M 5 & 6: Harmonics and Contrast

Question 1

define harmonics

Answer 1

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

2 X the fundamental or driving frequency

Question 2

what creates harmonics

Answer 2

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

Question 3

what does wave distortion depend on

Answer 3

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

Question 4

are harmonic frequencies linear

why

Answer 4

no

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

Question 5

are fundamental frequencies linear

why

Answer 5

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

Question 6

where does harmonics work the best

Answer 6

with higher intensities since there is more attenuation

Question 7

how does bulk modulus change w/ increasing density

how is velocity effected

Answer 7

increases

increases

Question 8

if only density increases how does velocity change

Answer 8

decreases

Question 9

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

Answer 9

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

Question 10

3 ways harmonics improves the image

Answer 10

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)

Question 11

how is penetration effect w/ harmonics

Answer 11

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

Question 12

how does harmonics effect the contrast of the image

Answer 12

will appear to have more contrast

Question 13

function of bandpass filtration

Answer 13

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

Question 14

whats the downside of bandpass filtration

Answer 14

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

Question 15

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

Answer 15

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

Question 16

what is pulse inversion

Answer 16

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)

Question 17

how does pulse inversion work

Answer 17

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

Question 18

which type of resolution is compromised w/ pulse inversion

which type is improved or maintained

Answer 18

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

Question 19

what are contrast agents

what are they used for

Answer 19

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)

Question 20

how does contrast increase the intensity of returning echos

Answer 20

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

Question 21

requirements for contrast agents (size, stability)

Answer 21

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

Question 22

why do the gas bubbles in contrast give off strong reflection

Answer 22

due to impedance mismatch b/w the gas and blood

Question 23

purpose of shell used in contrast agents

Answer 23

keeps the gas from dissolving in the blood

Question 24

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

75%?

25%?

Answer 24

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

Question 25

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

Answer 25

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

Question 26

what is Sono CT/compound imaging/spatial compounding

Answer 26

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

Question 27

why does shadowing from a stone appear darker w/ harmonics

Answer 27

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

Question 28

does CW or PW have a narrower bandwidth

Answer 28

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

Question 29

size of the contrast agent

Answer 29

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

Question 30

air reflects what % of sound

Answer 30

99.9%

Question 31

when should you use harmonics

Answer 31

anytime penetration allows... as much as possible

Question 32

describe coded excitation

Answer 32

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

Question 33

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

Answer 33

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

Question 34

which part of the receiver is important w/ coded excitation

Answer 34

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

Question 35

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

Answer 35

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

Question 36

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

Answer 36

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

Question 37

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

Answer 37

blood reflectors

Question 38

why are echos from blood rejected w/ conventional scanning are do we resolve this w/ coded excitation

Answer 38

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

Question 39

how is colour doppler added to our 2D image

Answer 39

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

Question 40

another name for coded excitation

Answer 40

b-flow

Question 41

advantages of coded excitation compared to colour doppler which gives us more info, colour doppler or coded excitation

Answer 41

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

Question 42

other advantages of coded excitation

Answer 42

- 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

Question 43

how does coded excitation allow for better penetration

Answer 43

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

Question 44

2 types of harmonics

Answer 44

tissue or native harmonics | contrast harmonics

Question 45

another name for fundamental frequency

Answer 45

characteristic frequency

Question 46

which is more intense, fundamental or harmonic frequency

Answer 46

fundamental

Question 47

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

Answer 47

yes

Question 48

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

Answer 48

the harmonic energy generated is proportional to the fundamental energy squared H proportional to F^2 (like intensity is proportional to amp^2)

Question 49

does harmonics affect dynamic range

Answer 49

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

Question 50

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

Answer 50

wave shaping

Question 51

what controls the amount of harmonics generated by tissue

Answer 51

transmit power, specifically the MI

Question 52

Describe how wave shaping works Another name for wave shaping

Answer 52

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

Question 53

How must the frequency change to accomplish wave shaping/coding

Answer 53

Frequency must be lowered

Question 54

Types of contrast agents

Answer 54

Oral, vascular or tissue specific

Question 55

Is there a better contrast resolution with harmonics? Why

Answer 55

Yes We have less dynamic range so more contrast

Question 56

Why don’t we use air as a contrast agent

Answer 56

Highly soluble in blood

Question 57

Can sonographers inject contrast material?

Answer 57

No

Question 58

What’s one way to enhance shadowing?

Answer 58

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

Question 59

Name for the peak and lowest amplitudes when related to density

Answer 59

Peak: compression Lowest: rare-fraction

Question 60

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

Answer 60

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