Ch 4 Transducers Flashcards
What is a transducer?
-Transducers are a device that converts electric energy into sound energy
-Some transducers can be attached to a phone/tablet (POCUS)
What is the piezoelectric effect?
-A phenomenon where an electric field (voltage) results when crystal materials are mechanically deformed
-The ability of certain materials to generate an electric charge in response to applied mechanical stress
Why is the piezoelectric effect important?
B/c it allows us the ability to convert electrical energy into sound energy (mechanical pressure) + vice versa
What is a common material used in u/s crystals?
Lead zirconate titanate
What is the process of “poling”?
It is how piezoelectric is made - in a strong electric field at high temperatures
What is curie point?
When the crystals are heated at about 350 celsius, then cooled to allow the elements to compress + expand
List other synonyms for piezoelectric elements?
Crystal, active element or transducer element
When a voltage is applied to the piezoelectric elements, will the thickness of the element increase or decrease?
Either! Depends on the polarity of the voltage
What is operating frequency (Fo)?
It is equal to the driving voltage for transducer operation
(is also called the resonance frequency)
What is the operating frequency determined by with CW + PW?
CW:
-driving voltage
PW:
-propagation speed
-crystal thickness
How thick are u/s elements?
0.2 - 1 mm thick
What are wide-bandwidth transducers?
-They can be used to operate a probe at more than 1 frequency
-Probe is driven at 1, 2 or 3 selectable frequencies by voltage pulses
-They have push button frequency switching
-They allow for harmonic imaging
Do higher pressure portions of the sound wave travel faster or slower than lower pressure portions?
Faster
(higher = faster, lower = slower)
Do the sound waves change shape as it travels through tissue?
Yes! The change in sinusoidal shape introduces harmonics
Explain harmonic imaging?
-Harmonic imaging improves our images by sending pulses of the fundamental/operating frequency into the body + then receives the 2nd harmonic frequency (which is double the frequency that was sent in)
-The 2nd harmonic echoes are 2x the fundamental frequency
-Harmonic imaging improves our lateral resolution
How does harmonic imaging reduce artifacts + improve our imaging?
-Removes high amplitude noise
-Enhances near field resolution
-Helps when imaging isoechoic lesions with a shallow depth
List the 4 layers of the transducer from front to back?
-Lens (focus’s beam)
-Matching layer (improves transmission by reducing acoustic mismatch)
-Piezoelectric element (converts electrical energy into sound energy)
-Backing/damping material (reduces SPL + PD which improves axial resolution)
What is the backing/damping material?
-Mixture of plastic or epoxy resin attached to the rear face of the probe element
-Reduces # of cycles in a pulse (ringdown)
-Reduces pulse duration + SPL which improves our axial resolution
Do CW transducers have a damping/backing material?
Nope
List 2 advantages + 2 disadvantages of the damping/backing material?
Advantages:
-improves resolution (spatial + axial)
-broadens bandwidth (wider range of frequencies emitted)
Disadvantages:
-reduces amplitude known as “efficiency” (ability to transfer energy from 1 form to another)
-reduces sensitivity (ability to detect weak echoes)
What is the matching layer?
-Very thin material on the probe face to improve sound transmission across the element tissue boundary
-Often multiple matching layers are used to improve the sound transmission over a large range of frequencies
-Allows more energy to exit the front of the element into the patient, rather than being lost as heat
What is gel (a coupling medium) used for?
-Eliminates air b/w probe + skin
-Gel removes acoustic impedance mismatch b/w the matching layer + the skin
-It creates less reflection + more transmission
What is the acoustic lens?
-Focus’s the beam
-Creates another acoustic impedance mismatch b/w the matching layer + the pt
What is CMUT?
Capacitive micromachined u/s transducers:
-new class of probes that are composed of thousands of microscopic silicon drums/elements
-they have an acoustic impedance much lower than ceramic elements
-ex. butterfly iQ (handheld u/s device)
List advantages to using CMUT?
-Broader bandwidth
-Improved detail resolution
-Greater # of electronic components in probe
-Less energy loss
-Some have 3D imaging
-Cost effective (3 probes in 1)
What is the sound beam shape?
-3D space consisting of a lateral, axial + elevation plane
-The area of beam convergence is the natural focus
Differentiate the near + far fields/zones?
Near (fresnel zone):
-region from probe to the min beam width
-beam width decreases with increasing distance from probe
Far (fraunhofer zone):
-region beyond the min beam width
-beam width increases with increasing distance from probe
(think of a glass that is narrower at the top + wider at the bottom)
What is the near zone length (NZL)?
-Aka focal length
-Distance from the probe to the natural focus
-Determined by size + Fo of the elements
-Increases with increasing frequency + with an increase in size of the elements or # of crystals (aperture)
(think it is from the top of the glass to the center of it)
If aperture doubles, what happens to the NZL?
Increases by a factor of 4
What is aperture?
Size/number of elements on the probe
If frequency doubles, what happens to the NZL?
Also doubles
(directly related)
Focusing beams improves what kind of resolution?
-Lateral (beam width) resolution
-Only in the near zone
-Beam width decreases in focal region + widens in region beyond it (think narrow in middle of glass then wider at bottom)
How is sound focused?
-Using a lens
-By phasing
-Using curved crystals (opposed to flat)
At the focus (focal region), is beam width increased or decreased?
Decreased
Signal intensity + strength is greatest where?
At the focus or slightly below it (shallower)
(m/c slightly below it)
Where would we get the best + worst lateral resolution in regards to focus?
Best: at or before focus
Poor: in far zone or beyond focus
What does lateral resolution help us distinguish?
Echoes placed side by side
(can see 2 structures side by side if at focus, but can only see 1 structure when above/below focus)
Define lateral resolution?
The min distance that 2 structures are separated side by side or perpendicular to the sound beam
(best at focus or just before it)
