ultrasound physics

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

2. •
A. Function
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–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
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–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)
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Bandwidth :
•
It is uncommon for a TXR to emit a sound beam with
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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
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highest and the lowest frequency emitted from the TXR
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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
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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 :
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Quality factor = resonant frequency (MHz)/ bandwith (MHz)
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When Q-factor is low (imaging probe):
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1. damping is effective
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2. pulse length & duration are short
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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)
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B) Pulsed transducers : The pulse repetition frequency (PRF)
is determined by the
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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
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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
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As sound travels, the width of the beam changes :
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» starts out at exactly the same size as the transducer
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diameter, gets progressively narrower until it reaches its
smallest diameter (focal point), then it diverges.
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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.
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2. a decrease in focal depth (the focus is shallower).
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3. a reduction in the size of the focal zone.
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Effective mainly in the near field and the focal zone.
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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
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focus. Also called focal length or near zone length.
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Near Zone (Fresnel Zone) is the region or zone in between
the
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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:
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At the end of the near zone, the beam diameter is ½ the
•
transducer diameter.
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At two near zone lengths, the beam diameter is equal to
•
the transducer diameter.
•

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

25. •
Sound Beam Divergence
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Describes the spread of the sound beam in the deep far zone.
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It is determined by 2 factors :
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1) TXR diameter and 2) ultrasound frequency
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@ 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
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Crystals : Scanhead contains one active element.
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Steering : The active element is moved by a motor, oscillating
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crystal or mirror through a pathway, automatically creating a
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scan plane.
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Focusing (Conventional or Fixed): curvature of the PZT or an
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acoustic lens focuses the beam at a specific depth

28. •
Transducer Arrays
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Array is a collection of active elements in a single transducer.
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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:
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1- linear array (also called sequential array)
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2- phased array
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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
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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
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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
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pulser voltage.
•
Effect upon image : slide (32)
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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.
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3- The brightness of the entire image changes.
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4- Increasing transducer output improves signal-to-noise
ratio.
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5- increasing TXR output affects patient exposure
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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.