Transducers and Frequency


Understanding the effects of increasing or decreasing frequency. There are roughly 5-7 Questions on the exam about change in frequency.

The main center frequency of a continuous wave transducer is determined by the frequency of the electrical signal. That’s it. Very simple. The machine determines the frequency.

For Pulsed wave transducers there are 2 characteristics of the piezoelectric crystal that determine the main (center) frequency.

  • The Propagation Speed of crystal. Directly related to frequency.
  • The thickness of the crystal. Inversely related to frequency.

When the thickness decreases the frequency increases. When the propagation speed increases the frequency increases.

So what relationships do we notice with the frequency of the transducer. Mainly resolution and penetration.

We are not able to increase the propagation speed of the crystal or the thickness. These are inherit in the design of the transducer. We can use transducers that are designed to be able to generate several different frequencies. Or we can change which transducer we use.

Increasing the frequency

Increasing the frequency of the pulse will have what effect? Resolution will increase. Temporal resolution will increase because you are sending more signals so you will be able to detect more objects in more locations.

Lateral resolution will increase due to the fact that the near zone length (nzl=d²/4λ) increases. We know that frequency is inversely related to wavelength. So if wavelength goes down then the frequency goes up. This is why we see the increase in lateral resolution.

Axial resolution is equal to 1/2 the Spatial Pulse Length (SPL)

Spatial Pulse Length: SPL= # of cycles x λ

So the main relationship we see here is that if the wavelength (λ) goes up then the spatial pulse length (SPL) increases which means that the axial resolution decreases. Axial resolution has an inverse relationship with the SPL.

The detractor is that with an increase in frequency you will have an increase in attenuation. A high frequency signal is absorbed more readily than a low frequency signal.

Decreasing the frequency

Decreasing the frequency will have what effect? Resolution will decrease. We see the opposite effect as to increasing the frequency. Temporal resolution decreases because we are sending less signals and will not be able to resolve fast moving structures.

Lateral resolution will decrease due to the fact that the NZL decreases.

Axial resolution will decrease because we have low frequency which is inversely related to wavelength. So we have a long wavelength which is related to the SPL. A long SPL means we have low axial resolution. When the pulse is long, structures that are near each other cannot be resolved.

So why would we use a low frequency transducer?

  • lower absorption in tissue
  • greater physical penetration depth in tissue 
  • stronger particle deflections 
  • significantly better acoustic penetration and power in bone
  • a nearly homogeneous near field and strongly divergent far field 

So when you look at this list you can see that penetration and resolution at depth is the benefit.

We sacrifice resolution to penetrate and image deep structures. The reason for the better depth imaging is that absorption is decreased in low frequency. This is the main reason high frequency transducers cannot penetrate.

One other benefit/detractor is that the near field beam thickness is generally the same all the way to the focus. Though at the focus the beam diverges greatly. More so than a high frequency transducer.

 


Transducer Frequency

Question 1
If the Near Zone Length (NZL) decreases what happens to the Lateral Resolution?
A
It is increased.
B
Lateral resolution and Near Zone Length (NZL) are not related.
C
It is decreased.
D
It stays the same.
Question 2
What is the formula for Axial resolution?
A
axial resolution = 1/2 the Pulse Repetition Frequency (PRF)
B
axial resolution = 1/2 the Spatial Pulse Length (SPL)
C
axial resolution = 1/4 the Spatial Pulse Length (SPL)
D
axial resolution = 1/2 the wavelength
Question 3
If the wavelength increases what happens to the Spatial Pulse Length (SPL)?
A
The wavelength is unrelated to the Spatial Pulse Length (SPL)
B
Increases
C
Decreases
D
Stays the same
Question 4
For Pulsed Wave transducers what is one characteristic that determines the center operating frequency?
A
The thickness of the backing material.
B
Spatial Pulse length.
C
The Propagation Speed of the medium.
D
The thickness of the crystal.
Question 5
What effect does lowering the frequency have on temporal resolution?
A
Increases
B
Frequency and temporal resolution are not related
C
Stays the same
D
Decreases
Question 6
If we increase the operating frequency of the transducer, what effect does it have on our image resolution?
A
Operating frequency and image resolution are independent of each other.
B
Increases
C
Decreases
D
Stays the same.
Question 7
A great benefit to a low frequency transducer is what?
A
Better Axial Resolution
B
Shorter wavelength
C
Reduced absorption.
D
A longer Spatial Pulse Length (SPL)
Question 8
For a Continuous Wave transducer what determines the main frequency?
A
Wavelength of the voltage applied.
B
Frequency of the voltage applied.
C
Amount of dampening.
D
Backing material.
Question 9
For Pulsed Wave transducers what is one characteristic that determines the center operating frequency?
A
The Propagation Speed of the crystal.
B
Thickness of the backing material.
C
Spatial Pulse length.
D
The Propagation Speed of the medium.
Question 10
The formula for Saptial Pulse Length is what?
A
SPL = # of cycles x λ (wavelength)
B
SPL = d² (daimeter squared) / 4λ (wavelength)
C
SPL = c (propagation speed) x λ (wavelength)
D
SPL = # of cycles x f (frequency)
There are 10 questions to complete.
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I am the founder and lead Instructor here at ExamRefresh.com. I strive to help those that are entering the field of Ultrasound to be the best that they can be. Ultrasound is my passion.

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Comments

  1. :) I kind have got it, temporal resolution still has me a bit confused though … but I am getting it!!!!

    • Just equate temporal with time. If you increase the depth it will take longer for the echo to go and come back. So it will take more time. That means worse temporal resolution. More scan lines, more pulses, etc. They all take more time so that means worse temporal resolution.
      High frequency – short pulse – better temporal resolution.

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