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Ultrasound Physics > Harmonics and Contrast Resolution > Flashcards

Harmonics and Contrast Resolution Flashcards

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1
Q

Conventional approaches to overcoming poor signal

A

Low frequency probe

Different imaging
applications

higher transmit power

Invasive Technique

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2
Q

weak reflections from Rayleigh scattering

A

cause poor signal to noise ratio, meaning that signal can be both nondiagnostic or not detected

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3
Q

The idea of contrast imaging

A

not to increase the backscattering surface but use a contrast agent which helps increases the autistic impedance mismatch of blood

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4
Q

How to increase the amount of backscatter (two ways)

A

Increase the surface of the reflector

increase the acoustic impedance mismatch

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5
Q

Amount of reflection

A

Based on acoustic impedance mismatch

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6
Q

how does the contrast agent increase the acoustic impedance mismatch within the blood?

A

Result of high compressibility and low density of gas

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7
Q

Contrast agents improve the reflectivity of blood by how much

A

30DB

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8
Q

what does harmonics do

A

Transmit beams at a lower frequency and receive at a higher frequency

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9
Q

advantage of higher transmit frequency

A

Improved resolution

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10
Q

First harmonic

A

Fundamental frequency (operating frequency)

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11
Q

second-harmonic

A

frequency that is twice the fundamental freqency

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12
Q

Transmitting at a lower frequency allows for better ________while receiving a6t a higher frequency allows for better _______

A

Penetration, resolution

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13
Q

Harmonics

A

multiple of the operating frequency

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14
Q

2nd harmonic frequency

A

2*transmit frequency

If transmit frequency was 4, 2nd harmonic frequency would be 8

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15
Q

high frequency….

A

Inadequate penetration

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16
Q

low frequency….

A

Poor resolution

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17
Q

what are the two mechanisms for producing harmonic signals

A
  • Non-linear propagation through tissue producing ‘native’ or ‘tissue harmonic imaging’
  • Resonance of contrast agents producing contrast harmonic imaging
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18
Q

Broadband transducers

A

Make harmonic imaging possible

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19
Q

when harmonic imaging was first developed

A

Used contrast agents

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20
Q

Non-linear response of tissue

A

Distorts the transmitted wave, producing harmonic energy

21
Q

what does harmonic imaging use

A

2nd harmonic

22
Q

with compression and refraction

A

Density of the medium changes and results in a change in propagation velocity

23
Q

Change in propagation velocity

A

Nonlinear

24
Q

with compression

A

Propagation speed increases

25
Q

with refraction

A

Propagation speed decreases

26
Q

what occurs to the amplitude of each successive higher harmonic

A

Amplitude decreases

27
Q

therefore the amplitude of the 2nd harmonic

A

is weaker then the fundamental signal

28
Q

Harmonics are best near….

A

The focus

29
Q

With increasing depth, there is what

A

Increasing beam attenuation, decreasing the pressure.

rate of decreasing harmonic energy will be faster than the fundamental frequency

30
Q

In the near field and far field….

A

Fundamental imaging may outperform harmonic imaging

31
Q

Attenuation of the harmonic signal is

A

higher because of increased frequency

32
Q

Harmonic beam is ______compared to fundamental beam

A

Narrower

33
Q

Narrow harmonic beam helps to improve what?

A

Lateral resolution

34
Q

Beam intensity is _____in the nearfield, which reduces what?

A

Much weaker, artifacts

35
Q

to produce harmonic imaging what must be prsent

A

enough depth

36
Q

what is the cycle of events of harmonic imaging

A
  • Fundamental beam sent into body
  • Wave travels deep into body
  • areas of density affect refraction and compression
  • harmonic energy is produced
  • transducer bandwidth detects harmonic imaging
37
Q

how does harmonic imaging affect axial resolution

A

Degraded axial resolution
-longer pulse duration which means longer spatial pulse length and therefore a decreased axial resolution because there is an overlap of signals

38
Q

Bandwidth is_________to PD

A

Inversely proportional

39
Q

Narrower bandwidth requires a ______pulse and ______cycles in the wave

A

longer, increased

40
Q

Advantage: decreased grating lobes

A

lower energy grating lobes produce much weaker harmonic signals, reducing the energy in the grating lobes
-Weaker grating lobes result in improved LATERAL resolution and less lateral translation of off axis energy into the main beam

41
Q

clutter reduction

A

Major advantage of harmonic imaging

42
Q

how does harmonic imaging help reduce clutter

A

reverb and clutter is created by large acoustic impedance mismatches from specular reflectors
-Harmonic signal is much weaker then fundamental signal which causes a large reduction in the strong signals responsible for artifacts

43
Q

how does harmonic imaging eliminate clutter and reverb artifacts

A

Intensity of each reverberation is lower which produces less harmonic signal
most artifacts are caused by specular reflectors in near field, where the beam is still converging (therefore weaker harmonic energy)

44
Q

Phase aberration

A

Distortion of the beam caused by differences in speed of sound in inhomogenous tissue

45
Q

how is beam aberration reduced by harmonic imaging

A
  • Focusing of lower frequencies is less affected y speed of sound then higher frequencies
  • Higher frequencies=shorter wavelengths
  • Less time delay and more destructive interference
46
Q

Overall advantages to harmonic imaging

A
  • Increased lateral resolution
  • decreased phase aberration
  • decreased clutter artifacts
  • Decreased grating lobes
47
Q

over disadvantages to harmonic imaging

A

-decreased axial resolution

48
Q

Pulse/phased inversion harmonics

A

-Peak of the harmonic wave occurs at the same time as both the peak and the minima of the fundamental wave

49
Q

how does pulse/phase inversion imaging work

A

By transmitting multiple lines with different phases and then adding the resulting lines together, the fundamental energy adds destructively while the harmonic data adds constructively. As a result the harmonic energy gets stronger and there is no need to degrade the axial resolution to help eliminate the fundamental energy.

Ultrasound Physics (10 decks)

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