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MIDS 139 (Core Exam) > Ch 4 Transducers > Flashcards

Ch 4 Transducers Flashcards

1
Q

What is a transducer?

A

-Transducers are a device that converts electric energy into sound energy
-Some transducers can be attached to a phone/tablet (POCUS)

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2
Q

What is the piezoelectric effect?

A

-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

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3
Q

Why is the piezoelectric effect important?

A

B/c it allows us the ability to convert electrical energy into sound energy (mechanical pressure) + vice versa

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4
Q

What is a common material used in u/s crystals?

A

Lead zirconate titanate

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5
Q

What is the process of “poling”?

A

It is how piezoelectric is made - in a strong electric field at high temperatures

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6
Q

What is curie point?

A

When the crystals are heated at about 350 celsius, then cooled to allow the elements to compress + expand

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7
Q

List other synonyms for piezoelectric elements?

A

Crystal, active element or transducer element

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8
Q

When a voltage is applied to the piezoelectric elements, will the thickness of the element increase or decrease?

A

Either! Depends on the polarity of the voltage

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9
Q

What is operating frequency (Fo)?

A

It is equal to the driving voltage for transducer operation

(is also called the resonance frequency)

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10
Q

What is the operating frequency determined by with CW + PW?

A

CW:
-driving voltage

PW:
-propagation speed
-crystal thickness

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11
Q

How thick are u/s elements?

A

0.2 - 1 mm thick

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12
Q

What are wide-bandwidth transducers?

A

-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

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13
Q

Do higher pressure portions of the sound wave travel faster or slower than lower pressure portions?

A

Faster

(higher = faster, lower = slower)

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14
Q

Do the sound waves change shape as it travels through tissue?

A

Yes! The change in sinusoidal shape introduces harmonics

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15
Q

Explain harmonic imaging?

A

-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

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16
Q

How does harmonic imaging reduce artifacts + improve our imaging?

A

-Removes high amplitude noise
-Enhances near field resolution
-Helps when imaging isoechoic lesions with a shallow depth

17
Q

List the 4 layers of the transducer from front to back?

A

-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)

18
Q

What is the backing/damping material?

A

-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

19
Q

Do CW transducers have a damping/backing material?

A

Nope

20
Q

List 2 advantages + 2 disadvantages of the damping/backing material?

A

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)

21
Q

What is the matching layer?

A

-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

22
Q

What is gel (a coupling medium) used for?

A

-Eliminates air b/w probe + skin
-Gel removes acoustic impedance mismatch b/w the matching layer + the skin
-It creates less reflection + more transmission

23
Q

What is the acoustic lens?

A

-Focus’s the beam
-Creates another acoustic impedance mismatch b/w the matching layer + the pt

24
Q

What is CMUT?

A

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)

25
Q

List advantages to using CMUT?

A

-Broader bandwidth
-Improved detail resolution
-Greater # of electronic components in probe
-Less energy loss
-Some have 3D imaging
-Cost effective (3 probes in 1)

26
Q

What is the sound beam shape?

A

-3D space consisting of a lateral, axial + elevation plane
-The area of beam convergence is the natural focus

27
Q

Differentiate the near + far fields/zones?

A

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)

28
Q

What is the near zone length (NZL)?

A

-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)

29
Q

If aperture doubles, what happens to the NZL?

A

Increases by a factor of 4

30
Q

What is aperture?

A

Size/number of elements on the probe

31
Q

If frequency doubles, what happens to the NZL?

A

Also doubles

(directly related)

32
Q

Focusing beams improves what kind of resolution?

A

-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)

33
Q

How is sound focused?

A

-Using a lens
-By phasing
-Using curved crystals (opposed to flat)

34
Q

At the focus (focal region), is beam width increased or decreased?

A

Decreased

35
Q

Signal intensity + strength is greatest where?

A

At the focus or slightly below it (shallower)

(m/c slightly below it)

35
Q

Where would we get the best + worst lateral resolution in regards to focus?

A

Best: at or before focus
Poor: in far zone or beyond focus

36
Q

What does lateral resolution help us distinguish?

A

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)

36
Q

Define lateral resolution?

A

The min distance that 2 structures are separated side by side or perpendicular to the sound beam

(best at focus or just before it)

MIDS 139 (Core Exam) (16 decks)

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