How are harmonic images created?
They are created from sound reflections that are TWICE the frequency of the probe.
For example: 2 MHz pulse is the FUNDAMENTAL frequency of the probe. The HARMONIC frequency would be 4 MHz.
Where do harmonic frequency waves arise from?
They arise from non-linear behavior of waves as they travel through the tissue.
What is another name for harmonic frequency?
Second harmonic frequency.
How do waves vary from a fundamental frequency to harmonic frequency?
- Waves move slightly faster in compression and slightly slower in rarefraction.
- The peak travels faster than the trough.
- So a miniscule amount of energy is transferred from the fundament to harmonic frequency during its travel.
When does the strength of the harmonic wave grow?
It grows as sound travels in the tissue.
What is the best use of harmonic imaging?
It is best used in deeper structures.
Why might artifacts occur with fundamental frequency?
Artifacts occur in the first few cm of tissues due to the strong signal and many superficial layers distorting the beam.
Harmonic signals do not even exist in superficial layers which reduces artifacts because?
Harmonic signals cannot distort early in the path.
What does less distortion lead to?
It results in a cleaner image from strong beams in deeper tissue.
How is a harmonic image created?
It is created using only echoes that are two times the initial frequency.
ie: 2 MHz yeilds 4 MHz harmonic.
Why are microbubbles injected into the vascular system?
Microbubbles, also known as contrast agents, are injected to create stronger reflectors.
What are microbubbles?
Gas bubbles encased in a shell.
Why do microbubbles create stronger reflections than RBC’s?
They are larger than RBC.
What are some of the contrast requirements?
- Safe
- metabollically inert
- long lasting
- Strong ultrasound reflector.
- Can pass through capillaries.
How are contrast harmonics formed?
They are formed as sound travels through a tissue.
What type of behavior do microbubbles have when they are hit with sound waves?
Non-linear behavior.
What is the response of microbubbles to high and low pressure variation (compression, rarefraction) of energy in sound waves?
Microbubbles grow and shrink in response.
When are greater harmonics formed?
During reflection.
What will happen to a microbubble when it is hit with the high pressure of a sound wave?
The bubble will shrink, and pressure in the bubble increases so it wont shrink.
What will happen to a microbubble when it is hit with the low pressure of a sound wave?
The bubble expands, and pressure in the bubble decreases.
What is the term used to describe when bubbles expand more than they shrink?
nonlinear.
What is resonance?
The nonlinear, uneven change in the size of the bubble.
What does nonlinear behavior create?
Hermonic frequency.
What does the mechanical index (MI) estimate?
The amount of harmonics created.
What does MI equal?
MI= peak rarefraction pressure in soundwave
If the strength of the wave increases (higher and lower pressure, amplitude), what will happen to the harmonics?
Increase.
If the frequency goes down, what will happen to the MI and harmonics?
Increase.
As the MI increases, what will happen to the harmonics?
Increase.
What does a low frequency and a high amplitude do to harmonics?
Will increase harmonics.
Low MI (<.1)
No harmonics.
Moderate MI (0.1-1.0)
Small amunt of harmonics
High MI (>1.0)
High amount of harmonics.
Describe shaken saline:
Causes bubbles with no shell, so they are short lived.
What do flexible shells do?
They make it easier for the bubble to get through capillaries.
Contrast harmonics are stronger than what?
Tissue harmonics.
