Flashcards in Chapter 17 Deck (39):
What is harmonic imaging?
the creation of image from sound reflections at twice the freq of the transmitted sound
What is the formula for harmonic imaging?
twice the fundamental freq
What is the freq of sound created by the transducer and transmitted into the body called?
Where do harmonic freq sound waves arise from?
What is the fundamental image?
the image created by processing reflections that have the same freq as the transmitted sound
What is the harmonic image?
the image created by processing reflections that are twice the fundamental freq
What are the two form of harmonics?
tissue harmonics and contrast harmonics
Why does harmonic imaging improve poor quality images?
b/c harmonic freq waves undergo less distortion that fundamental sound waves
What is linear behavior?
proportional or symmetrical. linear systems respond in an even manner
what is nonlinear behavior?
irregular or disproportionate. the system is nonlinear when it behaves unevenly
What are the main differences between linear and nonlinear?
linear is symmetrical and nonlinear is asymmetrical
When is tissue harmonics created?
when a miniscule amount of energy is converted from the fundamental freq to the harmonic freq. this conversion create tissue harmonics during transmission
What creates the nonlinear behavior in tissue harmonics?
sound travels at slightly uneven speeds through soft tissue-faster through compressions and slower through rarefactions
What two strong artifacts arise with tissue harmonics?
the beam is very strong
the many different superficial anatomic layers distort the sound beam
What does the use of harmonics increase?
signal to noise ratio
Do strong or weak sound beams create tissue harmonics?
Beams that are most likely to create harmonics are least likely to create ___?
When is tissue harmonics created?
during transmission and in the tissues
What is pulse inversion harmonics?
an imaging technique specifically designed to utilize harmonic reflections, which are distortion free, while eliminating distotred fundamental reflections
What is the disadvantage of pulse inversion harmonics?
that two pulses are transmitted down each scan line and twice the number of pulses are required to create each image. frame time is doubles, frame rate is halved and temporal resolution is reduced
What is power modulation harmonic imaging?
similar to pulse inversion and its designed to augment harmonic reflections
How hoes power modulation work?
the harmonics are extracted by sending one strong pulse and one weak pulse down each scan line
Another word for contrast agents?
What are contrast agents?
gas bubbles encapsulated in a shell.
What five standards do contrast agents need to meet?
strong reflector of US
small enough to pass through capillaries
Why were contrast agents designed?
to create strong reflections that actually light up blood chambers, vessels, or other anatomic regions.
Why are contrast harmonics created?
b/c microbubbles act in a nonlinear manner when struct by sound waves
When are contrast harmonics created?
during reflection as energy is converted from the fundamental freq to the harmonic freq
Bubbles ___ to a greater extent than they ____
What is the uneven behavior of contrast harmonics called?
When does the Mechanical index increase?
with lower freq
and stronger sound waves (substantial pressure variation) such as amplitude
What is the formula for mechanical index
peak rarefaction pressure/square root of freq
__ mechanical index sound beams do not create harmonics b/c the microbubbles expand and contract evenly in a linear fashion
High mechanical index is considered what?
great than 1.0
What happens when the MI is less than 0.1?
higher freq sound
low beam strength
bubbles expands very little
What happens when the MI is between 0.1-1.0?
lower freq sound
higher beam strength
bubbles expand moderately
What happens when the MI is greater than 1.0?
extreme nonlinear behavior
lowest freq sound
highest beam strength
bubble expands greatly
what are the important characteristics of contrast agents when used with harmonic imaging?
the nature of the outer shell
the gas that fills the microbubble