Week 2: Transducer design Flashcards
What is a transducer?
device converting non-electrical into electrical signals
i.e. electrical transmitted pulses into US pulses & US echo to electrical signals
in the piezoelectric effect; materials expand or …. when a postive (lead, ….. & titanium) or a …. (O2 ions) voltage is applied
1) contract
2) Zirconium
3) negative
When is a drop produced in relation to the probe and material?
When material is stretched or compressed by external force
Describe Dipolar molecules
+ve charge one end, -ve at the other
What does +ve poles move towards, and what does this do to alignment?
moves towards -ve electrodes. If voltages change across element so does alignment = effecting thickness
What is the most common material used for manufacturing medical imaging PZT probes?
synthetic polycrystalline (crystalline grains)/ ceramic material (lead, zirconate, titanate)
Why is this PZT plate/probe used (crystalline grains)?
- high sensitivity
- cope w/ large acoustic powers
What is an advantage of crystalline grains PZT?
- can be formed to a curve or flat shape
- elements fired to make rigid (original powder form)
After firing, +ve charged lead, zirconium and titanium ions and -ve charged O2 ions are …… so each …. contains +ve net and -ve net charge. They are fixed…. but equal distance apart.
1) arranged
2) net
3) opposite
What are PZT divided into and polarised (orientated in the same direction)?
regional numbers (AKA domain)
How is PZT polarised to make a uniform direction? (image 3.3a - no net polarisation)
- materials heated >200 degrees C, applying strong electrical field across it = poling
- Makes the dipoles (in the domain) switch to direction closest to direction of poling electric field
- when cooled ‘frozen’ to form near orientation –> parallel to poling field.
what are the advantages of modified PZT?
- increased BW
2. Increased sensitivity
With modified PZT, there are narrow spaces/channels filled with polymer (image 3.34). What does this help with?
the columns can vibrate more efficiently, reduced density of polymers resulting in reduced overall density and reduced Z
Why is the thickness of the plate chosen to be 1/2 of λ at the centre f of transmit pulse?
makes it expand and contract strongly when alternating voltages at that f are applied = 1/2 Resonance
Why does 1/2 resonance occur (producing a large amplitude)?
reverberating wave traveling back and forth across the plate will travel exactly 1 λ when arriving back at starting position.
What are PDVF (plastic polyvinyldifluroide) used in and its properties?
used in: membrane hydrophones
- wide BW
- short impulse response
- greater focusing
- increased sensitivity
What elements are single crystal transducers doped with?
either 1) lead, magnesium and niobium (PZT-PT)
2) lead, zinc and niobium (PZN-PT)
What does single crystal transducer do that ceramic material PZT transducer can’t?
during polarisation of ceramic PZT, structure limits alignment of domains
Single crystal - has no grain boundaries can be poled to give perfect alignment of dipoles
What advantages does single crystal transducer have?
- wider BW
- improved axial resolution
- -> used for multi-f and harmonic imaging
mirocrmachined US transducers (MUT’s) have better …., wider …. providing shorter US pulses, this improving ….. …..
1) Z
2) BW
3) axial res
What does this image show and what is required?
capacitive micro-machined transducer (CMUT)
steady bias voltage between electrodes
What can happen if increasing the bias voltage in CMUT?
increased beyond a limit (collapse voltage limit) causes membrane to contact with the base = electrical breakdown or damage
When is the > efficiency obtained?
when membrane is close to base (collapse voltage). Solved with placing small structure beneath membrane
What is this image?
peizoelectric micro-machined US transducer (PMUT)
How does PMUT (2-22MHz) function in T and R?
T = voltage applied leads to vibration of membrane R = deflection of membrane due to incoming pressure waves = lateral changes in dimension, which produce electrical signal at electrodes
What can CMUT probes be used for?
- HI
- high f small-scale (intravascular imaging)
What does ‘reasonce’ mean?
- element vibrates strongly at f, thickness = 1/2 a λ
- f where material/structure vibrates naturally
image HERE –> 1/2 max transducer output -3dB BW
generated waves will be partially ….. from front and back faces and …… within the element
1) reflected
2) vibrate
in piezoelectric element, thickness governs ….. ….
resonant f
What is this image showing (Hugyens principle)?
- row of sources, spherical waves from each interface forming a series of wavefronts
- waves propagate out
- parallel to surface forming plane waves
- non-parallel wave parts interfere destructively and cancel each other out
Linear arrays have …. elements and a λ ~ ….. width and have wide …..
1) 128
2) 1.3
3) Field of view (FOV)
Linear arrays allow…? and separate elements …?
a) elements to be narrow for wide of angles in scan plane
b) to prevent cross talk
Why is a matching layer on a transducer needed?
if PZT directly contacted w/ patient ~80% wave power is reflected back = poor sensitivity
IMAGE HERE
what does the intensity of reflected wave depend on when talking about matching layer?
Z of tissue
Z of matching layer between impedance of PZT …. and impedance of tissue (t) should be equal to what equation?
1) element
√ (Zpzt x Zt)
What should 100% transmission through matching layer that only occurs at 1 f, thickness be in relation to λ?
‘thickness equal to 1/4 of λ’
IMAGE HERE
While matching layer improves sensitivity at centre f, it also acts as…?
- f filter
- reducing BW
e. g. -3dB transducer with 1/4 λ, around 60% centre f is achieved.
How can the reduced BW be improved?
- larger BW transducer, also improved axial res.
what can multiple matching layers do?
- changes in Z from PZT to skin - effective over different f
- improves BW
How can composite PZT help with matching layers?
- close narrow spaced channels & filled w/ polymer = reduced Z than PZT alone = reduced matching problem
Why is a backing layer required?
prevents excess vibration
what does reduced vibration do w/ backing layer properties?
- shorter US pulses generated
- improve axial res
How can a backing layer help with long pulses?
- “dampens”/reduced ringing in materials with high Z characteristics and ability to absorb US
why is a (increased f = shorter λ & shorter pulse) short pulse often desirable in US imaging?
increases axial res
What is pulse length (PL) dependent on?
f
What is this imaging showing?
IMAGE HERE
1) T pulse sent out, spacial PL (SPL) with 2 objects = 1/2 SPL apart
2) pulse coming into 1st boundary, some US is reflected back (# sent back = Z mismatch properties)
3) T pulse reaches 2nd boundary causing more US reflection, w/ R pulse going back t transducer
4) R pulse looks like 1 long pulse (2 reflect pulses sent back) = appears as just 1
What does thin element cause in relation to PL & f?
greater resolution, poor penetration
What does thick element cause in relation to PL & f?
greater penetration, poor res
In wider beams (i.e. BW), what can this effect?
lateral res - 2 objects cannot be separately identified
In narrow beams (i.e. BW), what does this mean with lateral res?
2 objects can be separately identified
Single element transducer beam shape dependent on what?
diameter of element & λ of pulse
How can the lens on the transducer help when imaging?
focus the beam
in Array transducer, several element are used to form the beam what does this affect?
- beam maybe focused
beam shape
What is not the same in array transducers?
T and R beam shape
In broadband transducers (wide BW = produce short pulses), -3dB range of f over which….
output is at leas 1/2 of the maximum
IMAGE here
whats the advantage of broadband transducers?
- used in HI - decreased f signal 1st T and harmonic is R
- pulses can be broken down down to different component f
- able to T and R over wide range of f
What can broadband transducers also be used in?
- multi-f imaging - transducer operate a different centre f, user select f to optimise penetration and res
- f compounding - different f to create different depths of image
What does HI require?
a fundamental f
what is HI due to?
non-linear propagation (changes in pulse spectrum)
if a fundamental f (F0) is sent out/T, what is R?
Echoes returned at harmonic f, 2f0, 3f0 etc
When HI is forming an image, which pulse does it this with?
2f0
What are the advantages of HI?
- narrow beam = suppresses artefacts (side lobes)
- reduced acoustic noise i.e. reverberations
What is assumed with non-linear propogation?
- C is a fixed speed (1540ms)
- a linear relationship w/ amplitude of wave at source & amplitude elsewhere in the beam
High pressures >1MPa result in what?
picture breaks down and non-linear propogation is noticeable
speed of wave travels related to….., which either …. or ….. local speed
1) local particle velocity
2) increases
3) decreases
What does non-linear propogration result in?
some energy in the pulses being transferred from fundamental f to it’s harmonics
What happens to the medium at high pressure amplitudes?
becomes compressed increasing stiffness (K) and C
What can the increased K and C result in?
compression catches up with rarefaction resulting in a shock front and increase the pressure
