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)
