ultrasound physics ch 17
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what is harmonic frequency?
Harmonic imaging is the creation of an image from sound reflections at twice the frequency of the transmitted sound.
What do harmonic frequency of sound waves arise from?
IF the fundamental frequency is 2 Mhz what is the harmonic freguency?
If the harmonic frequency is 6 MHz, what is the fundamental frequency?
What are the two forms of harmonics that are important to diagnostic sonography?
tissue harmonics and contrast harmonics
What are three words that mean the same thing as linear?
What are three words that mean the same thing as nonlinear?
What is tissue harmonics?
it is when a sound wave travels in the body, a very small amount of energy is converted from the fundamental frequency to the harmonic frequency.
Is sound faster in compressions or rarefractions?
Is sound slower in compressions or rarefractions?
What are the two main reasons why artifacts appear within the first few centimeters of tissue?
- 1.) the beam is very strong
- 2.) the many different superficial anatomic layers distort the sound beam
Where do tissue harmonics develop? (superficial or deep)
Deeper in the tissue
Do harmonic tissue distort?
No, because they are not created or exist at superficial depths.
Does harmonics increase or decrease the signal to noise ratio?
Does each one create harmonics?
weak sound beams --
intermediate sound beams --
strong sound beams --
- weak sound beams --- no
- intermediate sound beams -- tiny amount of tissue harmonics
- strong sound beams -- significant amounts
Does tissue harmonics create artifacts, side lobes or grating lobes?
No very unlikely, most of the energy continues to propagate along the main beam.
Are tissue harmonics present as sound leaves the transducer?
No, they are created deeper in the tissues
WHat does pulse inversion harmonics do?
It separates the harmonics from the fundamental frequencies more successfully.
What is another word for contrast agents?
What are contrast agents?
gas bubbles encapsulated in a shell. THey are injected in the circulation or ingested. They have a different acoustic fingerprint than blood or tissue.
Do contrast agents have strong or weak reflections?
They have strong reflections that actually "light up " blood chambers, vessels, or other anatomic regions.
What five requirements must contrast agents meet?
- 1.) safe
- 2.) metabolically inert
- 3.) long lasting
- 4.) strong reflector of ultrasound
- 5.) small enough to pass through capillaries
When is tissue harmonic created?
when are contrast harmonics created?
What happens when a microbubble is within a sound beam?
the bubble grows and shrinks in relation to pressure changes.
When exposed to high pressure what happens to a microbubble?
the bubble shrinks and the pressure inside the bubble increases.
Does rarefraction or does compression have high pressure?
compression has high pressure
Does rarefraction or does compression have low pressure?
rarefraction has low pressure
When exposed to low pressure what happens to a microbubble?
the bubble expands.
Can microbubbles shrink or expand more?
bubbles expand more than they can shrink
What is more important in harmonics?
(when the bubble expands or when it shrinks)
And when does this happen?
when it expands is more important
happens during peak rarefraction
WHat does MI stand for?
What two things does the mechanical index depend on?
- 1.) the frequency of the transmitted sound
- 2.) the rarefraction pressure of the sound wave
Harmonic production increase with what two things?
- 1.) lower frequency sound
- 2.) stronger sound waves
WHat are two characteristics of contrast agents that are important?
Why are they important?
- 1.) the nature of the outer shell
- 2.) the gas that fills the microbubble
They determine the contrast agents stability and its longevity while circulating in blood.
What must the shells of the microbubble do?
- 1.) trap the gas
- 2.) designed to be flexible so they can change shape.
What must the gas in the microbubbles do?
1.) smaller gas molecules can leak through the shell and shrink the bubble
What type of harmonics is stronger?
(contrast harmonics or tissue harmonics)
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