- A transducer is a device that converts one form of energy to another. Piezoelectric ultrasound transducers convert electrical energy to ultrasonic energy and vice versa. Common transducer materials include lead zirconate titanate and polyvinylidene difluoride. - The thickness of the transducer crystal determines its resonant frequency. Thinner crystals have higher frequencies while thicker crystals have lower frequencies. Transducers can emit ultrasound at frequencies other than the resonant frequency but at lower intensities. - Transducers can be operated in pulsed or continuous wave mode. Pulsed mode is used for medical ultrasound to produce images. Phased array transducers use variable time delays between transdu

1. US TRANSDUCERS (2)
Dr. Kamal Sayed MSc US UAA
Function/frequency/bandwidth/focusing/arrays/
soundbeams/Output/ OK

2. •
A. Function
•
–A transducer is a device that can convert one form of energy
into another.
•
–Piezoelectric USs convert electrical energy into ultrasonic
energy, and vice versa.
•
Piezoelectric means pressure electricity
.
•
–The piezoelectric effect of a transducer is destroyed if heated
above its Curie temperature.

3. •
US TXR materials include lead-zirconate-titanate (PZT) ,
plastic polyvinylidene difluoride (PVDF), and the
new monocrystalline transducers.
US TXR materials include lead-zirconate-titanate (PZT) ,
plastic polyvinylidene difluoride (PVDF), and the
new monocrystalline transducers
PZT = P ; for Lead {Lead is a chemical element with the symbol
[Pb] (from the Latin plumbum) and atomic number 82.)
Z ; for Zirconate
T ; for titanate

4. High-frequency voltage oscillations are produced by the scanner’s
•
front end and sent to the US TXRr over coaxial cables.
•
–Transducer crystals do not conduct electricity, but each side is
coated with a thin layer of silver that acts as an electrode.
•
–The electrical energy causes the crystal to momentarily change
shape (i.e., expand and contract).
•
–The nonconducting crystal changes shape in response to
a voltage placed on its electrodes.
•
–This change in shape of the crystal increases and decreases the
pressure in front of the transducer, thus producing ultrasound
waves (transmitter).
•

5. •
–When the crystal is subjected to pressure changes by the
returning ultrasound echoes, the pressure changes are
converted back into electrical energy signals.
•
–Voltage signals from returning echoes are transferred from
the receiver to a computer, which are then used to create
ultrasound images.
•
–Transducers may be operated in either pulsed or continuous-
wave mode.
•
–Virtually all of medical ultrasound makes use of pulsed
transducers.

6. •
B. Frequency
•
–The thickness of a PZT determines the
resonant frequency of the transducer.
•
–Transducer crystals are normally manufactured so that
their thickness (t) is equal to one-half of the wavelength (λ)
(i.e., t = λ/2).
•
–Changing the thickness of the crystal changes the frequency
but not the ultrasound amplitude or velocity.
•
The matching layer thickness is one-fourth the wavelength
of sound in that material and is referred to as quarter-wave
matching.

7. •
–A thickness to 1 mm and a velocity of sound of 4,000 m/s
has a resonant frequency f = v/λ = v/(2 × t), or 2 MHz.
•
–High-frequency transducers are thin, and low-
frequency transducers are thick.
•
–Transducers also emit ultrasound energy at frequencies
other than the resonant frequency but at a lower intensity.
•
–Clinical scanners can drive their transducers at several
different transmit frequencies.

8. •
Bandwidth and Quality Factor (Q-F)
•
Bandwidth :
•
It is uncommon for a TXR to emit a sound beam with
•
only a single pure frequency. Rather, the pulse is more
like a sound ‘click’ and contains a range of frequencies
•
below and above the main frequency.
•
The bandwidth is the range of frequencies between the
•
highest and the lowest frequency emitted from the TXR
•
See slide (9)

10. •
–The bandwidth is related to the range of frequencies
generated by the crystal.
•
–The bandwidth determines @ the purity of sound and
@the length of time a sound persists, or (ring down time).
•
Narrow bandwidth TXRs produce a relatively pure frequency.
•
Pure sounds (narrow bandwidth) will persist for a long time.
•
Wide bandwidth transducers produce a wider
range of frequencies.
•
–Sounds with a broad range of frequencies last for only a very
short time.

11. •
Quality Factor : A unitless number representing the degree of
damping.
•
Imaging transducers are low-Q transducers when
•
compared to therapeutic transducers because imaging
•
transducers use backing material.
•
The Q-factor of typical imaging transducers can be
•
approximated by the number of cycles in the pulse
•
produced by the transducer (approximately 2 - 4).

12. •
Equation :
•
Quality factor = resonant frequency (MHz)/ bandwith (MHz)
•
When Q-factor is low (imaging probe):
•
1. damping is effective
•
2. pulse length & duration are short
•
3. bandwidth is wide
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4. axial resolution is improved
•
•
•

13. •
Transducer (TXR) Frequencies
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A) Continuos wave TXRs : Sound wave's frequency equals the
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The frequency of the voltage applied to the PZT by the
•
machine's electronics.
•
slide (20)
•
B) Pulsed transducers : The pulse repetition frequency (PRF)
is determined by the
•
number of electrical pulses the US machine delivers to the
•
active element.
•
see slides (7/8)
•

16. •
The thickness of the PZT crystal equals ½ of
•
The wavelength of sound in the crystal.
•
The thickness of the matching layer is ¼ of the
•
wavelength of sound in the matching layer.
•
The frequencies used in therapy are typically
between 1.0 and 3.0 MHz (1 MHz = 1 million cycles
per second).

17. •
Sound Beams
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Beam width
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RULE: Narrow beams create better images
•
As sound travels, the width of the beam changes :
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» starts out at exactly the same size as the transducer
•
diameter, gets progressively narrower until it reaches its
smallest diameter (focal point), then it diverges.
•
Focus or Focal Point is the location where the beam reaches
its minimum diameter.
•

18. •
Focusing
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Results in: 1. a narrower “waist” in the US beam.
•
2. a decrease in focal depth (the focus is shallower).
•
3. a reduction in the size of the focal zone.
•
Effective mainly in the near field and the focal zone.
•
Electronic Focusing Phased array technology provides
dynamic, variable (adjustable)focusing or multifocusing
•
slide (19)

20. •
Focal Depth : is the distance from the transducer face to the
•
focus. Also called focal length or near zone length.
•
Near Zone (Fresnel Zone) is the region or zone in between
the
•
transducer and the focus.
•
Sound beams converge in the near zone
•
Far Zone (Fraunhofer Zone) is the region or zone deeper than
the focus, beyond the near field.
•
Sound beams diverge in the far zone.

21. •
The near field of the ultrasound beam is adjacent to the
transducer and is the region used for ultrasound imaging.
•
–The near field is also called the Fresnel zone.
•
–The length of the near field is r2/λ, where r is the transducer
radius and λ is the wavelength.
•
–For a 10-mm-diameter transducer operating at 3.5 MHz, the
near field extends ~6 cm in soft tissue.
•
–Doubling the transducer size increases the near field length
fourfold.
•
–Doubling the transducer frequency halves the wavelength,
which doubles the extent of the near field.

22. •
Focal Zone is the region surrounding the focus where the
beam is “sort of narrow” & the picure is relatively good
•
For an unfocused continuous wave disc transducer:
•
At the end of the near zone, the beam diameter is ½ the
•
transducer diameter.
•
At two near zone lengths, the beam diameter is equal to
•
the transducer diameter.
•

24. •
Focal Depth
•
Definition : is the distance from transducer to the focal point.
•
It is determined by two factors:
•
1. TXR diameter and 2. frequency of the ultrasound.
•
Shallow focus beams have small diameter & low frequency
•
Deep focus beams have large diameter & high frequency
•
Equation : focal length (focal depth) in cm =
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TXR diameter (squared) X frequency/6

25. •
Sound Beam Divergence
•
Describes the spread of the sound beam in the deep far zone.
•
It is determined by 2 factors :
•
1) TXR diameter and 2) ultrasound frequency
•
@ larger diameter crystals produce higher frequency sound &
produce beams that diverge less in the far field (narrow
beam) in addition to improved lateral resolution in far field.
•
@ Smaller diameter crystals produce lower frequency sound
& wider beam in the far field in addition to degraded lateral
resolution in far field. Slide (23)

27. •
Mechanical Scanning
•
Crystals : Scanhead contains one active element.
•
Steering : The active element is moved by a motor, oscillating
•
crystal or mirror through a pathway, automatically creating a
•
scan plane.
•
Focusing (Conventional or Fixed): curvature of the PZT or an
•
acoustic lens focuses the beam at a specific depth

28. •
Transducer Arrays
•
Array is a collection of active elements in a single transducer.
•
Element is a single slab of PZT cut into a collection of
separate
pieces called elements
•
Channel is when the electronic circuitry is connected to each
element.

29. •
Phased Arrays
phased meaning Adjustable focus or multi-focus; achieved
electronically.
•
Crystals, Steering & Focusing : is a collection of electric
pulses, separated by miniscule (extremely small) time delays,
(10 ns) is delivered to all of the transducer’s elements in
various patterns. The patterns focus & steer the US beam
•
during transmission.
•
Thus, focusing and steering are
electronic.
•
•
•

30. •
Transducers can produce an ultrasound beam in two way:
•
1- linear array (also called sequential array)
•
2- phased array
•
As a general rule, if the shape at the top of the images
matches the shape at the bottom of the image it is a
sequential array (linear).
•
If the shapes are different (e.g. rectangular at the top and
curved at the bottom) it is a phased array.
•
Slides (31/32/33)
•

34. The convex, linear and sector transducers with different sound beams.
.

35. •
Convex probes (also called curved linear probes) have a
curved array that allows for a wider field of
view. Convex probes are primarily used for abdominal scans
due to their wider & deeper view.
•
A curvilinear probe uses lower frequency ultrasound allowing
a deep penetration and a wide depth of field, which is
excellent for viewing intra-abdominal structures.

36. •
How to hold the transducer
•
Hold the transducer like a pencil, not a flashlight. This gives
your wrist the greatest range of motion for moving in different
planes.
•
Hold the transducer close to the contact surface with the
patient. ...
•
Let the fourth and fifth fingertips of your scanning hand
contact the skin

37. •
Phased Arrays are arrays of US TXRs that fire individual
elements on the array in a specific sequence in order to direct
the sound wave in a specific direction. ... A phased array US
TXR typically will have a smaller footprint than a linear or
curved array, but can still image a large area.
•
With these transducers, a trapezoidal image
shape is produced.

38. •
A phased array US TXR is typically 2-3 cm long, consisting of
64-128 elements. It is a smaller assembly than a
sequential array and can be either linear or curvilinear. ...
Small delays in element firing allow for electronic field
steering and focusing without moving the ultrasound probe.
•
phased array usually means an electronically scanned array, a
computer-controlled array

39. •
Transducer Output
•
Synonyms : output gain, acoustic power, pulser power,
transmitter output, energy output.
•
Is determined by the excitation voltage from the pulser. And is
adjusted by sonographer.
•
Piezoelectric crystal vibrates with a magnitude related to
•
pulser voltage.
•
Effect upon image : slide (32)
•
1- When transducer output changes, every pulse transmitted
to the body changes.
•

40. Low Output (LEFT image) High Output (RT image)
Low Output High Output

41. •
2- All reflections from structures in the body also change.
•
3- The brightness of the entire image changes.
•
4- Increasing transducer output improves signal-to-noise
ratio.
•
5- increasing TXR output affects patient exposure
•
6- Excessive output degrades axial resolution.

42. •
Diagnostic ultrasound transducers often have
better axial resolution than lateral resolution,
•
although the two may be comparable in the focal region of
strongly focused. 11.
•
At this depth, the effective beam diameter is approximately
equal to half the transducer diameter.