Real-Time Considerations Flashcards
What is real-time scanning?
Anything that happens before you hit freeze
What is each image made up of?
Multiple frames per second
What is each frame made up of?
Multiple scan lines
What must a scan line wait for before sending out the next pulse?
All echoes to be received from the selected depth
What is the limitation to the amount of time it takes for pulses to be sent out?
The depth and number of focal zones the echoes must return from.
What is frame rate?
The number of times the sweep of sound is produced by the transducer (frames per second).
What type of resolution does frame rate relate to and how?
Temporal resolution.
As frame rate increases so does the temporal resolution.
What does temporal resolution allow? (3)
- Moving structures to be imaged
- Overcome motion artifacts
- Survey quickly
How many frames per second is the human eye capable of detecting?
15-20 fps
What is the common number of frame rates and how many scan lines are included?
30-60 fps (can vary)
120 scan lines
What are the factors that affect frame rate? (4)
- Depth of field
- Sector angle
- Number of lines
- Number of focal zones
How does depth affect frame rate? (2)
More depth = slower frame rate
Less depth = faster frame rate
How does sector angle affect frame rate? (2)
Narrow sector = faster frame rate
Wider sector = slower frame rate
How does the amount of scan lines affect frame rate?
Higher frame rate = less scan lines (less time to wait for each line’s echoes to return)
Lower frame rate = more scan lines
What is interpolation?
The use of information averaging in between the scan lines (made up info).
How does the amount of scan lines effect resolution?
Less scan lines = more interpolation, therefore worse spatial resolution
What type of resolution may improve with less scan lines?
Temporal resolution may improve but at the cost of spatial resolution
What is the equation to find the frame rate?
FR = c / (2 x Depth x # of lines per frame)
C= speed of sound in tissue
When do we need a higher frame rate?
When imaging structures that move at a faster rate so the motion can be accurately displayed
What are 3 factors that have an inverse relationship with frame rate?
- Lines per frame
- Number of foci
- Depth
What is the range ambiguity equation?
FR = 77,000 / (D (cm) x LPF x #foci x FR)
Why is 77, 000 significant in the range ambiguity equation?
Because it is 1/2 of 154,000cm/sec
What happens when the range ambiguity equation is larger than 77,000 cm/sec, when can this happen?
When lines are shot out faster than the echoes can return.
Impossible in 2D but possible in echo.
How long does it take for sound to travel 1 cm out and return in a scan line?
13 microseconds
What is the equation for the time to produce an image (1 frame)?
LPF x 13 (μs) x Depth (cm)
What is the reciprocal of the time it takes to produce an image? (With equation)
Frame rate
1/ LPF x 13μs x D(cm)
What is an F-number?
The ratio between the length of the focal zone and the aperature
What are the weak, moderate, and strong F-numbers?
Weak = Greater than 6
Moderate = 2 - 6
Strong = Less than 2
What does aperture refer to?
The size of a crystal or the number of crystals fired together in an array
What is “scan line density”?
The number of lines per degree or number of lines per centimetre
What scan line density is needed for appropriate resolution?
Approximately 1 scan line per degree
What happens if the SLD is too high or too low?
Too high = overwriting
Too low = Increased interpolation
How is interpolation limited?
By sectoring down to increase the number of scan lines
How does high def zoom effect resolution and SLD?
Using high def zoom will give better spatial and temporal resolution because the area of interest is smaller (increased SLD).
What type of resolution does SLD effect?
Spatial
What factors impact SLD and therefore the spatial resolution? (3)
- # scan lines
- # focal zones
- Size of the sector
What is cine-loop?
The machine stores 10-60 fps that can be reviewed after freezing.
What allows the machine to have the cine-loop feature?
A large memory capacity (RAM).
What is the advantage of cine-loop?
The visualization of tiny structures and quick movements
What is a freeze frame?
The continuous display of a single frame in the memory on the monitor
How is a freeze frame accomplished?
The same image is read over and over and the raster display continuously writes the image onto the display
What is the cathode ray tube doing when an image is frozen?
Refreshing 30 frames per second
What is a channel?
Each independent crystal and delay mechanism in an array (# of elements = number of channels)
How many channels are usually in a system?
48, 64 or 128
How do channels impact the image?
More channels = more control over steering, focus, aperture, NZL.
What is dynamic dampening?
The SPL (ringing) is limited by sending out a voltage pulse of opposite polarity immediately after the initial pulse is fired in order to counteract it.
What are side lobes?
Weaker, off-axis vibrations that radiate away from the main beam
What type of transducers are side lobes limited to?
Single disc mechanical
What is the main contributor to the creation of side lobes?
Radial vibration
What is the damage of side lobes?
They can falsely place reflectors in the image as if they came from the main beam because the machine assumes all reflections return from the centre.
How are side lobes compensated for?
An insulating ring is added to the element to suppress the radial mode vibrations
What type of transducers create fewer and less intense side lobes?
Broadband (low Q)
What are grating lobes?
The same as side lobes but are limited to array transducers
What is responsible for grating lobes?
The vibration of the crystals resulting in “crosstalk”
What are the means of reducing grating lobes?
- Apodization
- Sub-dicing
- Harmonics
- Filling the space between the crystals with air
What is apodization?
Stronger voltages are sent to the centre crystals and weaker ones are sent to the end crystals in the hopes that attenuation will not allow the weaker echoes to return.
What is sub-dicing?
Each element is divided into smaller pieces or sub-elements that will act as one crystal
How does sub-dicing work?
The sub-elements are cut to a dimension of less than a wavelength then the grating lobes will be 90 degrees or greater to the main beam
