Transducers Flashcards
Ultrasound transducers
Convert electric energy into ultrasound energy and vice versa
Piezoelectric element
A material/element when deformed by pressure produce a voltage
Reverse piezoelectric
The production of pressure when voltage deforms materials
Piezoelectric elements
Quartz
Synthetic crystals (ceramics)
- PZT
- Barium
How are piezoelectric elements made
Placed in strong magnetic field at high temp. This realigns molecular dipoles. Then cooled
Curie point
The temperature in which the magnetic properties of a solid can be changed
PZT curie point 350
What would happen if you brought the crystal back to the curie point but without the magnetic field ?
It would lose its piezoelectric properties
Transducer assembly
Case Damping Element 3 matching layers Gel
Natural frequency of piezoelectric element formula
Propagation speed of the element divided by 2x thickness (wavelength)
Propagation speed of PZT
4 mm/us
Thickness of element
0.2-1 mm
True or false:
Thinner elements have lower frequencies
False. Thinner elements have higher frequencies. Think of smaller bells
How many cycle US pulse does 1 cycle of alternating voltage create
2-3
Longer alternating voltage 5-30 is what
Doppler technique
What does Fvolt = Fo mean
One transducer can have more that one frequency based on the selected voltage
The transducer is driven at one of 2 or 3 selectable frequencies by voltage pulses with the selected frequency
Frequency must fall within the bandwidth of the transducer
Multi-hertz operation
2 or 3 frequencies in the same element
What does damping do
Good and bad
Good: - Decreases n (thus decreasing PD and SPL) - Image resolution - Bandwidth
Bad:
- Decreases amplitude
- Decreases sensitivity
Image resolution is inversely proportional to depth. If you need to go further, you must decrease:
PRF
If you don’t decrease PRF when going into deeper structures, what can happen?
Range ambiguity/echo misplacement
What are piezocomposites
Other materials added to Decrease z
Increase bandwidth
Increase sensitivity & resolution
Damping material reduces
Cycles per pulse - faster decay time
What are some unwanted things that happens with a damping material
Reduces amplitude - weaker sound out, weaker echo in
Decreases sensitivity (ability to detect weaker echoes)
Continuous wave ultrasound do not have damping material true or false
True Not needed because pulses are not used. Higher efficiency Better sensitivity Worse resolution
The case/housing unit
Absorbs energy from sides of crystal
Matching layer
Reduces reflection of ultrasound at the transducer-element surface (reduce impedance)
Want more transmission, less reflection
How many matching layers are used and thickness of each
1-3 layers are used to reduce the large differences in impedance.
Thickness of each = 1/4 wavelength
Beam definition
The width of a pulse as it travels away from the transducer
Fresnel zone
Near zone
What does the near zone depend on
- Size of aperture
2. Operating frequency
Fraunhofer zone
Far zone
Beam width decreases with increasing distance from transducer
Near zone
Beam width increases with increasing distance from transducer
Far zone
Aperture
Element size/ width
What does beam width affect
- Resolution of signal at that depth
2. Intensity of the sound beam at that depth (intensity is not uniform within the beam)
At the focus, what is the size of the beam width?
Wb= 1/2 aperture size
At what point is the beam width the same as the element width
At double the NZL
If aperture increases x2 NZL
increases x4
If frequency increases by 4x NZL increases
4x
Focal length
The distance to the focus from the transducer
What do you do if you want a high frequency disk transducer to look at a superficial structure. Do you want a large or small footprint
Smaller because near zone length will be shorter, focus higher
What kind of resolution is better
Smaller - more fine details
Low level of disinfection for transducer example
Non-critical - contacts skin
When is a high level of disinfection of probe required
Semi critical- mucous membranes
When is sterilization of probe required
Critical- device enters tissue
High level disinfection methods
Cidex
Recert
What are NOT suitable disinfection methods
Bleach
Ammonia
Alcohol based solutions
Sprays
What are low level disinfection methods
5% hydrogen peroxide
CAVI wipes
Preempt
Invasive transducers
Transvag
Transrectal
Transesophageal
Catheter mounted
Why are invasive transducers good
Get much closer to the tissue
Can have high frequency without worrying about attenuation
Focus is only accomplished where?
Near zone
How can sound be focused? (3)
- Curved transducer elements
- Lens
- Phasing
The limit to which a beam can be narrowed depends on (3)
- Wavelength
- Aperture
- Focal length
Mechanical scanning
Mechanical transducer
Single element
Sector image
Historical, obsolete
Automatic/electronic scanning
Live scanning
Many frames per min, looks like a live scan
Requires arrays
Arrays
Rectangular crystals
Assembled in a row
Two main types:
- Linear (sequenced/phased)
- convex (sequenced/phased)
Linear array
Firing groups of elements at same time
Each group acts like a single element - produce a pulse and receive an echo
Thus each group produces a scan line
In linear sequence array what is the aperture
Width of a group (not element)
In linear sequence array, what would be the width of the image?
The length of array
Rectangular image
Linear image consists of
Parallel scan lines
Travelling in same vertical direction
LPF
(scan) Lines Per Frame
128 element array fired in groups of 4 = how many scan lines
125
Increasing scan lines increases
Density and image quality
What type of array produces a modified sector image
Convex array / curvilinear transducer
Phased array
Straight line of elements
Elements fired with a small time difference
What is the time delay between firing of each element in phased array?
< 1 us time delay
In phased array, a wave always travels _____ to its ______
A wave always travels perpendicular to its wavefront
A new waveform creates by secondary wavelets happens according to what principle
Huygen’s principle
Phasing (3)
- Steers beam
- Provides electronic control of location of focus
- Generates echoes from a specific location with several viewing angles
In phased array, how do we move the focus closer to the transducer?
Increase the delay, which increases the curvature
In phased array, how do we move the focus deeper/ further from the transducer?
Decrease curvature/ delay
How do we achieve curvature in phased array
Fire outside crystals first, middle crystals last
What increases resolution but there is a trade off with temporal resolution (reduces frame rate)
Multiple foci where multiple frames focused at a different depth
“Montage” of overlaid images
Further applications for phasing (6)
- Spatial compounding
- Clears out cysts
- Sharpens boarders, smoother image
- Assess large areas through small windows
- Vector array
- Variable aperture
Spatial compounding
Hit same object from different angles
More pulses per image
Vector array
Linear array + phased array
- phasing applied to linear sequence array.
Delays are the same for each group
Beam sent out in angled direction = parallelogram shaped display
Used in Doppler US
Phasing can be applied to each element group in a linear sequences array to
- Steer pulses in various directions
2. Initiate pulses at various starting points across the array
Variable aperture
When only some elements of a phased array are used to generate a pulse
In variable aperture, smaller groups produce ______
And larger groups produce ______
smaller groups - Short focal length
Larger groups - foci located at increasing depth
Side lobes
Additional beams / artifact resulting from single element transducers
Grating lobes
Additional beans resulting from arrays (multi-element structure)
They are weak beams but can hit a strong reflector and produce strong echoes (diaphragm, bone, gas)
Artifacts - look like ‘amniotic bands’
3 things that fix grating lobes
- Apodization
- Subdicing
- THI
How does Apodization reduce grating lobes?
Reduces amplitude of voltage to outside element. Weaker beam going out, weaker echo coming back. Less artifact
How does Subdicing fix grating lobes?
Subdicing of each element into a group of smaller crystals reduces inter-element interactions
ONLY for grating lobe artifacts
How does THI fix grating/side lobes?
Grating/ side lobes are too weak to produce harmonic signals
What are the three aspects of imaging resolution? Which are attributed to the transducer and which are attributed to the instrument?
- Detail/spatial (transducer)
- Contrast (instrument)
- Temporal (instrument)
What are the three types of resolution?
Axial
Lateral
Elevational
Minimum reflector separation along scan line to produce separate echoes
Axial resolution
Minimum reflector separation perpendicular/across scan line to produce separate echoes
Lateral resolution
Minimum reflector separation perpendicular to scan plane to produce separate echoes
Elevational resolution
All that THI fixes
- Side lobes
- Grating lobes
- Partial volume artifact (section thickness artifact)
How do you improve LR
- Decrease beam width
2. Focusing
Lateral resolution is equal to
Beam width
What is the lateral resolution at the focus?
1/2 the aperture
What is partial volume artifact / section thickness
Filling in of anechoic structures. Shows echoes from outside the intended scan plane
How can you reduce the axial resolution
Decrease SPL via:
- Decreasing wavelength (increase frequency)
- Decrease n (damping layer)
True or false
Axial resolution is 3mm
Structures 2 mm apart are seen as two structures
False , seen as one structure. If structures are 3mm apart or more it will be seen as two structures
Contrast resolution
Being able to separate two shades of gray
Being able to separate echoes in time
Temporal resolution - dependent on instrument. Lag = poor TR
Diagnostic ultrasound uses frequency range
2 to 20 MHz
Lower range of ultrasound necessary when (2)
- Obese (depth)
2. Transcranial (High attenuation)
Higher range of ultrasound appropriate when imaging (3)
Breast
Thyroid
Peds
Frequencies up to 50 MHz may be used for:
Intravascular
Ophthalmologic
Dermatologic
Volumetric imaging used for
Obstetrics
Breast
What is 4D imaging
3D imaging plus time
Delaying the echoes coming to the transducer allows for (3$
- Steering
- Focusing
- Aperture
Concentric rings of piezoelectric material - what is it called and why don’t we use it anymore
Annular arrays
Not used anymore because we can do the same with phasing, electronic focusing
What type of voltage is the transducer driven by
Alternating current
Other names for natural vibrational frequency (3)
Natural frequency
Operating frequency
Resonance frequency
What determines the operating frequency of an element
The thickness of the piezoelectric crystal determines its natural frequency
Transducer bean scanned by sequencing (3)
- Linear array
- Convex array
- Vector array
NOT phased array
Transducer beam scanned by phasing (2)
- Phased array
2. Vector array
Transducer beans focused by phasing
- Linear array
- Convex array
- Phased array
- Vector array
Where is the damping material found?
Attached to the rear face of the transducer
Why is the matching layer required
Transducer elements are 20x the impedance of the tissues by itself, this would create a large reflection at the skin and very little waves would be transmitted into the body
Where is the matching layer located
At the surface of the transducer
What is the beam profile
Beam profile describes the shape of the main beam also referred as hourglass shaped.
Sound beam narrows from the aperture to the transition (focal) point then diverges through the far zone
What is self-focusing effect or natural focus seen in the beam profile?
Self focusing effect is the natural narrowing of the sound beam in a non-focused single element transducer
What size is the width of the beam at 2x the near zone length
The size of the aperture
Why is focusing of the beam required
Focusing improves resolution and increases intensity of the sound beam
What are the methods of focusing?
Mechanical - lens or curved element
Electronic - phasing
Two methods of real time scanning
Mechanical
Electronic/automatic
Major difference between mechanical and electronic transducers
Mechanical has a moving part to steer the beam, electronic transducer does not
Two modes of activation to produce a beam
- Sequencing
2. Phasing
How do sequenced arrays work?
A sequenced array applies voltage pulses to a group of elements in succession to form scan lines
How does a phased transducer work?
A phased array applies voltage pulses to all the elements with short time delays to steer the beam
How is the beam directed to the right?
Voltage is applied with time delays in rapid progression from left to right directing the beam to the right, based on Huygen’s principle
What is vector array, why is it useful
Vector array is a linear phased array, converts a linear rectangular format into a sector-like FOV
How is electronic focusing achieved
By using a curved pattern of phased delays, an increase or decrease in the curvature or the delay pattern moves the focus shallower or deeper, respectively
What is the limit to multiple foci?
Temporal resolution reduces
What is variable aperture, focusing?
To focus deeper, you need a bigger aperture
For a shallow focused you need a smaller aperture
Therefore the transducer fires only the elements needed depending on the distance of the focus
For closer natural focusing, smaller groups of elements are fired
What is dynamic aperture
As the depth changes, the aperture changes to maintain a constant focal width
Huygens principe
Huygens principe is that every point on a wavefront can be considered a source for secondary wavelets. These wavelets combine to form a wavefront that heads in a direction perpendicular to the combined wavelets
What is spatial compounding
The use of phasing to strike objects from multiple angles.
Images are compounded to produce an average image which will reduce artifacts and sharpen boarders
What is the limit for spatial compounding
Temporal resolution
Difference between grating lobes and side lobes
Grating lobes involve multi element transducers side lobes are for disk transducers
What is resolution
The ability to distinguish echoes in terms of space (detail) time (temporal) and strength (contrast)
What is axial resolution
Ability to separate interfaces that line along the beam axis (one on top of each other)
How can the operator improve the axial resolution?
Reducing SPL either with wavelength or number of cycles in a pulse.
Increase frequency
How is the transducer built to improve axial resolution
Damping layers decrease number of cycles per pulse
What is lateral resolution?
Lateral resolution is the ability to separate interfaces that lie perpendicular to the beam
How can the operator improve lateral resolution?
The more narrow the beam, the better the lateral resolution
So we can reduce the beam diabetes by applying the focus to the area of interest (1/2 the aperture)
When you decrease beam width and improve lateral resolution, what other parameters are affected
Increases intensity due to focusing
Decreases ability to penetrate due to higher frequency
Lateral resolution formula
LR = Wb
What is elevational resolution?
Elevational resolution is the ability to separate interfaces that lie perpendicular to the beam axis (one in front of the other)
How can the operator improve elevational resolution
higher frequency
THI to lower beam
Which type of focusing cannot be used with a single element transducer?
Only arrays may be electronically focused
Single element transducers are fixed focused by mechanical means which include crystal shaping or the use of an acoustic lens or mirror.
Temporal resolution
Ability to separate closely spaced events in time
What determines frame rate? (2)
Image depth
Lines per frame
Divergence in the far field is determined by (2)
Crystal diameter/thickness (also aperture)
Frequency
What will increase the near zone length (2)
Large crystal diameter/aperture
High frequency
What will decrease beam divergence in the far field (2)
Large crystal diameter/aperture
High frequency
Synonyms for axial resolution (3)
Range
Depth
Radial
Which transducer is also referred to as a sector or vector transducer
Linear phased array
Which transducer produces a pie-shaped image?
Phased array
At the face of a single, unfocused transducer, the beam diameter is ??? to the element diameter
At the face, beam diameter=element diameter
At the distance of 1 NZL in an unfocused transducer, the beam diameter is equal to _____ of the diameter of the element
Beam diameter is one half of the diameter of the element at 1 NZL of an unfocused, single element transducer
At 2 NZL of an unfocused transducer, the beam diameter is ??? to the element diameter
At a distance of two near zone lengths, the beam diameter equals the element diameter
Which transducer is best for imaging deep structures in the abdo
Curved sequenced array
Frame rate equation
frame rate = PRF/lines per frame
