Harmonics And Contrast Agents Flashcards
What is the Fundamental Frequency?
Frequency of sound transmitted into the body by the transducer; main frequency of the transducer.
What is the Harmonic Frequency?
Twice the fundamental frequency; resonant frequency.
What is the Fundamental Image?
Image created by main frequency.
What is Harmonic Imaging?
The creation of an image from sound reflections at twice the frequency of the transmitted sound.
Define Nonlinear Behavior.
Means irregular or disproportionate, asymmetrical; difference of speed between compression (fast) and rarefaction (slow); creates a ‘spilling’ of energy.
Define Linear Behavior.
Means proportional or symmetrical; responds in an even manner.
What are Tissue Harmonics?
Resonance of tissue created by the conversion of a miniscule amount of energy from the fundamental frequency to the harmonic frequency as it travels through the body.
What are Pulse Inversion Harmonics?
Most commonly used technique that separates fundamental frequency from harmonic frequency by transmitting 2 inverted pulses down each scan line.
True or False: Harmonic frequencies undergo more distortion than fundamental frequencies.
False.
Fill in the blank: The 1st Harmonic is _______.
Twice the fundamental frequency.
What happens to harmonics in superficial layers?
Do not develop in very superficial layers.
Where do harmonics develop?
In deeper/intermediate layers.
What is an advantage of harmonic imaging?
Increases S/N ratio and is distortion free.
What types of artifacts do harmonic frequencies avoid?
Do not create lobe artifacts.
What is the significance of the main axis in harmonic imaging?
Arise from main axis, strongest part of the sound beam.
What occurs when fundamental signals exhibit linear behavior?
Destructively interfere and completely cancel each other out, leaving only the harmonic portion.
What is the effect of harmonic imaging on image quality?
Improves image quality because harmonic frequencies undergo less distortion.
What is Power Modulation?
A technique used to augment harmonic reflections while eliminating distorted fundamental reflections.
What is the disadvantage of using Power Modulation?
Temporal Resolution is reduced.
How does the Power Modulation technique work?
It sends 2 consecutive pulses down each scan line, with the second pulse being 2x the strength of the first.
What does the first, weaker pulse in Power Modulation contain?
No harmonics.
What happens during reception in Power Modulation?
Reflections from the first pulse are amplified to DOUBLE, cancelling out the fundamental frequency of the second pulse’s reflections.
What do microbubbles do in medical imaging?
Create strong reflections that ‘light up’ blood chambers, vessels, or other anatomic regions.
What are the requirements for microbubbles?
- Safe
- Metabolically inert (inactive)
- Long lasting
- Strong reflector of ultrasound
- Small enough to pass through capillaries
What are microbubbles strong scatterers of?
Sound.
Why do microbubbles resonate when exposed to frequencies of 2-4 MHz?
Because they are the same size as RBCs.
What are the two characteristics of microbubbles?
- The outer shell should be flexible
- The gas molecule that fills the microbubble should be large enough to remain trapped within the bubble.
Fill in the blank: Microbubbles are gas bubbles encapsulated in a _______.
[shell].
What is the Mechanical Index (MI)?
A number that estimates the amount of contrast harmonics produced; depends on the frequency and peak rarefaction pressure.
What are the three classifications of beam strength?
- Weak
- Intermediate
- Strong
What is a summary of tissue harmonics?
Not present as sound leaves the transducer.
How are contrast harmonics created?
- Created deeper in the tissues
- Created in the tissues during transmission
What phenomenon occurs during sound in rarefactions?
Much stronger than tissue harmonics; created during reflection as energy is converted from the fundamental frequency to the harmonic frequency.
What nonlinear behavior affects microbubbles when struck by sound waves?
Nonlinear changes in size of microbubbles or resonance: bubbles expand to a greater extent than they shrink.
What is the importance of peak rarefaction pressure in relation to contrast harmonics?
It expands the bubble and is most important with regard to contrast harmonics.
List the advantages of using contrast harmonics.
- Small pressure variation
- Higher frequency
- Less bioeffects
- Increased spatial resolution
- Increased contrast resolution
- Reduces artifact
What is the relationship between frequency and Mechanical Index?
Inversely related.
What is the relationship between peak rarefaction pressure and Mechanical Index?
Directly related.
What is the amount of tissue harmonics created primarily influenced by?
Nonlinear behavior in the speed of sound: sound in compressions travels faster than in rarefactions.
What are the characteristics of beams that create harmonics?
Primarily created along the beam’s main axis; least likely to create artifacts.
What is the effect of higher Mechanical Index?
- Large pressure variation
- Lower frequency
- More bioeffects
Fill in the blank: _______ refers to the strength of the beam in ultrasound.
[Beam Strength]
What is the Mechanical Index (MI)?
A number that estimates the amount of contrast harmonics produced; depends on the frequency and peak rarefaction pressure.
What are the three classifications of beam strength?
- Weak
- Intermediate
- Strong
What is a summary of tissue harmonics?
Not present as sound leaves the transducer.
How are contrast harmonics created?
- Created deeper in the tissues
- Created in the tissues during transmission
What phenomenon occurs during sound in rarefactions?
Much stronger than tissue harmonics; created during reflection as energy is converted from the fundamental frequency to the harmonic frequency.
What nonlinear behavior affects microbubbles when struck by sound waves?
Nonlinear changes in size of microbubbles or resonance: bubbles expand to a greater extent than they shrink.
What is the importance of peak rarefaction pressure in relation to contrast harmonics?
It expands the bubble and is most important with regard to contrast harmonics.
List the advantages of using contrast harmonics.
- Small pressure variation
- Higher frequency
- Less bioeffects
- Increased spatial resolution
- Increased contrast resolution
- Reduces artifact
What is the relationship between frequency and Mechanical Index?
Inversely related.
What is the relationship between peak rarefaction pressure and Mechanical Index?
Directly related.
What is the amount of tissue harmonics created primarily influenced by?
Nonlinear behavior in the speed of sound: sound in compressions travels faster than in rarefactions.
What are the characteristics of beams that create harmonics?
Primarily created along the beam’s main axis; least likely to create artifacts.
What is the effect of higher Mechanical Index?
- Large pressure variation
- Lower frequency
- More bioeffects
Fill in the blank: _______ refers to the strength of the beam in ultrasound.
[Beam Strength]
What is the term for the behavior exhibited by sound at lower frequencies and higher beam strength?
Nonlinear behavior
Nonlinear behavior occurs when transmitted sound propagates through tissue.
What is the range of Mechanical Index (MI) for higher harmonics?
0.1 to 1.0
Higher harmonics are associated with some resonance and nonlinear behavior.
Fill in the blank: The _______ is the strongest type of harmonics created during reflection off of microbubbles.
Contrast Harmonics
What happens to bubbles during extreme nonlinear behavior?
Bubbles expand greatly
This behavior is observed when contrast agents are present and with MIs greater than 0.1.
True or False: Higher MI values indicate weaker harmonic signals.
False
What type of sound is created during transmission in tissue?
Tissue Harmonics
Tissue harmonics result from the nonlinear behavior of transmitted sound beams.
What is the Mechanical Index (MI) threshold for the highest harmonics?
> 1.0
Fill in the blank: Lower frequency sound is associated with _______ beam strength.
Lower
What is affected by the microbubble’s shell and the gas within it?
Stronger harmonic signal
What phenomenon occurs when sound propagates in tissue?
Resonance
Resonance can occur as sound waves interact with tissues, especially at certain frequencies.
What characterizes the behavior of bubbles during transmission?
Bubble expands moderately
What is the relationship between beam strength and harmonic signal strength?
Higher beam strength leads to stronger harmonic signals.
What type of behavior is characterized by the creation of harmonics during microbubble disruption?
Extreme nonlinear behavior
Fill in the blank: The _______ signal is weaker when lower frequency sound is present.
Tharmonic
What occurs only when contrast agents are present?
Creation of contrast harmonics
