PHYSICS OF TRANSDUCERS FOR IMAGING AND DOPPLER ...

DLM 14 - PHYSICS OF TRANSDUCERS FOR IMAGING AND DOPPLER
Benjamin Adeyemi MB M.Sc
North Middlesex University Hospital
London
Objectives
• To understand the properties of the basic ultrasound pulse as applied to 2D and Doppler imaging
• To understand the design and development of the modern day phased array transducer, the
piezo-electric materials and physical construction of a phased array transducer as applied to
clinical echocardiography, and how it functions effectively to improve image quality
• To understand the principles behind the development of the modern day transducer, its clinical
applications, and improvement in technology
• To understand broadband imaging, why image texture is different in harmonic imaging, and how
this knowledge can be used effectively to optimise imaging techniques
• To understand recent technological advances in the design of transducers.
1. Introduction
The development of ultrasound transducers has been driven by efforts to improve image quality. Image
quality is largely determined by beam width, frame rates, the length of the ultrasound pulse (l/bandwidth),
and sensitivity.
Reducing beam width and increasing frame rates have benefited from previous developments that have
have largely concentrated on electronic beam forming techniques such as focussing to reduce beam
width and improve lateral resolution, faster signal processing to improve display of the ultrasound image,
and increasing frame rates using faster digital processing. Most modern machines now have greatly
increased frame rates.
In recent years, manufacturers have concentrated on other aspects of transducer design, and have
made substantial improvements which include:
1. Using acoustics and vibration properties such as electromechanical properties of PZT and other
ceramics, and impedance matching to improve sensitivity and axial resolution.
2. Developments in transducer materials and impedance matching to improve sensitivity and
bandwidth, with greater penetration and increased dynamic range, and improved axial, spatial
and contrast resolution.
3. Focussing in the elevation plane which controls slice thickness and produces higher resolution.
4. Focussing in both elevation and lateral planes with improved image quality in 3D applications.
5. Further improvements in digital signal processing which affects all aspects of image quality.
6. Wider imaging field distribution for 3D imaging

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2. The Basic Ultrasound pulse and Pulse generation
A typical ultrasound pulse consists of cycles of oscillating amplitudes (figure 1a), and contains a
spectrum of frequencies (bandwidth) dominated by a centre frequency.
A high frequency transducer generates a short pulse (1-3 cycles), producing a wide bandwidth
(broadband), this gives accurate distance information with good axial resolution, and is ideal for
diagnostic 2D imaging (figure 1a).
Figure 1a. Short Pulse
1 – 3 cycles
Wide bandwidth
A low frequency transducer generates a long pulse about 5 – 30 cycles (figure 1b ), producing a narrow
bandwidth, this gives accurate frequency information but poor axial resolution, therefore used typically
for Doppler imaging (figure 1b).
Figure 1b. Long Pulse
5 – 30 cycles
Narrow bandwidth
The ultrasound pulse occurs in microseconds therefore difficult to measure. The pulse generated by the
transducer is sent to tissues, and the reflected echo is received by the same transducer within a time
interval determined by the depth or range of the reflector (Figure 1c). A brief pulse generates a single
echo allowing the delay to be measured, with continuous transmission individual echoes can’t be
identified.
Figure 1c.

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3. Structure of Phased Array Transducers
All transducers have a thin piezoelectric ceramic plate often made of lead zirconate titanate (PZT), a
matching layer, and a backing layer with a single lens across the transducer arrays. (figure 2a). The
piezoelectric plate generates and detects ultrasound waves. Some piezoelectric materials (e.g quartz)
occurs naturally, but (PZT) is a synthetic ceramic material commonly used. The curved cylindrical lens
after the matching layer is used to focus the beam in the elevation plane and improve electronic
focussing, producing the greatest transmitted amplitude and receive sensitivity at the focal zone.
Figure 2a: Cross-section of a typical phased array transducer
Figure 2b: Cross-section of a typical phased array transducer
4. PZT and generation of ultrasound pulse
The properties of piezoelectric materials are that they deform in response to an electrical voltage, and
generate electrical voltages when stretched or compressed by an external force. Newer versions of PZT
have been developed to improve sensitivity and produce larger acoustic power, and other piezoelectric
materials are being developed to generate stronger electrical signals when receiving echo pulses with
greater sensitivity and penetration and wider bandwidth, producing better axial resolution.
PZT generates and transmits ultrasound imaging pulses when fired by rapidly changing voltages. In
order to transmit an ultrasound pulse, the oscillating voltage of the required frequency is applied across

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the PZT plates making it vibrate at this frequency sending the ultrasonic pulse into tissues. During
reception, the pressure variation of returning echoes cause the PZT plate to contract and expand
generating voltage variations across the plate that form an electronic version of the received echo
signal.
For the PZT plate to vibrate most strongly, its thickness must be exactly half the wavelength of the
signal produced by the vibrating frequency produced ie “half-wave resonance”. Resonance occurs
because the ultrasound wave propagating across the thickness of the PZT plate reverberates within the
plate. If the thickness of the PZT is equal to half the wavelength of the vibrating frequency, it will travel a
full wavelength within the PZT before arriving at its starting point, this means that it will be in phase with
the original wave and add constructively to produce a greater output. A PZT plate with thickness that is
equal to half the wavelength at the required centre frequency will resonate and produce a larger output
at this frequency.
PZT crystals transmit impulses 1% of the time, and receive impulses 99% of the time. The electrical
pulse applied across the PZT plate typically consists of 1-3 cycles of oscillating voltages of peak to peak
amplitude of as much as 200 to 300 Volts determined by the output power control. The frequency and
duration of the oscillating pulse is determined by the transducer centre frequency and pulse length.
Excess “internal ringing”
The main disadvantage of PZT is that it has a high density therefore high characteristic acoustic
impedance up to 20 times higher than that of soft tissue. This results in up to 80% reflection of the
ultrasound energy at the PZT-tissue interface, multiple prolonged internal reverberation (ringing), and
very long pulses with ultimately poor axial resolution, poor sensitivity and reverberation artifarct.
Backing layer
A backing (damping) layer with a high characteristic acoustic impedance and ability to absorb ultrasound
is used behind the PZT plate to reduce unwanted ringing (figure 2a). Modern transducers have a
backing layer with impedance somewhat lower to give a useful reduction in ringing without lowering
sensitivity too much, but prevent total absorption of ultrasound. The remaining ringing is removed by
adding a matching layer (figure 2a).
Apart from the ringing problem, poor sensitivity is made worse because only 20% of the ultrasound
wave’s power would be transmitted through the front PZT-tissue interface. To correct this problem a
single “impedance matching layer” is bonded to the front face of the PZT plate to increase transmission
across the front face of the PZT by nearly 100%, and increase the efficiency of the transmitted wave as
it exits the transducer surface (figure 2a). This is done effectively by ensuring that the matching layer
has a thickness equal to a quarter of a wavelength, and an impedance equal to :
Impedance of PZT x Impedance of Tissue
This is called the geometric mean of the impedances.
Figure 2c shows reverberation within PZT plate that produces multiple transmission into tissues
resulting in reinforced signals and a larger amplitude pulse. Multiple reflections back into PZT cancel
out the original (top) reflection into backing layer. The most efficient (near 100%) transmission through
the matching layer only occurs when its thickness is exactly one-quarter of a wavelength of its vibrating
frequency.

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Figure 2c: Quarter-wave length matching layer
Matching layer and bandwidth
Although the matching layer improves sensitivity, it gives high sensitivity at a single frequency, and good
performance only over a narrow range of frequencies. It also acts as a filter therefore reducing
bandwidth. Figure 3 below shows the bandwidth or range of frequencies for a transducer. A -3db
bandwidth is the range of frequencies over which the transducer is most efficient. A transducer with a
matching layer will have a -3db bandwidth of about 60% of the centre frequency, therefore a 3MHz
transducer with a matching layer will have a -3dB lower frequency of 1.8MHz. and an upper frequency of
4.2MHz.
Figure 3: Transducer bandwidth
5. Improvements in efficiency of Transducers
A large transducer bandwidth is produced by a short pulse, and is crucial for good axial resolution.
Transducer bandwidth can be increased by using two or more matching layers of different thickness.
Improvements in backing layers and introduction of multiple matching layer technology have led to
transducers with wider bandwidths. Up to -3dB bandwidth with greater than 100% centre frequency is
now available.
Larger transducer bandwidths are now a common feature in modern day transducers. This allows a
choice of operating frequencies according to penetration or resolution required. Modern day transducers
can now operate at three frequencies for example a 1, 2 and 3MHz transducer would need to have a
centre frequency of 2MHz and a bandwidth of 2MHz which is 100% of the centre frequency. Axial

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resolution of bandwidths generated at the upper and lower frequencies is however less than if the whole
transducer bandwidth were used.
Figure 4: Broadband Transducer bandwidth
6. Improvements in PZT and transducer design
The Piezoelectric (PZT) material is the most effective determinant of beam penetration and image
quality. For many years, there has been slow progress in the development of PZT crystals with superior
electromechanical properties. Recently transducer design has been focussed on development of new
types of PZT materials (e.g. PureWave crystals, as developed by Philips Medical Ltd)) with improved
electromechanical properties compared to old PZT ceramics or PZT composites. These are being been
used with specially designed matching layers and backing materials producing dramatic improvements
in efficiency, sensitivity and bandwidth.
“Composite PZT” is made by cutting closely spaced narrow channels through a solid plate of PZT
ceramic and filling them with an inert polymer, this has a lower characteristic acoustic impedance than
PZT itself, therefore produces transducers with greater sensitivity and wider bandwidth, and alleviates
the problems associated with matching.
PureWave crystals are more efficient than PZT ceramic or PZT composites with electro-mechanical
properties improved by up to 68 – 85%, and ten times the ability to deform in the presence of an
electrical field. When combined with multiple matching layers and backing material, they greatly
increase bandwidth (1 - 5MHz), and sensitivity at transmission and reception, with improved dynamic
range, greater penetration, greater clarity of images, and greater uniformity throughout the entire image
field. They provide better endocardial border delineation in difficult-to-image patients, with significant
benefits in both tissue and contrast harmonics applications. Improved sensitivity means higher
frequencies can be used with better image resolution, and provides the main benefit for contrast
harmonics allowing the detection of bubbles more easily. There is also better sensitivity at the lower
frequency spectrum for colour and spectral Doppler frequencies.

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Figure 5a: Broadband versus Pure-wave crystal technology
Figure 5b: PureWave crystal transducer with large bandwidth

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7. Broadband Transducers and imaging frequencies
These are now used on modern ultrasound scanners. They have a large bandwidth, so they can
transmit and receive pulses with several different center frequencies (Pulse spectra shown as dashed in
figure 3). A large transducer bandwidth is crucial for good axial resolution and harmonic imaging. It
allows the operator a choice of transmitting and receiving frequencies, and to toggle between them
according to the penetration or resolution required.
Typical imaging frequencies in adult echocardiography can be anything from 1.0MHz to 5MHz.
Compared to a lower frequency band, a higher frequency band will have shorter near zone, more
divergent beam, greater side-lobes, poor penetration, but better axial & near field resolution. Attenuation
or energy loss is greatly increased at higher frequencies, and explains the poor beam penetration.
Beam intensity at a particular depth is lower in the higher frequency transducer due early beam
divergence.
A lower frequency band will have a longer near zone, less divergent beam, better penetration, and
poorer axial & near field resolution therefore ideal for Doppler imaging. Narrower beam width results
from a less divergent beam, therefore better lateral resolution. The beam characteristics are also ideal
for harmonic imaging which requires transmitting at low frequencies, and receiving at higher
frequencies. The short wavelengths associated with higher frequencies lead to improved resolution of
images. The aim is to choose the optimum frequency band for each particular application. This is a
compromise that ensures that the best resolution is obtained while allowing echoes to be received from
the required depth.
8. Phased Array Transducers (Beam steering and focusing)
Phased array transducers have typically 128 rectangular elements or arrays arranged with individual
connections. All arrays are used to transmit and receive beams for every scan line, and each scan line
represents the axis of the transmission-receive beam. The larger the arrays on a phased array
transducer, the lower the resonation, and the lower the transmitting frequency.
Beam steering
These transducers allow the beam to be moved electronically with the benefit of being able to change
the shape and size of the beam to suit imaging needs. The beam former is the part of the electronics of
the scanner that determines the shape size and position of the interrogating beams by controlling the
delay period of signals to and from the transducer array elements. During transmission it generates the
electronic signals that pulse each array and during reception combines the individual echo sequences
from all the elements into a single echo sequence.
The beam is angled and then swept as a sector electronically by firing all the arrays as a complete
group, with a small delay (< 1 microsecond) called “Phasing”, the time delay is also changed with each
successive transmitted echo signal, so that the wavefronts are inclined, and beam direction is
continuously changed and perpendicular to transducer face (figure 6a). However angling the wave-front
gives rise to side lobe artefacts affecting near field resolution (figure 6b).

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Figure 6a.
Electronic Steering of a Phased Array Sector Transducer
Sector scan
format of a
phased array
probe
Each waveform merges to form a compound wave,
generating a sector beam.
Constructive
interference
from wavelets
generating
sector beam
Huygen’s principle
This states that every point on a wavefront can be considered as a new point source emitting a
spherical wave of the same frequency and phase.
The width of the source producing the sound wave from a phased array probe is greater than the
wavelength of the wave, producing a wave that propagates at 90 0 to the source ie in form a beam. Each
small source generates a sound wave with same frequency and amplitude, and are in phase with each
other The spherical waves from each source propagate outwards with parts of the wave parallel to the
surface of the source. Some interfere destructively and cancel out, while others interfere constructively
and align to form a plane wavefront generating the ultrasound beam.
Figure 6b.
Typical Ultrasound Beam From a Phased Arrray Transducer
Note!! side lobe artefacts.
These grow stronger at
larger steering angles

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Beam focusing
The ultrasound beam is focussed during transmission and reception. Like beam steering, transmit
focussing is achieved by using an electronic delay in firing the arrays resulting in a wavefront curvature
that directs the beam to a focal zone, which is the zone of highest beam intensity.
Figure 6c.
FOCAL
ZONE
The same electronic time delay in receiving early echoes is then used during reception allowing echoes
from different depths to arrive at the same time. This is achieved by adjusting the delay periods during
the receive phase allowing late arriving echoes also to be in focus. This is called Dynamic Receive
Focussing.This reduces beam width and improves lateral resolution, but has no effect on focussing in
the elevation plane. This is achieved by using a curved cylindrical lens producing higher resolution.
Multi-zone Focusing is achieved by dividing each individual scan line into two or more sections, and
interrogating each section with a separate transmission pulse producing transmission foci at different
depths. This allows the operator to select two or more or multiple focal zones. This improves lateral
resolution, however temporal resolution and frame rate is affected due to the need to remain longer on
each scan line while interrogating several zones. Parameters such as centre frequency and pulse length
are not affected, but high frequencies within the pulse spectrum will be affected by attenuation because
several focal zones are selected at greater depths.
Phased array transducers have been established for many years and used in all modern echo
transducers. An effective phased array transducer will need to have the right balance between number
of elements and crystal size. More advanced phased array transducers for three-dimensional imaging
have now up to 3000 much smaller elements compared to 128 elements of standard two-dimensional
phased arrays.

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9. Transducer design and development
Figure 7.
LARGE
PROBE
2D Phased Array
Up to 128 elements
Field of view
(Widely used for 2D echo)
A1.5D Array enables
dynamic
focusing in the
elevation plane with better
resolution & dynamic range
(Potentially useful for 3D
volume reconstruction
imaging)
3D Matrix Array
3000 Elements
Real-time
(Live 3D echo)
10. The Three Dimensional Matrix Array
This transducer represents a breakthrough in the design of ultrasound transducers. Reduction in PZT
crystal size has made it possible for up to 3000 arrays to form a three dimensional (3D) image, that is
also ideal for two-dimensional bi-plane imaging. Using a pyramidal burst of ultrasound (Raster scanning),
a real-time dynamic 3D image is formed. The main limitation is due to variable distances between
individual scan planes with less structural information available, and less resolution. The transducer is
bulky, with images limited to a pyramid or cone shape, and acquisition restricted by small acoustic
windows and chest wall movement. However improvements in digital signal processing, and high speed
acquisition with rapid data processing, resolution and image quality has been improved. Improvements in
beam forming techniques have made it possible for electronic steering and beam focusing in both the
lateral and elevation planes, and 3D volume quantification. Beam focusing results in a narrow slice
thickness, with superior resolution.
Benjamin Adeyemi

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