Module 2 - Transducers, Ultrasound modes and Beam forming. Flashcards
What happens when voltage is applied to a piezo electric crystal ? (PZT or LZT in the case of ultrasound transducers)
It produces strain (3 reversed) across opposite faces of the crystal.
What is strain?
the change in shape due to applied stress (this can be voltage or pressure) It is proportional to applied voltage.
What is the piezo electric effect?
is the production of a voltage on the crystal when it is deformed by returning ultrasound pulses or waves.
What is the inverse piezo electric effect?
The production of a dimensional change in the crystal when a voltage is applied. This effect is used to produce ultrasound pulses or waves.
What happens if the frequency of a voltage applied to a PZT crystal changes?
The CW ultrasound wave possesses this new frequency.
What happens to an LZT crystal when a voltage spike is applied? (in terms of oscillation and frequency)
It will oscillate at it’s natural (resonance) frequency.
What determines the natural (resonance) frequency of an LZT crystal?
The crystals physical dimensions. The thickness of the crystal is equal to precisely ½ the wavelength of the resonance oscillation produced.
What are the equations for determining the natural resonance frequency of a crystal given it’s dimensions?
λ = 2L and if the speed of sound in the piezo-electric material is c = f λ then we can determine the frequency as f = c/ λ = c/2L Where L = LZT thickness f = frequency Hz
What is the duty factor (DF) of an LZT crystal?
The duty factor (DF) or duty cycle is the time that the LZT crystal produces a sound wave pulse compared to the total time.
What is the Pulse Repetition period (PRP)?
the time delay between each voltage pulse to the LZT crystal. Measured in seconds.
What is the pulse repetition frequency (PRF)?
The pulse repetition frequency, PRF or PRR, is the reciprocal of the pulse repetition period. The PRF is measured in Hz, and may also be called the pulse repetition rate (PRR). It is the total number of pulses transmitted each second. The PRR is limited by the time it takes ultrasound to travel in tissues as the machine must not transmit again until all detectable echoes from the previous pulse have been received.
What is the equation relating PRP and PRF?
PRF = 1/PRP
What effect does depth of penetration have on the PRF?
If the depth of penetration is small a high PRF can be used so the frame rate will also be high. Conversely a large depth of penetration will cause the machine to use a low PRF.
What is a transducer?
A device that transforms one kind of energy into another. IN an ultrasound transducer the piezo-electric crystal acts as both as an input and output transducer utilizing the inverse and normal piezo electric effect.
List the components of an ultrasound transducer.
LZT crystal (transducer), Electrodes, Matching layer, damping block, housing, insulating material.
What are the role of the electrodes in an ultrasound probe?
Electrodes are connected to the front and back of the crystal surface to provide the excitation pulse and to transmit the output voltage signal upon the crystal receiving the ultrasound echo.
What is the role of the damping block in an ultrasound probe?
A damping block is attached to the rear surface of the transducer to reduce the time duration of the ultrasound wave following the applied voltage spike. It also;
- Absorbs ultrasound waves of the back of the transducer
- Has a similar acoustic impedance to the crystal to optimise transmission and increase energy removal
- has a sloped rear surface to increase the attenuation path length and prevent reflection
- constructed of highly absorbing material
What is the role of the matching layer in an ultrasound probe?
The matching layer optimises transition of the sound from the transducer to the patient. The acoustic impedance of the LZT crystal differs significantly from that of skin/soft tissue. The layer of matching material is added to more nearly match the acoustic impedance of the human skin so that as much of the ultrasound energy as possible passes into the patient.
Note the thickness of the PZT is generally 1/2 the wavelength of the ultrasound produced and the thickness of the matching layer is generally 1/4 the wavelength of the ultrasound produced and is sometimes called the quarter wave matching layer.
The matching layer further dampens the transducer and so contributes to broadening the bandwidth and shortening the pulse duration.
What is the role of the transducer housing in an ultrasound probe?
Maintains electrical safety and is made of plastic.
What is the role of the insulating material in an ultrasound probe?
An acoustic and electrically insulating material is placed between the piezoelectric crystal and the transducer housing to reduce the risk of electric shock and isolate the crystal from any vibration due to movement of the transducer.
What is transducer damping?
a material (damping element, damping material) bonded to the back of the piezoelectric element of a transducer to limit the duration of vibration.
Why is a transducer damped?
To shorten the impulse response of the transducer to generate short signals.
Why is it important for the transducer to generate short signals? (eg why is a transducer damped explained)
The backing material has the effect of decreasing ringing of the PZT crystal. (limiting duration of vibration/shortening impulse response) It therefore shortens the spatial pulse length and so improves axial resolution (this is why the backing material is used). The backing material also has the effect of widening the bandwidth and therefore decreasing the quality factor. (because q factor is the central frequency divided by the bandwidth) Thus, diagnostic transducers are wide bandwidth, low quality factor transducers.
List why is it important for the transducer to generate short signals?
decreasing ringing of the PZT crystal limiting duration of vibration/shortening impulse response shortens the spatial pulse length improves axial resolution widening the bandwidth decreasing the quality factor
What is the bandwidth of an ultrasound transducer?
The range of frequencies produced by the transducer
How do you calculate the Q factor of an ultrasound transducer?
Central frequency (natural/resonance)divided by the bandwidth.
Define sensitivity of a transducer
The ability of the transducer to detect reflected ultrasound and generate an electrical signal.
What is the central frequency of a transducer?
The frequency noted on a transducer is the central or center frequency and depends primarily on the backing material. Highly damped transducers will respond to frequencies above and below the central frequency.
What happens when a transducer is highly damped?
Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range (bandwidth) provides a transducer with high resolving power. Less damped transducers will exhibit a narrower frequency range and poorer resolving power, but greater penetration.
What is the equation for calculating bandwidth?
B = fU – fL (Hz)
If the resonance frequency is denoted by f0 and the lower and upper half-power (or half-energy) frequencies are written as fL and fU respectively, then the bandwidth, B, is defined as (above)
What is the equation for determining quality factor?
This is a measure of the sharpness of the resonance, and can be expressed relating bandwidth to the resonance frequency.
Q = f0/B = f0/(fU - fL)
The Q-factor is thus a dimensionless quantity that is large if the damping is small, and small if the damping is large.
What are the range of desired Q factors?
For CW situations you want an ultrasound transducer with Q around 700 to 800, while pulse ultrasound transducers have a Q in the range 2 to 3.
Explain what is meant by the spatial pulse length, SPL, and state approximately how many wavelengths this involves
The spatial pulse length, SPL, is the pulse length in space. SPL = 2-5 λ.
What is echo time?
the total time for a round trip of the ultrasound pulse.
What is the equation used to determine echo time and how can it be manipulated?
Δt = x/c = 2d/c
a total distance, x, with x = 2d, (out and back)
Where c = 1540 m/s
d = c Δt/2
Why is the echo time and thus distance travelled calculations important?
From this echo time measurement, the distance d is determined, and this is the basis of all pulse wave (PW) diagnostic ultrasound imaging.
Why is maximum depth important and what equation is used to calculate it?
The received echo can only be sensibly recognised and interpreted if it returns to the transducer during the next ‘off’ time of the duty cycle. The maximum depth of a reflection interface that can be unambiguously calculated correctly is given by
dMAX = (c PRP)/2 = c/(2 PRR)
Differentiate between the piezo-electric effect and the inverse piezo-electric effect
The inverse piezo-electric effect is the production of a dimensional change in the crystal when a voltage is applied. This effect is used to produce ultrasound pulses or waves.
The piezo-electric effect is the production of a voltage on the crystal when it is deformed by returning ultrasound pulses or waves.
What other way can you ensure a LZT crystal oscillates at a frequency, say, 10 MHz?
Apply a voltage across it pulsing at a frequency of 10 000 000Hz
Why is damping used in a diagnostic ultrasound transducer, and what forms of damping are incorporated?
Damping is used to produce a pulse of the correct duration, PD, and pulse length, SPL.
The damping used is a combination of mechanical and electronic damping.
What are typical pulse durations?
A typical transmit pulse lasts 3-5 wavelengths (Spatial pulse length) and so the pulse duration will be 3-5 times the period of the ultrasound wave. (The period is the duration of a single cycle) The overall time is referred to as the pulse duration.
A spatial pulse length is about 3-5 wavelengths.
And as such a pulse duration is about 3-5 periods.
What are typical Spatial Pulse lengths
A typical transmit pulse lasts 3-5 wavelengths (Spatial pulse length)
What are typical q-values?
Q-factor. This is a measure of the sharpness of the resonance, and can be expressed relating bandwidth to the resonance frequency.
Q = f0/B = f0/(fU - fL)
The Q-factor is thus a dimensionless quantity that is large if the damping is small, and small if the damping is large.
For CW situations you want an ultrasound transducer with Q around 700 to 800, while pulse ultrasound transducers have a Q in the range 2 to 3
why is it important to have a series of ultrasound pulses in diagnostic imaging and not a continuous sinusoidal wave exiting the transducer and passing into the patient.
The duration of the transmit pulse will be extended and so the image quality will be poor. The machine will not be able to resolve objects close together as the returning echoes will be received continuously.
What effect does transmit pulse length have on resolution?
The shorter the transmit pulse, the better the resolution
How can a short transmit pulse be achieved?
a) transmitting only a few cycles and b) using as high ultrasound frequency as possible. This is because the higher the frequency the shorter the period and so the total pulse duration (PD) will be shorter.
T = 1/f PD = T x (3-5)
Bandwidth B = 1/PD
This means as the pulse gets shorter the bandwidth gets larger and vice versa.
In other words, if you want a very short pulse a bigger range of frequencies must be combined
