Lesson 4 (Part 2) Flashcards
Where are harmonics generated?
In the tissue
What is an example of an aftifact?
Anterior Reverberation
- main bang
Anterior Reverberation
Reflections between the transducer face and skin surface cause multiple horizontal lines of echoes to appear in the near field
- can also occur at the ribs
What are some drawback of harmonics? (3)
- Decreased penetration
- Worse Axial Resolution
- Narrower bandwidths
Why is decreased penetration a draw back with harmonics?
Higher frequency means more attenuation
Why does axial resolution weaken with harmonics?
Because the transducer must fit in the bandwidths of both the fo and 2fo, therefore, the bandwidth on each will be small
- compared to the overall bandwidth of the transducer
What does it mean if you have a narrower bandwidth? (3)
- Longer SPL
- Larger (worse) axial resolution
- Worse resolution
How can you solve the problems associated with narrower bandwidths?
Pulse Inversion Harmonic Imaging
Pulse Inversion Harmonic Imaging
2 pulses are sent out back to back, one at a regular pulse and the other at a inverted pulse
Where are harmonic signals produced in?
Tissues
What happens to the echo of the fundamental signal?
Destructive interface
- they will cancel each other out and end up with no signal
What happens to the echo of the harmonic signal?
Non sinusoid, meaning when combined they wont cancel each other out harmonic signal will be preserved
What does wider bandwidth allow for? (3)
- Shorter SPL
- Smaller (better) axial resolution
- Better resolution
What happens to the frame rate with sending out multiple pulses?
It decreases
What is another word for detection?
Demodulation
Detection
Is the conversion of echo voltages from radio frequency form to amplitude form
Compression
The process of decreasing the differences between the smallest and the largest echo amplitudes to a usable range
What function does compression perform?
Dynamic range
Dynamic range
The power ratio of the largest to smallest amplitude that a system can handle
- relationships between the weakest and the strongest echos expressed in decibles
What is the units for dynamic range?
Decibles
- dB
What is the formula for power ratio?
Voltage ratio^2
What does 60dB dynamic range mean?
That the strongest echo is 10^6 times stronger than the weakest echo
What is the dynamic range for amplifiers?
100-170dB
What is the dynamic range for displays?
30dB
What is the dynamic range for human vision?
20dB
What do greater values indicate?
The ability to detect weaker echoes
- greater sensitivity
What can the larges power approximately be?
100x the smallest
- for our viewing of the display
What do amplifiers amplify?
The weaker signals compared to the larger ones
How is compression adjusted?
It is operator dependent
What does controlling the compression allow for?
The reduction of dynamic range by assigning some weak echo amplitude values to zero or by assigning some of the strongest to max
How does compression reduce dynamic range?
With selective amplification
- contributes with contrast resolution
Contrast resolution
Being able to separate two different echoes as two different shades of gray
What does compression affect?
Our contrast
What happens when you decrease dynamic range? (2)
- More contrast
2. Grainier image
What happens when you increase dynamic range? (2)
- Less contrast
2. Smoother image
