Module 1: Transducer Design Flashcards
What does a transducer do in terms of energy?
Converts energy from one form to another
What is the transducer in terms of singals? What causes it to do it?
- Piezoelectric crystal or element
- Signal conversion device
What is a crystal in terms of the ultrasound probe?
Piezoelectric material
What is the element in the U/S probe
Another name for piezoelectric crystal
What is the scan head in the ultrasound probe?
Another name for transducer (or the sonography)
What is the probe?
Another name for transducer
What is the transducer assembly?
The housing and internal circuitry
A single element in the ultrasound probe can be used to do what?
Send and receive sound
Some of the more common parts of the U/S probe is what?
- Housing
- Backing material
- Crystal
- Matching layer
What does the housing do in the ultrasound probe?
Contains all probe components
What is the backing material in the ultrasound probe made of?
Mixture of metal, plastic, or epoxy bonded to the back of the crystal
What is the crystal in the ultrasound probe?
A ceramic element that has piezoelectric properties
What is the matching layer in the ultrasound probe used for?
Used to reduce sound reflection from the skin and enhance sound transmission
The three main parts of the ultrasound probe?
- Crystal
- Backing material
- Matching layer
Who discovered the piezoelectric principle?
Jacques and Pierre curie
What does the piezoelectric principle explain?
How some materials can convert electric energy to mechanical and vice versa
Which natural materials have piezoelectric properties (and used in early days)?
Quartz, lithium sulphate, Rochelle salt, and tourmaline
What crystals are used in modern day equipment?
Man made ceramics like
1. Lead zirconate
2. Lead titanate
3. Barium titanite
4. Lead zirconate titanate (PZT)*
5. Polyvinylidene fluoride (PVFD)
What is the piezoelectric (direct) effect?
When a mechanical pressure deforms the crystal which changes the orientation of the electric dipoles producing a small electric voltage
What is the reverse (indirect) effect?
Where an electric voltage changes the orientation of the dipoles causing the crystal to expand and contract
What are electric dipoles?
Essentially the molecules within the crystal and the have a positive charge at one end and a negative charge at the other end
What else can electric dipoles be influenced by?
Electrical or magnetic fields
Normally the dipoles are in what kind of alignment?
Random alignment
What does the random alignment of dipoles do for ultrasound?
They make the crystal inefficient for vibration when an electrical current is applied
If the dipoles are in alignment then what happens?
The vibration of the crystal will be much improved
When the crystal vibrates we must consider what?
The different modes of vibration that may occur
In early probes the crystal was what? This accounted for what:
Disc shaped and could vibrate in a thickeness mode or a radial mode
What are the three modes of vibration for modern day probes?
- Thickness
- Length
- Width
For modern probes what are the most desirable mode?
Thickness
Synthetic materials are used in the production of what? This does what?
Synthetic materials are used in the production of the crystal so that more pure product can be develop
How do we make substance align properly to enhance the piezoelectric properties for ultrasound production?
Currie temperature
What happens when we heat something above the Currie temperature?
The bonds between the molecule weaken
What is the Currie temperature for PZT?
350 degrees
What is the Currie temperature process?
- Heat above the Currie temp to weaken bonds
- Substance is subjected to an electrical field then the dipoles will align accordingly
- The substance is then cooled and the bonds strengthen
Heat can polarize what?
The dipoles
Reheating can do what?
Potential depolarize as well
In terms of frequencies the crystal determines what?
Frequencies a probe can emit
What are the different kinds frequencies a crystal can emit?
- Resonant frequency
- Driving frequency
- Operating frequency
- Harmonic frequency
What is resonant frequency? How is it determined?
The one at which the crystal likes to ring at. It is determined by the crystal material and thickness and is also called the fundamental frequency
What is driving frequency?
It is determined by the AC voltage sent to the crystal. If the voltage is altered then the crystal can be forced to ring at a different frequency then the fundamental
What is operating frequency?
The one that you are using to scan
What is 2nd harmonic frequency?
Two times the resonant frequency
The frequency of the crystal relates to what?
The propagation speed of sound and the thickness of sound
The thickness of crystal which determines its resonance frequency is equal to what?
1/2 wavelength
To build a 4 MHZ probe we do what?
We find the wavelength for 4 MHz and divide it by 2
If we double the thickness of the crystal what do we do to frequency?
It will half the frequency
When calculating the crystal thickness it is important to use what as the constant?
The speed of sound in the crystal as the constant
What is another name for backing material?
Damping block
What is the purpose of backing material?
- Reduce the SPL which will improve axial resolution.
- Reduce the amplitude of the wave and reducing sensitivity
- Absorbs sound so that reflections don’t occur from behind the crystal
What is backing material usually made of?
Epoxy resin with metal powder (tungsten) that is stuck on back of the crystal
As we increase the amount of damping material what will we do?
Shorten the length of the pulse.
To absorb sound so that reflections don’t occur the z value of the damping material must be what?
Comparable to the element, typically around 2-3 cycles per pulse
What is dynamic damping?
Electronic means to suppress the ringing of the crystal.
How do we apply dynamic damping?
We use an inverse wave
What is the matching layer? And why is it important?
Bridging the impedance mismatch between the crystal and the skin is quite large and without the matching layer much of the sound would return to the probe before entering the patient.
The matching layer has a z value between the crystal and skin to do what?
Reduce the amount of the reflection
What do we need to remember about cutting the matching layer?
To cut the thickness of the matching layer to 1/4 lambda
1/4 lambda helps create destructive interference of waves that do what?
Reverberate between matching layers
How many matching layers are there?
Typically more than one layer is used since there are multiple frequencies that come out of the probe
Many matching layers will do what?
It will accommodate multiple frequencies thus improving the transmission and reception of a wide bandwidth of frequencies
What is also considered a matching layer between the probe and the skin?
The gel we use.
What is the Z value of gel?
Somewhere between the last matching layer of the probe and the skin
What is probe excitation?
The way that the probe is excited with voltage helps determine the driving frequency
What is spike voltage method?
- Older technology for probe excitation
- Uses a direct current (DC) to vibrate the crystal
What is burst voltage method?
Newer technology for probe excitation
How does Spike voltage work?
The current from the pulsed hits the crystal where one spike is equal to one pulse.
In spike voltage method the driving/operating frequency is always equal to what?
The resonant frequency
DC current is also called what?
“Saw tooth” voltage because of the appearance
What is frequency bandwidth?
The range of frequencies that are produced by a pulse
Normally when the crystal is stimulated it will ring at what?
Resonant frequency and there is a very small range of frequencies produced
When we dampen the crystal to shorten the length of the pulse what happens?
A greater range of frequencies are emitted from the probe
If we were to graph the bandwidth what would we see?
We would see the range of frequencies that are produced on the X axis with the resonant frequency having the highest amplitude (y axis)
Why do we see a range of frequencies when we graph the bandwidth?
The crystal thickness and material helped determine the most efficient frequencies to ring at
The shorter the pulse what happens to the bandwidth?
The wider the bandwidth
this is more desirable for better resolution
Having a wider bandwidth means what?
We have more options for what driving frequency we can choose
This is on top of better resolution
The frequencies that we can drive the probe are limited by what?
The size of the bandwidth and attenuation
Any frequencies that have an amplitude of less than half of the resonant frequencies are what?
Too weak to be used by the system
The usable bandwidth is referred to as what?
The 6dB bandwidth.
What is 6dB equal to?
1/2 the amplitude or 1/4 intensity of the resonant frequency
It is the damping material that effects the bandwidth just as it effects what?
The SPL
If we increase the amount of damping then we should do what?
Shorten the pulse and increase the range of frequencies that are emitted
remember that the sensitivity is reduced
What is fractional bandwidth?
A common way to express the bandwidth of the probe
A probe with a Frequency bandwidth over 80% is considered to be what?
Broadband design
What is the formula for frequency bandwidth?
FB = Bandwidth/ frequency
What is quality factor or Q?
Another term for describing the bandwidth of a probe
Q or quality factor is a reciprocal of what?
Frequency bandwidth
It is desirable to have a low Q for what?
2D scanning and it gets higher as we use
1. Doppler
2. Pulsed doppler
3. CW doppler
To optimize the 2D image we use more what?
Damping to shorten the pulse for better axial resolution
Modes that require more sensitivity will benefit from what?
A narrower bandwidth or higher Q (CW doppler)
How does an increase in bandwidth affect q factor?
A lower Q factor
