Ch. 17 Physics: Harmonics and Contrast Agents Flashcards
the creation of an image from sound reflections at twice the frequency of the transmitted sound.
Harmonic imaging
Where does harmonic imaging come from?
nonlinear behavior
What are the Two forms of harmonics are important in diagnostic sonography?
Tissue Harmonics
Contrast Harmonics
Why is harmonic imaging important to poor quality images?
harmonic frequency waves undergo less distortion than fundamental waves, therefore improving poor quality images.
What are the Two types of sound beam behavior?
Linear
Nonlinear
What is a linear sound beam behavior?
Proportional or symmetrical, responding in an even manner
What is nonlinear sound beam behavior?
Irregular or disproportionate, behaves unevenly
Harmonic frequency is _____
Nonlinear
Energy is transferred from fundamental to harmonic frequency as sound travels through tissue speeding up (compression) and slowing down (rarefaction).
Tissue harmonics
A sound wave is a series of ____________ and ___________.
Compressions
rarefactions
Sound travels faster during what?
Compressions
sound travels slower during what?
Rarefactions
In fundamental imaging, artifacts will be present for what two reasons?
The beam is very strong
Many different superficial anatomic
layers distort the sound beam
True or false:
Tissue harmonic signals do exist at extremely superficial depths.
False
They do not. They develop deeper in the tissue
Harmonics increases the ____________ ratio.
signal-to-noise
Where in the sound beam does max harmonics occur?
Mid-field
Where in the sound beam do no harmonics occur?
Near and far field
The relationship between the sound beam strength and harmonic creation is ________.
nonlinear
Weak beams =
no harmonics
intermediate beams =
miniscule amount of harmonics
Strong beams =
intense harmonics
A tiny amount of energy is converted into harmonics from ……
fundamental energy.
True or false:
Only the strong central part of the beam will make harmonics.
True
Beams that are most likely to create harmonics are least likely to create ______.
artifacts.
What are the Two imaging techniques that are designed to enhance harmonic reflections?
Pulse inversion harmonics (most common)
Power modulation harmonics
What separates the fundamental frequency from the harmonic with greater success than filters achieved?
Pulse inversion harmonics
An imaging technique specifically designed to utilize harmonic reflections, while eliminating distorted fundamental reflections.
Pulse inversion harmonics
In pulse inversion harmonics, what is the second pulse sent down the scan line?
an inverted copy of the original.
What happens when Fundamental signals destructively interfere?
cancel each other out
How do Harmonic signals interfere with pulse inversion harmonics?
constructively
What is the disadvantage of pulse inversion harmonics?
decreased temporal resolution and frame rate
another technique to isolate the harmonic sound beam.
Power modulation harmonics
What are the two beams that are sent out for power modulation harmonics?
A weak beam
A beam 2x as strong
During reception of the power modulation harmonic, reflections from the strong pulse are ________________________ from the second reflection
doubled and subtracted
Power modulation harmonics ___________ frame rate and temporal resolution.
decreases
also known as microbubbles (encapsulated air)
Contrast agents
Because of their different acoustic properties they create strong reflections that “light up” the structure being imaged.
contrast agents
How are contrast agents taken?
injected or swallowed
What are the 5 requirements of contrast agents?
Safe
Metabolically inert
Long lasting
Strong reflector of ultrasound
Small enough to pass through capillaries
When sound interacts with contrast agents, much __________ harmonics are created.
stronger
How do microbubbles act when struck by a sound wave?
non linear
Contrast harmonics are created during ……
reflection
True or false :
Contrast harmonic signals are weaker than tissue harmonics
False
They are stronger
a number that allows for the estimation of contrast harmonics that will be created by a sound beam.
Mechanical index (MI)
What two factors does mechanical index rely on?
Frequency
Peak rarefaction pressure
MI and harmonic production increases with:
Lower frequency sound
Stronger sound waves (substantial pressure variation)
The contrast agents need either_________ or _________to create high Mechanical Indices.
Time or strength
MI and frequency are __________ related.
inversely
MI and Peak Negative Pressure are _____________ related.
Directly
represents the potential amount of contrast harmonics produced.
MI
Low mechanical index sound beams means
no harmonics
MI less than 0.1
Intermediate mechanical index means
relatively small amount of harmonics
MI 0.1 to 1.0
High mechanical index means
substantial harmonic signals
MI greater than 1.0
Very strong harmonic reflections from bubble disruption
determines the behavior of the bubbles (linear or nonlinear)
MI
A low MI creates what kind of behavior?
Linear
