Physics 3 - Harmonics - Module 5

The frequency the crystal likes to ring at

What is the resonant frequency

one-half wavelength

What is the calculation of the resonant frequency

Resonant/center frequency

What does this image represent

Compressibility

What has the most impact on the speed of the wave

Sine wave

What is the increase and decrease of density shown in

Change in stiffness

Changing in density =

Change in intensity

Change in stiffness=

Increase in density means an increase in stiffness which will increase the intensity of

Explain how an increase in density will increase the intensity

Decrease in density means and increase in compressibility which causes a decrease in intensity

Describe how a decrease in density results in a decrease in intensity

Decrease in intensity

Increased in compressibility =

Wave distortion

As sound travels through tissue with changes of stiffness it will result in:

False, it is random

T/F: the process of wave distortion is linear

True

T/F: every time we emit sound we get a non linear process

More options for frequencies that we can ring at

What does damping material give us

Beam dynamics or the result of the wave propagating through tissue

What is harmonics

HD TV

What is harmonic comparative to

Fundamental frequency

What is the frequency that we are driving at

True

T/F: you can still scan with harmonics turned off but we just wont get back the other harmonics frequency (will just get back the fundamental frequency)

Harmonic frequencies

What is produced due to the ultrasound beam going through wave distortion

2x the amplitude

What is the amplitude of harmonic frequencies usually

Will have more energy to get back, will have a stronger signal

What is the significance of the harmonic frequency being 2x the amplitude

Resonate more

Because harmonic are 2x the amplitude that can:

There is not just a west harmonic that comes back, there is the 2nd, 3rd, and so on

Explain how many harmonic waves we get back

1st and 2nd one only

Which harmonic wave that we get back do we use

As frequency increase, absorption also increases so the harmonic waves past the 2nd one are attenuated too much

Why do we only use the 1st and 2nd harmonic wave

True

T/F: harmonics happens naturally and every time that we scan. The button on the machine just determines how we use it

Since harmonics happens all the time, harmonics still occurred with older machines but we just didn't know what to do with it until later (so there wasn't a button on the machine)

Explain how harmonics was or wasn't used in the past on older machines

Intensity of the beam, distance travel, and the nature of the tissue

What 3 things does the wave distortion depend on

Linear

Fundamental waves are:

Non-linear

Harmonic frequencies are:

Due to dependencies (intensity of the beam, the distance travelled, and the nature of the tissue)

Why is harmonic frequencies non linear

Bc no dependant on anything

Why are fundamental frequencies linear

The most intense part of the beam

What part of the beam is harmonics best produced by

No, will be random and slightly different

Do we get back the exact same range of frequencies with harmonics

High pressure: tissue compressed and high velocity (because more dense)

Describe the top wave

Low pressure: tissue expanded and lower velocity (bc more compressible)

Describe the bottom wave

Starting with fundamental frequency and compressing into harmonic frequency

What does this image show

2

Which has the most harmonics

Pure tone

What does this image represent

Harmonic beam is narrower, grating lobes are eliminated, and reverb is reduced or eliminated

What are the 3 ways that harmonics improves the image

The most intense part of the beam

Where is harmonics the best

Where harmonics is the best

What does this image represent

Improve lateral resolution and a little improvement in elevational resolution

What improves due to harmonics coming from the narrowest part of the beam

Adipoziation (less cross talk) and subdicing (cut the crystals to less than a wavelength but still act as 1)

What are 2 other things that reduce grating lobes

Bc harmonics only picks up the intense part of the beam and grating lobes are the weaker sides of the beam

Explain how harmonics decreases grating lobes

Bc the beam needs to be more powerful with harmonics, and at first we didn't know if it was too powerful or not for OB scanning (later found out it was okay to use harmonics with OB)

Explain why when harmonics first came out it was not used on OB scans

Cystic structures bc getting a narrower and more intense beam= better lateral and elevational resolution

What is an example of something that would be more resolved with harmonics

Beam gets gradually more intense until it is the most intense at the middle and then fans out, gradually becoming less intense

Describe the intensity of the beam as it travels down

Near field, weak harmonics

What is 1 and what are the harmonics like here

Mid field, the best harmonics

What is 2 and what are the harmonics like here

Far field, harmonics are attenuated faster than they are produced (no/worse harmonics)

What is 3 and what are the harmonics like here

There is not a lot of harmonics at the near field bc in order to get harmonics the wave need to propagate and distort (not enough time for this in the near field)

Explain why harmonics is the best at the near field/why harmonics decreases reverberation

Main bang artifact (reverberation artifact right at the top of the image)

What type of reverberation artifact does harmonics greatly reduce/eliminate

Superficial structures

What exams or structures may it be helpful to turn off harmonics due to it not going through enough distortion in the near field

Could also result in main bang, so have to decide which is better

Although turning off harmonics may help with visualization of superficial structures, what do you risk by turn it off

Turn harmonic frequency down or turn it off for more penetration and visualization of the far field (remember, lower frequency=better penetration)

Explain what adjustments you can make to the harmonics to get a better visualization of the far field

Bandpass filtration

What was developed to allow us to use harmonics to our advantage

Eliminates the fundamental frequency and allow the harmonic signal to pass through the beam former

What does the Bandpass filtration do

Beam former

What most the harmonic signal pass through in order for us to make use of

Tuning a radio to accept only the frequency that you want to hear

What is the Bandpass filtration analogous to

Will have a narrower beam which is better for fitting through the ribs and also helps resolve the cystic structures in the heart (atria, ventricles, vessels)

Explain why harmonics greatly assists the resolution of echoes

Bandpass filtration

What was the first method for us to be able to use harmonic signal

Pulser inversion

What was the 2nd method used for us to be able to use the harmonics signal

Wide range of frequencies that can come out of the machine

Wider bandwidth =

Shorter pulse, shorter SPL which = better axial resolution

What does a wider bandwidth mean for pulse lengthier and SPL and resolution

Axial

What resolution is improved with a wider bandwidth

The fundamental frequency and the second harmonic bandwidths must fit within the overall transducer bandwidth without overlapping

What is required for Bandpass filtering to work

Makes the bandwidth narrower

what is the major disadvantage of Bandpass filtering

Longer pulses and longer SPL which = bad axial resolution

What does a narrower bandwidth result in

If they were to overlap then when the machine deletes the fundamental frequency it would also be deleting the portion of the harmonic frequency which would result in missing information

Explain why the fundamental and second harmonic bandwidths can not overlap with Bandpass filtering to work

How Bandpass filtering works

What does this image represent

Axial resolution is our best resolution so Bandpass filtering would actually make the image worse bc decreasing axial resolution

Why is having a narrower bandwidth so bad

If we want to first 2 and get back 4, the 2 and 4 would have to fit in the bandwidth without overlapping

Explain the bandwidth requirements using a C6-2 transducer

More absorption and scatter

Increase in frequency =

Turn down the frequency of harmonics to get better penetration instead of turning it off so you still have the benefits of harmonics

Instead of turning harmonics completely off when you want more penetration, what is an adjustment that you can make

Pulse inversion

How can we maintain the axial resolution while still getting the benefits of harmonics (less reverb, improving lateral resolution, get ride of main bang and grating lobes)

A technique that can filter out the fundamental and leave only the harmonic signal while maintaining a wide bandwidth

Explain what pulse inversion is

A pulse is sent out followed by the inverse of that pulse

Describe in laymen's terms what is happening with pulse inversion

When the first pulse and inverted pulse are added together they will cancel each other out bc they are the SAME (ex/ if you send out 2 you will get 2 back bc fundamental echoes are not dependent on anything)

Describe what happens with pulse inversion on fundamental echoes (why it doesn't work with fundamental frequencies)

Destructive interference

What is it called when the fundamental echoes cancel each other out

Harmonics is NOT LINEAR so we will get back a random number (that is close to double of what we sent out). When these are added together, they will NOT cancel each other out bc they are not the exact same

Explain why pulse inversion works with harmonics

False, we do NOT get destructive interference bc they wont be the same number

T/F: when we apply pulse inversion to harmonics we get total destructive interference

Bc the echoes are slightly out of phase so wont cancel each other

Explain why pulse inversion works with harmonics in laymen's terms

True

T/F: without harmonics, pulse inversion would just cancel the pulses out and there would be no signal

Pulse inversion with fundamental frequency

What does this image represent

Pulse inversion with harmonics

What does this image represent

Keep pulse short, gets all the benefits of harmonics, while not sacrificing the axial resolution

What is the benefits of pulse inversion

Decreases temporal resolution

What is the disadvantage of pulse inversion

Bc now we have to send out 2 pulse in order to inverse and add them together

Why does pulse inversion decrease the temporal resolution

Yes!! The little bit of decrease of temporal resolution is worth the better resolution we get from harmonics

Is pulse inversion still worth it even though it decreases the temporal resolution

Harmonics

What is define by the decrease in the distance between peak refractable pressures

Harmonics

The distance between peak refractional pressures=