Exam 3 - Lesson 5

Question 1

Role of a transducer?

Answer 1

Converts electrical energy (voltage) to mechanical energy

Question 2

What is Piezoelectricity?

Answer 2

property of crystals to generate voltage (electrical energy) to vibrate back & forth

Question 3

What is the Reverse Piezoelectric Effect?

Answer 3

pressure generated when fluctuating voltage applied causing expansion in thickness (alternates btwn +/-)

Question 4

Manufactured Piezoelectric material

Answer 4

Ceramics - PZT ( Lead Zicronate Titanate)

* Impedance of PZT is 30 - 40 Rayls

Question 5

What is Poling?

Answer 5

strong electric field applied to PZT material at high temps. Molecules slide and align themselves

Question 6

Cure point

Answer 6

Temp above which Pzt material loses its poling. Material becomes depoled so it can be used

Question 7

What is the active element in a transducer?

Answer 7

The crystal

Question 8

What determines the operating fq in PW?

Answer 8

Thickness of the crystal

Question 9

Crystal thickness formula

Answer 9

λPZT / 2 …. thickness = half wavelength in PZT

Question 10

What does the diameter of the crystal tell us?

Answer 10

sound beam width

Question 11

Crystal width vs. fq

Answer 11

Inverse

Question 12

Backing/ Damping material

Answer 12

dampens crystal vibrations to limit ringing.

Question 13

If you increase damping, what will also increase?

Answer 13

Bandwidth (larger range of fq), Axil Res

Question 14

How does backing affect impedance?

Answer 14

backing Z = PZT so prevents reflection

Question 15

advantages of damping/ backing?

Answer 15

Shortens SPL, Increases axil res, and increases bandwidth

Question 16

Matching layer

Answer 16

protects crystal & helps transmission from crystal / skin by matching impedances (z)

Question 17

Lens

Answer 17

focuses ultrasound beam to reduce slice thickness

Question 18

Transducer fq in CW

Answer 18

Transducer fq (5 MHz) = drive voltage fq (5 Mhz)

Question 19

Transducer fq in PW

Answer 19

short bursts (pulses) applied to crystal

Question 20

What is the operating fq in PW determined by?

Answer 20

Propagation speed (c) & thickness of crystal

Question 21

If you have a thinner crystal, what will your operating fq be?

Answer 21

higher fq ; shorter pulse

Question 22

Pulse length/ fq / bandwidth

Answer 22

Damping limits ringing - shortens SPL - widens bandwidth

Question 23

How to get transducer bandwidth?

Answer 23

Max fq - Min fq

Question 24

What is Quality Factor?

Answer 24

pureness of the beam (no range of fq’s)

Question 25

What do Imaging transducers need to emit for quality factor

Answer 25

short pulses (SPL) with wide bandwidths = low Q factor (wide range in fq)

Question 26

What has the best quality factor?

Answer 26

CW bc always ringing at same fq & has a narrow bandwidth. The closer you are to no variation, the increased quality factor.

Question 27

Transducer sensitivity

Answer 27

ability to detect reflectors at given depth (penetration)

Question 28

How are bandwidth and sensitivity related?

Answer 28

Inverse bc broad bandwidth = shorter pulse = high fq = less penetration or sensitivity

Question 29

CW beam shape

Answer 29

single flat disc (unfocused) crystal, no lens | Acoustic fq = voltage fq

Question 30

What does every beam start out as in CW?

Answer 30

every beam starts out at the same diameter as the crystal

Question 31

What is the narrowest point (natural focus) equal to?

Answer 31

Narrowest point = 1/2 crystal diameter

Question 32

Near Zone Length (NZL)

Answer 32

distance from transducer face to point of min. beam width

Question 33

NZL formula

Answer 33

NZL = diameter of transducer ^2 / 4x wavelength

Question 34

How is NZL related to diameter and fq?

Answer 34

^^ NZL = ^^ D/ Fq

Question 35

Far field (fraunhofer)

Answer 35

region deeper than natural focus where beam diverges

Question 36

Qualities of far field?

Answer 36

More uniform, less intensity = poor sensitivity & resolution

Question 37

Huygens Principle

Answer 37

sound waves propagate in spherical ripples. Most of the intensity is in the center of focus zone *what forms hourglass shape