2. SOUND BEAM Flashcards
What is a sound beam?
Answer: A sound beam is the acoustic energy emitted by the transducer.
The small wavelets within a beam collide and the constructive or destructive interference results in the formation of sound beam.
What is near zone?
Answer: Near zone is the region of sound beam in between the transducer and the focus or focal point. Near zone is also called near zone length and Fresnel zone.
What are the two factors that affect the near zone length and the focal depth?
Answer: The two factors that affect the near zone length or focal depth are:
- the diameter of the piezoelectric crystal
- the frequency of the ultrasound wave
The larger diameter crystals produce ultrasound beams with longer near zone length and deeper focus.
The smaller diameter crystals produce ultrasound beams with shorter near zone length and shallower focus.
The higher frequency ultrasound waves have longer near zone length and deeper focus.
The lower frequency ultrasound waves have shorter near zone length and shallower focus.
Two ultrasound transducers are used during an exam.
Transducer A has a frequency of 5 MHz and the diameter of the PZT crystal is 8 mm. Transducer B has the frequency of 2.5 MHz and the diameter of the PZT crystal is 4 mm. Which transducer will have longer near zone length and deeper focus?
Answer: Transducer A will have longer near zone length and deeper focus.
The larger diameter crystals produce ultrasound beams with longer near zone length and deeper focus.
The higher frequency ultrasound waves have longer near zone length and deeper focus.
Two ultrasound transducers are used during an exam.
Transducer A has the frequency of 7 MHz and the diameter of the PLI crystal is 10 mm. Transducer B has the frequency of 3 MHz and the diameter of the PZT crystal is 5 mm. Which transducer will have shorter near zone length and shallower focus?
Answer: Transducer B will have shorter near zone length and shallower focus.
The smaller diameter crystals produce ultrasound beams with shorter near zone length and shallower focus.
The lower frequency ultrasound waves have shorter near zone length and shallower focus.
What is Focus or focal Point?
Answer: The focus or focal point is the area where the ultrasound beam reaches its smallest diameter. The images obtained at the focal point are of better quality.
What are the methods used to change the focus?
Answer: The focus of an ultrasound beam can be changed using lenses, mirrors, curved elements, or by the use of electronic focusing.
What are the advantages of using lenses for changing focus?
Answer: The advantages of using lenses for changing the focus are:
- it produces a shallower focus
- sound can be converged more rapidly
What are the disadvantages of using lenses for changing focus?
Answer: The disadvantages of using lenses for changing the focus are:
- It decreases the efficiency of the transducer.
- It causes some surface heating on the transducer.
- It creates another impedance mismatch between the matching layer and the skin.
What are the advantages of using curved surface focusing?
Answer: The advantages of using curved surface focusing are: g it eliminates the absorption of heat energy from lens
- it eliminates the acoustic impedance mismatch
- newer materials are more flexible
What are the disadvantages of using curved surface focusing?
Answer: The disadvantages of using curved surface focusing are that piezoelectric crystals are very brittle and are difficult to construct.
What is the effect of focusing the ultrasound beam?
Answer: By focusing the ultrasound beam the area diminishes and the intensity and power of the beam increases at the focal point.
What is Fraunhofer zone?
Answer: Fraunhofer zone is also called far zone, it is there that starts at the focal point where the ultrasound beam has the narrowest diameter and extends beyond it.
- The sound beam expands to the size of the transducer diameter in the far zone.
If the diameter of an ultrasound beam produced by a piezoelectric crystal is 10 mm at a depth of twice the near zone length, then what is the diameter of piezoelectric crystal producing the ultrasound wave?
Answer: 10 mm
- The diameter of ultrasound beam is equal to the diameter of the piezoelectric crystal at a depth twice the near zone length.
What happens to the ultrasound beam when it travels deep in the far zone? What is the effect of large diameter crystals and small diameter crystals on the sound beam?
Answer: When ultrasound beam travels deep in the far zone, it tends to diverge or spread out.
- Large diameter crystals produce sound waves that diverge less in the far zone.
- Small diameter crystals produce sound waves that diverge more in the far zone.
What is the effect of frequency on the sound beam?
Answer: The higher frequency sound waves will create less divergence and lower frequency sound waves will create more divergence of the sound beam in the far zone.
Which transducer will produce the most divergent sound beam?
a) 5 cm and 5MHz
b) 2 cm and 2.5MHz
c) 8 cm and 8MHz
Answer: b. 2 cm and 2.5MHz
The smaller diameter crystals and lower frequency sound waves will create more divergence of the sound beam in the far zone.
Which transducer will produce the least divergent sound beam?
d) 5 cm and 5MHz
e) 2 cm and 2.5MHz
f) 8 cm and 8MHz
Answer: c. 8 cm and 8MHz
The larger diameter crystals and higher frequency sound waves will create less divergence of the sound beam in the far zone.
What is Diffraction?
Answer: Diffraction is the spread out of sound beam when the sound wave travels away from the transducer. Diffraction is more pronounced when the sound wave is produced by a small source. The diffraction pattern is similar to a wedge shape. The wedge shape is similar to a wave produced by a motorboat when travelling in water.
The wedge shape wavelet is also called a Huygens wavelet.
What is Huygens’ Principle?
Answer: Huygens states that an ultrasound wave produced by an ultrasound transducer is made up of thousands of tiny wavelets of sound, each of which is wedge shaped. As these tiny sound wavelets travel away from transducer, they interfere with each other and produce an hourglass shaped ultrasound beam.
What does Huygens’s principle explain?
Answer: Huygens principle explains why an ultrasound beam produced by an ultrasound transducer has the shape of an hourglass rather than a wedge shape.
What is the beam shape for continuous wave and pulse wave?
Answer: For continuous wave, the beam shape is consistent as long as the signal is on. For pulse wave, the beam shape varies depending on the time it had been traveling.
What is the beam width for an unfocused transducer?
Answer: For an unfocused transducer, the beam width is half the diameter of the crystal at the focus.
True or False. Unfocused transducers do not have a focus?
Answer: False
The diameter of unfocused ultrasound beam is half the diameter of the PZT crystal at the focal point.
