Module 5: Real Time Considerations Flashcards
Real time scanning create an image that consists of what?
Multiple frames per second and each frame is made up of multiple scan lines
During real time consideration each scan line must wait for what?
All echoes to be received from the selected depth prior to the next pulse being sent out
What is the limitation for depth for real time scanning?
The limitation is the depth through a greater number of focal zones will increase the time it takes for each frame to be produced also
What is frame rate?
The number of times the sweep of sound is produced by the transducer (frames per second)
What is frame rate related to?
Temporal resolution in that if the frame rate increases then so does temporal resolution
Good temporal resolution gives use the ability to do what?
Image moving structures, overcome motion artifacts and surgery quickly
The human eye is capable of detecting flicker at what?
15-20 fps
Frame rates in imaging are what?
~30-60 fps and include ~120 lines
The factors affecting frame rate are what? 4
- Depth of field
- Sector angle
- Number of lines
- Number of focal zones
What is the formula for Frame rate?
FR = ((77,000)/(LPF x depth x # of foci))
Can also be called what?
Range ambiguity equation
The time to produce an image or 1 frame is what?
LPF x 13microseconds x depth(cm)
The frame rate is the reciprocal of the time to produce an image therefore what formulas can we see?
What is scan line density?
The number of lines per degree of number of lines per centimetre
What is the importance of line density ?
~1 degree is needed for appropriate resolution
Too low density needs what?
More interpolation and if its too high then overwriting will occur
Aside from the axial, lateral and elevational resolution we need to consider scan lines as another aspect of what?
Spatial resolution
Things that effect the scan line density have an impact on the details of what? 3
The images we see such as
1. # of scan lines
2. Number of focal zones
3. Size of the sector
What is cine loop?
When the system stores numerous frames into the memory (~10-60 fps) that can be reviewed after freezing
What does the machine need to run Cine loop?
Large memory capcity
What is the advantage of a large capacity?
It can improve the efficiency of seeing tiny structures missed during the real time interrogation like valve motion in the heart during cine loop
What is freeze frames ability?
The continuous display of a single frame in the memory on the monitor
How is freeze frame accomplished?
The same image is read over and over and the raster display continually writes the image onto the display
What is a CRT?
A CRT is still refreshing 30 frames per second even though the images is frozen
What are side lobes?
Secondary projections of ultrasound energy that radiates away from the main beam
Side lobes are limited to what?
Single disc mechanical probes
What is a key contributor for side lobes?
Radial mode vibration
What can side lobes do? Why?
- Falsely place reflectors in the image as if they came from the main beam
- The machine assumes that all reflections must come from the center
To compensate for side lobe artifacts, what do we use?
Insulating ring is added to the element to suppress the radial mode vibration
What are grating lobes?
Like side lobes but are specific to array probes
What is the assumption made by the machine with the result being an artifact?
The same as side lobes
The vibration of the crystals is again responsible for what during grating lobes?
Crosstalk
What do we use to reduce grating lobes?
Apodization
What is sub dicing?
- Another means of eliminating the impact of grating lobes
- Each elements is divided into smaller pieces or sub elements that will act as one crystal
For grating lobes if the sub elements are cut to a dimension less than a wavelength than what happens?
Grating lobes will project at 90 degrees or greater to the main beam
What does channels refer to?
The number of elements in the array?
Systems usually have how many channels?
48, 64, 128
More channels means what in terms of beam characteristics?
More precise control
What’s the moderate range for frame rate?
5-40
