CH 17 - Harmonics and Contrast Agents Flashcards
the creation of an image from sound reflections at twice the frequency of the transmitted sound
harmonic imaging
is the fundamental frequency transmitted or received
transmitted
twice the transmitted frequency
harmonic frequency
harmonic frequency sound waves arise from _____ behavior
nonlinear
another name for harmonic frequency
second harmonic frequency
the image created by processing reflections that have the same frequency as the transmitted sound
fundamental image
the image created by processing reflections that are twice the fundamental frequency
harmonic image
two forms of harmonics that are important in diagnostic sonography
tissue harmonics
contrast harmonics
harmonic imaging is most useful in improving what
image quality
because harmonic frequency waves undergo less distortion than fundamental sound waves
linear systems respond in an even manner
linear behavior
other words for linear
proportional or symmetrical
a system is nonlinear when it behaves unevenly
nonlinear behavior
other names for nonlinear
irregular or disproportionate
does harmonic frequency sound arise from linear or nonlinear behavior
nonlinear
as a sound wave travels into the body, a small amount of energy is converted from the fundamental frequency to the harmonic frequency. this conversion creates _____ during transmission
tissue harmonics
why does sound travel slightly at uneven speeds through soft tissue
travels faster as it comprises and slower through rarefactions
this variation in speed creates ____ in the biologic media
tissue harmonics
the variation in speed alters the shape of the sound beam and a small amount of energy is transferred from ___ frequency to ___ frequency
fundamental to harmonic
the strength of the harmonic wave (increases/decreases) as it travels through tissue
increases
what is the significance of the gradual development of a harmonic signal?
harmonic signals cannot distort because they do not exist in the superficial depths
the use of harmonics (increases/decreases) signal to noise ratio?
increases
why do strong artifacts appear within the first few centimeters of tissue during fundamental imaging?
the beam is very strong and the many different superficial anatomic layers distort the sound beam
Is there another form of nonlinear behavior that further minimizes distortion with tissue harmonics?
the relationship between the sound beam strength and harmonic creation is nonlinear
what would happen if it were linear?
weak beams would create weak tissue harmonics. medium strength beams would create medium harmonics, and strong beams would create strong harmonics
Why do only strong sound beams create harmonic signals?
harmonics are only produced in the sound beams main axis, where the beam is strong
beams that are most likely to create harmonics are least likely to create _____
artifacts
tissue harmonics are not present as sound leaves the transducer. Where are they created?
in deep tissues during transmission
what is the behavior of harmonics?
nonlinear
harmonic frequencies are not created at the skins surface. this avoids what?
distortion and reduces noise
an imaging technique that is specifically designed to utilize harmonic reflections, which are distortion free, while eliminating distorted fundamental reflections
pulse inversion harmonics
this type of imaging creates images from the portion of the reflected sound that has the same frequency as the transmitted sound
fundamental imaging
type of imaging that creates images only from the harmonic, which is twice the frequency of the transmitted sound
harmonic imaging
instead of filters, we now use _________ to separate the harmonics from the fundamental frequencies
pulse inversion harmonics
in pulse inversion harmonics, two consecutive ultrasound pulses are transmitted down each scan line. what are these pulses
first is a typical pulse
second is an inverted copy that is out of phase with the first
wherever a compression exists in the first pulse, a _______ exists in the second pulse
rarefaction
the reflections from these two pulses are combined where
in the receiver
since the two waves are linear and one is out of phase, what happens to the waves
they destructively interfere and completely cancel each other out
when these waves cancel out, the fundamental portions disappear but what portion stays?
the harmonic portion
disadvantage of pulse inversion harmonics
2 pulses are transmitted down each scan line and twice the number of pulses are required to create each image which means the time required to create a Fram e is doubled, the frame rate is halved, and the temporal resolution is reduced
does pulse inversion harmonics increase or decrease temporal resolution
decrease
What is power modulation in harmonic imaging?
With power modulation, the harmonics are extracted by sending one strong pulse and one weak pulse down each scan line.
The fundamental portions of each wave completely cancel each other out and leave only the harmonic portion of the reflection. The harmonics create an image with less distortion, the frame rate is halved, and temporal resolution is decreased
gas bubbles that are encapsulated in a shell that are injected or ingested
contrast agents or microbubbles
what do contrast agents do
light up blood chambers, vessels, or other anatomic regions for diagnostic purposes
5 requirements that contrast agents must meet
safe metabolically inert long lasting strong reflector of ultrasound small enough to pass through capillaries
are contrast harmonics created during transmission or reflection
reflection
contrast harmonics are created during reflection as energy is converted from ____ frequency to ____ frequency
fundamental to harmonic
what nonlinear behavior of a micro bubble creates contrast harmonics?
resonance
what is resonance
when bubbles expand and compress when exposed to sound beams
when exposed to high pressure, bubbles ____
compress
when exposed to low pressures, the bubbles ___
expand
bubbles expand (more or less) than they shrink
more
because bubbles expand more than they shrink, this uneven, nonlinear change, a small amount of energy is transferred from the ___ frequency to the ___ frequency
fundamental to harmonic
what does rarefaction of a sound wave do to a bubble
expands the bubble
What is mechanical index?
a calculated number that is related to the likelihood of harmful bio effects from cavitation
depends on the frequency of the transmitted sound and the rarefaction pressure of the sound wave
effects the amount of contrast harmonics produced
Higher MI
large pressure variation low frequency strong harmonics nonlinear behavior high beam strength bubble greatly expands
Lower MI
small pressure variation higher frequency low beam strength bubble doesn't expand much linear behavior backscatter no harmonics
Why are micro bubbles strong scatterers of sound waves?
they resonate (oscillate) when exposed to sound in the 2-4 MHz range
Is there another form of nonlinear behavior of a micro bubble that creates contrast harmonics?
yes
the relationship between the mechanical index and the harmonic creation is nonlinear
(if it was linear, a low MI would create weak harmonics, etc)
Why don’t low MI sound beams create harmonics?
micro bubbles expand and contract evenly in a linear fashion
why do beams with a high MI create harmonics?
the bubbles expand and may burst (extreme nonlinear behavior) which creates strong harmonic reflections
What are the most important characteristics of contrast agents when used with harmonic imaging?
the nature of the outer shell
the gas that fills the microbubble
How do the strengths of tissue and contrast harmonics compare?
contrast harmonics created by micro bubbles are much stronger than tissue harmonics
Summary of contrast harmonics
created during reflection
nonlinear behavior
related to MI
affected by micro bubble’s shell and the gas within it
tissue harmonics
transmission
occurs during sound propagation
results from nonlinear behavior of sound beam
weak harmonic signal
contrast harmonics
reflection
occurs when contrast agents are present and when MI greater than 0.1
nonlinear behavior from microbubble
strong harmonics
