Chapter 8: Transducers

Question 1

Transducer

Answer 1

Any device that converts one form of energy into another

Question 2

Electrical energy from the system is converted into sound during transmission

Answer 2

The reflected sound pulse is converted into electricity during reception

Question 3

7 components of basic transducer

Answer 3

Case 
Electrical shield
Acoustic insulator 
PZT or active element 
Wire 
Matching layer 
Backing material or damping element

Question 4

Piezoelectric Materials

Answer 4

-convert sound into electricity (visa versa)
-aka ferroelectric
-quartz and tourmaline are found in nature
-lead zirconate titanate (PZT) is a synthetic used in clinical transducers
Aka- ceramic, active element, crystal

Question 5

Piezoelectric Effect

Answer 5

Property of certain materials to create voltage when pressure is applied to them (mechanically deformed)

Question 6

Reverse Piezoelectric Effect

Answer 6

Piezoelectric materials change shape when a voltage is applied to them

Question 7

Case

Answer 7

Metal/plastic cylindrical tube that protects the internal components of transducer from damage and insulted patient from electric shock (do NOT use with cracked case)

Question 8

Acoustic Insulator

Answer 8

Uncouples internal components of transducer from the case (prevents vibrations in case affecting PZT)

Question 9

Electrical shield

Answer 9

Helps prevent electrical noise from contaminating clinically important electrical signals

Question 10

Wire

Answer 10

Electrical connection between PZT and US system.
Active element must have electrical connection
-transmission: voltage from the system excites the crystal producing sound
-reception: reflected sound wave deforms the crystal producing a voltage (do not use with frayed wire

Question 11

Element/ backing material

Answer 11

Rule- short pulses create more accurate images

• material bonded to PZT to reduce its ringing
• restricts the extent of PZT deformation crystal damping enhances axial resolution
• epoxy resin impregnated with metal powder
• too much makes transducer insensitive

Question 12

Active element

Answer 12

1/2 wavelength thick

-piezoelectric crystal

Question 13

Matching layer

Answer 13

1/4 wavelength thick

• different impedances result in reflections at boundaries
• designed with impedance between active element and skin
• may use multiple matching layers (impedance of each layer is less and less from PZT to face of probe)
• same with coupling gel- further inc% sound transmitted inside/outside body
• PZT > matching layer > gel > skin

Question 14

Decreased sensitivity

Answer 14

During reception, transducers with backing material are less able to convert low level sound reflections into meaningful electrical signals (reduces active element vibration during transmission and reception)

Question 15

Wide bandwidth

Answer 15

Bandwidth- range of frequencies between highest and lowest emitted by Tx; main frequency is called the center, resonant, primary, or natural frequency
-imaging probes produce pulses that are wide bandwidth (or broadband)
[PZT is restricted - pulse is short duration click vs long steady tone]
-therapeutic US/CW Doppler- no backing material = narrow bandwidth
Long duration = events - narrow bandwidth
Short duration = events - wide bandwidth
Example:
3 MHz main frequency produces a sound pulse with range of 1 MHz-5MHz bandwidth = 4 MHz (5-1)

Question 16

Low quality factor

Answer 16

Quality factor- unitless # related to extent of damping; inversely related to bandwidth and directly related to pulse length
Q factor= main frequency/ bandwidth
-imagining probes use backing material - have a wide bandwidth and are therefore low q; therapeutic/CW - narrow bandwidth and high q
Damping & wide bandwidth = lower Q factor
No damping & narrow bandwidth = higher Q factor
Example:
3 MHz tx with 4 MHz bandwidth has a Q factor of 3/4 or .75

Question 17

Imaging Transducers

Answer 17

Pulses w/ short duration & length 
Backing material to limit ringing 
Reduced sensitivity 
Wide bandwidth 
Lower Q-factor 
Improved axial resolution

Question 18

Non-imaging transducers

Answer 18

CW or pulses w/ long duration & length 
No backing material 
Increased sensitivity 
Narrow bandwidth 
Higher Q-factor 
Cannot even create an image

Question 19

Piezoelectric Materials (pt.2)

Answer 19

1. Depolarization - loss of piezoelectric properties destroyed by exposure to high temperature, above the curie point
2. Sterilization - complete destruction of all microorganisms by exposure to extreme heat, chemicals or radiation
   [instruments that penetrate the skin; Tx should never be sterilized using dry heat, moist heat, or chemicals bc this could cause damage]
3. Disinfection - application of chemical agent to reduce or eliminate infectious organisms on an object; attempts to reduce microbial load
   [low level disinfection for less critical instruments that simply come in contact with intact skin; endocavitary probes require HLD-high level disinfection; most critical require sterile covering]

Question 20

Piezoelectric Materials (pt.3)

Answer 20

The piezoelectric properties of PZT (lead zirconate titanate) do not occur spontaneously. These are created by polarization - exposing the material to a strong electrical field while being heated to a substantial temperature known as the curie temp (curie point) - approx 360 degrees C / 680 degrees F

Question 21

Transducer Frequencies

Answer 21

Frequency produced by a transducer depends on whether the sound is continuous or pulses wave

Continuous wave:
Electrical frequency =acoustic frequency
A CW ultrasound system produces continuous electrical signal that excites the active element. Frequency of sound emitted is equal to frequency of the electrical signal.