Physics Quiz Ch 16, 17 Flashcards
Sound with a frequency of 5 MHz is created by a transducer. What is the fundamental frequency?
5 MHz. The fundamental frequency is the frequency of sound created by the transducer.
Sound with a frequency of 3 MHz is created by a transducer. What is the harmonic frequency?
6 MHz. The harmonic frequency is twice the fundamental frequency.
How are harmonics created?
The image created by processing reflections that are twice the fundamental frequency.
*It is useful in improving poor quality images because harmonic frequency waves undergo less distortion than fundamental sound waves.
Where are harmonics created?
Harmonics are created in the tissues
What type of behavior creates harmonics?
Nonlinear behavior-irregular or disproportionate; behaves unevenly
Dynamic range of information, does it increase or decrease as it is processed?
Decreases the more it is processed
What does compression do to dynamic range?
Compression reduces the dynamic range of a signal without introducing errors.
Requirements:
-the largest signal remains the largest
-the smallest signal remains the smallest
-the range of signals is reduced
Wide dynamic range vs narrow dynamic range
Wide dynamic range: -more shades -many choices -gray scale -low contrast Narrow dynamic range: -fewer shades -few choices -black and white (bistable) high contrast
Two forms of harmonics
- Tissue harmonics
2. Contrast harmonics
Where are harmonics mainly located a long the beam?
Harmonics are produced only in the sound beam’s main axis, where the beam is strong. They are not created in the weaker side lobes.
The numerical value of the mechanical index, and therefore, harmonic production, increases with..
- Lower frequency
- Stronger sound waves (large pressure variation)
*Mechanical index is directly related to peak rarefactional pressure and inversely related to frequency.
Contrast harmonics
- created during reflection off of micro bubbles
- occurs only when contrast agents are present and with MIs greater than 0.1
- results from nonlinear behavior of micro bubbles
- stronger harmonic signal
Low MI: less than 0.1
- no harmonics
- backscatter
- linear behavior
- higher frequency sound
- low beam strength
- bubble expands very little
Higher MI: 0.1 to 1.0
- some harmonics
- resonance
- nonlinear behavior
- lower frequency sound
- higher beam strength
- bubble expands moderately
Highest MI: greater than 1
- strongest harmonics
- bubble disruption
- extreme nonlinear behavior
- lowest frequency sound
- highest beam strength
- bubble expands greatly
5 requirements for contrast agents
- safe
- metabolically inert
- long lasting
- strong reflector ultrasound
- small enough to pass through capillaries
Pressure variations
Lower MI:
- small pressure variation
- higher frequency
Higher MI:
- large pressure variation
- lower frequency
Pulse inversion harmonics
Is an imaging technique designed to utilize harmonic reflections, which are distortion free, while eliminating distorted fundamental reflections
Two consecutive ultrasound pulses are transmitted down each scan line, one positive and one negative
Disadvantage of pulse inversion harmonics
Two pulses are transmitted down each scan line and twice the number of pulse are required to create each image.
The time required to create a frame is doubled, the frame rate is halved, and temporal resolution is reduced.
When the bubbles expand, will they expand with a low MI or a high MI?
high MI
Tissue harmonics
- created during transmission in tissue
- occurs as sound propagates in tissue
- results from nonlinear behavior of transmitted sound beam
- weaker harmonic signal
