Flashcards in Module 2 practice exam Deck (16)
The natural frequency of a LZT transducer element is determined by:
a. the length of the element
b. the thickness of the element
c. the material the element is composed of
d. the density of the element
e. the speed of sound in LZT
The correct answer is: the thickness of the element
The M-mode image display comprises the following:-
a. amplitude on the horizontal axis, time on the vertical axis
b. distance/depth on the horizontal axis, time on the vertical axis
c. amplitude on the horizontal axis, distance/depth on the vertical axis
d. time on the horizontal axis, distance/depth on the vertical axis
e. time on the horizontal axis, amplitude on the vertical axis
The correct answer is: time on the horizontal axis, distance/depth on the vertical axis
A pulse of ultrasound is produced from an array based transducer. The firing sequence of the elements used to generate the ultrasound pulse starts from one end of the group of active elements and finishes at the other, and the pattern of the timing delays between firing the successive elements is decremental. What would such a firing pattern achieve?
a. Beam steering only
b. Beam focussing within the scan plane only
c. Beam focussing within the elevation plane only
d. Beam steering and beam focussing within the scan plane
e. Beam steering and beam focussing within the elevation plane
The correct answer is: Beam steering and beam focussing within the scan plane
How is the elevation plane focussing altered in a curved linear array transducer?
a. By increasing the time delays between successive pairs of elements.
b. By decreasing the time delays between successive pairs of elements.
c. Neither of the above.
The correct answer is: Neither of the above.
In an electronically focused array the ultrasound beam:
a. is parallel in the near field and diverges in the far field
b. diverges in the near field and is parallel in the far field
c. converges in the near field and is parallel in the far field
d. is parallel in the near field and converges in the far field
e. converges in the near field and diverges in the far field
The correct answer is: converges in the near field and diverges in the far field
Explain how beam steering is achieved using a curved linear array transducer.
Beam steering for a curved linear array transducer is achieved by using the elements in groups to produce a pulse of ultrasound and detect any echoes. The groups of elements selected starts from one end of the transducer, moving along the array by one element for each successive pulse of ultrasound. This allows an adequate beam profile to be generated by having a larger effective transducer diameter, and maintains an adequate line density by moving the centre of the beam only one element width along the scan plane.
What is the main difference in the shape of the scan plane image between a curved array and a phased array?
The phased array image starts at the top (transducer end) at a single point. The curved array has a broader extent at the transducer end. Both give rise to sector display that widens with depth.
What is meant by the term bandwidth in relation to the ultrasound pulse? And how does the bandwidth relate to the quality factor?
The bandwidth refers to the range of frequencies present in the transmitted pulse. A high quality factor is associated with a narrower bandwidth.
How does a strongly focussed beam profile compare to a medium focussed beam profile, assuming that only the focal depth is changed and all other parameters remain the same.
The beam width of the strongly focussed beam is narrower at the focus, but beyond the focus the beam is more divergent and hence wider than a medium focussed beam.
What is the main advantage of an annular array compared to a linear array?
With an annular array the elevation plane focus and the scan plane focus can be adjusted electronically to be the same. In a modern linear array the elevation plane focus is fixed by the mechanical, external, lens.
Describe the construction of the transducer elements in an annular array transducer.
The transducer elements comprise concentric rings or annuli of piezo-electric crystal, centred round a central disc element. Each element is electrically and acoustically isolated to ensure independent operation. The elements are all of the same width and thickness.
What are side lobes? Write brief notes to address why they occur and how they present within the ultrasound beam profile.
Sidelobes – Additional beams that occur either side of the desired ultrasound beam. They are generally weaker than the main beams by a factor of 100 (20dB) they are still capable of contributing echoes and can therefore cause artifacts.
Since a machine cannot know that the echo from the object comes from a sidelobe, both within the scan plane and the elevation plane, it displays it across the midline of the beam.
As the beam is scanned the object will be displayed in its correct location by the main beam and also to the left and right of it’s true location by the sidelobes. This is referred to as side lobe artifact.
Transducers are designed so that the side lobe intensity is much lower than that of the main beam.
Explain the benefits of using dynamic aperture control and how this technique is implemented.
Dynamic aperture control is used to improve the beam profile and lateral resolution for the superficially focussed beams. This is achieved by reducing the number of active elements used when detecting echoes from superficial structures, increasing the number of elements with increasing echo depth.
What determines the resonant frequency of the ultrasound transducer? Explain.
The thickness of the piezoelectric crystal determines the resonant frequency as half wavelength resonance occurs when the thickness is equal to half the wavelength. This is calculated based on the velocity of sound in the piezoelectric crystal = resonant frequency x wavelength = resonant frequency x 2 x crystal thickness.
A voltage pulse produces an ultrasound pulse that lasts 8 micro-seconds. This voltage pulse is repeated every 1 milli-second. What is the duty cycle for this transducer? Remember to show your workings.
Duty factor = pulse duration / pulse repetition period = 0.000008 s / 0.001 s = 0.008