Test 3 Flashcards
(84 cards)
What does the scan converter do?
It writes data to memory/pixel (“picture element”)
What are two types of scan converters?
- Analog→continuous
- Digital→Discrete steps (with clock)
What are some examples of a analog scan converter?
- Actual BP
- Tidal volume
- EKG
- Acutal body temperature
What is the scan converter directly related to?
The nyquist limit
What does the nyquist limit mean?
“speed limit”
It is the highest Doppler frequency or velocity that can be measured without the appearance of aliasing.
Formula for nyquist limit is:
Nyquist limit (Hz)=PRF (Hz)/2
What are two ways to avoid aliasing?
- Raise the nyquist limit
- Reduce the Doppler shift
What happens when the sample volume is deep?
The PRF is low and the Nyquist limit is low.
- Velocity is sampled fewer times each second, and the system is unable to accurately measure high velocities.
- Deeper sample volumes have a lower pulse repetition frequency and create aliasing.
What happens when the sample volume is shallow?
The PRF is high, velocity is sampled many times each second, and the Nyquist limit is high.
Sampling always needs to be…..
greater or equal to 2x per cycle.
What does the scan converter subsitute?
It substitutes high frequency for low frequency.
How is digital format demonstrated?
It is demonstrated by bits and pixels.
What is the basic storage device?
“bit”
What are the 2 levels for a bit?
On & off/
(0 or 1)
What does n equal?
n=#bits
what does 2n equal?
The number of combinations.
What does the scan converter convert from?
It converts from binary (base 2) to decimal (base 10) and vice versa;
What is interpolation?
It is a method of constructing new stimulated data points to fill in the gaps.
What does the scan converter allow you to do?
Read/write zoom
What is the goal of fill-in interpolation?
To fill in the gaps of missing data in a way that cannot be detected by the observer.
It is a form of preprocessing.
It improves visibility to see around the boarders of round structures.
Describe Write:
Image you get before you freeze.
It is clear.
It zooms to an area of interest and writes to full SC matrix.
Describe read:
Image you get after you freeze.
It is blurry.
Read from SC matrix only. Area of interest after “freeze”