crystal’s propagation speed (mm/s)
2 x thickness (mm)
Note: The US system determines the PRF; the PW US
crystal determines FREQUENCY of sound
In CW US, the frequency of sound is determined by
electrical voltage applied to the element
Probe Construction
Probe Construction
- referred to as the probe, the scanhead, or
transducer assembly. Most commonly referred to
as the transducer & is comprised of the following:

Active Element

Damping Material (backing material)

Matching Layer (facing material )

Wiring

Insulating Case
Probe Construction
Active Element
- piezoelectric crystal or composite

single-element transducer - disk shaped

linear array transducer - rectangular prism

annular array - doughnut-shaped rings
Damping Material (Backing Material)
- composed of epoxy resin impregnated with
tungsten bonded to the back of the elements
to reduce the # of cycles in the pulse

 PD & SPL   axial resolution

Z backing material = Z of the crystal

Note: Dynamic damping - electronic means to
suppress the ringing by applying a voltage of
opposite polarity to the crystal after the
excitation pulse
Damping Material
- limits the crystal from ringing & absorbs
any energy emitted in a backwards
direction
Rear surface of the backing material is
slanted to prevent reflection of sound
energy into the crystal
Limiting the amount of ringing of the
crystal,  the transducer’s bandwidth
Bandwidth
- range of frequencies above & below
the main (resonant) frequency
 difference
between the highest & lowest
frequency found in a pulse
 measured
in MHz
Bandwidth
bandwidth - “purer” transducer
frequency
 Narrow
material  the bandwidth because
it  PD & SPL which in turn  resolution
 Damping
 Shorter
pulses = wider bandwidth & lower
Q factor
 Imaging
transducers have wide bandwidth
Multihertz transducers have a broad
bandwidth subdivided into 2 or more
frequency ranges for transmission and
reception
To change to a different frequency,
the operator just pushes a button
Quality Factor
(Q Factor or Mechanical Coefficient)
High Quality Factor: Crystal rings for a
long time (CW transducers), bandwidth is
narrow & poor axial resolution
Low Quality Factor: Crystal rings for a
very short time (PW transducers),
bandwidth is broad & good axial resolution
We use low Q-factor with a value of 2 to 3
Q-factor = operating frequency  bandwidth
Q-factor = Resonating Frequency (MHz)
Bandwidth (MHz)
Matching Layer (facing material)
 Thin
layer of aluminum powder in epoxy
resin in front (facing) of the crystal

 the impedance difference between
the crystal & the skin
Piezoelectric elements have:
Z values > Zsoft tissue
Z PZT = 20X Zsoft tissue
- creates a large reflection of the sound
with very little transmission into the body
Matching Layers
2
layers - each with a slightly different Z
the Z mismatch to ; permitting
better transmission between crystal & skin
 Causes
matching layer thickness
= ¼  of crystal’s resonating frequency
Wiring
 Carries
electrical pulse to the crystal
 Transmits
voltage from the receiving
crystal back to the US unit
Each crystal requires electrical contact
Insulating Case
 Plastic
or metal casing around transducer
 Protects:



Sonographer & Pt. from electrical shocks
Keeps outside interference/electrical noise
from entering
Protects the transducer’s components
Sound Beam Formation
Sound Beam Formation
We do not want the
sound beam coming
from the transducer to
be non-directional
(diffraction) like a light
bulb.
Diffraction causes the
sound beam to spread
out as the waves move
further from the
transducer
Huygen’s Principle
We want the sound beam to be directional like a
flashlight. So, the design of the transducer
permits the sound beam to follow Huygen’s
Principle which states that all points on a wave
are considered a point source for the production
of spherical secondary wavelets.
These wavelets combine to produce a new wave
front that determines the direction of the sound
beam.
The resulting effect of the destructive and constructive
interference of the sound wavelets is a sound beam
that is hourglass-shaped with most of the energy
transmitted along the main central beam.
Huygen’s Principle explains why the sound beam
shape does not immediately demonstrate diffraction.
divergence  with  diameter crystals
Sound Beam Shape

Sound beam produced by the transducer is
hourglass-shaped

At its starting point, the sound beam =
transducer’s diameter

As the sound travels, the width of the
beam changes

Becomes narrower until it reaches its
smallest diameter; then it begins to diverge
Sound Beam Points of Interest
 Focus
(focal point)
 Focal
length (near zone length, near
field length, focal length or focal depth)
 Focal
 Near
 Far
zone (focal area or focal region)
zone (Fresnel zone or near field)
Zone (Fraunhofer Zone or far field)
Focus (focal point)
crystal
 Narrowest
=
area of beam diameter
½ the crystal’s diameter
 Region
with highest beam intensity
Sound beam
focus
Focal length (near zone length, focal length,
near field length, or focal depth)
crystal
- the distance from the crystal to
the beam’s focus.

The focal length zone is related to
wavelength and crystal radius or
diameter.

As frequency or crystal diameter
(aperture) ⇧, focal length ⇧.

At 2X the near zone length,
beam width = crystal diameter
focal length
focus
Focal zone (focal area or focal region)
- the region surrounding
the focus that has a
narrow beam
This area has the
maximum sensitivity,
intensity, and best
lateral resolution of
the beam
crystal
Focal zone
Near zone (Fresnel zone or near field)
crystal
- the region between the
transducer & focus

This is where additional
focusing can be added

Longer near zones = more
additional focusing
Near zone
Far Zone (Fraunhofer Zone or far field)
crystal
- region beyond the near
field where beam starts to
diverge & the intensity is
more uniform
⇧
 (or crystal diameter)
⇩ widening of the Far Zone
Far Zone
Note
Near & far field shapes are influenced by
transducer frequency & crystal diameter
⇧ frequency or crystal diameter (aperture) =
⇧ length of the near field &
⇩the amount of divergence in the far field
Focusing
Focusing

creates a narrower beam over a specified
region, resulting in improved image resolution
Focusing is only performed in the near field
⇧ frequency (or crystal diameter) produces a
narrower beam & ⇧focal length

focusing ⇩ the focal zone by bringing the focus
closer to the crystal

Results in ⇩ resolution distal to the focal zone
4 Methods of Focusing
1.
External focusing
2.
Internal focusing
3.
Electronic focusing
4.
Acoustic mirrors
External Focusing

Acoustic lens placed in front
of the crystal to focus the
sound beam at a predetermined focal zone
 Curvature
of the lens
determines the focal zone
LENS
Internal Focusing
 Piezoelectric
elements
are shaped concavely
to produce a focused
beam
 Curvature of the crystal
determines the focal zone
CRYSTALS
Electronic Focusing
 Uses
the interference phenomena by
delaying (phasing) the electrical pulses to
each crystal to cause the wave fronts to
converge at variable focal points
 The
rate of delay in electronic pulses
determines the focal zone
Electronic Focusing
Acoustic mirrors
 Used
to focus the beam by the ultrasound
beam is directed back toward a curved
acoustic mirror that reflects the sound
beam outward
 Curvature
focal zone
of the mirror determines the
Resolution
Resolution

Capability of making individual parts
of closely adjacent things distinct

3 aspects of resolution in imaging
1) Temporal
2) Contrast
3) Detail
Temporal
 Ability
to distinguish closely spaced
events in time
 Relates
to the US imaging
equipment’s frame rate.
Contrast

ability of the equipment’s gray scale
display to distinguish between
echoes of slightly different intensities
Detail
- ability to distinguish 2 adjacent objects
as separate objects rather than 1 merged
object.
 Measured
in millimeters (mm)
 A function
of the transducer
 the resolution #, the better the
image quality
 The
Detail resolution is subdivided into 3 categories
– (LARD – longitudinal,
axial, range, depth)
 Longitudinal
 Lateral
(LATA - lateral, angular,
transverse, azimuthal)
 Elevational
(Slice Thickness)
Longitudinal –
(LARD - longitudinal, axial, range, depth)

Ability to distinguish 2 structures that
are laying one on top of the other;
parallel to the path of sound travel

Commonly called axial resolution
Axial Resolution
 Determined
 Shorter
by SPL
pulses improve resolution
Axial (LARD) resolution (mm) = SPL (mm)
2
Axial (LARD) resolution (mm) = # of cycles x 
2
2 Ways to Improve Axial Resolution

Use a transducer with damping material (less
cycles)

Use a higher frequency transducer (shorter )
Note: axial resolution is typically < 1.0 mm &
remains constant along the sound path
Explain why that would be logical
Axial (LARD)
Detail resolution is subdivided into 3 categories
– (LARD – longitudinal,
axial, range, depth)
 Longitudinal
 Lateral
(LATA - lateral, angular,
transverse, azimuthal)
 Elevational
(Slice Thickness)
Lateral Resolution – LATA
lateral, angular, transverse, azimuthal
 Resolution
perpendicular to beam path
 Minimum
distance that 2 structures lying
next to each other can be separated &
still produce 2 distinct echoes
 Lower
# (mm) = better the resolution
Lateral resolution
 Nearly
equal to (but slightly >) beam
diameter
 Beam
diameter varies along path (with
depth); so, lateral resolution varies
depending on its location along the
beam
 Is
always best at the focus (beam is the
narrowest)
Lateral Resolution

 with focusing ( beam diameter)

with a higher frequency transducer
(longer near field & less divergent far field)
Lateral Resolution
A
B
C
D
E
Detail resolution is subdivided into 3 categories
– (LARD – longitudinal,
axial, range, depth)
 Longitudinal
 Lateral
(LATA - lateral, angular,
transverse, azimuthal)
 Elevational
(Slice Thickness)
Elevational (Slice Thickness)
 Thickness
of the scanned tissue
perpendicular to the scan plane
 AKA - section thickness, Z-axis,
elevational axis, or out-of-plane focusing
 Accomplished by the attaching a curved
lens that has a fixed focal depth. The
curve of the lens is from front to back of
the transducer - different from a curved
lens used for LATA resolution
Elevational Resolution
 Slice
thickness is usually the size of the
scanhead close to the array, narrows
down to a few mm. at the lens focal
distance, & then broadens at beyond the
focal distance
 Worst measure of resolution for array
transducers


except for annular array transducers
annular arrays have a cone-shaped beam that
focuses in 3 dimensions
Elevational Resolution

slice thickness  spatial resolution
(ability to detect & display adjacent
entities)
 Cause

of slice thickness artifact
ability to detect small low-contrast
lesions
Resolutions Compared
Side view
Lateral resolution
Slice Thickness
AXIAL RESOLUTION
Front view