Lesson 3 (Part 2) Flashcards
What do longer paths have? (2)
- Greater attenuation
- Later arrival times
- the later it arrives the more amplified it is
What is an increase of gain with depth known as?
Time gain compensation slope
What is the TGC slope measured in?
dB/cm x MHz
What is the TGC slope for soft tissue?
1 dB/cm x MHz
- 0.5 dB/cm x MHz each way
What is the max the TGC slope can be?
100dB
- 1x10^10 x
What is lateral compensation used for?
Adjacent structures that have different attenuations
What is the drawback for amplification?
It also amplifies noise
What does the amplification of noise result in? (2)
- Grainier signals
2. Weaker echos from the electric current
What does attenuation depend on?
Frequency
What does TGC compensate for?
The effect of attenuation on an image
What determines max imaging depth? (2)
- Attenuation
2. Max amplifier
What is the max amplifier determined by?
Noise
Why does the max amplifier determine imaging depth?
Because if it was turned up past too much then the noise would take over the screen and it would not be readable
What happens to the echo voltages after amplification?
They are digitalized
ADCs
Analog-to-digital converter
Analog-to-digital converter
Convert the voltage from an analog to the digital form
- goes from voltage to numbers
What does analog mean?
Proportional
What does digital mean?
In the form of discrete numbers
- binary numbers
What happens to the echo voltages after they get digitalized?
They are replaced by a series of numbers and further manipulation of the echoes is accomplished as digital signal processing
- they are digital delays now
What does ADC do?
It interrogates the incoming voltages at the regular intervals
What does ADC determine?
Its values at each interrogation instant
What must the interrogation rate be?
Twice the highest frequency involved
Why must the interrogation rate by twice the highest frequency involved?
In order to preserve the harmonics
What happens to the echo voltages after digitizing?
The echo voltages pass through digital delay lines to accomplish reception dynamic focuses and steering functions
What are echo delays the same as?
Pulse delays
- but they are digital delays
What does echo delays allow? (2)
- Focus
2. Steering
What happens after echo delays?
The echo voltages are added together in the adder (summer) to produce the resulting scan line
What does the summer (adder) do?
Produces the scan line
What are 2 other components that are accomplished as part of the summing process?
- Reception apodization
2. Dynamic aperture functions
What is the beam former responsible for? (5)
- Electronic beam scanning
- Steering
- Focusing
- Apodization
- Aperture functions with arrays
