UNIT 5 ULTRASOUND TRANSDUCERS

Question 1

What is a Transducer?

Answer 1

Answer: The transducer is a device that converts one form of energy into another form of energy such as a light bulb, loudspeaker, and electric motor. In diagnostic ultrasound, the ultrasound transducer converts electrical energy into pressure energy and pressure energy into electrical energy.

Question 3

What is a piezoelectric effect?

Answer 3

Answer: Piezoelectric Effect is the property of certain materials to create a voltage when they are mechanically deformed. The piezoelectric effect is the process by which pressure energy is converted into electrical energy•
Piezoelectric crystals are used in ultrasound transducers. During reception phase, the returning ultrasound echoes strike the piezoelectric crystals in the ultrasound transducer. The piezoelectric crystals vibrate and create electrical signals. The ultrasound machine then processes these electrical signals and converts them into an ultrasound image.

Question 4

What is the reverse piezoelectric effect?

Answer 4

Answer: When an electric voltage is applied to a piezoelectric material, it deforms or changes shape. This is called the reverse piezoelectric effect.
During transmission phase, ultrasound system produces electrical signals which excite the piezoelectric crystals in the transducer and produce ultrasound waves.

Question 5

Name the different piezoelectric materials which are natural and possess piezoelectric properties?

Answer 5

Answer: The piezoelectric materials which are natural and possess the piezoelectric properties are, Quartz, Tourmaline, and Rochelle salt.

Question 6

Name the different piezoelectric materials which are synthetic and possess piezoelectric properties?

Answer 6

Answer: The piezoelectric materials which are synthetic, and possess piezoelectric properties are Lead zirconate titanate (PZT), barium titanate, lithium sulfate, lead metaniobate, and ammonium dihydrogen phosphate.

Question 7

What are the other names used for PZT?

Answer 7

Answer: The other names used for PZT are crystal, ceramic, active element.

Question 8

Which ceramic material with piezoelectric properties is most commonly used in ultrasound transducers?

Answer 8

Answer: The lead zirconate titanate (PZT) is a synthetic ceramic material with piezoelectric properties. It is most used in transducers because of its greater efficiency and sensitivity.

Question 9

What is the trade name for lead zirconate titanate?

Answer 9

Answer: The trade name for lead zirconate titanate is PZT-5.

Question 10

What are the advantages that lead zirconate titanate has over other ceramic materials?

Answer 10

Answer: The advantages that lead zirconate titanate has over other ceramic materials are that lead zirconate titanate is easy to shape, effective at low-voltage, and is inexpensive.

Question 11

The propagation speed of sound in PZT crystal is about 3-5 times greater than in soft tissue?

Answer 11

Answer: True
It is true that the propagation speed of sound in PZT crystal is about 3-5 times greater than in soft tissue.

Question 12

What is an active element?

Answer 12

Answer: The active element is also known as piezoelectric element, piezoelectric crystal or PZT. The function of the active element is to produce ultrasound pulses during the transmission phase and receive the returning echoes from the body and convert them into electric signals during the reception phase.
The active element produces ultrasound pulses when electric voltage is applied. These ultrasound pulses are transmitted into the body. When ultrasound pulses strike a structure in the body, they reflect back and return to the transducer. When returning echoes strike the active element, it changes its shape and produces electrical signals. These electrical signals are processed by the ultrasound machine and converted into ultrasound images.
The most common piezoelectric material used as an active element is known as lead zirconate titanate (PZT-5).

Question 13

What is Polarization?

Answer 13

Answer: Polarization is the process in which material is exposed to strong electrical fields while being heated to a substantial temperature to create PZT.

Question 14

What is the temperature at which PZT is polarized called?

Answer 14

Answer: The temperature at which PZT is polarized is called Curie temperature or Curie point.

Question 15

What is Depolarization?

Answer 15

Answer: The process of destroying piezoelectric properties of PZT material is called Depolarization.

Question 16

What occurs when the temperature of a PZT crystal is elevated above the Curie point?

Answer 16

Answer: Depolarization
When a piezoelectric material is heated above the Curie point, it loses its piezoelectric properties and becomes depolarized.

Question 17

What is Curie temperature?

Answer 17

Answer: The Curie temperature (or Curie point) is the temperature at which a piezoelectric material loses its piezoelectric properties. This occurs because the material’s internal dipoles, which are responsible for generating an electric charge when mechanical stress is applied, become disordered at high temperatures.
The Curie temperature is typically around 300°C to 400°C (572°F to
752°F), depending on the specific composition.
In summary, when a piezoelectric crystal is exposed to high temperatures, it will become depolarized and lose its piezoelectric properties permanently. This temperature is called Curie temperature.

Question 18

What is an acoustic insulator?

Answer 18

Answer: Acoustic insulator is a thin barrier of cork or rubber that prevents vibrations in the case from inducing an electrical voltage in the piezoelectric crystal in the ultrasound transducer.
An acoustic insulator is a material or structure that prevents or minimizes the transmission of sound waves from one area to another. Its primary function is to block or absorb sound, thus reducing noise or preventing unwanted acoustic energy from spreading.
An acoustic insulator plays a crucial role in controlling sound transmission, particularly in medical ultrasound devices, by preventing the unintended spread of sound waves, thereby improving efficiency and clarity.

Question 19

What is an electrical shield?

Answer 19

Answer: An electrical shield is a thin, metallic barrier lining the inside of the transducer case which prevents electrical interference from contaminating the signals used to create ultrasound images.

Question 20

What determines the frequency of sound emitted from a pulse
wave transducer?

Answer 20

Answer: The frequency of ultrasound wave emitted from a pulse wave transducer is determined by the thickness of the piezoelectric crystal and the propagation speed of the crystal.

Question 21

What is the relationship between the thickness of the piezoelectric crystal and the frequency of ultrasound wave produced?

Answer 21

Answer: In a pulsed wave transducer, there is an inverse relationship between the PZT thickness and frequency of ultrasound wave produced.
The thicker the PZT crystal, lower the frequency of the ultrasound wave produced. The thinner the PZT crystal, higher the frequency of the ultrasound wave produced.

Question 22

Q222. If the thickness of a crystal doubles, operating frequency for pulse wave mode:
a) doubles
b) quarters
c) halves
d) quadruples

Answer 22

Answer: c. halves

f= C/ 2 x thickness

Operating frequency of a transducer is inversely proportional to the thickness of the piezoelectric crystal.

Question 23

Which piezoelectric crystal will produce sound waves with higher frequency?
a) 5 mm thick, 3 cm diameter, 5.0 mm/us propagation speed
b) 3 mm thick, 5 cm diameter, 3.0 mm/us propagation speed
c) 2 mm thick, 5 cm diameter, 5.0 mm/us propagation speed

Answer 23

Answer: C 2 mm thick, 5 cm diameter, 5.0 mm/us propagation speed
The primary frequency in a pulsed wave transducer is determined by the thickness and propagation speed of the crystal. Thin piezoelectric crystals with faster propagation speed produce higher frequency sound waves.
The diameter of the PZT crystal does not affect the frequency of the sound wave produced.

Question 24

Which of the following crystals will produce sound waves with the lowest frequency?
a) thin and with a low speed
b) thin and with a high speed
c) thick and with a high speed
d) thick and with a low speed

Answer 24

Answer: d. thick and with a low speed
The crystal that will produce sound waves with the lowest frequency is the one with the largest thickness.
In ultrasound transducers, the frequency of the sound waves is inversely related to the thickness of the piezoelectric crystal.
The one with the greatest thickness will generate sound waves with the lowest frequency. This is because the sound wave has a longer wavelength in thicker crystals, which corresponds to a lower frequency.

Question 25

How many piezoelectric crystals are used in continuous wave Doppler?

Answer 25

Answer: Two piezoelectric crystals are used in continuous wave Doppler, one for transmitting the ultrasound waves and the other for receiving the returning echoes.

Question 26

Continuous wave transducers are commonly used in which applications?

Answer 26

Answer: Continuous wave transducers are commonly used in surgical, therapeutic, and Doppler applications.

Question 27

How is the frequency of sound determined in continuous wave transducer?

Answer 27

Answer: The frequency of sound in continuous wave transducer is determined by the frequency of the electrical signal that excites the piezoelectric crystal. The frequency of ultrasound wave is equal to the frequency of the electrical signal applied. If the frequency of electrical signal applied is 5 MHz, then the frequency of ultrasound wave produced will be 5 MHZ

Question 28

Two PZT crystals are made from the same material. The thicker crystal will make a continuous wave transducer with a lower frequency.

Answer 28

Answer: False The thickness and propagation speed of the piezoelectric crystal do not determine the frequency of ultrasound wave in continuous wave transducer. The frequency of the continuous wave transducer will be the same as the electrical frequency that excites the PZT in continuous wave transducer.

Question 29

If the frequency of the electrical excitation voltage of a pulsed wave transducer is 2.5 MHz, then the operating frequency of the transducer will be 2.5 MHz?

Answer 29

Answer: False In pulse mode, the operating frequency of a transducer is determined by the thickness of the piezoelectric crystal and the speed of sound within the crystal material rather than the frequency of the electrical excitation voltage. In pulse mode, the transducer's operating frequency depends on the physical properties (thickness and sound speed) of the piezoelectric crystal, not the frequency of the applied electrical signal.

Question 30

In pulsed wave transducer the diameter of the active element of a transducer helps to determine the frequency of the sound produced by the transducer.

Answer 30

Answer: False It is the thickness of the active element that helps to determine the frequency of the sound produced by the transducer, not the diameter of the active element.

Question 31

Two PZT crystals are made from the same material. The thicker crystal will make a pulsed wave transducer with a higher frequency.

Answer 31

Answer: False The thickness of the PZT crystal, and the frequency of the sound produced are inversely proportional. The pulsed wave transducer with thicker crystal will produce low frequency ultrasound waves.

Question 32

In a pulsed wave transducer, what is the relationship between the PZT speed of sound and frequency?

Answer 32

Answer: Directly related There is a direct relationship between the PZT speed of sound and frequency. Faster speed PZT crystals produce higher frequency sound waves and slower speed PZT crystals produce lower frequency sound waves.

Question 33

In a pulsed wave transducer, what is the relationship between the PZT thickness and frequency?

Answer 33

Answer: Inversely related There is an inverse relationship between the PZT crystal thickness, and the frequency of sound waves produced. The thicker PZT crystals produce lower frequency sound waves and the thinner PZT crystals produce higher frequency sound waves.

Question 34

What is the PZT thickness and speed of sound in high frequency pulsed wave ultrasound transducers?

Answer 34

Answer: Thinner PZT and higher sound speeds In high frequency pulsed wave transducers, thinner PZT crystals and high sound speed PZT crystals are used.

Question 35

What is the PZT thickness and speed of sound in low frequency pulsed wave ultrasound transducers?

Answer 35

Answer: Thicker PZT and lower sound speeds In low frequency pulsed wave transducers, thicker PZT crystals and low sound speed PZT crystals are used.

Question 36

What is a matching layer?

Answer 36

Answer: Matching Layer is a material placed in front of the active element to decrease the reflection of the sound waves at the transducer-tissue interface and increase the transmission of sound waves between the piezoelectric element and the body. Matching Layer also protects the active element.

Question 37

What is the function of the matching layer?

Answer 37

Answer: The function of matching layer is to decrease the impedance difference between the transducer face and the skin and help to increase the transmission of ultrasound waves in the body. The impedance difference between the transducer crystal and the soft tissue is so large that most of the ultrasound pulses will be reflected back at the skin surface. The matching layer provides intermediate impedance, which allows transmission of ultrasound waves into the body.

Question 38

The impedance of a transducer active element is 1,850,000 Rayls, and the impedance of the skin is 1,350,000 Rayls. What is the acceptable impedance for the matching layer? a) 1,250,000 Rayls b) 1,300,000 Rayls c) 1,900,000 Rayls d) 1,626,000 Rayls

Answer 38

Answer: d. 1,626,000 Rayls The impedance of the matching layer in a transducer is typically chosen to minimize the reflection of sound waves between the active element (piezoelectric crystal) and the skin. For optimal transmission, the impedance of the matching layer should be intermediate between the impedance of the active element and the impedance of the skin, and ideally follow the geometric mean of the two impedances.

Question 39

What is the impedance difference between the PZT crystal and the skin?

Answer 39

Answer: The impedance of PZT crystal is 20 times greater than the impedance of the skin.

Question 40

What additional component is used by the sonographers to improve the transmission of ultrasound waves into the body?

Answer 40

Answer: Ultrasound Gel Ultrasound gel is used by sonographers to improve the transmission of ultrasound waves into the body. The impedance of ultrasound gel is in between the impedance of the matching layer and the skin.

Question 41

What is damping material and what is its function?

Answer 41

Answer: The damping material is made of tungsten fiber embedded in an epoxy mixture. The damping material is attached to the back of piezoelectric crystal. The damping material decreases the ringing of the piezoelectric crystal which helps to produce ultrasound pulses of short duration and smaller spatial pulse length. The short pulses create images of better quality. The damping material shortens the ultrasound pulses but increases the bandwidth of the pulse and decreases the Q factor.

Question 42

What is the function of the backing material?

Answer 42

Answer: When a PZT crystal is excited it continues to ring resulting in long pulses. The longer pulses degrade the quality of image therefore longer pulses are not needed in the diagnostic ultrasound imaging. The function of backing material or damping material is to absorb the vibrations of the PZT crystals and reduces the number of cycles in a pulse. The backing material shortens the ultrasound pulses produced by the active element. Short pulses improve the quality of the images produced by the ultrasound machine.

Question 43

In an imaging transducer, what is the purpose of attaching the backing material to the PZT crystal? a) increase the bandwidth b) improve the image quality c) decrease the Q factor d) decrease the sensitivity

Answer 43

Answer: b. improves the image quality The backing material shortens the ultrasound pulses produced by the active element. Short pulses improve the quality of the images produced by the ultrasound machine.

Question 44

What are the consequences related to the use of a backing material in a transducer?

Answer 44

Answer: The use of backing material in imaging transducers results in decreased sensitivity of the transducer, wide bandwidth, and low-quality factor.

Question 45

What is the effect of decreased sensitivity due to backing material on a transducer function?

Answer 45

Answer: The disadvantage of decreased sensitivity due to backing material is that the transducer becomes less responsive to incoming low amplitude sound waves.

Question 46

What kind of backing material is used in continuous wave transducers?

Answer 46

Answer: Continuous wave transducers have no backing material. They are usually air backed.

Question 47

What is the effect of backing material on duty factor?

Answer 47

Answer: When a piezoelectric crystal does not have a backing material, it vibrates for a long time. The backing material inhibits the ringing of the piezoelectric crystal and shortens the pulse duration. Shorter pulse duration decreases the time that the system is on and therefore decreases the duty factor.

Question 48

Q248. What is the effect of backing material on quality factor?

Answer 48

Answer: The backing material decreases the quality factor or Q factor of a transducer. The backing material reduces the quality factor (Q factor) of a transducer by damping the crystal's vibrations. This results in a wider bandwidth and shorter pulse, improving image resolution but reducing the ability to sustain oscillations at a single frequency.

Question 49

What is the effect of backing material on spatial pulse length?

Answer 49

Answer: The backing material shortens spatial pulse length and pulse duration, which improves the axial resolution. When an electric current is applied to the piezoelectric crystal, it starts vibrating. The backing material or damping material is applied to the back of the piezoelectric crystal. It tends to decrease the ringing of the piezoelectric crystal and shortens the spatial pulse length. Without backing material, the crystal will vibrate for a long time and produce a longer pulse.

Question 50

What is the effect of damping material on pulse repetition period?

Answer 50

Answer: The pulse repetition period is not affected by the application of damping material. The pulse repetition period is the time from the beginning of a pulse to the beginning of the next pulse. The pulse repetition period changes when the imaging depth is changed. The damping material has no effect on pulse repetition period.

Question 51

What is the Bandwidth of a pulse?

Answer 51

Answer: The bandwidth of a pulse is the range of frequencies that are present within the pulse. The bandwidth of a pulse is calculated by subtracting the lowest frequency within a pulse from the highest frequency within a pulse.

Question 52

The main frequency of a transducer is 3.5 MHz. The transducer produces pulses with frequencies ranging from 2 MHz to 6 MHz. What is the bandwidth of the transducer?

Answer 52

Answer: The bandwidth of the transducer is 4 MHz. The bandwidth of a pulse is calculated by subtracting the lowest frequency within a pulse from the highest frequency within a pulse. bandwidth = highest frequency - lowest frequency bandwidth = 6 MHz - 2 MHz bandwidth = 4 MHz

Question 53

What is the relationship between the bandwidth and the quality factor?

Answer 53

Answer: There is an inverse relationship between the bandwidth and the quality factor. As the bandwidth increases, the quality factor decreases. quality factor = primary frequency /bandwidth When the bandwidth is wide, the Q factor is low as in pulse wave transducers. When the bandwidth is narrow, the Q factor is high as in continuous wave transducers.

Question 54

A transducer with a wide bandwidth is likely to have good axial resolution?

Answer 54

Answer: True A transducer with a wide bandwidth is likely to have good axial resolution. A wide bandwidth is equivalent to a short spatial pulse length and short pulse length improves the axial resolution.

Question 55

What is the relationship between pulse duration and bandwidth?

Answer 55

Answer: Inversely Related There is an inverse relationship between pulse duration and bandwidth. The transducers with shorter duration ultrasound pulses have wider bandwidth and transducers with longer duration ultrasound pulses have narrower bandwidth.

Question 56

What is Quality factor or Q factor?

Answer 56

Answer: Quality Factor also called Q Factor represents the degree of damping or shortening of ultrasound pulse in an ultrasound transducer. It is calculated by dividing the primary frequency of a transducer by its bandwidth. quality factor = primary frequency/bandwidth The quality factor is a unitless number.

Question 57

The main frequency of a transducer is 6 MHz The transducer produces pulses with frequencies ranging from 2 MHz to 5 MHz. What is the Q factor of the transducer?

Answer 57

Answer: The Q factor or quality factor of the transducer is 2. The quality factor of a transducer is determined by dividing the main frequency by its bandwidth.

Question 58

What is the effect on image when Q factor is low?

Answer 58

Answer: When Q factor is low, damping is substantial, pulse length and pulse duration is short, bandwidth is wide, and image is good.

Question 59

What is the relationship between pulse length and the Q-factor?

Answer 59

Answer: Directly Related. There is a direct relationship between the pulse length and the Q factor. A shorter pulse has a lower Q factor, and a longer pulse has a higher Q factor.

Question 60

Which type of transducer has a greater Q factor: therapeutic or imaging?

Answer 60

Answer: Therapeutic The transducers used for therapeutic ultrasound have greater Q factor compared to imaging transducers.

Question 61

Which type of transducer has a greater bandwidth: continuous wave or imaging?

Answer 61

Answer: Imaging The imaging transducer has wider bandwidth compared to the continuous wave transducer.

Question 62

Continuous wave Doppler uses how many piezoelectric crystals?

Answer 62

Answer: Continuous wave Doppler uses two piezoelectric crystals, one for transmitting and the other for receiving the returning echoes.

Question 63

Transducers with continuous wave operation are commonly used in which applications?

Answer 63

Answer: Transducers with continuous wave operation are commonly used in surgical, therapeutic, and Doppler applications.

Question 64

What kind of backing material is used in continuous wave transducers?

Answer 64

Answer: Continuous wave Doppler transducers have no backing materials. They are usually air backed.

Question 65

Which transducer would be best to image superficial structures?

Answer 65

Answer: Small diameter and high frequency transducer will be best to image superficial structures.

Question 66

Which transducer would be most useful for imaging superficial structures?

Answer 66

Answer: High frequency and short focus will be most useful for imaging superficial structures.

Question 67

Which transducer will be most useful for good penetration on an obese patient?

Answer 67

Answer: Low frequency and long focus transducer will be most useful for good penetration on an obese patient.

Question 68

What techniques can be used to improve performance of a multi hertz transducer?

Answer 68

Answer: Harmonic imaging, dynamic frequency tuning, frequency fusion or frequency compounding are used to improve the performance of a multi hertz transducer.

Question 69

What are the charactenses of imaging transducers?

Answer 69

Answer: The characteristics of the imaging transducers are: • produce ultrasound pulses with short duration and short pulse length • use backing material to limit ringing • have reduced sensitivity to the low amplitude returning echoes • wide bandwidth • lower Q-factor • better axial resolution

Question 70

All of the following correctly describe an imaging transducer except: a) low Q factor b) wide bandwidth c) damped d) high sensitivity e) short pulses

Answer 70

Answer: d. high sensitivity Imaging transducers have low sensitivity to low amplitude returning echoes.

Question 71

What are the characteristics of a non-imaging transducers?

Answer 71

Answer: The characteristics of the non-imaging transducers are: • produce continuous sound waves or pulses with long duration and long spatial pulse length • use no backing material • have increased sensitivity to low amplitude returning signals • narrow bandwidth • higher Q-factor • does not produce an image

Question 72

All of the following correctly describe a non imaging transducer except:

Answer 72

a) low sensitivity b) high Q factor c) narrow bandwidth d) no damping e) longer pulses Answer: a) low sensitivity Non imaging transducers have high sensitivity to low amplitude returning echoes.