Module 5 & 6 : Harmonics and Contrast Flashcards
(38 cards)
1
Q
how are harmonic frequencies produced
A
- produced as a result of WAVE DISTORTION
2
Q
what is harmonics
A
- essentially beam dynamics
- result of wave propagating through tissue
3
Q
what is a longitudinal mechanical wave
A
- particle motion same as direction of the wave
4
Q
what is a transverse mechanical wave
A
- particle motion perpendicular to direction of wave
5
Q
what 3 things is wave distortion dependent on
A
- intensity of the beam
- distance travelled
- nature of tissue
6
Q
are harmonic frequencies linear or non linear
A
- non linear due to dependancies
7
Q
what does non linear mean
A
- kind of random
- doesn’t happen the same way every time
8
Q
are fundamental waves linear or non linear
A
- linear
- what we send out of the machine
9
Q
how are harmonic frequencies produced
A
- the tail end of the wave becomes distorted from the front end of the wave causing the tissue to compress
- when the tissue is compressed the density increases and the bulk modulus increases which increases the speed of sound in soft tissue
- this causes the peak rarefaction of pressure to speed up as the troughs slow down due to the tissue stiffness to decrease which increases the frequency of the wave
10
Q
what 4 ways does harmonics benefit the image
A
- the harmonic beam is NARROWER since they are best produced from the most intense part of the beam = improve lateral resolution
- grating lobes are eliminated as they are too weak to produce harmonics
- reverberation is greatly reduced or eliminated = fundamental frequency causes main being, harmonics doesnt start at probe so no main bang
- also increases contrast
11
Q
are harmonic waves always present
A
- yes just not listening for them
12
Q
what is bypass filtration
A
- used to eliminate the fundamental frequency and allow fro the harmonic signal to pass through the beam former
- like tuning a radio
13
Q
what is needed for bandpass filtering to work
A
- the fundamental frequency and second harmonic bandwidths must fit within the overall transducer bandwidth without overlapping
14
Q
what shape do the bandwidths need to be in order for bandpass filtering to work
A
- the need to be more narrow
15
Q
what does a narrow bandwidth result in
A
- a longer pulse will increase SPL that will negatively impact the axial resolution
16
Q
what is pulse inversion
A
- technique that can filter out the fundamental and leave on the the harmonic signal while maintaining a wide bandwidth
- physically removing the fundamental frequency
17
Q
how does pulse inversion work
A
- a pulse is sent out then a second pulse that is the inverse of the first
- when the fundamental echoes return from the first pulse and the inverted pulse they cancel out
- when the harmonic echoes return from the first pulse and the inverted pulse they do not completely destructively interfere so the resultant wav is the harmonic signal
18
Q
what is sacrificed when pulse inversion is used
A
- because you have to send out two pulses the temporal resolution is decreased
19
Q
what is a contrast agent
A
- liquid suspensions that can be injected into the blood stream to improve the return of echoes for enhanced visualization of blood flow or tissue
20
Q
what must the size of contrast agents be
A
- small enough to pass through capillaries but large enough to give back echoes
21
Q
what other physical attribute must be considered with the bubbles in regards to the heart
A
- must be stable enough to make it through the heart for several cycles to allow for adequate imaging
22
Q
what are the majority of contrast agents made of
A
- microbubbles of gas that are contained within a protein, lipid or polymer shell
23
Q
what is the purpose of the gas in the contrast agents
A
- creates a large impedance mismatch (increase echo return back)
24
Q
what is the purpose of the shell in contrast agents
A
- keeps the gas from dissolving into the solution
25
what are the 2 benefits of contrast
- improve lesion detection and characterization
| - increase doppler signals
26
what machine characteristics is important to consider when scanning with contrast
- output power
| - MI < 0.4 cavitation shouldn't occur
27
what happens to the bubbles at 100% power
- bubbles collapse and gas dissolves
28
what happens to bubbles at 75% power
- produce intense harmonic contrast between the contrast and the tissue
- better resolution
29
when using contrast what shows an increase in vascularity
- the echogenicity will be much brighter when there is increased vascularity due to the increase of contrast traveling through the vessels
30
What is a new way to visualize blood flow without Doppler
- coded excitation / B-flow
31
How does coded excitation work
- ultrasound pulses are digitized to help reduce noise and improve the overall imaging quality
32
How does coding help differentiate between real echoes and noise
- when teh pulses are coded the returning echoes will have teh same code no matter how weak they are while the noise signals will be an analog signal the machine will take the coded echoes and amplify them to match the rest of the tissue improving penetration
33
Why are blood echoes rejected in normal 2D scanning
- they are very weak and therefore are rejected during the compression portion of the 5 steps of teh receiver
34
What is the difference in how color Doppler is produced vs B-flow
- traditional color is overlaid the original 2D image by sending out a second pulse
- B-flow with coded excitation gives us simultaneous tissue flow without the second pulse
+ also giver full field of view no need for color box
35
Is the farm rate higher with odor Doppler or coded excitation
- coded excitation
| - better temporal resolution
36
What is one downside to B-flow
- when image is frozen cannot tell direction of flow
| - color gives better info on quality of flow
37
What does an increase in RBCs result in for the signal with coded excitation
- increase brightness of signal
38
What are 4 other improvements that occur with coded excitation
- improved sensitivity and suppress unwanted signal components
- Improved spatial resolution
+ subtle lesion identification
- improved penetration / contrast
+ subtle contrast differences
- technically difficult patients
