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”
What is pre-processing?
It is conditioning prior to storing in SC
What is post-processing?
Conditioning after storing in SC
What are some output devices?
- Montior
- Raster,interlaced
- Hardcopy
- Digital storage
What are some recording techniques?
- Multiimage camera
- Video tape
- SVHS
- VHS
How many lines does SVHS have?
400
How many lines does VHS have?
200
What does the monitor do?
It interlaces which increases frame build up.
If you write an image from the transducer to memory what kind of interlacing is yield?
Vertical
If you read the image from memory to the monitor what type of interlacing will you yield?
Raster
What does a Roster scan do?
It converts vertically collected data.
What is the rastor scan problem?
Frametime= 1/30 sec.
What is the inverse of time?
frequency
1/ (1/30)=
30 Hz
When can the eyes detect changes?
<60 Hz which is called the strobe effect.
Gray scale imaging was initially made possible with the use of what?
Scan converters
What do scan converters do?
FIrst store information and later display it.
What is the storage image information in the scan converter called?
Writing.
What is called when the image data is transfered from the scan converter for display on the monitor?
Read.
What does the scan converter translate the infromation to and from?
It translates infromation from the spoke formate into the video format.
How were scan converted originally constructed?
From analog technology.
Modern_________ scan converters use computer technology.
Digital.
What was the first type of scan converter and made gray scale imaging possible?
Analog scan converter
What are the advantages of digital scan converters?
- Uniformity
- Stability
- Durability
- Speed
- Accuracy
What are the two important elements of digital scan converters?
- Pixel
- Bit
What is the smallest building block of a digital picture?
Pixel.
What is a bit?
It is the smallest amount of computer memory.
A byte is a group of….
eight bits
A word of computer memory consists of…..
two bytes or bits
What is prepocessing?
It is the manipulation of image data before storage in the scan converter.
What is postprocessing?
It is the manipulation of image data after storage.
What does the amplifer do?
It corrects for amplitude loses with attenuation.
It raises all echo signals proportionally.
What is demodulation?
It is the process of extracting the low doppler frequency from the transducer’s carrier frequency.
What does the demodulator do signals?
It seperates phased shifted signals into two parts.
What does mixing a demodulator to a baseband do?
It changes into a range humans can hear.
20 Hz to 20 Khz
What do wall filters do?
They eliminate low frequencies associated with slow moving objects.
What is the doppler equation ?
Fd=(2)(v)(f0)(cosø)/C
Fd=doppler shift
f0= transducer frequency
v=velocity
ø= angle of insonation
c= spped of US in soft tissue
How do the speaks present themselves?
One speaker displays forward flow.
One speaker displays reverse flow.
How do the speakers aid in learning?
- Begin eith normal patients
- listen to signal
- Correlate to on screen waveform
- Confirm
- Profress to abnormal patients
What is Fast Fourier Transformation bins?
It is a mathematical technique for separating spectrum of fd to descrete velocity bins.
What is the frequency porporitonal to?
velocity
What is the amount of frequency porpotional to?
Gray scale brightness
What is an artifact?
It is not a 1-to-1 correspondence between image and object being scanned.
What is the assumptions of an artifact?
The reflector lies along the thin beam line.
What is echo porportional to?
echogenicity of objects.
related to impedance.
What are the 3 classifications of attenuation?
- Locational
- Attentuation
- Doppler
What are the type of locational artifacts?
- Speed error
- Refraction
- Reverberation
- Grating lobe/side lobe
- Mirror image
- Multiple reflections
- Slice thickness artifact
Describe locational speed error
should be 1540 m/s.
if less→shallower than actual
if greater→deeper than actual
Describe location refraction?
It occurs when a sound wave strikes a boundary that has a different progragation speed.
2° media of different Z→Snell’s law
Describe locational reverbation:
Appears on the display as multiple,equally spaced echoes caused by the bouncing of the sound wave between two strong reflectors positioned parallel to the ultrasound beam.
What are the two types of reverberation?
- Comet Tail→Anatomic structure
- Ring down→Gas bubbles
What is the solution of locational grating lobe?
Apodization which reduces power/energy delivered to outer elements.
What does a mirror image need?
A mirror.
e.g. tibia
What is slice thickness artifact?
Its when the transducer picks up outer curvature of bladder in elevation plane which looks like sludge/thrombus.
What are some formes of attentuation?
- Shadowing
- below highly attenuating structures
- e.g. bone, dense tissue
- below highly attenuating structures
- Enhancement (brightening)
- blwo lowly attenuating structures
- e.g. full bladder, fluid filled cysts.
- blwo lowly attenuating structures
What happens if the nyquist samples <2x the during a cycle?
- Loss of increased frequencies
- “wrap-around” effect
- “Apparent” reversal of flow.
How should you avoid wrap around
- Increase velocity scale
- Decrease baseline
- Decrease frequency probe
- Increase 90°
- Use higher PRF mode
How do you avoid NL aliasing?
- Increase PRF
Equation for PRF?
PRF=1/PRP
How do you get a shallower sample depth?
- Decrease PRP
- Increase PRF
- Increase NL
What is grating lobe?
It happens when there is energy outside of the beam.
