C1A: Transducer Evolution Flashcards
Did old static scanners used one or multiple crystals? And what shape was the crystal?
One crystal which was disc shaped
Do modern days probes use a single or an array of crystals?
An array of multiple crystals
How was the sound steered in early probes?
It was mechanically steered (the crystal had to rotate)
Is the focal zone fixed for mechanical transducers?
Yes
What is the names of the earliest probe technology where the crystal was mechanically moved across a track?
Linear translation
Describe a wobbler probe?
A wobbler probe has a fixed element on a motor that rotates back and fourth
What type of image does a wobbler probe produce?
Sector type image
What is the main downside of the construction of a wobbler probes?
They were filled with fluid and the moving parts produced air bubbles
With mechanical transducers what are the two items you can’t change?
Frequency
Focus
How do mechanical probes obtain a linear FOV?
By using a reflector or acoustic mirror to direct sound into the patient
How does an oscillating mirror probe work? What about oscillating element
It has a mirror that moved and a stationary element… opposite for oscillating element
How do rotating wheel probes work?
They have several crystals that spin and only pulse when the element passes the FOV
Why was the annular array probe one of the biggest advances in probe technology?
You had the ability to change the focus
How was an annular array probes steered?
Mechanically
How are annular array probes constructed?
It uses concentric discs to allow focusing of the beam
How does firing the different consecutive layers of an annular array probe effect the focus?
The wider the diameter of the concentric layer, the deeper the focal point and vice versa
How are annular array probes focused?
Electronically
But they’re still mechanically steered
What is one of the most significant problems with annular array probes?
The production of grating lobes
What are grating lobes called in mechanical probes?
They’re called side lobes and occur due to radial vibration
What are grating lobes caused by in annular away probes?
Caused by length and width vibration
What are grating lobes?
low energy beams that radiate away from the main axis and contribute to artifacts
How is the element arranged in electronic array probs?
They have many small elements aligned in a linear row
What are the two main types of arrays?
Phased array and Phased linear array
What is phasing?
Phasing is using nanosecond time deals to change when the voltage stimulate the crystal
Whats the purpose of phasing?
allows for beam focusing and steering
Whats another term that describes steering the sound beam?
Vector Scanning
How will an unsteered beam travel in comparison to a steered beam?
The unsteered beam will travel in a straight line, the steered beam with be angled one way or another.
What determines which way the beam angles?
Whichever side of the probe is stimulated first, the sound will travel away from the side (e.g. if L side is stimulated first, the beam will travel to the R side)
How are the elements fired in a sector phased array probe?
All crystals are fired at nearly the same time to steer the beam across the face of the probe
What is the range of the FOV for the sector phased array probes?
90-30 degrees
Approximately how many crystals does a sector phased array probe have?
~128
Approximately how many crystals does a linear array probe have?
~64-200 elements arranged in a row
What shape is the FOV for a linear array probe?
Rectangular but can also be trapezoidal if scanning in vector mode
Do linear array probes have scan line divergence?
No
Are linear array probes associated with higher or lower frequencies?
Higher
Why is it bad to fire only a single element when scanning?
Firing a small source results in more divergence which causes a broad beam.
A broad beam gives poor resolution and sensitivity (specifically, we have poor lateral resolution)
Does firing a single element effect our ability to focus and steer the beam?
Yes, it takes the ability away completely
What is our overall field of view composed of?
Each individual scan line firing
What effect does firing multiple elements in a group or segment result in?
A larger group means less divergence so better resolution (lateral) and sensitivity.
It also allows us to steer and focus the beam electronically.
What is the downside of fining multiple elements in a group of segment?
We have fewer scan lines which reduces spatial resolution and can cause gaps between beams
How do we fix gaps caused by firing elements in groups?
We use segmental sequencing
How does segmental sequencing work?
We fire a group of crystals to create a beam and then we fired a second group which is only one crystal down the row.
This causes the beams to overlap so their are no empty spaces between the beams
How are the crystals in a curved linear array probe different than a linear array probe?
It‘s rectangular shaped elements are curved on one end instead of flat
What is the advantage of a curved linear probe?
It has a large footprint and FOV and has good near field and far field resolution
What are the disadvantages of a curved linear probe?
Resolution in the far field can be degraded as the distance between beams in increased
How do we correct for grating lobes?
Apodization
What is apodization?
A process in which the voltages of the crystals are gradually weakened from inner to outer… this reduces cross talk
(e.g. outside crystals dont ring as much as the inner ones)
What are the two ways to obtain 3D and 4D images?
Freehand, using a traditional array probe
AND
Volumetric probe
How do volumetric probes work?
The beam is swept over the area of interest to get in info needed for 3D/4D (can be mechanically or electronically swept)
How are 1.5D probes different than 1D probes?
1.5D probes allow you to focus the beam in the x, y AND z planes which reduce artifacts (especially beam width artifacts) and improves the image
How are the elements arranged in 1.5 D probes?
There are 3 rows of crystals
1D probes are used for what type of scanning?
2D
What kind of backing to CW probes have? And what kind of voltage do they use?
They’re air backed and have a continuous AC driving voltage
What is the ‘focal zone’ or sample volume for CW?
Zone of sensitivity
What type of bandwidth is associated with electronic array probes?
Wide bandwidth
What limits the number of frequencies that be can drive the probe to ring at?
The size of the bandwidth and attenuation
What type of frequencies are too weak to be detected by the probe?
Any frequencies that have an amplitude of less than half of the resonance frequency
What is another name for the usable bandwidth?
The 6 dB bandwidth
When referring to bandwidth, what value is equal to half of the amplitude or 1/4 the intensity of the resonant frequency?
6 dB
A drop in what number is equal to a reducing of half the amplitude?
6 dB
Whats the equation for fraction bandwidth?
FB= Bandwidth/Frequency
A probe with a faction bandwidth of what value is considered to be a broadband design?
80%
What is another term to describe bandwidth?
Quality Factor or Q factor
What is the equation for Q factor?
Q= Frequency/Bandwidth
What kind of Q factor do we want for 2D scanning? And for Doppler?
Low Q factor for 2D (because we want a short pulse and a wide bandwidth)
High Q factor for Doppler (because we want a long pulse and a narrow bandwidth)
Whats the relationship between pulse length and bandwidth?
Inverse
Short pulse = wide bandwidth
Long pulse = narrow badwidth
Whats the relationship between bandwidth and Q factor?
Inverse
Modes requiring more sensitivity (CW Doppler) will have what type of bandwidth?
Narrow bandwidth (which means higher Q factor)
2D probes are used for what type of scanning?
3D and 4D
What happens to the focal point if the outer crystals of a probe are fired before the inner crystals?
The focal point will be closer to the probe
What are two distinct qualities of a vector probe?
1) they are capable of expanding the probes field of view
2) they allow the operator to steer the Doppler colour box
Does segmental sequencing increase the number of scan lines we’re sending out
Yes
