Harmonics and Contrast Resolution

Question 1

Conventional approaches to overcoming poor signal

Answer 1

Low frequency probe

Different imaging
applications

higher transmit power

Invasive Technique

Question 2

weak reflections from Rayleigh scattering

Answer 2

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

Question 3

The idea of contrast imaging

Answer 3

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

Question 4

How to increase the amount of backscatter (two ways)

Answer 4

Increase the surface of the reflector

increase the acoustic impedance mismatch

Question 5

Amount of reflection

Answer 5

Based on acoustic impedance mismatch

Question 6

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

Answer 6

Result of high compressibility and low density of gas

Question 7

Contrast agents improve the reflectivity of blood by how much

Answer 7

30DB

Question 8

what does harmonics do

Answer 8

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

Question 9

advantage of higher transmit frequency

Answer 9

Improved resolution

Question 10

First harmonic

Answer 10

Fundamental frequency (operating frequency)

Question 11

second-harmonic

Answer 11

frequency that is twice the fundamental freqency

Question 12

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

Answer 12

Penetration, resolution

Question 13

Harmonics

Answer 13

multiple of the operating frequency

Question 14

2nd harmonic frequency

Answer 14

2*transmit frequency

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

Question 15

high frequency….

Answer 15

Inadequate penetration

Question 16

low frequency….

Answer 16

Poor resolution

Question 17

what are the two mechanisms for producing harmonic signals

Answer 17

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

Question 18

Broadband transducers

Answer 18

Make harmonic imaging possible

Question 19

when harmonic imaging was first developed

Answer 19

Used contrast agents

Question 20

Non-linear response of tissue

Answer 20

Distorts the transmitted wave, producing harmonic energy

Question 21

what does harmonic imaging use

Answer 21

2nd harmonic

Question 22

with compression and refraction

Answer 22

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

Question 23

Change in propagation velocity

Answer 23

Nonlinear

Question 24

with compression

Answer 24

Propagation speed increases

Question 25

with refraction

Answer 25

Propagation speed decreases

Question 26

what occurs to the amplitude of each successive higher harmonic

Answer 26

Amplitude decreases

Question 27

therefore the amplitude of the 2nd harmonic

Answer 27

is weaker then the fundamental signal

Question 28

Harmonics are best near....

Answer 28

The focus

Question 29

With increasing depth, there is what

Answer 29

Increasing beam attenuation, decreasing the pressure. | rate of decreasing harmonic energy will be faster than the fundamental frequency

Question 30

In the near field and far field....

Answer 30

Fundamental imaging may outperform harmonic imaging

Question 31

Attenuation of the harmonic signal is

Answer 31

higher because of increased frequency

Question 32

Harmonic beam is ______compared to fundamental beam

Answer 32

Narrower

Question 33

Narrow harmonic beam helps to improve what?

Answer 33

Lateral resolution

Question 34

Beam intensity is _____in the nearfield, which reduces what?

Answer 34

Much weaker, artifacts

Question 35

to produce harmonic imaging what must be prsent

Answer 35

enough depth

Question 36

what is the cycle of events of harmonic imaging

Answer 36

- 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

Question 37

how does harmonic imaging affect axial resolution

Answer 37

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

Question 38

Bandwidth is_________to PD

Answer 38

Inversely proportional

Question 39

Narrower bandwidth requires a ______pulse and ______cycles in the wave

Answer 39

longer, increased

Question 40

Advantage: decreased grating lobes

Answer 40

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

Question 41

clutter reduction

Answer 41

Major advantage of harmonic imaging

Question 42

how does harmonic imaging help reduce clutter

Answer 42

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

Question 43

how does harmonic imaging eliminate clutter and reverb artifacts

Answer 43

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)

Question 44

Phase aberration

Answer 44

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

Question 45

how is beam aberration reduced by harmonic imaging

Answer 45

- 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

Question 46

Overall advantages to harmonic imaging

Answer 46

- Increased lateral resolution - decreased phase aberration - decreased clutter artifacts - Decreased grating lobes

Question 47

over disadvantages to harmonic imaging

Answer 47

-decreased axial resolution

Question 48

Pulse/phased inversion harmonics

Answer 48

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

Question 49

how does pulse/phase inversion imaging work

Answer 49

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.