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Physics 3 > M 5 & 6: Harmonics and Contrast > Flashcards

M 5 & 6: Harmonics and Contrast Flashcards

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1
Q

define harmonics

A

essentially beam dynamics, its the result of the sound wave propagating through tissue

2 X the fundamental or driving frequency

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2
Q

what creates harmonics

A

wave distortion, specifically changes in wave compressibility and density which occur as the US beam propagates and produces multiple harmonic frequencies

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3
Q

what does wave distortion depend on

A

intensity of the beam, distance traveled and nature of the tissue

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4
Q

are harmonic frequencies linear

why

A

no

they are non-linear due to their dependencies (harmonic frequency is what we get back)

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5
Q

are fundamental frequencies linear

why

A

yes

the fundamental frequency is what we send out is the ‘leading edge’ so when it strikes the molecules for the first time they were previously stationary so theres no distortion

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6
Q

where does harmonics work the best

A

with higher intensities since there is more attenuation

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7
Q

how does bulk modulus change w/ increasing density

how is velocity effected

A

increases

increases

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8
Q

if only density increases how does velocity change

A

decreases

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9
Q

how does wave distortion and the compression of a sound wave work

A

at the peak amplitude theres increased density and stiffness (compressed) w/ faster velocities, and at the lowest amplitude theres decrease density, stiffness (expansion) and slower velocities…. the difference in the velocities cause the particles to move closer to one another and the wave is compressed, producing a higher frequency/harmonic frequency

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10
Q

3 ways harmonics improves the image

A
  1. the harmonic beam is narrower since they are best produced from the most intense part of the beam, e.g. better lateral resolution than the fundamental frquency
  2. grating lobes are eliminated b/c they’re too weak to produce harmonics
  3. reverb is greatly reduced/eliminated at the face of the probe (main bang) b/c harmonics/wave distortion doesn’t happen until deeper into the tissue (also helped my the matching layer)
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11
Q

how is penetration effect w/ harmonics

A

it’s reduced due to more attenuations so we need to use more power when scanning w/ harmonics

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12
Q

how does harmonics effect the contrast of the image

A

will appear to have more contrast

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13
Q

function of bandpass filtration

A

electronically eliminates the fundamental frequency and allows the harmonic frequency to pass through the beam former

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14
Q

whats the downside of bandpass filtration

A

the fundamental and second harmonic bandwidths both must fit w/in the overall probe bandwidth w/o overlapping…. this means that the individual bandwidths need to be narrower which will cause the SPL to be longer, resulting in worse axial resolution

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15
Q

what happens if we keep the SPL short and try to use the bandpass filter

A

the bandwidth of the fundamental and harmonic frequencies will overlap and the bandpass filter will cut out part of the harmonic frequency and the image is not formed properly

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16
Q

what is pulse inversion

A

a technique that can filter out the fundamental frequency and leave only the harmonic signal while keeping a wide bandwidth (as there’s no need to fit both signals in the overall probe bandwidth)

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17
Q

how does pulse inversion work

A

2 pulses are sent out, the initial pulse, followed by the inverse of that pulse…. since the fundamental frequencies are linear, when the echos return from both pulses, they will cancel out.

but the process of harmonics is non-linear so the 2 returning echos won’t cancel out and we have a resultant signal

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18
Q

which type of resolution is compromised w/ pulse inversion

which type is improved or maintained

A

temporal is reduced (but not significant)….. b/c we have to send out 2 pulses instead of 1

axial is maintained and lateral is improved

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19
Q

what are contrast agents

what are they used for

A

liquid suspensions injected into the blood stream, most are micro bubbles of gas w/in a shell

improve the intensity of returning echos for enhanced visualization of blood flow or tissue (lesion detection and characterization)

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20
Q

how does contrast increase the intensity of returning echos

A

through stable cavitation, the gas bubbles resonate and produces its own sound source

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21
Q

requirements for contrast agents (size, stability)

A

they need to be small enough to pass through capillaries but large enough to give back echos

must be stable enough to make it through the heart for several cycles to provide enough imaging time

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22
Q

why do the gas bubbles in contrast give off strong reflection

A

due to impedance mismatch b/w the gas and blood

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23
Q

purpose of shell used in contrast agents

A

keeps the gas from dissolving in the blood

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24
Q

what happens to the gas bubbles in contrast if we scan at 100% power

75%?

25%?

A

the collapse and dissolve

the bubbles produce very intense harmonic echos which increase the contrast b/w the contrast agent and the tissue

bubbles produce resonant frequency

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25
Q

an MI of what value w/ contrast will cause the bubbles to pop

A

0.4 MI, which means 0.4 MI is 100% power in this case

26
Q

what is Sono CT/compound imaging/spatial compounding

A

combination of beam steering and frame averaging that can improve SNR and help reduce artifacts

27
Q

why does shadowing from a stone appear darker w/ harmonics

A

theres more attenuation (reflection, refraction, absorption, etc) w/ harmonics so the shadow looks darker

28
Q

does CW or PW have a narrower bandwidth

A

CW (because we use a longer pulse, see doppler module)

29
Q

size of the contrast agent

A

~ 7 micrometers, the same size, or smaller than a RBC

30
Q

air reflects what % of sound

A

99.9%

31
Q

when should you use harmonics

A

anytime penetration allows… as much as possible

32
Q

describe coded excitation

A

newer technology that has the ability to visualize blood flow in 2D w/o the need for doppler and the artifact associated w/ it

33
Q

how is the transmitted/received pulse different w/ coded excitation

advantages

A

the transmitted pulses are digitized and coded:
we send out a coded the pulse and the retuning echos will have the same code which tells the machine that the echo is real…. if the retuning pulse doesn’t have the code, it’s disregarded as noise.

reduces noise, improves image quality and SNR

34
Q

which part of the receiver is important w/ coded excitation

A

compression: this refers to ignoring echos below our threshold level/noise level and only using the echos b/w the threshold and saturation level

helps get rid of fake echos and improve SNR

35
Q

problem w/ compression and how does coded excitation account for this

A

real low level echos may be rejected as noise if they don’t meet the threshold… coded excitation will accept all echos that return as code so it will incorporate these weaker but real, low level echos in the image

36
Q

describe the process of coded excitation for each scan line in the B-flow image

A
  1. transmit coded sound waves
  2. decoder enhances flow signal
  3. flow and tissue displayed as in B-mode
37
Q

detecting what kind of reflectors in B-mode is key w/ coded excitation

A

blood reflectors

38
Q

why are echos from blood rejected w/ conventional scanning

are do we resolve this w/ coded excitation

A

the echos are very weak and below our threshold

code the pulse so that real echos can be differentiated from noise and so that the beam former no longer relies on intensity of returning echos to determine noise

39
Q

how is colour doppler added to our 2D image

A

its a separate image thats transposed onto the 2D, this is why FR goes down so much w/ colour

40
Q

another name for coded excitation

A

b-flow

41
Q

advantages of coded excitation compared to colour doppler

which gives us more info, colour doppler or coded excitation

A
  1. simultaneous tissue and flow w/o an over lay like colour doppler
  2. full field of view
  3. no separate firing of scan lines (only 1) so higher frame rate than colour doppler (one or the best benefits)

colour doppler… we can’t tell direction of flow w/ coded excitation

42
Q

other advantages of coded excitation

A
  • better sensitivity, suppresses unwanted signal components… allows you to scan w/ a higher frequency and still penetrate
  • better spatial res, contrast and penetration which helps w/ technically difficult patients
43
Q

how does coded excitation allow for better penetration

A

it can separate out the noise from the real weaker echos the are returning from deeper in the tissue that would have otherwise gotten cancelled out by the compression part of the receiver

44
Q

2 types of harmonics

A

tissue or native harmonics

contrast harmonics

45
Q

another name for fundamental frequency

A

characteristic frequency

46
Q

which is more intense, fundamental or harmonic frequency

A

fundamental

47
Q

are 3rd, 4th, and 5th harmonics also produced as a result of wave distortion

A

yes

48
Q

relationship b/w the strength of the harmonic beam/energy and the fundamental wave

A

the harmonic energy generated is proportional to the fundamental energy squared

H proportional to F^2
(like intensity is proportional to amp^2)

49
Q

does harmonics affect dynamic range

A

yes, it reduces it b/c the signals are weaker

50
Q

name for the process of filtering the fundamental frequency at reception to avoid noise and clutter

A

wave shaping

51
Q

what controls the amount of harmonics generated by tissue

A

transmit power, specifically the MI

52
Q

Describe how wave shaping works

Another name for wave shaping

A

It controls the shape and duration of the transmitted pulse in order to filter out the fundamental frequency

Coded excitation

53
Q

How must the frequency change to accomplish wave shaping/coding

A

Frequency must be lowered

54
Q

Types of contrast agents

A

Oral, vascular or tissue specific

55
Q

Is there a better contrast resolution with harmonics?

Why

A

Yes

We have less dynamic range so more contrast

56
Q

Why don’t we use air as a contrast agent

A

Highly soluble in blood

57
Q

Can sonographers inject contrast material?

A

No

58
Q

What’s one way to enhance shadowing?

A

Turn off compound imaging, which sweeps the sound beam across the face of the probe and averages the frames to reduce shadowing and enhancement

59
Q

Name for the peak and lowest amplitudes when related to density

A

Peak: compression

Lowest: rare-fraction

60
Q

What happens when you decrease the distance b/w the peak refraction pressure?

A

you are increasing the frequency and producing harmonics (FOR SONOG CANADA)

Physics 3 (5 decks)

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