Module 4 Harmonic Imaging

Front 1

what are harmonics?

Back 1

multiples of the frequency(5mhz transducer has harmonics of 10, 15, etc)
-a change in the way particles compress generates harmonics

Front 2

how are harmonic frequencies generated?

Back 2

w/ passage of the ultrasound wave, wheras the surrounding medium is much less so.

Front 3

Where do harmonic frequencies originate?

Back 3

originate from generated bubbles, not from the transducer
-or from the medium through which the beam is traveling

Front 4

how small do bubbles have to be to to cross the pulmonary capillary bed?

Back 4

have to be <6.6 micrometers

Front 5

what are some problems w/ contrast agents?

Back 5

-FDA has limited contrast to the LV function studies
-a byproduct of passing an ultrasound wave through bubbles is that sound wave destroys the bubbles which are the source of harmonics

Front 6

What does THI stand for?

Back 6

tissue harmonic imaging

Front 7

Explain 1st, 2nd and 3rd harmonics?

Back 7

If scanning w/ a 3.5mhz transducer, it is the 1st harmonic
-the second harmonic is 7mhz, and 10.5 is the 3rd

Front 8

what are the ways of producing harmonics?

Back 8

-bubbles
-using what is being put out by the transducers

Front 9

what is the frequency of sinusoidal waves?

Back 9

-single frequency equal to the number of cycles per second

Front 10

Explain(in terms of the sinusoidal wave), how harmonics are produced?

Back 10

-sinusoidal wave propigates as a series of pressure variations
-higher pressure=faster wave which results in a change in the wave shape(it's now non-sinusoidal), and generation of harmonic frequencies.
-as pressures increase(increased output), there are more harmonics genrerated.
-only areas of the beem w/ sufficient energy will generate harmonics, so there is effective narrowing of the beam.

Front 11

what is non-linear propegation?

Back 11

-in regions of higher pressure, the wave travels faster and changes shape, so it's now non-sinusoidal, and harmonic frequencies are generated.

Front 12

why does the effective beam width narrow when harmonics are generated?

Back 12

-as pressures increase, output is increased, and there are more harmonics generated.
-only areas of the beam w/ sufficient energy will genreate harmonics
-this results in a narrow effective beam.

Front 13

how do the harmonic sigals compare to the fundamental frequencies?

Back 13

-10-20db lower
-because energy at 3rd and 4th harmonics is expodentially weaker, they are too weak to be useful

Front 14

what does generation of harmonics depend on?

Back 14

-acoustic pressure
-frequency
-non-linearity co-efficient
-density of the medium
-velocity in the medium
-distance of propegation

Front 15

what portion of a beam doesn't genreate harmonics?

Back 15

peripheral portions of the beam

Front 16

why do manufacturers want to put out higher amplitude waves? why isn't this practical?

Back 16

-because low amplitude waves do not produce harmonics
-the higher amplitude the wave, the better harmonics
-this has potential for biohazardous effects

Front 17

what areas do not produce harmonics?

Back 17

-peripher of a wave
-low amplitude waves
-first few centimeters
-lower pressure regions in the far field

Front 18

HOw is information w/ harmonics processed?

Back 18

-w/ regular harmonics, both fundamental and harmonic frequcny must fit into the transducer bandwidth w/out overlapping, so longer pulses are used
-as pulselength increases, bandwidth decreases.
-amplifier is tuned to process higher frequencies, so it filters out the fundamental frequencies leaving the harmonics to be displayed

Front 19

how are the bandwidths kept from overlapping?

Back 19

-SPL is lengthened so the bandwidths are narrowing preventing overlapping

Front 20

what are the advantages of harmonic imaging?

Back 20

-decreased clutter, grating lobes, side lobes, etc.
-decreased reverberation
-reduction of acoustic noise
-fluid spaces have less fill in
-improved lateral resolution due to decreased effective beam width
-effect of subcutaneous fat in lg. patients is reduced

Front 21

what happens to fat w/ fundamental frequencies?

Back 21

-fat tends to defocus the beam and distort echos from deeper structures

Front 22

what is another name for transient echo imaging?

Back 22

stimulated acoustic emision(SAE)

Front 23

what is transient echo imaging? what is it useful for?

Back 23

-under the influence of ultrasound, gas bubbles can break allowing gas to diffuse into the surrounding tissues quite easily
-useful for slow moving blood

Front 24

what are microbubbles?

Back 24

-tiny gas bubbles surrounded by a thin shell

Front 25

what is a solution to to keep bandwidths from overlapping?

Back 25

pulse inversion harmonics

Front 26

what is a downside to increasing PRF?

Back 26

causes a reduction of frame rate

Front 27

differnetiate btw normal imaging and harmonic imaging?

Back 27

Normal:
-ist echo cancells out w/ second echo(because the second one is inverted)

Harmonic:
-2nd echo amplifies 1st echo

Front 28

w/ tissue harmonics, what are the 2 ways of producing harmonics?

Back 28

-regular one-pulse, one echo train methond
-use of 2 equal but opposite pulses for each line of site. In this case, the fundamental frequencies will cancel, and the harmonic echos will be constructively added.

Front 29

What are harmonic frequencies?

Back 29

Frequencies which are a multiple of the fundamental or lowest frequency.

Front 30

Describe what happens to bubbles as they encounter a sound wave?

Back 30

The bubbles change diameter and shape in response to the changing pressures of the sound wave. If the pressure changes are of sufficient magnitude, harmonic frequencies will be emitted.

Front 31

Why is contrast injected into the patient?

Back 31

So that cardiac and other vascular structures and associated flow features can be adequately demonstrated. This contrast can also perfuse some soft tissues and improve imaging there too.

Front 32

Why is imaging of longer duration events sometimes difficult with bubble studies?

Back 32

The ultrasound beam tends to destroy the bubbles.

Front 33

What is the source of the harmonic frequencies in bubble studies?

Back 33

The bubbles, as they react with a high pressure sound wave

Front 34

What is the source of the harmonic frequencies in THI (tissue harmonic imaging)?

Back 34

The tissue as it interact with the passage of a high pressure sound wave.

Front 35

How are harmonic generated in soft tissue?

Back 35

-As a low pressure wave passes through soft tissue a sinusoidal (sine) wave is generated with a relatively pure (single frequency) wave.
-As pressure increases, differences in propagation speed in various parts of the sound wave are introduced.
-high pressure=increased speed of sound, and opposite w/ low pressure.
-This changes the shape of the once-sinusoidal wave, and also gives rise to the harmonic frequencies.

Front 36

Compare the terms linear and non-linear propagation

Back 36

Linear propegation:
-When low pressure fundamental frequencies are used, fundamental frequency echoes are returned.

NOn-linear propegation:
-When high pressure fundamental frequencies are used, various harmonic frequencies are generated.

Front 37

Why are 3rd and 4th harmonic frequencies not currently used?

Back 37

As subsequent harmonics are significantly weaker than lower harmonics, they cannot be currently processed.

Front 38

What is the down-side of using non-pulse inversion harmonics?

Back 38

Because pulse lengths are increased to reduce bandwidths, there is a concomitant reduction in axial resolution.

Front 39

Why do harmonic produce better spatial resolution?

Back 39

-Harmonics are, by definition higher frequency sound waves, and so the better axial and lateral resolution characteristics are realized.
-Because harmonics are generated only from the highest pressure parts of the beam, no harmonics are generated toward the periphery of the beam, thereby effectively narrowing the beam.

Front 40

Why are longer pulse lengths generally used in non-pulse inversion harmonics?

Back 40

Both transmitted and received bandwidths must 'fit' within the transducer bandwidth.
- In order to cause a narrowing of these bandwidths, the pulse lengths are increased.

Front 41

In transient echo imaging, what causes the chaotic appearances that helps track blood flow?

Back 41

The disintegration of the gas bubbles.

Front 42

Describe the basic process of pulse inversion harmonics.

Back 42

-two pulses instead of one are generated.
-the second pulse is an inversion of the first(180 degrees out of phase w/ the first)
-Echos from the 2 opposite echos cancel eachother out(effectively destructively interfere.
-the harmonics from each of the two pulses are summed and so constructively interfere improving the harmonic signal.

Front 43

There is something to be given up when using each of the two types of harmonics. What are they?

Back 43

-In regular harmonics axial resolution is worse than with pulse inversion.
-With pulse inversion, frame rate is lower due to the increased dwell time.

Front 44

Which generates stronger harmonics, contrast agents, or tissue harmonic imaging?

Back 44

contrast agents

Front 45

One type of harmonics is considered a filtering technique and the other a subtraction technique. Which is which.

Back 45

Regular, non-pulse inversion=a filtering technique because the fundamental is filtered out leaving only the harmonics.

Pulse inversion=a subtraction technique because the fundamental frequencies are removed from the bandwidth by cancellation.